Method for producing fe-ni-based alloy thin sheet, and fe-ni-based alloy thin sheet

Abstract

The present invention provides an Fe-Ni-based alloy thin sheet which is capable of achieving both good flatness and etching warpage resistance.　The present invention specifically provides a method for producing an Fe-Ni-based alloy thin sheet that obtains a thickness of 0.5 mm or less, the method including: a finish cold rolling step in which finish cold rolling with a draft of 5% to 50% is applied to a cold rolling material that contains, in mass%, 0.01% or less of C, 0.5% or less of Si, 1.0% or less of Mn, and 28-52% of Ni, with the balance being made up of Fe and inevitable impurities, so as to form a thin sheet; a recrystallization annealing step in which annealing is performed on the thin sheet after the finish cold rolling at the temperature of 800°C to 1000°C inclusive; a tensile stress application step in which shape correction is performed on the thin sheet after the recrystallization annealing step under the tension of 200 MPa to 600 MPa so as to impart a tensile stress to the thin sheet; and a strain relief annealing step in which strain relief annealing is performed on the thin sheet after the tensile stress application step at a temperature of 400°C to 750°C inclusive. The present invention also specifically provides the Fe-Ni-based alloy thin sheet.

Description

Fe-Ni alloy thin plate and manufacturing method thereof

[0001] The present invention relates to a method for producing an Fe—Ni alloy thin plate and to an Fe—Ni alloy thin plate.

[0002] Various studies have been conducted in the past to improve the performance of Fe—Ni alloy thin plates used in metal masks, lead frames, etc. One known method for manufacturing such metal masks and lead frames is to subject an Fe—Ni alloy thin plate to etching. Various studies have been conducted in the past to improve the etching accuracy. For example, Patent Document 1 describes a method in which an Fe—Ni alloy containing 32 to 38 wt % of Ni is cold-rolled, and then subjected to shape correction by imparting a strain of 0.4 to 3% elongation, followed by heating at a sheet temperature of 550 to 690°C and a tension of 2 kgf / mm ２ Patent Document 2 discloses a method for producing an Fe—Ni-based low thermal expansion alloy thin plate having excellent plate shape and thermal shrinkage resistance, characterized by performing stress relief annealing under the following conditions: Patent Document 2 also discloses a method for producing a low thermal expansion alloy thin plate in which a hot-rolled material is cold-rolled and annealed at least once, and the cold rolling rate in the cold rolling before final recrystallization annealing is 90% or more, the annealing temperature in the final recrystallization annealing is 850° C. or more, and the final cold rolling rate is 30% or less, in order to improve the etching rate and etching precision of the Fe—Ni—Co-based low thermal expansion alloy thin plate.

[0003] Furthermore, in Patent Document 3, the applicant of the present application has proposed an Fe—Ni-based alloy thin plate having a thickness of 0.5 mm or less, which has both good flatness and excellent etching warpage resistance, comprising a finish cold rolling process in which a cold rolling material is subjected to finish cold rolling at a rolling reduction of 5% to 50% to form a thin plate, a final annealing process in which the thin plate after the finish cold rolling is annealed at a temperature of 800° C. or higher and 1000° C. or lower, and a tensile stress imparting process in which the thin plate after the final annealing process is subjected to shape correction under a tension condition of 200 to 400 MPa to impart tensile stress.

[0004] JP 10-204541 A JP 2003-253398 A JP 2021-014639 A

[0005] Due to the increasing complexity and precision of recent products, the Fe-Ni thin plates used as raw materials for these products are required to have a wide width, high flatness on the etched surface, and suppress warpage after etching. Shape correction using a roller leveler or tension leveler has traditionally been effective in improving flatness, but residual stress imparted to the thin plate during shape correction is released during etching, causing warpage, which is problematic. One method known to reduce warpage due to such residual stress is stress relief annealing, in which the thin plate is annealed at a temperature below its recrystallization temperature to remove strain. However, as product shapes become more diverse and complex, stress relief annealing alone cannot completely remove the effects of processing strain remaining in the thin plate, and warpage still tends to occur. While the invention described in Patent Document 1 can improve thermal shrinkage resistance and flatness, it does not take into consideration suppressing warpage that occurs when wide thin plates are half-etched, leaving room for further study. Furthermore, the invention described in Patent Document 2 is an invention that can increase the (200) plane integration degree on the rolled surface and thereby improve etching precision, but no description can be found of suppressing warpage in a process after finish rolling, and there remains room for further investigation into further suppression of warpage and improvement of flatness.

[0006] Furthermore, the invention described in Patent Document 3 is an excellent invention that can achieve both good flatness and excellent etching warpage resistance. For example, in the case of metal masks, high-precision pattern formation is required, so half-etching to various depths in the thickness direction is performed on the metal mask material. On the other hand, the deeper the etching, the greater the risk of warpage due to the influence of residual stress. Therefore, there is a demand for a material that is less susceptible to deformation such as warpage even when half-etched to various depths. Therefore, an object of the present invention is to provide a method for producing an Fe—Ni-based alloy thin plate having a thickness of 0.5 mm or less, which has even better flatness and etching warpage resistance.

[0007] One aspect of the present invention is a method for producing an Fe—Ni-based alloy thin plate, comprising: a finish cold rolling process in which a cold rolling material containing, by mass, C:≦0.01%, Si:≦0.5%, Mn:≦1.0%, Ni: 28-52%, with the balance being Fe and unavoidable impurities, is subjected to finish cold rolling at a rolling reduction of 5% to 50% to produce a thin plate; a recrystallization annealing process in which the thin plate after the finish cold rolling is annealed at a temperature of 800°C to 1000°C; a tensile stress imparting process in which the thin plate after the recrystallization annealing process is subjected to shape correction under a tension of 200 to 600 MPa to impart tensile stress; and a strain relief annealing process in which the thin plate after the tensile stress imparting process is subjected to strain relief annealing at a temperature of 400°C to 750°C, thereby obtaining an Fe—Ni-based alloy thin plate having a thickness of 0.5 mm or less. Another aspect of the present invention is an Fe—Ni-based alloy thin plate having a thickness of 0.5 mm or less, containing, by mass%, C: ≦0.01%, Si: ≦0.5%, Mn: ≦1.0%, Ni: 28 to 52%, with the remainder consisting of Fe and unavoidable impurities, wherein a sample having a length of 150 mm and a width of 30 mm is cut out from the Fe—Ni-based alloy thin plate, and the sample is etched from one side to remove half of the thickness of the sample, the amount of warpage is 10 mm or less, and the maximum floating height of the Fe—Ni-based alloy thin plate over a length of 800 mm is 1.5 mm or less.

[0008] According to the present invention, it is possible to provide an Fe—Ni alloy thin plate having a thickness of 0.5 mm or less, which can achieve both good flatness and etching warpage resistance.

[0009] An embodiment of the present invention will be described below. First, as mentioned above, the invention proposed by the applicant and disclosed in Patent Document 3 can achieve both good flatness and excellent etching warpage resistance. Based on this, the present applicant has discovered optimal manufacturing conditions that can achieve both good flatness and excellent etching warpage resistance. A method for manufacturing an Fe—Ni-based alloy thin sheet according to the present invention will be described below. In the present invention, for example, a hot-rolled material is prepared having a composition containing, by mass, C: ≦0.01%, Si: ≦0.5%, Mn: ≦1.0%, Ni: 28-52%, with the balance consisting of Fe and unavoidable impurities. The hot-rolled Fe—Ni-based alloy thin sheet having the composition specified in the present invention has the composition necessary to obtain the desired thermal expansion coefficient. The reason for specifying the above-mentioned composition range is as follows: [C: ≦0.01 mass%] C is an element that affects etchability. Since excessive C content inhibits etchability, the upper limit of C is set to 0.01%. The lower limit may be 0%, but is not particularly limited because significant amounts are included in the manufacturing process. [Si: ≦0.5 mass%, Mn: ≦1.0 mass%] Si and Mn are typically used for deoxidation purposes and are contained in trace amounts in Fe-Ni alloys. However, excessive content can easily cause segregation, so the Si content is set to 0.5% or less and the Mn content to 1.0% or less. The preferred Si and Mn contents are 0.1% or less and 0.5% or less. The lower limits for Si and Mn are not particularly limited, but can be set to, for example, 0.01% for Si and 0.05% for Mn. [Ni: 28-52 mass%] Ni is an element that adjusts the thermal expansion coefficient and significantly influences the low thermal expansion properties. A Ni content of less than 28% or more than 52% loses its effect of reducing the thermal expansion coefficient, so the Ni content range is set to 28-52%. The lower limit of the Ni content is preferably 30%, more preferably 32%, and even more preferably 34%. The upper limit of the Ni content is preferably 50%, more preferably 45%, and even more preferably 38%. The remaining components are Fe and unavoidable impurities. In this embodiment, a portion of the Ni can be substituted with Co in order to adjust the thermal expansion characteristics and to provide high strength.In order to make it easier to impart the above-mentioned effects to the material, the upper limit of Co is preferably set at 20%.

[0010] <Cold Rolling Material> In this embodiment, a strip-shaped cold rolling material having the above-described composition is prepared. This cold rolling material can be obtained, for example, by mechanically or chemically removing an oxide layer from a hot-rolled material having the above-described composition. Furthermore, the edges of the cold-rolled material may be trimmed to prevent defects such as cracks from occurring at the edges during cold rolling. If necessary, a homogenization heat treatment at approximately 1200°C may be performed before cold rolling.

[0011] <Intermediate Cold Rolling, Intermediate Annealing> In the present invention, one or more intermediate cold rolling steps may be performed to adjust the plate thickness before the finish cold rolling described below. In this embodiment, the case where intermediate cold rolling is performed is described. However, if the desired plate thickness can be adjusted after hot rolling, intermediate cold rolling may be omitted. Furthermore, the thin plate after intermediate cold rolling may be subjected to intermediate annealing to soften the work-hardened material and remove processing strain. The temperature for this intermediate annealing may be set to a temperature of 800°C or higher, which promotes recrystallization. If the annealing temperature is less than 800°C, the material may not be sufficiently softened, and the desired properties may not be obtained. The upper limit of the annealing temperature is not particularly limited, but since excessively high temperatures are likely to prevent the desired properties from being obtained, it can be set to approximately 1100°C. The heating holding time may be adjusted appropriately depending on the material composition and plate thickness. This intermediate annealing can be performed by continuously passing the thin plate after intermediate cold rolling through a heating furnace set to the desired temperature. For example, the intermediate cold-rolled thin sheet may be pulled out from the rolled state, passed through a heating furnace, and wound into a roll.

[0012] <Finish Cold Rolling> In the manufacturing method of this embodiment, the intermediate material after the intermediate cold rolling or intermediate annealing described above is subjected to finish cold rolling with a reduction ratio of 5% to 50% to obtain a thin sheet. By keeping the reduction ratio within the above range, the difference in elongation between the center and edges of the intermediate material is reduced, thereby suppressing the occurrence of excessive corrugation and facilitating adjustment to a flat shape after the shape correction process described below. If the reduction ratio is less than 5%, central elongation tends to occur, and the flatness of the central part of the thin sheet after shape correction tends to decrease. Furthermore, if the reduction ratio is less than 5%, recrystallization of the thin sheet tends to be excessively promoted in the recrystallization heat treatment described below, which tends to reduce the hardness of the thin sheet. If the reduction ratio is more than 50%, the edge corrugation tends to become stronger, and the flatness tends to decrease after shape correction. The lower limit of the rolling reduction ratio is preferably 15%, and the more preferable lower limit is 20%. Furthermore, the upper limit of the rolling reduction ratio is preferably 40%, and the even more preferable upper limit is 30%. Here, the number of passes in the finish cold rolling is preferably one pass. The manufacturing method of this embodiment can be applied to thin plates with a width of 300 to 1100 mm. The plate thickness suitable for this embodiment is 0.5 mm or less, preferably 0.3 mm or less, and more preferably 0.2 mm or less. There is no particular lower limit for the plate thickness, but if it is too thin, it will be unsuitable for half etching, so it can be set to 0.02 mm, for example. The plate thickness is preferably 0.05 mm or more, and more preferably 0.08 mm or more.

[0013] <Recrystallization Annealing Step> In this embodiment, a recrystallization annealing step is performed on a thin sheet (finish cold-rolled thin sheet) that has undergone finish cold rolling, in which annealing is performed at a temperature of 800 ° C to 1000 ° C. This step sufficiently reduces the proof stress of the sheet while removing processing strain caused by rolling, making it easier to further improve flatness in the tensile stress application step described below. Here, if the temperature during recrystallization annealing is less than 800 ° C, processing strain cannot be sufficiently removed, and if the temperature is more than 1000 ° C, the hardness of the thin sheet tends to decrease too much, which is not preferable. The preferred lower limit of the recrystallization annealing temperature is 850 ° C, and the preferred upper limit of the recrystallization annealing temperature is 950 ° C.

[0014] <Tensile Stress Application Step> In the manufacturing method of this embodiment, a tensile stress application step is performed on a thin sheet (recrystallization-annealed thin sheet) that has undergone a recrystallization annealing step. This step corrects excessive edge corrugations and center elongation remaining in the recrystallization-annealed thin sheet, significantly improving flatness. In particular, the present invention is characterized by minimizing the bending stress imparted by shape correction using a tension leveler and performing correction primarily using tensile stress. Conventionally, to remove such processing strain, shape correction using a tension leveler or the like to impart bending stress has been performed, followed by stress relief annealing, in which the thin sheet is heat-treated at or below the recrystallization temperature. However, the processing strain introduced into the thin sheet by shape correction that imparts bending stress cannot be sufficiently removed even by stress relief annealing, potentially resulting in a decrease in the flatness of the thin sheet. The tensile stress application step of this embodiment makes it possible to improve flatness without imparting additional processing strain to the thin sheet and without imparting bending stress through the recrystallization annealing step described above. The shape correction can be performed using conventional shape correction devices such as a stretcher, roller leveler, or tension leveler (a tension leveler is used in this embodiment). However, since the correction is performed mainly by tensile stress as described above, when using a tension leveler, for example, it is not necessary to perform pressing using a leveler roll (straightening roll). Furthermore, when pressing using a leveler roll, it may be performed with a pressing amount equal to or less than the plate thickness. Here, the tension during shape correction can be set to 200 to 600 MPa. If the tension is less than 200 MPa, the correction will be insufficient. If the tension exceeds 600 MPa, there is a possibility that new shape defects will occur due to excessive tension. The preferred lower limit of the tension is 250 MPa, the preferred upper limit is 500 MPa, the more preferred upper limit is 450 MPa, and the even more preferred upper limit is 400 MPa. In the manufacturing method of the present invention, a thin plate having excellent flatness and etching warpage resistance can be manufactured by the stress relief annealing step described below, even without performing temper rolling (light reduction rolling), and therefore temper rolling can be omitted before and after the tensile stress imparting step.

[0015] <Strain relief annealing step> In this embodiment, the thin sheet (shape-corrected thin sheet) after the tensile stress imparting step is subjected to a strain relief annealing step, in which the thin sheet is annealed at a temperature of 400°C to 750°C, to obtain the Fe—Ni-based alloy thin sheet of the present invention. This "strain relief annealing step," which is not disclosed in Patent Document 3, is essential to the present invention. The tensile stress imparting step of the present invention is a step that imparts as little bending stress to the sheet material as possible compared to conventional methods. However, since a stress difference still remains between the surface layer and the central portion of the sheet material, there is a risk that the above-mentioned residual stress will be released when deep etching is performed, resulting in excessive warpage. By performing the above-mentioned strain relief annealing step in this embodiment, the residual stress difference in the sheet material generated by the tensile stress imparting step is eliminated, resulting in an Fe—Ni-based alloy thin sheet that has excellent flatness and can suppress warpage due to etching. According to this manufacturing method of the present invention, the lower limit of the strain relief annealing temperature is preferably 500°C, and the upper limit of the strain relief annealing temperature is preferably 700°C.

[0016] Next, the Fe—Ni alloy thin sheet of the present invention obtained by the manufacturing method of the present invention will be described. <Warpage Amount> The Fe—Ni alloy thin sheet of the present embodiment is characterized in that, when cut into a sample size of 150 mm in the longitudinal direction (rolling direction) and 30 mm in the transverse direction (direction perpendicular to the rolling direction), the amount of warpage of the sample after removing half of the sheet thickness is 10 mm or less. This reduces residual stress at the center of the sheet thickness, making it possible to suppress sheet deformation even when etching is performed to a depth at the center of the sheet thickness where the stress balance is more disrupted. This allows for satisfactory etching while ensuring shape accuracy. Thus, if the amount of warpage after removing half of the sheet thickness is 10 mm or less, which is more susceptible to warpage than removing one-third of the sheet thickness as described in Patent Document 3, residual strains at complex shapes and deep cross-sectional positions can be considered to be released, making the sheet suitable for a variety of etching processes. A more preferable upper limit of the amount of warpage is 7 mm, and an even more preferable upper limit of the amount of warpage is 5 mm. In this embodiment, a sample was cut so that the longitudinal direction coincided with the rolling direction to prepare a cut sample with a length of 150 mm and a width of 30 mm. Half of the sheet thickness was then removed by etching from one side, and one longitudinal end (top end) of the cut sample was then suspended in contact with a vertical surface plate. The horizontal distance between the vertical surface plate and the other end (bottom end) of the cut sample, which had moved away from the vertical surface plate due to warping, was measured as the amount of warpage. In this embodiment, a concave warpage of the etched surface was designated as "+" warpage, and a convex warpage of the etched surface was designated as "-" warpage. The amount of warpage was evaluated as an absolute value, with smaller absolute values ​​being preferable. In this embodiment, when a sample was cut to a size of 150 mm in the width direction (perpendicular to the rolling direction) and 30 mm in the length direction (rolling direction), it was preferable that the amount of warpage after removing half of the sheet thickness of the sample be 10 mm or less (more preferably 7 mm or less, and even more preferably 5 mm or less).

[0017] <Floating Height> The Fe—Ni-based alloy thin plate of this embodiment can also have a lower maximum floating height at an 800 mm length than conventional products. Specifically, when the length of the Fe—Ni-based alloy thin plate is 800 mm, the maximum floating height is 1.5 mm or less. As such, the thin plate obtained by implementing the manufacturing method of the present invention described above has excellent flatness even in wide thin plate materials, such as those with a width of 800 mm or more (preferably 900 mm or more, more preferably 1000 mm or more), and tends to exhibit effects such as improved dimensional accuracy in etching processing. A more preferable maximum floating height value is 1.2 mm or less, and an even more preferable maximum floating height value is 1.0 mm or less. The lower limit of the maximum floating height is not particularly limited, but since it is extremely difficult to manufacture a completely flat shape, the lower limit of the maximum floating height may be set to, for example, about 0.1 mm. The floating height can be measured by cutting the thin plate to a length of 800 mm, placing it on a horizontal surface plate, and measuring the floating height of the thin plate using a laser displacement meter or the like.

[0018] <Steepness> The Fe—Ni-based alloy thin plate of this embodiment preferably has a maximum steepness of 0.7% or less at a length of 800 mm. An Fe—Ni-based alloy thin plate of the present invention that satisfies this requirement has better flatness. A more preferable maximum steepness value is 0.5% or less, and an even more preferable maximum steepness value is 0.3% or less. The lower limit of the maximum steepness is not particularly limited, but since, as with the floating height, it is extremely difficult to manufacture a completely flat shape, the lower limit of the maximum steepness may be set to, for example, about 0.01%. The steepness can be measured by calculating the steepness from data in which the floating height is recorded in a matrix at regular intervals in the plate width and plate length directions.

[0019] <Tensile Strength, 0.2% Yield Strength, and Elongation> The Fe—Ni-based alloy thin plate of this embodiment preferably has a tensile strength of 400 MPa or more and a 0.2% yield strength of 200 MPa or more. Furthermore, when workability is important, the elongation may be set to 20 to 50%. This allows the Fe—Ni-based alloy thin plate of this embodiment to exhibit excellent flatness and etching warpage resistance, as well as superior plastic workability, and tends to be less susceptible to fractures or cracks during winding or plastic working. The preferred lower limit of the tensile strength is 450 MPa, and the preferred lower limit of the 0.2% yield strength is 250 MPa. The preferred lower limit of the elongation is 25%, and the preferred upper limit of the elongation is 45%. The tensile strength, 0.2% yield strength, and elongation properties can be determined by tests based on the methods specified in JIS-Z2241.

[0020] An Fe—Ni-based alloy having the composition shown in Table 1 was hot-pressed and hot-rolled to prepare a 3.0 mm-thick hot-rolled material. The hot-rolled material was chemically and mechanically polished to remove the oxide layer on the surface of the hot-rolled material, and then trimmed to remove cracks that had occurred during hot rolling at both ends in the material's width direction. The material was then divided into an inventive example and a comparative example. Each was subjected to intermediate cold rolling and intermediate annealing (approximately 900°C) to produce an intermediate cold-rolled material with a width of 1040 mm and a thickness of 0.2 mm. Both the inventive example and the comparative example were then subjected to one pass of finish cold rolling at a rolling reduction of 25% to form a thin sheet (finish cold-rolled thin sheet). The process shown in Table 2 was then carried out to produce Sample No. 1, an inventive example, and Samples Nos. 11 and 12, comparative examples. The recrystallization annealing for the inventive and comparative examples was performed at approximately 900°C, and shape correction was performed using a tension leveler, but without pressing using a leveler roll, under conditions of a correction tension of approximately 300 MPa, mainly tensile stress. The stress relief annealing temperature for Sample No. 1, an inventive example, and Sample No. 12, a comparative example, was approximately 630°C.

[0021] Various test pieces were taken from the prepared sample of Invention Example No. 1 and subjected to respective tests. The test results are shown in Table 2. To measure the warpage to confirm the effect of suppressing etching warpage, a cut sample 150 mm long and 30 mm wide was prepared, and half of the plate thickness was removed by etching from one side. The cut sample was then hung on a vertical upper plate and the amount of warpage measured and evaluated. The cut sample was taken from the center of the width direction of the prepared sample so that the length direction was the rolling direction. An aqueous solution of ferric chloride was used as the etching solution, and the test piece was corroded by spraying the etching solution at a temperature of 50°C. To confirm the effect of flatness, the maximum lift height and maximum steepness were measured using a three-dimensional shape measuring device on a test piece cut to a length of 800 mm and placed on a horizontal surface plate. The steepness was calculated from data recorded in a matrix of lift heights at fixed lengths (width: 21 mm, length: 8 mm). The tensile strength, 0.2% yield strength and elongation were measured according to the test methods specified in JIS-Z2241.

[0022]

[0023]

[0024] As shown in Table 2, the sample of Inventive Example No. 1, which was subjected to finish cold rolling, recrystallization annealing, shape correction, and stress relief annealing, exhibited a good etching warpage of 5 mm or less and good mechanical properties (tensile strength: 460 MPa or more, 0.2% yield strength: 280 MPa or more, elongation: 30% or more), and showed very excellent values ​​of maximum lift-up height of 0.8 mm or less and maximum steepness of 0.3% or less. This indicates that the Fe—Ni-based alloy thin sheet obtained by applying the present invention is suitable for various etching processes. On the other hand, the sample of Comparative Example No. 11, which was subjected to finish cold rolling, recrystallization annealing, and shape correction but not stress relief annealing, exhibited a large etching warpage, and the maximum lift-up height and maximum steepness were inferior to those of the Inventive Examples. Comparative Example No. For sample No. 12, the amount of warping due to etching was larger than that of the inventive example, and excessive selvage waves were generated in the thin plate, resulting in excessively large values ​​for the maximum floating height and maximum steepness, which indicated that the material was unsuitable for forming high-precision patterns by etching.

Claims

1. A method for producing an Fe-Ni based alloy thin plate, comprising: a finish cold rolling process in which a cold rolling material containing, by mass%, C:≦0.01%, Si:≦0.5%, Mn:≦1.0%, Ni: 28-52%, with the balance being Fe and unavoidable impurities, is subjected to finish cold rolling at a rolling reduction rate of 5% to 50% to produce a thin plate; a recrystallization annealing process in which the thin plate after the finish cold rolling is annealed at a temperature of 800°C to 1000°C; a tensile stress imparting process in which the thin plate after the recrystallization annealing process is subjected to shape correction under a tension condition of 200 to 600 MPa to impart tensile stress; and a strain relief annealing process in which the thin plate after the tensile stress imparting process is subjected to strain relief annealing at a temperature of 400°C to 750°C, thereby obtaining an Fe-Ni based alloy thin plate having a thickness of 0.5 mm or less.

2. A thin Fe-Ni alloy plate having a thickness of 0.5 mm or less, containing, by mass%, C: ≦0.01%, Si: ≦0.5%, Mn: ≦1.0%, Ni: 28-52%, with the remainder being Fe and unavoidable impurities, wherein a specimen having a length of 150 mm and a width of 30 mm is cut out from said thin Fe-Ni alloy plate, and said specimen is etched from one side to remove 1 / 2 of the thickness of said specimen, the amount of warping being 10 mm or less, and the maximum floating height of said thin Fe-Ni alloy plate over a length of 800 mm is 1.5 mm or less.