Austenitic stainless steel sheet and method for manufacturing the same

Abstract

To stably realize an austenitic stainless steel sheet reduced in uneven brightness.SOLUTION: In an austenitic stainless steel sheet, the arithmetical average height Sa of crystal grains (1) exposed to its surface is 0.15 μ or less. The austenitic stainless steel sheet is produced by controlling a temperature rise rate in a finish annealing step and a soaking time after that to specified ranges.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel sheet and a method for producing the same. [Background technology]

[0002] Stainless steel sheets are often used as exterior panels, and so their surfaces are generally required to have a beautiful appearance. However, due to the manufacturing method, for example, austenitic stainless steel sheets can have a surface with a band-like uneven gloss, which is a series of areas with different gloss across the width.

[0003] A known technique for suppressing the occurrence of such band-like uneven gloss is a method for manufacturing cold-rolled austenitic stainless steel sheet, in which an austenitic stainless steel slab containing δ-ferrite is heated in a heating furnace at 1200°C or higher for 40 minutes or longer in an atmosphere with a controlled oxygen concentration before hot rolling. This manufacturing method produces dense oxides throughout the slab, suppressing variations in oxidation and thereby suppressing the occurrence of unevenness on the surface of the austenitic stainless steel sheet (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 11-335740 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the above-mentioned conventional techniques, the occurrence of band-like uneven gloss may be insufficient depending on the influence of processes after hot rolling on the surface of the austenitic stainless steel sheet. Thus, the conventional techniques leave room for further study from the viewpoint of stably suppressing the occurrence of uneven gloss.

[0006] An object of one aspect of the present invention is to stably realize an austenitic stainless steel sheet with little uneven gloss. [Means for solving the problem]

[0007] In order to solve the above problems, an austenitic stainless steel sheet according to one aspect of the present invention has crystal grains exposed at the surface of the steel sheet having an arithmetic mean height Sa of 0.15 μm or less.

[0008] In order to solve the above-mentioned problems, one aspect of the present invention provides a method for producing an austenitic stainless steel sheet, comprising: an annealing step of finish-annealing a rolled austenitic stainless steel sheet; and a pickling step of cleaning the surface of the austenitic stainless steel sheet annealed in the annealing step with acid, wherein the annealing step comprises a heating step of heating the rolled austenitic stainless steel sheet so that the temperature of the rolled austenitic stainless steel sheet reaches a specific annealing temperature; and a soaking step of maintaining the temperature of the austenitic stainless steel sheet heated in the heating step at the annealing temperature, wherein the temperature rise rate of the austenitic stainless steel sheet heated in the heating step is 80°C / second or more, and the soaking time for maintaining the temperature of the atmosphere around the austenitic stainless steel sheet at the annealing temperature in the soaking step is less than 10 seconds. [Effects of the Invention]

[0009] According to one aspect of the present invention, an austenitic stainless steel sheet with little uneven gloss can be stably produced. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a view showing a photograph taken with a laser microscope of microscopic irregularities on an example of the surface of an austenitic stainless steel sheet according to an embodiment of the present invention. [Figure 2]FIG. 1 is a photograph taken with a laser microscope of microscopic irregularities on an example of the surface of an austenitic stainless steel sheet according to an embodiment of the present invention, and a diagram schematically showing an example of the shape of the irregularities in the photograph. [Figure 3] FIG. 1 is a diagram showing an example of phase distribution in the cross-sectional surface layer of a SUS304 sheet at 900°C when annealing is performed at a temperature increase rate of 20°C / sec with an end temperature of 1100°C. [Figure 4] FIG. 1 is a diagram showing an example of phase distribution in the cross-sectional surface layer of a SUS304 sheet at 1000°C when annealing is performed at a temperature increase rate of 20°C / sec with an end temperature of 1100°C. [Figure 5] FIG. 1 is a diagram showing an example of phase distribution in the cross-sectional surface layer of a SUS304 sheet at 1100°C when annealing is performed at a temperature increase rate of 20°C / sec with an end temperature of 1100°C. [Figure 6] FIG. 1 is a diagram showing an example of phase distribution in the cross-sectional surface layer of a SUS304 sheet at 1100°C when annealing is performed at a temperature increase rate of 100°C / sec with an end temperature of 1100°C. [Figure 7] FIG. 3 is a diagram showing temperature changes of a test specimen in a final annealing step in Example 1 of the present invention. [Figure 8] FIG. 2 is a diagram for explaining measurement points of the surface shape of crystal grains in an example of the present invention. [Figure 9] FIG. 2 is a diagram for explaining measurement points of the brightness of surface gloss in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Austenitic stainless steel plate] In an austenitic stainless steel sheet according to one embodiment of the present invention, the arithmetic mean height Sa of the crystal grains exposed on the surface of the steel sheet is 0.15 μm or less. First, the surface condition will be described.

[0012] [Surface condition] Fig. 1 is a photograph taken with a laser microscope of microscopic irregularities on an example surface of an austenitic stainless steel sheet according to an embodiment of the present invention. Fig. 2 is a photograph taken with a laser microscope of microscopic irregularities on an example surface of an austenitic stainless steel sheet according to an embodiment of the present invention, and a diagram schematically showing an example of the shape of the irregularities in the photograph. The lower diagram of Fig. 2 shows an example of the photograph taken with the laser microscope, and the upper diagram of Fig. 2 shows the shape of the irregularities on the surface along the straight line in the lower diagram.

[0013] As shown in Figure 1, there are crystal grains 1 and grain boundaries 2 on the surface of an austenitic stainless steel sheet. Crystal grains 1 are areas that appear brighter and are surrounded by grain boundaries 2. Crystal grains 1 are mainly composed of the austenite phase. Grain boundaries 2 are depressions that surround crystal grains 1. Grain boundaries 2 are also mainly composed of the austenite phase, but they are deeper or larger than the micro-depressions described below, and therefore appear darker than crystal grains 1.

[0014] The crystal grains 1 are preferably large from the viewpoint of fully expressing aesthetic optical properties such as metallic luster and brightness. Therefore, from the above viewpoint, it is preferable that the average grain size of the crystal grains 1 is 5 μm or more. The larger the average grain size of the crystal grains 1, the better from the viewpoint of expressing the above optical properties, but from the viewpoint that the effect of expressing the above optical properties substantially reaches a plateau, the crystal grains 1 may be 20 μm or less.

[0015] The crystal grains 1 and the grain boundaries 2 can be confirmed using a normal microscope. The average grain size of the crystal grains 1 only needs to appropriately represent the size of the crystal grains 1. For example, the average grain size of the crystal grains 1 can be represented by the average value of the circle equivalent diameter.

[0016] As shown in FIGS. 1 and 2, the crystal grain 1 has minute protrusions 11 and minute recesses 12. The minute protrusions 11 are convex portions of the irregularities formed on the surface of the crystal grain 1. The minute recesses 12 are concave portions of the irregularities formed on the surface of the crystal grain 1. The mechanism by which the crystal grain 1, the grain boundary 2, the minute protrusions 11, and the minute recesses 12 are formed will be described later.

[0017] The crystal grains 1, grain boundaries 2, micro-projections 11, and micro-depressions 12 can be confirmed by taking unevenness images using a laser microscope or a scanning electron microscope (SEM). Clusters of crystal grains with large micro-projections 11 and micro-depressions 12 (hereinafter collectively referred to as "micro-projections") are observed as areas with lower brightness due to gloss, and are recognized as gloss irregularities. The gloss irregularities may appear as linear irregularities (5 to 50 mm wide) along the rolling direction during the production of the austenitic stainless steel sheet. In conventional austenitic stainless steel coils, such gloss irregularities may be present in large numbers across the width of the coil.

[0018] The uneven gloss can be visually identified. The areas of the uneven gloss that have lower brightness are called "uneven areas," while the areas with a stronger metallic gloss and higher brightness are called "normal areas." The "uneven areas" tend to appear streaky and relatively dark.

[0019] In an embodiment of the present invention, an austenitic stainless steel sheet with reduced unevenness is realized. "Reduced unevenness" means that the change in brightness is reduced to the point where it is no longer visible to the naked eye. Such an austenitic stainless steel sheet in an embodiment of the present invention can be defined by the indices Sa and Sz that represent the surface state of the crystal grains.

[0020] [Sa and Sz] In an embodiment of the present invention, the crystal grains exposed on the surface of the austenitic stainless steel sheet have an Sa of 0.15 μm or less. Sa is the arithmetic mean height, which represents the average of the absolute values ​​of the height differences at each point relative to the average plane of the surface. From the viewpoint of reducing the occurrence of the above-mentioned unevenness, the smaller Sa is, the better. However, from the viewpoint of productivity or from the viewpoint of fully expressing the aesthetic effect of the metallic luster appearance, Sa may be 0.1 μm or more.

[0021] In an embodiment of the present invention, a small Sz of the crystal grains exposed on the austenitic stainless steel sheet is preferable from the viewpoint of suppressing the occurrence of uneven gloss, and from this viewpoint, it may be 1.5 μm or less. Sz is the maximum height, and represents the distance from the highest point to the lowest point on the surface. From the viewpoint of reducing the occurrence of the uneven portions, the smaller Sz is the better, but from the viewpoint of productivity or from the viewpoint of fully expressing the aesthetic effect of the metallic gloss appearance, it may be 1.0 μm or more.

[0022] Sa and Sz can be measured by photographing with the laser microscope and processing the obtained images. Sa and Sz can also be measured by controlling the temperature rise rate and soaking time in the final annealing, which will be described later.

[0023] [Glossiness] Furthermore, the austenitic stainless steel sheet according to the embodiment of the present invention, in which unevenness is reduced, can also be specified by the optical properties of the surface. That is, the specific lightness difference ΔL of the austenitic stainless steel sheet according to the embodiment of the present invention * It is preferable that the value is 40 or more from the viewpoint of further reducing the difference in brightness and further reducing unevenness.

[0024] Here, ΔL * L * 1 to L * It is the difference minus 2. L * L is the brightness when measuring the spectral total reflectance in the 8-degree direction (including specular reflection) on the surface of an austenitic stainless steel plate. * 2 is the brightness when the spectral diffuse reflectance is measured in the 8 degree direction (SCE) excluding specular reflection light on the surface of the austenitic stainless steel plate.

[0025] ΔL * From the viewpoint of the effect of reducing uneven portions, the larger the better, but from the viewpoint that the effect substantially reaches a plateau, it may be 50 or less.

[0026] L *Since L1 is the brightness when measuring the spectral total reflectance, the larger the value, the better from the viewpoint of increasing the brightness of the surface and enhancing the aesthetic appearance due to the metallic luster, and for example, it is preferably 80 or more. * 1 may also be 86 or less, since the effect essentially reaches a plateau.

[0027] L * Since L2 is the brightness when measuring the spectral diffuse reflectance, the smaller it is, the better from the viewpoint of the effect of reducing unevenness, and for example, it is preferably 45 or less. * 2 may also be 36 or more from the viewpoint that the effect substantially reaches a plateau or from the viewpoint of productivity, etc.

[0028] L * 1 and L * 2 can be measured using a commercially available or known spectrophotometer. * 1 and L * 2 can be achieved by controlling the temperature rise rate and soaking time in the final annealing, which will be described later.

[0029] [Method for manufacturing austenitic stainless steel sheet] A method for producing an austenitic stainless steel sheet according to one embodiment of the present invention comprises an annealing step of annealing a rolled austenitic stainless steel sheet for finishing, and a pickling step of cleaning the surface of the austenitic stainless steel sheet annealed in the annealing step with an acid. The rolled austenitic stainless steel sheet is obtained in the same manner as a known method for producing an austenitic stainless steel sheet including a hot rolling step.

[0030] In the production of austenitic stainless steel sheets, the annealing process includes an annealing process that is performed to remove internal strain due to work hardening, soften the structure, and improve workability in subsequent processes (such as cold rolling). Such an annealing process (also called an "intermediate annealing process") may be performed once or multiple times in the production of austenitic stainless steel sheets. The production method of the present embodiment may also include an intermediate annealing process.

[0031] On the other hand, the annealing process in the production of austenitic stainless steel sheets includes an annealing process that is carried out for the purpose of imparting the mechanical properties and surface condition required for the final product. The finishing annealing process specified in the embodiments of the present invention is an annealing process for such a purpose, and this annealing process (also referred to as the "final annealing process") and the subsequent pickling process (also referred to as the "final pickling process") are carried out for the purpose of finishing the austenitic stainless steel sheet.

[0032] [Final annealing process] In an embodiment of the present invention, the above-mentioned final annealing step includes a heating step of heating the rolled austenitic stainless steel sheet so that the temperature of the rolled austenitic stainless steel sheet reaches a specific annealing temperature, and a soaking step of maintaining the temperature of the austenitic stainless steel sheet heated in the heating step at the annealing temperature.

[0033] In an embodiment of the present invention, the temperature rise rate of the austenitic stainless steel sheet heated in the heating step is 80°C / second or more. A faster temperature rise rate in the heating step is more effective in terms of enhancing the effect of reducing the uneven portions described above. From this perspective, the temperature rise rate in the heating step is preferably 95°C / second or more, and more preferably 105°C / second or more. The temperature rise rate is determined by dividing the difference in surface temperature of the austenitic stainless steel sheet from the start to the end of the heating step by the time (heating time).

[0034] The temperature of the austenitic stainless steel sheet before being heated in the heating step (also referred to as the "annealing start temperature") is usually about 10 to 30°C, and the temperature may be within this range in this embodiment as well. The end temperature of the austenitic stainless steel sheet heated in the heating step of the final annealing step can be appropriately determined within a range that allows the purpose of the final annealing step to be achieved, and may be, for example, 1000 to 1150°C.

[0035] In the soaking step following the heating step in the final annealing step, the temperature of the atmosphere surrounding the austenitic stainless steel sheet is maintained at the final annealing temperature for a specific period of time. This specific period of time is also referred to as the "soaking time." In an embodiment of the present invention, the soaking time is less than 10 seconds. The temperature of the austenitic stainless steel sheet in the soaking step and the temperature of the atmosphere therein may be substantially the same temperature; for example, the temperature of the atmosphere may be within a range of ±10°C of the final annealing temperature of the austenitic stainless steel sheet.

[0036] The heating step and the soaking step in the final annealing step can be performed by using known heating devices alone or in appropriate combinations within the range where heating under the above conditions is possible. The heating device in the heating step and the heating device in the soaking step may be the same or different.

[0037] In the final annealing step of the embodiment of the present invention, the temperature rise rate in the heating step and the soaking time in the soaking step can be set independently. However, a faster temperature rise rate is preferable from the viewpoint of suppressing the progression of surface oxidation in the depth direction, and a shorter soaking time according to the temperature rise rate is preferable from the viewpoint of suppressing surface oxidation. Furthermore, the faster the temperature rise rate, the more difficult it tends to be to shorten the soaking time. Therefore, from the above viewpoint, it is preferable to appropriately set the soaking time according to the temperature rise rate.

[0038] Furthermore, if the heating rate is fast, the crystal grains on the surface of the austenitic stainless steel sheet tend to become smaller, but it is possible to adjust the size of the crystal grains appropriately by adjusting the soaking time (for example, to make the crystal grain size the same even when the heating rate is different). On the other hand, if the soaking time is too long, oxidation at the grain boundaries of the crystal grains will cause large depressions, which may reduce the overall surface brightness of the austenitic stainless steel sheet after the final annealing process.

[0039] From the above viewpoints, the temperature rise rate in the heating step of the final annealing step and the soaking time in the soaking step can be set. For example, from the above viewpoints, in an embodiment of the present invention, when the temperature rise rate is 80°C / sec or more and less than 95°C / sec, the soaking time is preferably less than 3 seconds. When the temperature rise rate is 95°C / sec or more and less than 105°C / sec, the soaking time is preferably 3 seconds or more and less than 5 seconds, and when the temperature rise rate is 105°C / sec or more, the soaking time is preferably 5 seconds or more and less than 10 seconds.

[0040] [Surface shape formation process] In the embodiment of the present invention, the specific surface state of the crystal grains is realized by the specific heating rate and soaking time described above. On the other hand, the heating rate in a normal annealing process is about 20°C / sec, and in this case, the minute irregularities in the crystal grains exposed on the surface of the austenitic stainless steel sheet become larger than those in the embodiment of the present invention. The formation of the microscopic surface shape in the annealing process will be explained below. First, the method for measuring the phase distribution will be explained.

[0041] [Method for measuring the crystal phase distribution in the surface layer] SUS304 test pieces measuring 80 mm x 10 mm were prepared and rolled to 1 mm. They were then annealed under the following conditions: (1) a heating rate of 20°C / s, with an end temperature of 1100°C; and (2) a heating rate of 100°C / s, with an end temperature of 1100°C. The C-sections (cross sections perpendicular to the sheet running direction) of the test pieces during and after annealing were analyzed using electron backscatter diffraction (EBSD) using an OIM4.0-CCD / ADV crystal orientation analysis system (TSL Solutions). The phase distribution of the crystalline structure in the C-sections of the test pieces was analyzed, and the state of the Cr-depleted zone was confirmed.

[0042] Measurements were taken in five fields of view per specimen. The magnification was 1800x. The measurement area in each field of view was a randomly set region of 50 μm x 15 μm. The specimens analyzed were BCC (body-centered cubic structure, martensite phase) and FCC (face-centered cubic structure, austenite phase).

[0043] It has been revealed that a martensite phase with a Cr depletion of approximately 10-12% occurs at the interface between the oxide scale and the base material of annealed SUS304 sheet due to a decrease in alloying elements (increase in the Ms point). EBSD analysis shows that the Cr-depleted layer appears as BCC.

[0044] Next, the process of forming the surface shape in a normal annealing process will be described.

[0045] <When the heating rate is 20°C / sec> Fig. 3 shows an example of the phase distribution in the cross-sectional surface layer of a SUS304 sheet at 900°C when annealing is performed at a heating rate of 20°C / s to an end temperature of 1100°C. In the following description of the surface shape formation process, it is assumed that the SUS304 sheet is cooled immediately after reaching the end temperature.

[0046] As shown in Figure 3, recrystallization progresses in the SUS304 sheet as the temperature rises during the annealing process. Meanwhile, oxide scale 31 forms in the surface layer (the dark half-tone area in the figure). Cr easily migrates from the base material to oxide scale 31, resulting in a relatively high Cr content. Directly below the formed oxide scale, a region where the Cr content in the base material is reduced, i.e., martensite phase 32, forms (the light half-tone area in the figure). This martensite transformation occurs more quickly at the grain boundaries indicated by the lines in the figure. This is because grain boundaries are easily oxidized due to the ease of diffusion. Therefore, in the surface layer of SUS304 (base material), recesses due to martensite phase 32 form at the grain boundaries of the austenite phase 33 (e.g., the area indicated by the arrow in the figure).

[0047] FIG. 4 shows an example of the phase distribution in the cross-sectional surface layer of a SUS304 sheet at 1000°C after annealing at a heating rate of 20°C / s and an end temperature of 1100°C. As shown in FIG. 4, as the temperature increases during the annealing process, the grains grow further. Furthermore, because diffusion is rapid at grain boundaries, martensite preferentially develops at the grain boundaries. Thus, even at the surface layer of the base material at 1000°C, depressions due to martensite phase 32 are formed at the grain boundaries of the austenite phase 33, as indicated by the arrows in the figure. At the surface layer of the base material, the depressions in the martensite phase 32 at 900°C are contained within the crystal grains that have grown due to the progress of recrystallization at 1000°C. The oxide scale 31 in the surface layer has slightly increased in thickness due to accelerated oxidation.

[0048] In actual manufacturing, the temperature rise curve in the heating process usually becomes gentler as it approaches the end temperature (target temperature). Therefore, it takes approximately 90 seconds to reach 1000°C. When the temperature of the SUS304 plate reaches 1050°C, the temperature rise rate becomes slower compared to around 900°C. Therefore, after 1050°C, the effect of soaking becomes stronger as the temperature rises. Oxidation caused by this soaking effect deepens the depressions caused by the martensite phase 32 at the grain boundaries in the surface layer.

[0049] FIG. 5 shows an example of the phase distribution in the cross-sectional surface layer of a SUS304 sheet at 1100°C after annealing at a heating rate of 20°C / s and an end temperature of 1100°C. As shown in FIG. 5, as the temperature increases during the annealing process, the grains grow further. Furthermore, at the grain boundaries, martensite preferentially develops due to diffusion. Therefore, even at the surface layer of the base material at 1100°C, recesses due to the martensite phase 32 are formed at the grain boundaries of the austenite phase 33. At 1000°C, the recesses in the martensite phase 32 formed at 900°C and 1000°C are contained within the crystal grains that have grown due to the progress of recrystallization. Furthermore, due to the accelerated oxidation, the thickness of the oxide scale 31 increases even further.

[0050] Thus, in the annealing process with a heating rate of 20°C / s, numerous concave portions due to the martensite phase 32 are formed in the crystal grains, with large and small concave portions formed deeper at the grain boundaries. The subsequent pickling removes the oxide scale 31 and the concave portions due to the martensite phase 32 in the surface layer. Therefore, after pickling, the austenite phase 33 is exposed on the surface, and larger micro-concavities and convexities are formed on the surface of the crystal grains on the SUS304 sheet. Furthermore, the higher the temperature and the longer the pickling time, the more likely Cr depletion due to oxidation progresses, resulting in deeper and larger concave portions formed at the grain boundaries.

[0051] <When the heating rate is 100°C / sec> In contrast, when the temperature is increased more rapidly during the annealing process, the micro-irregularities on the grain surfaces become smaller. Figure 6 shows an example of the phase distribution in the cross-sectional surface layer of a SUS304 sheet at 1100°C after annealing at a heating rate of 100°C / s and an end temperature of 1100°C. As shown in Figure 6, even when the temperature is increased at 100°C / s, the grains grow due to the progress of recrystallization caused by the temperature increase, and recesses due to martensite phase 32 are formed at the grain boundaries due to preferential martensite formation. However, when the temperature is increased at 100°C / s, the time for grain boundary oxidation during the heating process is short, and the recesses due to martensite phase 32 formed during the heating process are small, and the recesses at the grain boundaries are also less likely to grow. Furthermore, the thickness of the oxide scale 31 formed at the surface layer is less likely to increase. Therefore, when the oxide scale 31 and martensite phase 32 are removed by pickling after annealing, the micro-irregularities contained in the crystal grains on the surface of the SUS304 sheet are smaller, and the concave portions at the grain boundaries are also smaller, compared to those when annealing at a heating rate of 20°C / sec.

[0052] <Explanation of Micro-Unevenness Formation> As described above, the heating rate during annealing and the recrystallization process up to the reached temperature were variously confirmed, and the surface condition after pickling was investigated. As a result, it was found that the size of the minute irregularities in the crystal grains is influenced by the growth of the crystal grains from the start of recrystallization until the temperature is reached.

[0053] More specifically, when the heating rate is slow, (1) Cr depletion occurs at grain boundaries where the diffusion rate is fast in the early stages of recrystallization, (2) grain boundary migration under Cr depletion progresses (grain growth) as the temperature rises, (3) the time for grain boundary oxidation during the heating process is long due to the gradual heating, and (4) depressions depleted of Cr remain on the surfaces of the crystal grains.

[0054] In contrast, when the heating rate is fast, (1) Cr depletion occurs at grain boundaries with fast diffusion rates in the early stages of recrystallization, and (2) as the temperature rises, the process is the same as when the heating rate is slow until grain boundary migration under Cr depletion progresses (grain growth). However, (3) the rapid heating process shortens the time for grain boundary oxidation during the heating process, and (4) Cr-depleted depressions are less likely to form on the grain surfaces. As a result, the difference in the micro-irregularities on the grain surfaces is small, and there is no visible gloss unevenness from a macroscopic perspective, no reduction in brightness, and the surface brightness of the austenitic stainless steel sheet is uniform.

[0055] The brightness observations above were made on crystal grains with a certain average grain size of 16.9 to 18.5 μm. This was done to prevent brightness differences due to different grain sizes, as fine crystal grains tend to have many grain boundaries and appear dark. Furthermore, it was confirmed that when the heating rate was set to 80°C / sec or higher, which is slightly slower than 100°C / sec, minute irregularities were less likely to be generated on the crystal grain surfaces.

[0056] [Pickling process] In an embodiment of the present invention, the final pickling step is a step of pickling a steel sheet after the final annealing step to remove from the steel sheet any effects of oxidation in the final annealing step (oxide scale, martensite phase, etc.). The final pickling step and the intermediate pickling step described above can be carried out under known conditions that allow the removal by pickling of oxide scale present on the surface of the austenitic stainless steel sheet after annealing and the martensite phase beneath the oxide scale.

[0057] [Other processes] The embodiments of the present invention may include other processes as long as the effects of the embodiments of the present invention described above can be obtained. For example, the method for producing an austenitic stainless steel sheet in the embodiments of the present invention does not include rolling processes (the above-mentioned hot rolling and cold rolling) that substantially change the dimensions of the steel sheet (mainly the sheet thickness) and the annealing process associated therewith, after the final annealing process and the subsequent final pickling process.

[0058] On the other hand, in the manufacturing method according to the embodiment of the present invention, any process other than the process involving substantial changes in the dimensions and surface condition of the steel sheet may be included after the final annealing process. Examples of processes that can be performed after the final annealing process include a process for modifying the shape of the steel sheet and a temper rolling process for adjusting the properties of the steel sheet.

[0059] According to the manufacturing method of the embodiment of the present invention, an austenitic stainless steel sheet can be obtained in which unevenness is reduced to the point that it is not visible at the macro level.

[0060] [Features of the embodiment of the present invention] <Conventional features> In a typical manufacturing method for austenitic stainless steel sheets, a slab is produced by steelmaking, the slab is hot-rolled, and the resulting hot-rolled coil is annealed and pickled, then cold-rolled, and then annealed and pickled again. Cold rolling may be performed once or twice or more times. When cold rolling is performed twice or more times, annealing and pickling are usually performed after each cold rolling. Once the hot-rolled coil has been cold-rolled to the desired thickness, it is subjected to final annealing.

[0061] In general annealing, the rolled coil is heated at a temperature increase rate of about 20°C / second, and is immediately cooled after reaching the target material temperature. After annealing, the metal structure is recrystallized, and crystal grains of about 5 to 20 μm are confirmed by pickling.

[0062] In order to suppress unevenness across the coil width, measures have been taken in steelmaking, such as reducing or homogenizing the amount of delta ferrite or extending the heating time, but there is still room for improvement in the stability of the effect of suppressing unevenness in the final finished material.

[0063] Furthermore, various techniques are known for suppressing unevenness on the surface of a stainless steel sheet, such as those listed in (1) to (4) below. (1) A method of making uneven gloss caused by Ni segregation less noticeable by controlling the crystal grain size and orientation (Japanese Patent Application Laid-Open No. 2003-290881). (2) A method of improving uneven gloss by making the surface state after pickling into a uniform corrosion state (Japanese Patent Laid-Open Publication No. 2004-137538). (3) A method of producing fine and uniform crystal grains by limiting the ratio of the amount of sulfur to the amount of dissolved oxygen in a continuously cast slab to a predetermined range (Japanese Patent Laid-Open Publication No. 2009-028791). (4) A method of suppressing the occurrence of surface irregularities by generating dense oxides throughout the austenitic stainless steel slab and suppressing variations in oxidation (Patent Document 1).

[0064] <Features of the embodiment of the present invention> In contrast to these conventional techniques, embodiments of the present invention control the size of minute irregularities that cause dark areas that are visually recognized as uneven gloss on the surface of an austenitic stainless steel sheet. This suppresses the generation or growth of minute irregularities on the crystal grain surface after annealing and pickling, providing an austenitic stainless steel sheet that exhibits excellent surface quality with uniform brightness after cold rolling and annealing. That is, by setting the temperature rise rate during annealing to 80°C / sec or more, preferably 100°C / sec or more, a smooth state is achieved in which the generation of minute irregularities on the crystal grain surface after pickling is suppressed. Therefore, a stainless steel sheet can be produced that exhibits excellent surface quality with uniform brightness after cold rolling and annealing and pickling.

[0065] The austenitic stainless steel sheet according to the embodiment of the present invention has a sufficiently high brightness due to reflected light, and the change in brightness is reduced to such an extent that there is no gloss unevenness that is visible to the naked eye.

[0066] According to this configuration, by controlling the heating in the final annealing, an austenitic stainless steel sheet having uniform brightness without uneven gloss can be obtained. It becomes possible to stably supply stainless steel sheets exhibiting uniform metallic luster for exterior applications, and the use of such stainless steel sheets as exterior materials is expected to contribute to the creation of beautiful urban landscapes. Thus, embodiments of the present invention are expected to contribute to the achievement of the Sustainable Development Goals (SDGs).

[0067] 〔summary〕 As is clear from the above description, in the austenitic stainless steel sheet according to the embodiment of the present invention, the arithmetic mean height Sa of the crystal grains exposed at the surface of the steel sheet is 0.15 μm or less.

[0068] Furthermore, a method for producing an austenitic stainless steel sheet according to an embodiment of the present invention includes an annealing step in which a rolled austenitic stainless steel sheet is annealed for finish finishing, and a pickling step in which the surface of the austenitic stainless steel sheet annealed in the annealing step is cleaned with acid. The annealing step includes a heating step in which the rolled austenitic stainless steel sheet is heated to a specific annealing temperature, and a soaking step in which the temperature of the austenitic stainless steel sheet heated in the heating step is maintained at the annealing temperature. The temperature rise rate of the austenitic stainless steel sheet heated in the heating step is 80°C / second or more, and the soaking time in the soaking step in which the temperature of the atmosphere around the austenitic stainless steel sheet is maintained at the annealing temperature is less than 10 seconds.

[0069] As described above, according to the embodiment of the present invention, by controlling the heating in the finish annealing, an austenitic stainless steel sheet having sufficiently small irregularities in the crystal grains exposed on the surface of the steel sheet can be provided, and therefore, the embodiment of the present invention can stably realize an austenitic stainless steel sheet having little uneven gloss.

[0070] In the austenitic stainless steel sheet according to the embodiment of the present invention, the maximum height Sz of the crystal grains exposed at the surface of the steel sheet may be 1.5 μm or less, which is even more effective in suppressing a decrease in lightness at the surface.

[0071] In the austenitic stainless steel sheet according to the embodiment of the present invention, the lightness L * The brightness L when measuring the spectral diffuse reflectance at an 8-degree angle, excluding specular reflection, from the brightness of 1. * Lightness difference ΔL minus 2 * may be 40 or more. This configuration is even more effective in terms of making the brightness on the surface more uniform.

[0072] In the austenitic stainless steel sheet according to the embodiment of the present invention, the lightness L * 1 may be equal to or greater than 80. This configuration is even more effective from the viewpoint of enhancing the aesthetic appearance due to metallic luster.

[0073] In the method for producing an austenitic stainless steel sheet according to an embodiment of the present invention, the heating rate may be 80°C / sec or more and less than 95°C / sec and the soaking time may be less than 3 seconds, or the heating rate may be 95°C / sec or more and less than 105°C / sec and the soaking time may be 3 seconds or more and less than 5 seconds, or the heating rate may be 105°C / sec or more and the soaking time may be 5 seconds or more and less than 10 seconds. This configuration is even more effective in terms of fully achieving the effect of reducing uneven gloss.

[0074] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Example]

[0075] An embodiment of the present invention will be described below.

[0076] Example 1 A hot-rolled coil of SUS304 (3 mm thick) was annealed and pickled, then the surface was polished using a CG (coil grinder), and then cold-rolled from 3 mm to 1 mm using a cold rolling machine (ZM). In this way, a rolled SUS304 plate was prepared.

[0077] Next, the rolled SUS304 plate was cut into 80 mm x 10 mm pieces to prepare test specimens. Two test specimens were set in a high-frequency induction heating Formaster (manufactured by Fuji Electric Machinery Co., Ltd.). When current was passed through the coil in the high-frequency induction heating Formaster, magnetic lines of force were generated. As a result, eddy currents flowed in the test specimen placed inside the coil, and Joule heat was generated due to electrical resistance, heating the test specimen. In this way, the test specimen was annealed by heating under the following conditions: annealing temperature: 1100°C, heating rate: 101°C / sec, and soaking time: 2 seconds. Figure 7 shows the temperature change of the test specimen during the final annealing process in Example 1. In this way, test specimen 1a annealed under the above conditions was obtained.

[0078] Next, one of the test pieces 1a was immersed in nitric hydrofluoric acid, and the surface of the test piece was washed with nitric hydrofluoric acid, thereby obtaining a test piece 1 of a pickled SUS304 plate.

[0079] [Examples 2 to 7, Comparative Examples 1 to 5] Test pieces 2a to 12a and test pieces 2 to 12 were obtained in the same manner as in Example 1, except that the temperature rising rate and soaking time were changed as shown in Table 1 below.

[0080] 〔evaluation〕 [1] Average grain size The average grain size of the crystal grains was measured based on Japanese Industrial Standard JIS G0551 for each of test pieces 1 to 12. That is, the crystal grains of SUS304 on one surface of the test piece were photographed at 200x magnification using a microscope, and the obtained image was processed to calculate the individual circle equivalent diameters of the multiple crystal grains, and the average value was calculated to be the average grain size.

[0081] [2] Surface condition of crystal grains For each of test pieces 1 to 12, the arithmetic mean height Sa and maximum height Sz of the crystal grains on the surface of the test piece were measured at 4000x magnification using a laser microscope "LEXT OLS4100" (manufactured by Olympus Corporation). Sa and Sz were measured in 10 arbitrary fields of view per test piece, and the average of the obtained measurements was calculated and used as Sa and Sz of that test piece. Note that Sa and Sz were measured in a rectangular measurement area 11c inscribed in the largest crystal grain 1 in any one field of view, as shown in Figure 8. The shape of measurement area 11c was the rectangle with the aspect ratio closest to 1 and the largest area among the rectangles drawn to inscribe the crystal grains.

[0082] [3] Surface gloss brightness For each of test pieces 1 to 12, the lightness L when the spectral total reflectance (SCI: including specular reflection light) at an 8-degree angle on the surface of the test piece was measured using a spectrophotometer "CM-700d" or "CM-600d" (manufactured by KONICA MINORUTA) * 1. Brightness L when measuring the spectral diffuse reflectance (SCE: specular reflection excluded) at 8 degrees * 2, and the difference between them ΔL * (L * 1-L *2) were determined. As shown in Fig. 9, linear unevenness 20 observed in the sheet running direction was defined as the unevenness 20, and measurements were made in five measurement regions 30 within the unevenness 20 of one test piece. That is, relatively dark areas visually distributed in the rolling direction (sheet running direction) were regarded as unevenness 20, and five measurement regions 30 were randomly selected from within the unevenness 20. When multiple unevenness 20 were observed per test piece, measurement regions 30 were selected from each of the unevenness 20.

[0083] In test pieces 1 to 7, the unevenness was generally difficult to distinguish visually, but in these cases, the brightness was measured at the locations that appeared to be uneven (parts that appeared relatively dark).

[0084] [4] Appearance evaluation The appearance of each of the test pieces 1 to 12 was visually observed by a skilled technician and evaluated according to the following criteria. <Evaluation criteria> G: It is judged that there is no problem in practical use as a stainless steel sheet for exterior use, having an appearance with a uniform brightness of metallic luster. NG: It is judged that there may be practical problems when used as the above-mentioned stainless steel sheet for exterior use.

[0085] [5] Results For test pieces 1 to 12, the annealing conditions, average grain size, surface condition of the grains, brightness of the steel sheet surface, and appearance evaluation are shown in Table 1 below.

[0086] [Table 1]

[0087] [5] Consideration As is clear from Table 1, all of the test pieces 1 to 7 were excellent stainless steel sheets exhibiting metallic luster with uniform brightness. This is because Sa and Sz were sufficiently small and ΔL * and L * This is thought to be because the temperature rise rate in the heating step of the final annealing was 80°C / second or more, and the soaking time was short, at most 5 seconds.

[0088] On the other hand, all of the test pieces 8 to 12 were problematic in terms of appearance for practical use as stainless steel sheets exhibiting a metallic luster with uniform brightness. This is because Sa and ΔL * It is also considered that the temperature rise rate in the heating step of the final annealing was slow, less than 80°C / sec. [Industrial Applicability]

[0089] The present invention can be suitably used to provide an exterior material that exhibits metallic luster with uniform brightness. [Explanation of symbols]

[0090] 1 grain 2 grain boundaries 11 Micro-protrusions 11c, 30 measurement area 12 Micro recesses 20 Village 31 Oxide scale 32 Martensite phase 33 Austenite phase

Claims

1. The arithmetic mean height Sa of crystal grains exposed on the surface of the austenitic stainless steel plate is 0.15 μm or less, The maximum height Sz of crystal grains exposed on the surface of the austenitic stainless steel plate is 1.5 μm or less, Lightness L when measuring the spectral total reflectance at an 8-degree angle, including specular reflection ＊ The brightness L when measuring the spectral diffuse reflectance at an 8-degree angle excluding specular reflection from the brightness of 1 ＊ Lightness difference ΔL minus 2 ＊ is 40 or more, Said L ＊ 2 is 36 or more and 45 or less.

2. Lightness L when measuring the spectral total reflectance at an 8-degree angle, including specular reflection ＊ 2. The austenitic stainless steel sheet according to claim 1, wherein .gamma.-1 is 80 or more.

3. A method for manufacturing an austenitic stainless steel sheet, wherein the maximum height Sz of the crystal grains exposed on the surface is 1.5 μm or less, the brightness difference ΔL* obtained by subtracting the brightness L*2 when measuring the spectral diffuse reflectance at an 8-degree direction excluding specular reflected light from the brightness L*1 when measuring the spectral total reflectance at an 8-degree direction including specular reflected light is 40 or more, and said L*2 is 36 or more and 45 or less, The method includes an annealing step of annealing the rolled austenitic stainless steel sheet for finishing, and a pickling step of cleaning the surface of the austenitic stainless steel sheet annealed in the annealing step with an acid, the annealing step includes a heating step of heating the rolled austenitic stainless steel sheet so that the temperature of the rolled austenitic stainless steel sheet reaches a specific annealing temperature, and a soaking step of maintaining the temperature of the austenitic stainless steel sheet heated in the heating step at the annealing temperature, The temperature rising rate of the austenitic stainless steel plate heated in the heating step is 80°C / sec or more and less than 105°C / sec, the soaking time for maintaining the temperature of the atmosphere of the austenitic stainless steel sheet at the annealing temperature in the soaking step is less than 5 seconds; A method for manufacturing austenitic stainless steel sheets.

4. 4. The method for producing an austenitic stainless steel sheet according to claim 3, wherein the temperature rise rate is 80°C / sec or more and less than 95°C / sec, and the soaking time is less than 3 seconds.

5. 4. The method for producing an austenitic stainless steel sheet according to claim 3, wherein the temperature rise rate is 95°C / second or more and less than 105°C / second, and the soaking time is 3 seconds or more and less than 5 seconds.

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