Ferritic stainless steel having excellent ultra-thin rolling properties and method for manufacturing same

Abstract

The present invention relates to a ferritic stainless steel having excellent ultra-thin rolling properties, the ferritic stainless steel comprising, in wt%, 0.001% to 0.1% of carbon (C), 0.05% to 0.7% of silicon (Si), 0.05% to 1% of manganese (Mn), 15% to 19% of chromium (Cr), 0.001% to 0.1% of nitrogen (N), 0% to 0.01% of aluminum (Al), the remainder being Fe and unavoidable impurities, in a cross-section of a cast structure, the thickness of the ferritic stainless steel being less than or equal to 10 [mu] m, and the thickness of the ferritic stainless steel being less than or equal to 10 [mu] m. The number of inclusions having a total content of Al and Mg of more than 30 wt% and a length of more than 6 [mu] m in the thickness direction is 5 or less per 160 mm2.

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel having excellent ultra-thin rollability and a method for manufacturing the same. Background Art

[0002] The application of ultra-thin stainless steel in products that require both flexibility and rigidity, such as flexible solar cells and flexible displays, is gradually expanding.

[0003] The thickness of ultra-thin stainless steel is generally 0.01 mm to 0.08 mm. At this time, the stainless steel is rolled thin, and the strength is increased by work hardening, while ensuring flexibility due to the thin thickness. In this case, the ultra-thin rolling process uses a cold-rolled coil sheet with a thickness of 0.4 mm or more for ultra-thin rolling. However, during the rolling process, since the thickness significantly becomes thinner, plate fracture may occur. This plate fracture is presumably caused by hard and coarse inclusions present inside the coil sheet. Therefore, in the past, in order to prevent plate fracture, although low-speed operations were performed multiple times by reducing the reduction ratio, rolling speed, etc., there was a problem of poor productivity. Summary of the Invention

[0004] (I) Technical Problems to be Solved

[0005] An object of the present invention is to provide a ferritic stainless steel having excellent ultra-thin rollability and a method for manufacturing the same.

[0006] However, the technical problems to be solved by the present invention are not limited to the above technical problems, and those skilled in the art can clearly understand other technical problems not described through the following description.

[0007] (II) Technical Solution

[0008] The ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention may contain, by weight %: carbon (C): 0.001% to 0.1%, silicon (Si): 0.05% to 0.7%, manganese (Mn): 0.05% to 1%, chromium (Cr): 15% to 19%, nitrogen (N): 0.001% to 0.1%, aluminum (Al): 0% to 0.01%, the balance being Fe and inevitable impurities, wherein in the cross-section of the cast structure of the ferritic stainless steel, the number of inclusions having a total content of Al and Mg exceeding 30% by weight and a length in the thickness direction exceeding 6 μm is 5 or less per 160 mm 2 of the cross-section.

[0009] In addition, the inclusions in the ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention may contain silicon-containing oxides or silicon-containing sulfides.

[0010] In addition, in the ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention, the ratio of the average grain size in the outer region in the width direction of the stainless steel to the average grain size in the central region in the width direction may be 2 or less.

[0011] A method for manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention may include the following steps: casting a slab which, by weight%, contains: carbon (C): 0.001% to 0.1%, silicon (Si): 0.05% to 0.7%, manganese (Mn): 0.05% to 1%, chromium (Cr): 15% to 19%, nitrogen (N): 0.001% to 0.1%, aluminum (Al): 0% to 0.01%, the balance being Fe and inevitable impurities; heating the slab at 1150°C to 1250°C; hot-rolling the heated slab to produce a hot-rolled material; hot-rolling annealing the hot-rolled material at 750°C to 880°C; cold-rolling the hot-rolled material to produce a cold-rolled material; and cold-rolling annealing the cold-rolled material at 750°C to 880°C.

[0012] In the method for manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention, the hot-rolling may include hot-rolling the heated slab to a thickness of 2.5 mm to 5 mm.

[0013] In the method for manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention, the hot-rolling may include finish-rolling the heated slab at 800°C to 950°C.

[0014] In the method for manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention, the cold-rolling may include cold-rolling the hot-rolled material to a thickness of 0.4 mm to 1 mm in one pass, and then cold-rolling the material cold-rolled in one pass to a thickness of 0.01 mm to 0.2 mm in a second pass.

[0015] In the method for manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention, in the cross-section of the as-cast structure of the ferritic stainless steel, the number of inclusions having a total content of Al and Mg of more than 30% by weight and a length in the thickness direction of more than 6 μm may be 5 or less per 160 mm 2 5 or less.

[0016] In the method for manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention, the ratio of the average grain size in the outer region in the width direction of the stainless steel to the average grain size in the central region in the width direction may be 2 or less.

[0017] (III) Advantageous Effects

[0018] According to an embodiment of the present invention, a ferritic stainless steel having excellent ultra-thin rollability capable of preventing sheet breakage during cold rolling and a method for manufacturing the same can be provided.

[0019] The effects that can be obtained by the present invention are not limited to the above effects, and other effects not mentioned will be clearly understood by those skilled in the art from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A photograph of inclusions having a characteristic of causing sheet breakage during ultra-thin rolling. DETAILED DESCRIPTION

[0021] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the technical idea of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely describe the present invention to those skilled in the art.

[0022] The terms used in the present invention are only for explaining specific examples. Therefore, unless it is clearly stated in the context that it must be singular, the singular expression includes the plural expression. In addition, it should be noted that the terms "comprising", "including" or "having" used in the present invention are used to clearly specify the existence of the features, steps, functions, components or combinations thereof described in the specification, rather than to pre-exclude the existence of other features or steps, functions, components or combinations thereof.

[0023] In addition, unless otherwise defined, all terms used in this specification should be regarded as having the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. Therefore, in this specification, unless otherwise clearly defined, specific terms should not be interpreted in an overly ideal or formal meaning. For example, unless the context clearly indicates an exception, the singular expression includes the plural expression in this specification.

[0024] Furthermore, with respect to "about", "substantially", etc. in this specification, when manufacturing and material allowable errors inherent in the mentioned meaning are presented, they are used in the meaning of that value or close to that value, and are used to prevent illegal use by unethical infringers. Mentioning accurate or absolute numerical values helps to understand the disclosure of the present invention.

[0025] The ferritic stainless steel with excellent ultra-thin rollability according to an example of the present invention may contain, by weight%: carbon (C): 0.001% to 0.1%, silicon (Si): 0.05% to 0.7%, manganese (Mn): 0.05% to 1%, chromium (Cr): 15% to 19%, nitrogen (N): 0.001% to 0.1%, aluminum (Al): 0% to 0.01%, the balance of Fe and inevitable impurities.

[0026] Hereinafter, the reasons for defining the composition ranges of the respective alloying elements will be described. Hereinafter, unless otherwise specified, the unit is weight%.

[0027] The content of carbon (C) may be 0.001% to 0.1%.

[0028] C is an essential element for ensuring the strength of the steel grade. Considering this, the lower limit of the carbon content is defined as 0.001% or more, preferably 0.01% or more. However, when the carbon content exceeds 0.1%, a hard martensite phase is formed after cold rolling annealing and cooling, and plate fracture may occur during ultra-thin rolling. Therefore, the upper limit of the carbon content is defined as 0.1% or less, preferably 0.07% or less.

[0029] The content of silicon (Si) may be 0.05% to 0.7%.

[0030] Si is an element that can ensure strength and improve corrosion resistance. Considering this, the lower limit of the silicon content is defined as 0.05% or more, preferably 0.11% or more. However, when the silicon content is too high, brittleness is generated, and plate fracture may occur during ultra-thin rolling. In addition, Si is an element that can react with oxygen (O) or sulfur (S) to form inclusions of oxides or sulfides. However, silicon (Si) inclusions have the characteristic of being relatively easily stretched during rolling. However, silicon (Si) inclusions can become nuclei for aluminum (Al) or magnesium (Mg) inclusions. Therefore, the upper limit of the silicon content is limited to 0.7% or less, preferably 0.61% or less.

[0031] The content of manganese (Mn) may be 0.05% to 1%.

[0032] Mn refines the structure during hot rolling, thereby reducing the ratio of the grain size in the width direction. Considering this, the lower limit of the manganese content is defined as 0.05% or more, preferably 0.26% or more. However, when the manganese content exceeds 1%, a martensite phase may be formed after cold rolling annealing. Therefore, the upper limit of the manganese content is defined as 1% or less, preferably 0.85% or less.

[0033] The content of chromium (Cr) may be 15% to 19%.

[0034] Cr is an element that improves corrosion resistance. When the chromium content is less than 15%, a martensitic phase can be formed in the cold rolling and annealing range. Therefore, the lower limit of the chromium content is defined as 15% or more, preferably 15.1% or more. However, when the chromium content exceeds 19%, the steel may be overly hardened. The upper limit of the chromium content is defined as 19% or less, preferably 18.7% or less.

[0035] The content of nitrogen (N) can be from 0.001% to 0.1%.

[0036] N is an essential element to ensure the strength of the steel grade. Considering this, the lower limit of the nitrogen content is defined as 0.001% or more, preferably 0.01% or more. However, when the nitrogen content exceeds 0.1%, a hard martensitic phase is formed after cold rolling annealing and cooling, and plate fracture may occur during ultra-thin rolling. The upper limit of the nitrogen content is defined as 0.1% or less, preferably 0.07% or less.

[0037] The content of aluminum (Al) can be from 0% to 0.01%.

[0038] Al can be a factor that reacts with oxygen (O) to form oxide inclusions. Considering this, the aluminum content is controlled below 0.01%.

[0039] The remaining component is iron (Fe). However, in the normal manufacturing process, unwanted impurities inevitably mix in from raw materials or the surrounding environment, so these impurities cannot be excluded. These impurities are well known to those skilled in the normal manufacturing process, so all of their details are not specifically mentioned in this specification.

[0040] The ferritic stainless steel according to one embodiment can include 5 or fewer inclusions in the cross-section of the as-cast structure per 160 mm 2 each inclusion having a total content of Al and Mg exceeding 30% by weight and a length in the thickness direction exceeding 6 μm.

[0041] In addition, the inclusions may contain oxides or sulfides with a high silicon content that are easily stretched during rolling. Preferably, inclusions that do not thin or break during rolling and maintain a large size, such as oxides or sulfides with a high content of aluminum or magnesium, are minimized.

[0042] In the ferritic stainless steel, there are inclusions formed during casting. They are mainly oxides or sulfides with Al, Mg, Si, and Mn as the main components. In the usual stainless steel manufacturing process, as the thickness of the coiled sheet thins during hot rolling and cold rolling, the thickness of the inclusions also thins or they are broken inside. Assuming the thickness of the cold-rolled coiled sheet is 0.5 mm, the size of the existing inclusions is at the level of several μm to dozens of μm. Compared with the thickness of the coiled sheet, the size of the inclusions is small, so there will be no big problems during rolling. However, in the ultra-thin rolling where the thickness of the cold-rolled coiled sheet is at the level of 0.01 mm to 0.1 mm, the size of such inclusions is not small compared with the thickness of the whole coiled sheet.

[0043] In addition, during ultra-thin rolling, there are inclusions that do not significantly thin or are not broken and remain, so plate fracture may occur during rolling. For example, Figure 1 Fig. is an electron microscope photograph of an inclusion with a size of 5 μm observed in a cold-rolled coiled sheet with a thickness of 0.03 mm. The inclusions as described above remain with a size of about 1 / 3 of the whole thickness of the coiled sheet, which may cause plate fracture. According to an embodiment of the present invention, by ensuring that there are no remaining inclusions in the cold-rolled coiled sheet during ultra-thin rolling, plate fracture during ultra-thin rolling can be prevented.

[0044] Therefore, the inclusions with the sum of the contents of Al and Mg in the composition of the inclusions exceeding 30% should be restricted. This is because the inclusions with the sum of the contents of Al and Mg exceeding 30% by weight become hard and may still remain even after ultra-thin rolling. On the other hand, the size of the inclusions with the sum of the contents of Al and Mg being 30% by weight or less decreases due to ductility or breakage during the rolling process, so they may not affect plate fracture.

[0045] In addition, according to an embodiment of the present invention, the number of inclusions with a length along the thickness direction exceeding 6 μm in the ferritic stainless steel can be restricted to 2 5 or less per 160 mm. The inclusions with a length along the thickness direction of 6 μm or less have been reduced by stretching and breakage, so they have a relatively small size compared with the ultra-thin rolling thickness, and thus may have little effect on plate fracture. That is, when the number of inclusions with a length along the thickness direction exceeding 6 μm is controlled to 2 5 or less per 160 mm, the stress concentration sites during ultra-thin rolling are reduced, thereby preventing plate fracture during ultra-thin rolling.

[0046] In addition, for the ferritic stainless steel according to an embodiment of the present invention, the ratio of the average grain size in the outer region in the width direction of the stainless steel to the average grain size in the central region in the width direction may be 2 or less. When the difference in grain size along the width direction is large, stress may concentrate on one side during ultra-thin rolling. Therefore, the ratio of the average grain sizes is limited to 2 or less, preferably limited to 1.9 or less.

[0047] Next, a method for manufacturing a ferritic stainless steel with excellent ultra-thin rollability according to an example of the present invention will be described.

[0048] A method for manufacturing a ferritic stainless steel with excellent ultra-thin rollability according to an example of the present invention may include the following steps: casting a slab, which, by weight%, contains: carbon (C): 0.001% to 0.1%, silicon (Si): 0.05% to 0.7%, manganese (Mn): 0.05% to 1%, chromium (Cr): 15% to 19%, nitrogen (N): 0.001% to 0.1%, aluminum (Al): 0% to 0.01%, the balance being Fe and inevitable impurities; heating the slab at 1150°C to 1250°C; hot rolling the heated slab to manufacture a hot-rolled material; subjecting the hot-rolled material to hot-rolling annealing at 750°C to 880°C; cold rolling the hot-rolled material to manufacture a cold-rolled material; and subjecting the cold-rolled material to cold-rolling annealing at 750°C to 880°C.

[0049] The reasons for limiting the numerical ranges of the components of each alloy composition are as described above. The following will provide a more detailed description of each manufacturing step.

[0050] First, after manufacturing a slab that meets the alloy composition, a series of processes of reheating, hot rolling, hot-rolling annealing, cold rolling, and cold-rolling annealing can be performed.

[0051] First, the slab can be heated to a temperature of 1150°C to 1250°C in a hot-rolling heating furnace and then hot-rolled to a thickness of 2.5 mm to 5 mm to manufacture a hot-rolled steel sheet. When the heating temperature is too high, the internal grains may become too coarse, resulting in severe surface oxidation, which may cause surface defects and uneven distribution along the width direction, making it difficult to control the grain size along the width direction after subsequent hot rolling. In addition, when the temperature is too low, it is difficult to reabsorb inclusions during the heating process. The heating temperature can be limited to 1150°C to 1250°C, preferably limited to 1181°C to 1245°C.

[0052] In addition, when the hot-rolled thickness exceeds 5 mm, the remaining thickness must be thinned by cold rolling. In this case, sometimes inclusions that should be stretched along the length direction are broken and segmented instead of being stretched. That is, the inclusions must be made more stretchable at high temperatures. When the inclusions are broken and segmented, the inclusions will not be reabsorbed by the base metal during the following annealing process and will remain in a thick state, which may become a cause of plate fracture. In addition, when the hot-rolled thickness is less than 2.5 mm, the hot-rolling load increases, so the hot-rolled thickness is limited as described above.

[0053] In the hot rolling, finish rolling can be carried out at 800°C to 950°C.

[0054] When the finish rolling temperature is too low, sticking may occur on the surface of the slab during hot rolling, and the reabsorption of inclusions during hot rolling may be insufficient. However, when the finish rolling temperature is too high, coarse ferrite grains may be formed, and the finish rolling temperature can be limited as described above.

[0055] In addition, the hot-rolled material can be subjected to hot-rolling annealing at a temperature of 750°C to 880°C.

[0056] When the hot-rolling annealing temperature is low, the stress formed during hot rolling cannot be sufficiently removed, the workability may be reduced, and the reabsorption of inclusions may be insufficient. However, when the hot-rolling annealing temperature is too high, the edge part in the width direction may overheat, so the difference in the size of grains in the width direction may increase. The hot-rolling annealing temperature of the hot-rolled material can be limited to 750°C to 880°C, and preferably can be limited to 760°C to 865°C.

[0057] The hot-rolled material subjected to hot-rolling annealing can be cold-rolled and then cold-rolled annealed to manufacture a cold-rolled steel sheet. At this time, the cold-rolling annealing can be carried out at a temperature of 750°C to 880°C. When the cold-rolling annealing temperature is too low, the stress formed during cold rolling cannot be sufficiently removed, the workability for ultra-thin rolling is reduced, and the problem of insufficient reabsorption of inclusions may occur. When the cold-rolling annealing temperature is too high, the edge part in the width direction overheats, and the difference in the size of grains in the width direction may increase. The cold-rolling annealing temperature can be limited to 750°C to 880°C, and preferably can be limited to 755°C to 871°C.

[0058] In the cold rolling process, after the hot-rolled material is cold-rolled to a thickness of 0.4 mm to 1 mm at one time, in order to manufacture an ultra-thin cold-rolled coil sheet, it can also be cold-rolled to a thickness of 0.01 - 0.2 mm for the second time.

[0059] The reduction ratio during the first cold rolling can be from 60% to 92%, and the rolling speed can be from 50 meters per minute (mpm) to 700 meters per minute. In addition, the reduction ratio during the second cold rolling can be from 50% to 88%, and the rolling speed can be from 50 meters per minute to 700 meters per minute.

[0060] The ferritic stainless steel according to an embodiment of the present invention can prevent plate breakage without reducing the reduction ratio and the rolling speed, thereby improving the productivity of cold-rolled coil sheets.

[0061] In the method for manufacturing a ferritic stainless steel according to an embodiment of the present invention, the cold rolling may include cold rolling the hot-rolled material to a thickness of 0.4 mm to 1 mm for the first time and then cold rolling to a thickness of 0.01 mm to 0.2 mm for the second time.

[0062] In the method for manufacturing a ferritic stainless steel according to an embodiment of the present invention, in the cross-section of the as-cast structure, the total content of Al and Mg exceeding 30% by weight and the number of inclusions having a length exceeding 6 μm in the thickness direction may be 5 or less per 160 mm 2 5 or less.

[0063] In the method for manufacturing a ferritic stainless steel according to an embodiment of the present invention, the ratio of the average grain size in the outer region in the width direction of the stainless steel to the average grain size in the central region in the width direction of the stainless steel may be 2 or less, preferably 1.1 to 1.9.

[0064] Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are for more specifically illustrating the present invention, and the scope of the present invention is not limited to the following examples.

[0065] {Examples}

[0066] A slab having the alloy composition shown in Table 1 below was cast to a thickness of 220 mm. The cast slab was heated under the manufacturing conditions shown in Table 1 below, and the heated slab was hot-rolled to 3 mm and then hot-rolled annealed. After cold rolling the hot-rolled annealed material to 0.5 mm for the first time, it was cold-rolled annealed, and then cold-rolled to 0.1 mm and cold-rolled annealed at about 820 °C. Ultra-thin rolling was performed to a thickness of 0.05 mm.

[0067] For the obtained cold-rolled coil sheet specimens, Table 1 below shows: the number of inclusions per unit area in the cross-section of the as-cast structure having a total content of Al and Mg exceeding 30% by weight and a length exceeding 6 μm in the thickness direction, the ratio of the average grain size in the outer region in the width direction of the stainless steel to the average grain size in the central region in the width direction of the stainless steel, and whether plate breakage occurred.

[0068] In the present invention, inclusions existing in the cold-rolled coil for ultra-thin rolling and inclusions remaining after ultra-thin rolling are compared to analyze the types of inclusions that are not eliminated, that is, the types of inclusions that do not become thinner or are not broken during the rolling process and maintain a large size.

[0069] The inclusions are analyzed by Energy Dispersive X-ray Spectrometer in a scanning electron microscope. Arbitrary 160 mm of the cross-section of the cold-rolled coil is observed with an electron microscope 2 of the area, the number, size, and composition of inclusions with an oxygen content of 1% or more are measured, and the number of inclusions with a total content of more than 30% by weight of Al and Mg and a length in the thickness direction exceeding 6 μm in the cross-section of the casting structure per 160 mm 2 of the area are shown in Table 1. Herein, the size of the inclusion refers to the length in the thickness direction.

[0070] By measuring the average grain size observed from the cross-sections of the outer region and the central region in the width direction of the cold-rolled coil and then calculating their ratio, the ratio of the grain sizes is obtained. At this time, the observation area is set to at least 2500 μm 2 , covering all thickness layers in the thickness direction. The observation method can use an optical microscope or a scanning electron microscope, and the measurement unit is μm. Alternatively, the grain size is measured by the method defined in ASTM E112 to obtain the grain size number, and then converted into μm. The outer region in the width direction is the region 0 - 100 mm from the edge, and the central region in the width direction is the region ±100 mm from the center of the entire width of the coil.

[0071] [Table 1]

[0072]

[0073] Referring to Table 1, in the case of Examples 1 to 11 that satisfy the alloy composition and manufacturing conditions of the present invention, it has been confirmed that per 160 mm in the cross-section of the casting structure 2There are 5 or fewer hardened inclusions (for each inclusion, the total content of Al and Mg in the composition of the inclusion exceeds 30% by weight and the length in the thickness direction exceeds 6 μm), which remain even after ultra-thin rolling. Inclusions within the range described above become small enough during rolling due to stretching and breakage, etc., so as not to affect sheet fracture. That is, inclusions smaller than 6 μm are considered to have been reduced in size by stretching and breakage and are relatively small compared to the ultra-thin rolling thickness, and thus have little effect on sheet fracture. In addition, it has been confirmed that the ratio of the average grain size in the outer region in the width direction of the stainless steel to the average grain size in the central region in the width direction is 2 or less, indicating that the change in grain size in the width direction is small, thereby preventing fracture. On the other hand, in Comparative Example 1 where the alloy composition does not satisfy the present invention, the carbon content is 0.11%, and a hard martensite phase is formed after cold rolling annealing and cooling, and sheet fracture occurs during ultra-thin rolling. In the case of Comparative Example 2, it can be confirmed that the chromium content is 19.1%, the steel becomes too hard, the nitrogen content is 0.11%, and a hard martensite phase is formed after cold rolling annealing and cooling, and sheet fracture occurs during ultra-thin rolling. In addition, in the cases of Comparative Example 3 and Comparative Example 4, it can be confirmed that the aluminum contents are 0.03% and 0.11% respectively, which causes the formation of oxide inclusions. Comparative Example 3 and Comparative Example 4 each have inclusions with a total content of Al and Mg exceeding 30% by weight and a length in the thickness direction exceeding 6 μm in the cross-section of the casting structure every 160 mm 2 have 8 and 12 respectively, resulting in sheet fracture.

[0074] In addition, in the case of Comparative Example 5, it can be confirmed that the silicon content is 0.8%. The silicon content is too high, resulting in brittleness. Silicon (Si) inclusions become nuclei for aluminum (Al) or magnesium (Mg) inclusions, resulting in inclusions with a total content of Al and Mg exceeding 30% by weight and a length in the thickness direction exceeding 6 μm in the cross-section of the casting structure every 160 mm 2 having 9, and thus sheet fracture occurs.

[0075] In addition, in the case of Comparative Example 6, it can be confirmed that the manganese content is 1.22%, and a martensite phase is formed after cold rolling annealing, resulting in inclusions with a total content of Al and Mg exceeding 30% by weight and a length in the thickness direction exceeding 6 μm in the cross-section of the casting structure every 160 mm 2 having 8, and thus sheet fracture occurs.

[0076] In addition, in the case of Comparative Example 7 and Comparative Example 8, it can be confirmed that the cold rolling annealing temperatures are 870 °C and 900 °C respectively, which are outside the scope of the present invention. Therefore, the edge portion in the width direction overheats, the difference in the grain size along the width direction becomes larger, stress concentrates on one side during ultra-thin rolling, and the ratio of the average grain size in the outer region in the width direction of the stainless steel to the average grain size in the central region in the width direction is 2.2 and 2.5 respectively. Therefore, plate fracture occurs.

[0077] In addition, in Comparative Example 9, the slab heating temperature is 1145 °C, and inclusions are difficult to be reabsorbed during the heating process, resulting in 7 inclusions per 160 mm in the cross-section of the casting structure with a total content of Al and Mg exceeding 30% by weight and a length along the thickness direction exceeding 6 μm. 2 In the case of Comparative Example 10, it can be confirmed that the hot rolling annealing temperature is 740 °C and the cold rolling annealing temperature is 745 °C. The stress formed during hot rolling cannot be sufficiently removed, and the workability may be reduced. Not only is the reabsorption of inclusions insufficient, but also the stress formed during cold rolling cannot be sufficiently removed. Therefore, the workability for ultra-thin rolling is reduced, and the reabsorption of inclusions is insufficient, resulting in 7 inclusions per 160 mm in the cross-section of the casting structure with a total content of Al and Mg exceeding 30% by weight and a length along the thickness direction exceeding 6 μm. 2 Therefore, plate fracture occurs. In the case of Comparative Example 11, it can be confirmed that the slab heating temperature is 1140 °C and the hot rolling annealing temperature is 742 °C. It is difficult to reabsorb inclusions, and the stress formed during hot rolling cannot be sufficiently removed, resulting in reduced workability. 8 inclusions per 160 mm are formed in the cross-section of the casting structure with a total content of Al and Mg exceeding 30% by weight and a length along the thickness direction exceeding 6 μm. 2 Therefore, plate fracture occurs.

[0078] As described above, although exemplary embodiments of the present invention have been illustrated, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications can be made without departing from the concepts and scopes of the claims.

Claims

1. A ferritic stainless steel having excellent ultra-thin rollability, in weight %, the ferritic stainless steel comprises: carbon (C): 0.001% to 0.1%, silicon (Si): 0.05% to 0.7%, manganese (Mn): 0.05% to 1%, chromium (Cr): 15% to 19%, nitrogen (N): 0.001% to 0.1%, aluminum (Al): 0% to 0.01%, the balance Fe and unavoidable impurities, In the cross-section of the cast structure of the ferritic stainless steel, the number of inclusions having a total content of Al and Mg of more than 30% by weight and a length in the thickness direction of more than 6 μm is 5 or less per 160 mm 2 .

2. The ferritic stainless steel having excellent ultra-thin rolling properties according to claim 1, wherein, The inclusions comprise silicon-containing oxides or silicon-containing sulfides.

3. The ferritic stainless steel having excellent ultra-thin rollability according to claim 1, wherein, The ratio of the average grain size in the outer region in the width direction of the stainless steel to the average grain size in the central region in the width direction of the stainless steel is 2 or less.

4. A method for manufacturing a ferritic stainless steel having excellent ultra-thin rollability, which comprises the following steps: Casting a slab, in weight %, the slab comprises: carbon (C): 0.001% to 0.1%, silicon (Si): 0.05% to 0.7%, manganese (Mn): 0.05% to 1%, chromium (Cr): 15% to 19%, nitrogen (N): 0.001% to 0.1%, aluminum (Al): 0% to 0.01%, the balance Fe and unavoidable impurities; Heating the slab at 1150°C to 1250°C; Hot-rolling the heated slab to produce a hot-rolled material; Performing hot-rolling annealing on the hot-rolled material at 750°C to 880°C; Cold-rolling the hot-rolled material to produce a cold-rolled material; And Performing cold-rolling annealing on the cold-rolled material at 750°C to 880°C.

5. The manufacturing method of a ferritic stainless steel having excellent ultra-thin rollability according to claim 4, wherein, The hot-rolling includes hot-rolling the heated slab to a thickness of 2.5 mm to 5 mm.

6. The method for manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to claim 4, wherein, The hot-rolling includes finish-rolling the heated slab at 800°C to 950°C.

7. The method for manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to claim 4, wherein, The cold-rolling includes cold-rolling the hot-rolled material to a thickness of 0.4 mm to 1 mm in one pass, and then cold-rolling the once cold-rolled material to a thickness of 0.01 mm to 0.2 mm in a second pass.

8. The manufacturing method of the ferritic stainless steel having excellent ultra-thin rollability according to claim 4, wherein, In the cross-section of the cast structure of the ferritic stainless steel, the number of inclusions having a total content of Al and Mg of more than 30% by weight and a length in the thickness direction of more than 6 μm is 2 5 or less per 160 mm.

9. The manufacturing method of a ferritic stainless steel having excellent ultra-thin rollability according to claim 4, wherein, The ratio of the average grain size in the outer region in the width direction of the stainless steel to the average grain size in the central region in the width direction of the stainless steel is 2 or less.

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