EP4603616A1

EP4603616A1 - Ferritic stainless steel with excellent ultra-thin rollability and manufacturing method thereof

Info

Publication number: EP4603616A1
Application number: EP23907713.4A
Authority: EP
Inventors: Hyunggu KANG; Seungmin HAN; Youngjin Kwon; Nayeon CHU; Suhyeon SEOK
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-20
Filing date: 2023-12-20
Publication date: 2025-08-20
Also published as: EP4603616A4; CN120322583A; WO2024136430A1; JP2026500396A

Abstract

The present invention relates to a ferritic stainless steel having excellent ultra-thin rollability, comprising, in percent by weight (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), and the remainder of iron (Fe), and inevitable impurities, wherein in a cross-section of a cast structure of the ferritic stainless steel, the number of inclusions having a total content of the Al and the Mg exceeding 30wt% and a length exceeding 6 µm in a thickness direction is 5 or fewer per 160mm².

Description

[Technical Field]

The present invention relates to a ferritic stainless steel having excellent ultra-thin rollability and a method of manufacturing the same.

[Background Art]

The application of ultra-thin stainless steel is gradually expanding in products that require both flexibility and rigidity, such as flexible solar cells and flexible displays.
Ultra-thin stainless steel typically has a thickness of 0.01 mm to 0.08 mm, and in this case, and by rolling stainless steel thinly, strength is increased through work hardening while flexibility is ensured due to the thin thickness. In this case, ultra-thin rolling process is performed using cold-rolled coils having a thickness of 0.4 mm or more to perform ultra-thin rolling. However, during the rolling process, sheet fracture may occur due to the extremely thin thickness. Such a sheet fracture is considered to be caused by hard, coarse inclusions present inside the coil. In the conventional technology, in order to prevent sheet fracture, the reduction ratio and the rolling speed are lowered and multiple, low-speed operations are performed. However, this results in reduced productivity.

[Disclosure]

[Technical Problem]

The present invention provides a ferritic stainless steel having excellent ultra-thin rollability and a method of manufacturing the same.
The technical objectives of the present invention are not limited to the above, and other objectives that are not described above will be clearly understood by those skilled in the art from the above detailed description.

[Technical Solution]

A ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention include, in percent by weight (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), and the remainder of iron (Fe), and inevitable impurities, wherein in a cross-section of a cast structure of the ferritic stainless steel, the number of inclusions having a total content of the Al and the Mg exceeding 30wt% and a length exceeding 6 µm in a thickness direction is 5 or fewer per 160mm².
In the ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention, the inclusions may include a silicon-containing oxide or a silicon-containing sulfide.
In the ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention, a ratio of an average grain size in a widthwise outer region to an average grain size in a widthwise central region of the stainless steel may be 2 or less.
A method of manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to an embodiment of the present invention includes: casting a slab comprising, in percent by weight (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), and the remainder of iron (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-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-annealing the cold-rolled material at 750°C to 880°C.
In the method 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.
In the method according to an embodiment of the present invention, the hot rolling may include finish-rolling the heated slab at 800°C to 950°C.
In the method according to an embodiment of the present invention, the cold rolling may include performing primary cold rolling on the hot-rolled material to a thickness of 0.4 mm to 1 mm. and then performing secondary cold rolling on the primary cold-rolled material to a thickness of 0.01 mm to 0.2 mm.
In the method according to an embodiment of the present invention, the number of inclusions having a total content of the Al and the Mg exceeding 30wt% and a length exceeding 6 µm in a thickness direction may be 5 or fewer per 160mm² in a cross-section of a cast structure of the ferritic stainless steel.
In the method according to an embodiment of the present invention, a ratio of an average grain size in a widthwise outer region to an average grain size in a widthwise central region of the stainless steel may be 2 or less.

[Advantageous Effects]

According to an embodiment of the present invention, it is possible to provide a ferritic stainless steel having excellent ultra-thin rollability capable of preventing sheet fracture during cold rolling and a method of manufacturing the same.
The effects of the present invention are not limited to those described above, and other effects that are not described above will be clearly understood by those skilled in the art from the above detailed description.

[Description of Drawings]

FIG. 1 is a photograph of an inclusion with characteristics that cause sheet fracture during ultra-thin rolling.

[Modes of the Invention]

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The embodiments of the present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.
Also, the terms used herein are merely used to describe particular embodiments. An expression used in the singular encompasses the expression of the plural, unless otherwise indicated. Throughout the specification, the terms such as "including" or "having" are intended to indicate the existence of features, operations, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, operations, functions, components, or combinations thereof may exist or may be added.
Meanwhile, unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Thus, these terms should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.
The terms "about", "substantially", etc. used throughout the specification means that when a natural manufacturing and a substance allowable error are suggested, such an allowable error corresponds the value or is similar to the value, and such values are intended for the sake of clear understanding of the present invention or to prevent an unconscious infringer from illegally using the disclosure of the present invention.
A ferritic stainless steel having improved formability according to an example of the present invention includes, in percent by weight (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), and the remainder of iron (Fe), and inevitable impurities,
Hereinafter, the reason for limiting the compositional range of each alloy element will be described. Hereinafter, unless otherwise specified, the units thereof are expressed in weight percent (wt%).
The content of carbon (C) may be 0.001% to 0.1%.
C is an element required for ensuring the strength of the steel grade. Considering this, the lower limit of the carbon content is limited to 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 annealing and cooling, which may cause sheet fracture during ultra-thin rolling. Therefore, the lower limit of the carbon content is limited to 0.1% or less, preferably 0.07% or less.
The content of silicon (Si) may be 0.05% to 0.7%.
Si is an element that may ensure strength and improve corrosion resistance. Considering this, the lower limit of the silicon content is limited to 0.05% or more, preferably 0.11% or more. However, when the silicon content is excessive, brittleness may occur, which may cause sheet fracture during ultra-thin rolling. In addition, Si is an element that may react with oxygen (O) or sulfur (S) to form oxide or sulfide inclusions. However, silicon (Si) inclusions tend to easily elongate during rolling. However, since silicon (Si) inclusions may serve as nuclei of aluminum (Al) or magnesium (Mg) inclusions, the upper limit of the silicon content is limited to 0.7% or less, preferably 0.61% or less.
The content of manganese (Mn) may be 0.05% to 1%.
Mn may refine the structure during hot rolling, thereby reducing a ratio of grain sizes in the width direction. Considering this, the lower limit of the manganese content is limited to 0.05% or more, preferably 0.26% or more. However, when the manganese content exceeds 1%, a martensite phase may be formed after cold annealing, and thus the upper limit of the manganese content is limited to 1% or less, preferably 0.85% or less.
The content of chromium (Cr) may be 15% to 19%.
Cr is an element that improves corrosion resistance. When the chromium content is less than 15%, a martensite phase may be formed in the cold annealing section, and thus the lower limit of the chromium content is limited to 15% or more, preferably 15.1% or more. However, when the chromium content exceeds 19%, the steel may be excessively hardened, and thus the upper limit of the chromium content is limited to 19% or less, preferably 18.7% or less.
The content of nitrogen (N) may be 0.001% to 0.1%.
N is an element required for ensuring the strength of the steel grade. Considering this, the lower limit of the nitrogen content is limited to 0.001% or more, preferably 0.01% or more. However, when the nitrogen content exceeds 0.1%, a hard martensite phase may be formed after cold annealing and cooling, which may cause sheet fracture during ultra-thin rolling, and thus the upper limit of the nitrogen content is limited to 0.1% or less, preferably 0.07% or less.
The content of aluminum (Al) may be 0% to 0.01%.
Al may be a factor that may form oxide inclusions by reacting with oxygen (O). Considering this, the aluminum content is controlled to 0% or less.
The remainder is iron (Fe). However, since unintended impurities may inevitably be introduced from raw materials or the surrounding environment during a typical manufacturing process, this may not be excluded. Since such impurities may be well known to those skilled in the art during a typical manufacturing process, details thereof are not described in this specification.
A ferritic stainless steel according to an embodiment may include 5 or fewer inclusions per 160mm² in a cross-section of a cast structure, each inclusion having a total content of Al and Mg exceeding 30wt% and a length exceeding 6 µm in a thickness direction.
In addition, the inclusions may include an oxide or sulfide with a high silicon that is easily elongated during rolling, and inclusions that maintain a large size without being thinned or fractured during rolling, such as an oxide or sulfide with a high aluminum or magnesium content, may be minimized.
Ferritic stainless steels have inclusions formed during casting. The inclusions are typically oxides or sulfides having main components of Al, Mg, Si, and Mn. In general, in stainless steel manufacturing processes, the coil are subjected to hot rolling and cold rolling, and the thickness of the coil is reduced, and the inclusions also become thinner or fracture inside the coil. When the thickness of the cold-rolled coil is assumed to be 0.5 mm, the size of the conventional inclusions is on the order of several to several tens of micrometers, which is relatively small compared to the thickness of the coil, and thus does not significant affect the rolling process. However, in ultra-thin rolling in which the thickness of the cold-rolled coil is 0.01 mm to 0.1 mm, the size of the inclusions is not relatively small compared to the overall thickness of the coil.
In addition, there are inclusions that remain without being greatly thinned or fractured during ultra-thin rolling, which may cause sheet fracture during rolling. For example, FIG. 1 is an electron microscope image of a 5 µm-sized inclusion observed in a cold-rolled coil with a thickness of 0.03mm. Such an inclusion remain at a size of about 1/3 of the overall thickness of the coil and may cause sheet fracture. According to an embodiment of the present invention, sheet fracture during ultra-thin rolling may be prevented by preventing such residual inclusions from being present in the cold-rolled coil during ultra-thin rolling.
Accordingly, there is a need to limit inclusions in which the sum of the contents of Al and Mg among inclusion components exceeds 30%. This is because inclusions in which the sum of the contents of Al and Mg exceeds 30 wt% may become hard and may remain even after ultra-thin rolling. On the other hand, inclusions in which the sum of the contents of Al and Mg is 30 wt% or less may undergo size reduction due to elongation or fracture during rolling and thus may not affect sheet fracture.
In addition, an ferritic stainless steel according to an embodiment of the present invention may limit the number of inclusions having a length exceeding 6 µm in the thickness direction to 5 or less per 160 mm². Inclusions having a length of 6 µm or less in the thickness direction have already become smaller due to elongation and fracture, and thus have a relatively small size compared to the ultra-thin rolling thickness, and the influence on sheet fracture may be negligible. That is, when the number of inclusions having a length exceeding 6 µm in the thickness direction is controlled to 5 or less per 160 mm², the site in which stress is concentrated during ultra-thin rolling is reduced, so that sheet fracture during ultra-thin rolling may be prevented.
In addition, in a ferritic stainless steel according to an embodiment of the present invention, a ratio of an average grain size in a widthwise outer region to an average grain size in a widthwise central region of the stainless steel may be 2 or less. When the grain size variation in the width direction is significant, stress may be concentrated on one side during ultra-thin rolling, and thus the ratio of the average grain sizes is limited to 2 or less, preferably 1.9 or less.
Next, a method of manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to an example of the present invention is described.
A method of manufacturing a ferritic stainless steel having excellent ultra-thin rollability according to an example of the present invention includes: casting a slab comprising, in percent by weight (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), and the remainder of iron (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-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-annealing the cold-rolled material at 750°C to 880°C.
The reasons for limiting the numerical ranges of the respective alloy compositions are as described above, and the following provides details of manufacturing operations.
First, a slab satisfying the above described alloy composition may be prepared, and subjected to a series of processes including reheating, hot rolling, hot annealing, cold rolling, and cold annealing.
First, the slab may be heated in a hot rolling furnace at a temperature of 1150°C to 1250°C, and then hot-rolled to a thickness of 2.5mm to 5mm to produce a hot-rolled steel sheet. When the heating temperature is excessively high, the internal grains may become excessively coarse, sever surface oxidation may occur, causing surface defects, and the distribution in the width direction may become non-uniform, making it difficult to control the grain size in the width direction after the subsequent hot rolling. In addition, when the heating temperature is excessively low, it may be difficult for the inclusions to be reabsorbed during heating, and thus the heating temperature may be limited to 1150°C to 1250°C, preferably 1181°C to 1245°C.
In addition, when the hot-rolled thickness is greater than 5mm, the remaining thickness needs to thinned by cold rolling, but in this case, the inclusions that need to elongate in the longitudinal direction may instead fracture and become segmented. In other words, inclusions need to be easily elongated at high temperatures. When inclusions fracture, the inclusions may not be reabsorbed into the base metal during the annealing process described below and may remain in a thick state, causing sheet fracture. On the other hand, when the hot-rolled thickness is less than 2.5mm, the hot rolling load increases, and thus the hot-rolled thickness is limited as described above.
In the hot rolling, the finish rolling may be performed at 800°C to 950°C.
When the finish rolling temperature is excessively low, sticking may occur on the sheet surface of the slab during hot rolling, and the reabsorption of inclusions may be insufficient during hot rolling. However, when the finish rolling temperature is excessively high, coarse ferrite grains may be formed, and thus the finish rolling temperature may be limited as described above.
In addition, the hot-rolled material may be subjected to hot-annealing at a temperature of 750°C to 880°C.
When the hot-annealing temperature is low, the stress formed during the hot rolling may not be sufficiently removed, and thus the workability may be lowered and the reabsorption of inclusions may be insufficient. However, when the hot-annealing temperature is excessively high, a widthwise edge portion may be overheated, and the grain size variation in the width direction may become large, and thus the hot-annealing temperature of the hot-rolled material may be limited to 750°C to 880°C, preferably 760°C to 865°C.
The hot-rolled material subjected to the hot annealing may be cold-rolled and then cold-annealed to produce a cold-rolled steel sheet. In this case, the cold-annealing may be performed at a temperature of 750°C to 880°C. When the cold-annealing temperature is excessively low, the stress formed during the cold rolling is not sufficiently removed, which may result in poor workability for ultra-thin rolling, and insufficient reabsorption of inclusions. When the cold-annealing temperature is excessively high, a widthwise edge portion may be overheated, which may increase the grain size variation in the width-direction. Therefore, the cold-annealing temperature may be limited to 750°C to 880°C, preferably 755°C to 871°C.
In the cold rolling described above, the hot-rolled material is subjected to a primary cold rolling to achieve a thickness of 0.4 mm to 1 mm, followed by a secondary cold rolling to achieve a thickness of 0.01 to 0.2 mm to produce an ultra-thin cold-rolled coil.
In the primary cold rolling, the reduction ratio may be 60% to 92%, and the rolling speed may be 50mpm to 700mpm. In addition, in the secondary cold rolling, the reduction ratio may be 50% to 88%, and the rolling speed may be 50mpm to 700mpm.
The ferritic stainless steel according to an embodiment of the present invention may prevent sheet fracture without reducing the reduction ratio and the rolling speed, thereby improving the productivity of the cold-rolled coil.
The method of manufacturing a ferritic stainless steel according to an embodiment of the present invention may include the cold rolling including performing the primary cold rolling on the hot-rolled material to a thickness of 0.4 mm to 1 mm, followed by the secondary cold rolling to a thickness of 0.01 mm to 0.2 mm.
The method of manufacturing a ferritic stainless steel according to an embodiment of the present invention may provide the stainless steel in which the number of inclusions having a total content of Al and Mg exceeding 30wt% and a length exceeding 6 µm in a thickness direction is 5 or fewer per 160mm² in a cross-section of a cast structure of the ferritic stainless steel.
The method of manufacturing a ferritic stainless steel according to an embodiment of the present invention provides the stainless steel in which a ratio of an average grain size in a widthwise outer region to an average grain size in a widthwise central region of the stainless steel is 2 or less, preferably 1.1 to 1.9.
Hereinafter, the present invention will be described in more detail through embodiments. However, the descriptions of the embodiments are only for illustrating the implementation of the present invention, and the present invention is not limited by the descriptions of the embodiments.

{Embodiment}

A slab having an 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 annealed. The hot-annealed material was first cold-rolled to 0.5 mm, followed by cold-annealing, and then secondarily cold-rolled to 0.1 mm, followed by cold-annealing at approximately 820°C. Ultra-thin rolling was performed to a thickness of 0.05 mm.
For the cold-rolled coil specimen manufactured above, the number of inclusions having a total content of Al and Mg exceeding 30 wt% and a length exceeding 6 µm in the thickness direction per unit area in the cross section of the cast structure, the ratio of an average grain size in a widthwise outer region to an average grain size in a widthwise central region of the stainless steel, and whether sheet fracture occurred are shown in Table 1 below.
In the present invention, inclusions present inside a cold-rolled coil used for ultra-thin rolling and inclusions remaining after the ultra-thin rolling were compared to analyze the types of inclusions that were not eliminated, that is, inclusions that did not thin or fracture during rolling and maintain a large size.
The analysis of the inclusions was performed using an energy dispersive X-ray spectrometer in a scanning electron microscope. A random area of 160 mm² in the cross section of the cold-rolled coil was observed with an electron microscope, and the number, size, and composition of the inclusions having an oxygen content of 1% or higher were measured to list the number of inclusions having a total content of Al and Mg exceeding 30 wt% and a length exceeding 6 µm in the thickness direction per 160 mm² in the cross section of the cast structure in Table 1. Here, the size of the inclusion refers to the length in the thickness direction.

The ratio of grain sizes was obtained by measuring the average grain sizes observed in the cross sections of the outer region and the central regions in the width direction of the cold-rolled coil, and then calculating the ratio of the average grain sizes. In this case, the observation area was set to at least 2500 µm², covering the full thickness layer in the thickness direction, and the observation method may use an optical microscope or a scanning electron microscope, and the unit of measurement was micrometer (µm). Alternatively, the grain size may be measured by the method defined in ASTM E112 to obtain a grain size number, which may then be converted to micrometers (µm). The widthwise outer region is an area from 0 to 100 mm from the edge, and the widthwise central region is an area within ±100 mm from the center of the total coil width.

[Table 1]

	C	Si	Mn	Cr	N	Al	Heatin g temper ature of slab (°C)	Hot Anneal ing Tempe rature (°C)	Cold Anneal ing Tempe rature (°C)	Numbe r of inclusi ons in cross section of casting structu re (count per 160 mm²)	The ratio of grain sizes	Occurr ence of sheet fractur e
Embodi ment 1	0.04	0.31	0.52	16.1	0.03	0.003	1181	840	850	3	1.6	good
Embodi ment 2	0.04	0.25	0.85	18.7	0.03	0.004	1189	835	851	3	1.9	good
Embodi ment 3	0.01	0.61	0.52	15.1	0.07	0.002	1205	842	845	4	1.3	good
Embodi ment 4	0.07	0.31	0.62	16.2	0.01	0.003	1204	845	852	3	1.4	good
Embodi ment 5	0.06	0.35	0.81	16.1	0.04	0.002	1229	839	835	2	1.3	good
Embodi ment 6	0.01	0.11	0.26	15.9	0.01	0.001	1245	835	832	1	1.2	good
Embodi ment 7	0.04	0.31	0.52	16.1	0.03	0.008	1205	815	799	4	1.1	good
Embodi ment 8	0.03	0.31	0.52	16.2	0.03	0.006	1204	840	850	3	1.5	good
Embodi ment 9	0.04	0.31	0.62	16.1	0.02	0.003	1229	839	845	2	1.5	good
Embodi ment 10	0.04	0.28	0.52	16.3	0.03	0.003	1245	865	871	0	1.5	good
Embodi ment 11	0.04	0.31	0.45	16.1	0.03	0.009	1182	760	755	5	1.1	good
Compara tive example 1	0.11	0.55	0.29	18.5	0.03	0.005	1222	835	844	2	1.1	fractu re
Compara tive example 2	0.03	0.48	0.33	19.1	0.11	0.009	1218	841	844	3	1.1	fractu re
Compara tive example 3	0.04	0.31	0.55	16.3	0.03	0.03	1215	822	835	8	1.3	fractu re
Compara tive example 4	0.03	0.28	0.45	15.8	0.02	0.11	1218	826	811	12	1.4	fractu re
Compara tive example 5	0.04	0.8	0.61	17.2	0.03	0.005	1216	833	822	9	1.3	fractu re
Compara tive example 6	0.05	0.33	1.22	16.9	0.02	0.003	1218	833	821	8	1.1	fractu re
Compara tive example 7	0.04	0.28	0.55	16.3	0.03	0.002	1255	860	870	5	2.2	fractu re
Compara tive example 8	0.05	0.33	0.45	16.1	0.02	0.001	1225	890	900	5	2.5	fractu re
Compara tive example 9	0.04	0.32	0.48	16.2	0.02	0.001	1145	820	830	7	1.6	fractu re
Compara tive example 10	0.05	0.33	0.49	16.1	0.03	0.002	1189	740	745	7	1.5	fractu re
Compara tive example 11	0.04	0.33	0.52	16.2	0.03	0.001	1140	742	765	8	1.1	fractu re

Referring to Table 1, in the case of Embodiments 1 to 11 satisfying the alloy composition and manufacturing conditions according to the present invention, it was confirmed that five or fewer hardened inclusions, each having a total content of Al and Mg among the components of the inclusions exceeding 30 wt% and a length exceeding 6 µm in the thickness direction, were present per 160mm² in the cross-section of a cast structure, and remained even after ultra-thin rolling. Inclusions satisfying the above range were reduced in size due to elongation, fracture, etc. during rolling, thereby becoming small enough not to affect sheet fracture. That is, inclusions smaller than 6 µm in size are considered to have already reduced in size through elongation and fracture, and are relatively small compared to the ultra-thin rolling thickness, thereby having a negligible effect on the sheet fracture. In addition, it was confirmed that the ratio of an average grain size in a widthwise outer region to an average grain size in a widthwise central region of the stainless steel was 2 or less, indicating there was a small variation in grain sizes across the width direction, by which fracture was prevented. On the other hand, in Comparative Example 1, which did not satisfy the alloy composition according to the present invention, the carbon content was 0.11 %, leading to the formation of a hard martensite phase after a cold annealing and a cooling, which caused sheet fracture during ultra-thin rolling. In Comparative Example 2, the chromium content was 19.1%, resulting in excessive hardening of the steel, and the nitrogen content was 0.11%, which also led to the formation of a hard martensite phase after a cold annealing and a cooling, resulting in sheet fracture during ultra-thin rolling. In addition, in Comparative Examples 3 and 4, the aluminum contents were 0.03% and 0.11%, respectively, which promoted the formation of oxide inclusions, and therefore, Comparative Examples 3 and 4 had 8 inclusions and 12 inclusions per 160 mm², respectively, in the cross section of the cast structure, each inclusion having a total content of Al and Mg exceeding 30 wt% and a length exceeding 6 µm in the thickness direction, resulting in sheet fracture.
In addition, in Comparative Example 5, the silicon content was 0.8%, which was excessively high, causing brittleness, and the silicon (Si) inclusions served as the nuclei of aluminum (Al) or magnesium (Mg) inclusions, resulting in the presence of 9 inclusions per 160 mm² in the cross-section of the cast structure, each inclusion having a total content of Al and Mg exceeding 30 wt% and a length exceeding 6 µm in the thickness direction which caused sheet fracture.
In addition, in Comparative Example 6, the manganese content was 1.22%, which led to the formation of a martensite phase after cold annealing, resulting in the presence of 8 inclusions per 160 mm² in the cross-section of the cast structure, each inclusion having a total content of Al and Mg exceeding 30 wt% and a length exceeding 6 µm in the thickness direction, which caused sheet fracture.
In addition, in Comparative Examples 7 and 8, the cold annealing temperatures were 870°C and 900°C, respectively, which were outside the range of the present invention. As a result, the edge portion in the width direction was overheated, which increased the grain size variation in the width direction, and thus, during ultra-thin rolling, stress was concentrated on one side, and the ratio of an average grain size in a widthwise outer region to an average grain size in a widthwise central region of the stainless steel was 2.2 and 2.5, respectively, resulting in sheet fracture.
In addition, in Comparative Example 9, the slab heating temperature was 1145°C, at which it was difficult to reabsorb inclusions during heating, resulting in the presence of 7 inclusions per 160 mm² in the cross-section of the cast structure, each inclusion having a total content of Al and Mg exceeding 30 wt% and a length exceeding 6 µm in the thickness direction. In Comparative Example 10, the hot-annealing temperature was 740°C and the cold-annealing temperature was 745°C, and thus the stress formed during hot rolling was not sufficiently removed, resulting in poor workability, and the reabsorption of inclusions was insufficient. In addition, the stress formed during cold rolling was not sufficiently removed, resulting in poor workability for ultra-thin rolling. In addition, the insufficient reabsorption of inclusions resulted in 7 inclusions 160 mm² in the cross-section of the cast structure, each inclusion having a total content of Al and Mg exceeding 30 wt% and a length exceeding 6 µm in the thickness direction, which caused sheet fracture. In Comparative Example 11, the slab heating temperature was 1140°C, and the hot-annealing temperature was 742°C, and thus reabsorption of inclusions was difficult, and the stress formed during hot rolling was not sufficiently removed, resulting in poor workability, and the presence of 8 inclusions per 160 mm² in the cross-section of the cast structure, each inclusion having a total content of Al and Mg exceeding 30 wt% and a length exceeding 6 µm in the thickness direction, which caused sheet fracture.
While exemplary embodiments of the present disclosure have been described above with reference to the accompanying drawings, it will be understood by those of ordinary skill in the art that various modifications and changes can be made without departing from the spirit and scope of the present disclosure as defined by the appended claims.

Claims

A ferritic stainless steel having excellent ultra-thin rollability, comprising, in percent by weight (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), and the remainder of iron (Fe), and inevitable impurities,
wherein in a cross-section of a cast structure of the ferritic stainless steel, the number of inclusions having a total content of the Al and the Mg exceeding 30wt% and a length exceeding 6 µm in a thickness direction is 5 or fewer per 160mm².
The ferritic stainless steel having excellent ultra-thin rollability of claim 1, wherein the inclusions include a silicon-containing oxide or a silicon-containing sulfide.
The ferritic stainless steel having excellent ultra-thin rollability of claim 1, wherein a ratio of an average grain size in a widthwise outer region to an average grain size in a widthwise central region of the stainless steel is 2 or less.
A method of manufacturing a ferritic stainless steel having excellent ultra-thin rollability, comprising:
casting a slab comprising, in percent by weight (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), and the remainder of iron (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-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-annealing the cold-rolled material at 750°C to 880°C.
The method of claim 4, wherein the hot rolling includes hot rolling the heated slab to a thickness of 2.5 mm to 5 mm.
The method of claim 4, wherein the hot rolling includes finish-rolling the heated slab at 800°C to 950°C.
The method of claim 4, wherein the cold rolling includes performing primary cold rolling on the hot-rolled material to a thickness of 0.4 mm to 1 mm, and then secondary cold rolling on the hot-rolled material to a thickness of 0.01 mm to 0.2 mm.
The method of claim 4, wherein in a cross-section of a cast structure of the ferritic stainless steel, the number of inclusions having a total content of the Al and the Mg exceeding 30wt% and a length exceeding 6 µm in a thickness direction is 5 or fewer per 160mm².
The method of claim 4, wherein a ratio of an average grain size in a widthwise outer region to an average grain size in a widthwise central region of the stainless steel is 2 or less.