EP3556888A1

EP3556888A1 - Ferritic stainless steel with excellent ridging property and surface quality and manufacturing method therefor

Info

Publication number: EP3556888A1
Application number: EP17881883.7A
Authority: EP
Inventors: Soo Ho Park; Kye Man Lee
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2016-12-13
Filing date: 2017-07-04
Publication date: 2019-10-23
Also published as: CN110073022B; WO2018110785A1; JP2020510135A; KR20180068089A; JP7116064B2; EP3556888A4; US20190316237A1; KR101921595B1; CN110073022A

Abstract

Disclosed herein are a ferritic stainless steel having excellent ridging properties and excellent surface quality of a final product by modifying a microstructure of a central area of a cross-section by further performing cold rolling before hot annealing.

A ferritic stainless steel according to the present disclosure includes, by wt%, 0.005 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of surfur (S), 10 to 30% of chromium (Cr), 0.005 to 0.1% of nitrogen (N), 0.005 to 0.2% of aluminum (Al), and the remainder of iron (Fe) and other impurities, wherein a γ_max value is in the range of 20% to less than 50%, and a surface microgroove area ratio is 2.0% or less.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of Korean Patent Application No. 2016-0169696, filed on December 13, 2016 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND

1. Field

Embodiments of the present disclosure relate to a ferritic stainless steel having excellent ridging properties and excellent surface quality and a method of manufacturing the same, and more particularly, to a ferritic stainless steel having excellent ridging properties and excellent surface quality by improving a structure of the thickness center by further performing cold rolling before conducting hot annealing after hot rolling, and a method of manufacturing the same.

2. Description of the Related Art

In general, stainless steels are classified according to components or metal structures. According to the metal structures, stainless steels are classified into austenitic, ferritic, martensitic, and duplex stainless steels. Among these stainless steels, ferritic stainless steel excellent in corrosion resistance despite a small amount of expensive alloying elements, has been widely applied to various kitchen appliances, parts of automobile exhaust systems, construction materials, household appliances, and the like and is a steel grade requiring high glossiness when externally used.
However, ferritic stainless steel has a problem of ridging defects which are surface defects in the form of winkles parallel to a rolling direction caused during a forming process such as a deep drawing process. The ridging defects not only deteriorate an appearance of a product but also requires an additional polishing process after the forming process in the case of serious ridging defects causing increases in time and costs for manufacturing. Therefore, in order to expand the use of ferritic stainless steel, there is a need to improve ridging properties and obtain excellent surface quality.
Ridging defects are basically caused by development of columnar crystal in a cast structure. That is, when columnar crystal having a certain orientation remains without being broken during a rolling or annealing process, it is shown as ridging defects due to width and thickness deformation behavior thereof different from those of the surrounding recrystallized structure. Various attempts have been made to remove a structure causing ridging to eliminate ridging defects. The ridging defects may be reduced by decreasing a fraction of columnar crystal via an increase in an equiaxed crystal ratio or by adjusting processing parameters such as hot rolling temperature, hot rolling reduction ratio, and annealing temperature.
However, there has been almost no attempt to improve texture by performing symmetric rolling or asymmetric rolling of a hot rolled steel sheet coiled at a high temperature before annealing, and then performing continuous annealing.

Patent Document 1: Korean Patent Laid-Open Publication No. 10-2008-0061863 (Published on July 3, 2008 )
Patent Document 2: Korean Patent Laid-Open Publication No. 10-2014-0080348 (Published on June 30, 2014 )

DISCLOUSRE

TECHNICAL PROBLEM

Therefore, it is an aspect of the present disclosure to provide to a ferritic stainless steel having excellent ridging properties and excellent surface quality of a final product by modifying a microstructure of a central area of a cross-section by further performing cold rolling before conducting hot annealing.

TECHNICAL SOLUTION

According to an aspect of an embodiment, a ferritic stainless steel having excellent ridging properties and excellent surface quality includes, by wt%, 0.005 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of surfur (S), 10 to 30% of chromium (Cr), 0.005 to 0.1% of nitrogen (N), 0.005 to 0.2% of aluminum (Al), and the remainder of iron (Fe) and other impurities, wherein a γ_max value represented by Formula 1 below is in the range of 20% to less than 50%, $420 \times C + 470 \times N + 10 \times Mn + 180 - 11.5 \times Cr - 11.5 \times Si - 52.0 \times Al$
wherein C, N, Mn, Cr, Si, and Al refer to amounts of respective elements (wt%).
In addition, according to an embodiment of the present disclosure, the stainless steel may have a surface microgroove area ratio of 2.0% or less.
In addition, according to an embodiment the present disclosure, the stainless steel may have a ridging height of 12 µm or less.
In addition, according to an embodiment of the present disclosure, the stainless steel may have an r-bar value of 1.2 or greater.
A method of manufacturing a ferritic stainless steel having excellent ridging properties and excellent surface quality according to an embodiment of the present disclosure includes: preparing a slab including, by wt%, 0.005 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of surfur (S), 10 to 30% of chromium (Cr), 0.005 to 0.1% of nitrogen (N), 0.005 to 0.2% of aluminum (Al), and the remainder of iron (Fe) and other impurities and having a γ_max value represented by Formula 1 below and satisfying 20% to less than 50%; hot rolling the slab by reheating; coiling the hot rolled steel sheet; and cold rolling the coiled hot rolled steel sheet before conducting hot annealing, $420 \times C + 470 \times N + 10 \times Mn + 180 - 11.5 \times Cr - 11.5 \times Si - 52.0 \times Al$
wherein C, N, Mn, Cr, Si, and Al refer to amounts of respective elements (wt%).
In addition, according to an embodiment of the present disclosure, a coiling temperature may be 750°C or higher in the coiling of the hot rolled steel sheet.
In addition, according to an embodiment of the present disclosure, the cold rolling may be performed by asymmetric cold rolling.
In addition, according to an embodiment the present disclosure, the cold rolling or the asymmetric cold rolling may be performed with a reduction ratio of 30% or more.
In addition, according to an embodiment the present disclosure, the asymmetric cold rolling may be performed under rolling conditions of a speed ratio between upper and lower rolling rolls (V_h/V_l) of 1.25 or greater and a rolling shape factor (l/d) of 1.7 or greater.
In addition, according to an embodiment the present disclosure, a height of a stainless steel prepared by performing hot annealing, secondary cold rolling, and cold annealing after the asymmetric cold rolling may be 10 µm or less.
In addition, according to an embodiment the present disclosure, the method may further include hot annealing after the cold rolling.
In addition, according to an embodiment the present disclosure, the hot annealing is performed in a temperature range of 550 to 950°C for 60 minutes or less.
In addition, according to an embodiment the present disclosure, an average aspect ratio of a structure of the thickness center of a cross-section of the hot annealed steel sheet is 4.0 or less after performing the hot annealing.

ADVANTAGEOUS EFFECTS

According to the ferritic stainless steel and the method of manufacturing the same according to an embodiment of the present disclosure, occurrence of ridging defects may be inhibited on a surface of a product by lowering an aspect ratio of a band structure of a thickness center of a cross-section of a steel sheet by performing cold rolling before hot annealing.
In addition, according to the ferritic stainless steel and the method of manufacturing the same according to an embodiment of the present disclosure, excellent surface glossiness may be obtained due to a low surface microgroove area ratio of the surface of the steel sheet.
In addition, according to the ferritic stainless steel and the method of manufacturing the same according to an embodiment of the present disclosure, a ridging height may be reduced during a forming process due to a high r value together with excellent ridging properties.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects of the disclosure will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 is a photograph of a microstructure of a cross-section of a sample according to Comparative Example 3 parallel to a rolling direction according to an embodiment of the present disclosure;
FIG. 2 is an image of a surface of a sample according to Example 2 according to an embodiment of the present disclosure obtained by an optical microscope; and
FIG. 3 is an image of a surface of a sample according to Comparative Example 4 according to an embodiment of the present disclosure obtained by using an optical microscope.

BEST MODE

Reference will now be made in detail to the embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.
A ferritic stainless steel having excellent ridging properties and excellent surface quality according to an embodiment of the present disclosure includes, by wt%, 0.005 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of surfur (S), 10 to 30% of chromium (Cr), 0.005 to 0.1% of nitrogen (N), 0.005 to 0.2% of aluminum (Al), and the remainder of iron (Fe) and other impurities, wherein a γ_max value represented by Formula 1 below is in the range of 20% to less than 50%, $420 \times C + 470 \times N + 10 \times Mn + 180 - 11.5 \times Cr - 11.5 \times Si - 52.0 \times Al$
wherein C, N, Mn, Cr, Si, and Al refer to amounts of respective elements (wt%).

MODE OF THE INVENTION

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. These embodiments are provided to fully convey the concept of the present disclosure to those of ordinary skill in the art. The present disclosure may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. In the drawings, parts unrelated to the descriptions are omitted for clear description of the disclosure and sizes of elements may be exaggerated for clarity.
A ferritic stainless steel having excellent ridging properties and excellent surface quality according to an embodiment of the present disclosure includes, by wt%, 0.005 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of surfur (S), 10 to 30% of chromium (Cr), 0.005 to 0.1% of nitrogen (N), 0.005 to 0.2% of aluminum (Al), and the remainder of iron (Fe) and other impurities, wherein a γ_max value is in the range of 20% to less than 50%.
Roles and contents of the components included in the ferritic stainless steel according to the present disclosure will be described below. Moreover, % of each component described below means wt%.
The content of carbon (C) is in the range of 0.005% to 0.1%.
C is an element considerably affecting strength of a steel. When the content of C is too high, the strength of the steel is excessively increased, resulting in deterioration of ductility. Thus, the content of C is limited to 0.1% or less. However, when the content of C is too low, strength required for the steel may not be satisfied, so that C is added in an amount of 0.005% or more.
The content of silicon (Si) is in the range of 0.01% to 2.0%.
Si, as an element added for deoxidation of molten steel and stabilization of a ferritic phase, is added in an amount of 0.01% or more according to the present disclosure. However, when the content of Si is too high, the steel may be hardened, resulting in deterioration of ductility. Thus, the content of Si is limited to 2.0% or less.
The content of manganese (Mn) is in the range of 0.01% to 1.5%.
Mn, as an element effective for improving corrosion resistance, is added in an amount of 0.01% or more, preferably, 0.2% or more according to the present disclosure. However, when the content of Mn is too high, weldability may deteriorate due to rapid increase in generation of Mn-based fumes during welding and ductility may deteriorate due to formation of excessive MnS precipitates. Thus, the content of Mn is limited to 1.5% or less, more preferably 1.0% or less.
The content of phosphorus (P) is in the range of 0 to 0.05%.
P, as an impurity inevitably included in steel, is an element causing grain boundary corrosion during pickling or deteriorating hot workability. Thus, the content of P may be controlled as low as possible. An upper limit of the P is controlled to 0.05%.
The content of sulfur (S) is in the range of 0 to 0.005%.
S, as an impurity inevitably included in steel, is an element segregated on grain boundaries to cause deterioration of hot workability. The content of S may be controlled as low as possible. According to the present disclosure, an upper limit of S is controlled to 0.005%.
The content of chromium (Cr) is in the range of 10 to 30%.
Cr, as an element effective for improving corrosion resistance, is added in an amount of 10% or more according to the present disclosure. However, when the content of Cr is too high, manufacturing costs rapidly increase. Thus, the content of Cr is limited to 30% or less.
The content of nitrogen (N) is in the range of 0.005% to 0.03%.
N, as an element forming a nitride, is present as an interstitial type. When the content of N is too high, impact toughness and formability may deteriorate. Thus, the content of N is limited to 0.03% or less.
The content of aluminum (Al) is in the range of 0.005% to 0.2%.
Al, as a strong deoxidizer, serves to reduce the content of oxygen in molten steel and is added in an amount of 0.005% or more according to the present disclosure. However, when the content of Al is too high, non-metallic inclusion increases, causing sleeve defects in a cold rolled strip and deteriorating weldability. Thus, the content of Al is limited to 0.2% or less, preferably 0.15% or less.
A γ_max value is a well-known index of austenite stability corresponding to a maximum amount of austinite at high temperature. The γ_max value is calculated using Formula 1 below. In the present disclosure, the γ_max value is in the range of 20% to less than 50%. $420 \times C + 470 \times N + 10 \times Mn + 180 - 11.5 \times Cr - 11.5 \times Si - 52.0 \times Al$
When the γ_max value is less than 20%, transformation of an austenitic phase to a ferritic phase is not sufficiently accumulated, and recrystallization of a ferritic band is not promoted thereby during hot rolling, failing to improve ridging properties. Meanwhile, although the contents of austenite-forming elements such as C, N, Mn, and Ni may be increased to increase the γ_max value, steel may be hardened or manufacturing costs may be increased thereby. Thus, the γ_max value needs to be less than less than 50%.
When the ferritic stainless steel satisfies the above-described components and the γ_max value range, accumulation of strain energy is sufficient for recrystallization before annealing, and thus a texture advantageous for ridging properties and formability may be formed.
For example, the ferritic stainless steel according to an embodiment of the present disclosure may have a ridging height of 12 µm or less and an r-bar value of 1.2 or greater.
In addition, the ferritic stainless steel according to an embodiment of the present disclosure may have a surface microgroove area ratio of 2.0% or less. The surface microgroove area ratio is related to glossiness. The lower the microgroove area ratio, the higher the glossiness. The ferritic stainless steel according to the present disclosure with a surface microgroove area ratio of 2.0% or less may have a surface of high quality.
Hereinafter, a method of manufacturing a ferritic stainless steel having excellent ridging properties and excellent surface quality will be described.
In order to improve ridging properties and surface quality of a ferritic stainless steel, formation of a texture advantageous for formability needs to be promoted and a band structure causing ridging needs to be removed. In order to form the texture and remove the band structure, it is important to promote recrystallization during annealing of a hot rolled steel sheet. To this end, sufficient strain energy needs to be accumulated before the annealing. Although attempts have been made to reduce finishing temperature of hot rolling to accumulate strain energy in the hot rolled steel sheet, accumulation of the stain energy is not sufficient. Thus, according to an embodiment of the present disclosure, a texture advantageous for formability is formed by performing cold rolling before hot annealing to promote recrystallization resulting from accumulation of strain energy.
In general, a deformation state of a steel sheet during rolling may be expressed by two factors, shear deformation and planar deformation. In a conventional symmetric rolling, shear deformation acts on the surface of the steel sheet. Since a shear strain decreases toward a central layer due to intrinsic symmetric characteristics, the shear strain of the central layer is always 0. That is, planar deformation always acts on the central layer. According to the present disclosure, shear deformation may be applied to the thickness center of the steel sheet by applying asymmetric rolling thereto. There are many rolling parameters when applying asymmetric rolling. By optimizing these parameters, appropriate shear strains are respectively applied to all thickness layers to promote recrystallization, thereby changing the microstructure, resulting in a decrease in a ridging height important to surface quality of a final cold rolled product.
A method of manufacturing a ferritic stainless steel according to an embodiment of the present disclosure includes: preparing a slab including, by wt%, 0.005 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of surfur (S), 10 to 30% of chromium (Cr), 0.005 to 0.1% of nitrogen (N), 0.005 to 0.2% of aluminum (Al), and the remainder of iron (Fe) and other impurities and having a γ_max value satisfying 20% to less than 50%; hot rolling the slab by reheating; coiling the hot rolled steel sheet; and cold rolling the coiled hot rolled steel sheet before conducting hot annealing.
Strain energy for promoting recrystallization may be accumulated by further cold rolling the hot rolled steel sheet before conducting hot annealing.
The prepared slab is hot rolled by reheating before the cold rolling. The hot rolled steel sheet is coiled at a high temperature in a coiling machine (black coil). A coiling temperature for phase transformation from the austenitic phase to the ferritic phase while coiling after hot rolling may be 750°C or higher.
Meanwhile, in the method of manufacturing the ferritic stainless steel according to an embodiment of the present disclosure, the cold rolling of the coiled hot rolled steel sheet conducted before hot annealing may be performed by asymmetric cold rolling.
As described above, shear deformation may be caused in the thickness center of the steel sheet by performing asymmetric rolling according to the present disclosure. Since the microstructure is changed by promoting recrystallization via appropriate shear deformation acting on the thickness center, the ridging height important to the surface quality of the final cold rolled product may be reduced.
The asymmetric cold rolling may be performed under rolling conditions of a reduction ratio of 30% or greater, a speed ratio between upper and lower rolling rolls (V_h/V_l) of 1.25 or greater, and a rolling shape factor (l/d) of 1.7 or greater.
The speed ratio between upper and lower rolling rolls (V_h/V_l) needs to be 1.25 or greater to cause shear deformation in the thickness center during the asymmetric cold rolling. When the speed ratio is less than 1.25, shear deformation cannot be applied to the thickness center. Here, V_h is a speed of a fast roll and V_l is a speed of a slow roll.
The rolling shape factor (l/d) is required to be 1.7 or greater to cause shear deformation in the thickness center. When the rolling shape factor is less than 1.7, shear deformation cannot be applied to the thickness center. The rolling shape factor related to the size of the rolling roll and the reduction rate is an index applying shear deformation during rolling and defined by Formula 2 below. $l / d = 2 \sqrt{r (h_{0} - h)} / (h_{0} + h)$
In Formula 2, l is a length to which a contact arc between a roll and a steel sheet in a roll bite is projected, d is an average thickness of the steel sheet (d=(ho+h)/2), r is a radius of the rolling roll, ho is an initial thickness of the steel sheet, and h is a final thickness of the steel sheet.
The present disclosure is characterized in improving ridging properties and surface quality by adjusting the speed ratio between upper and lower rolling rolls, the reduction ratio, and the rolling shape factor (l/d), as a result of investigating relationships between the rolling parameters and the properties of a steel sheet such as ridging properties, formability, and surface quality during asymmetric rolling conducted as cold rolling before performing hot annealing.
The cold rolled or asymmetric cold rolled steel sheet may be subjected to hot annealing. The hot annealing may be performed at a temperature range of 550 to 950°C for 60 minutes or less. The hot annealing is performed to further improve ductility of the hot rolled steel sheet, precipitation of carbonitride and recrystallization may be induced thereby. To this end, the annealing may be performed at a temperature of 550°C or higher. However, when the annealing temperature is higher than 950°C or the annealing time exceeds 60 minutes, coarse crystal grains may deteriorate formability or ridging properties. Meanwhile, although a lower limit of the annealing time is not particularly limited, the annealing may be performed for 30 seconds or more to obtain sufficient effects.
The hot annealed steel sheet may have an average aspect ratio of 4.0 or less at the thickness center of a cross-section in a direction parallel to the rolling direction. The aspect ratio refers to a ratio of grain size of ferrite in the rolling direction to grain size of ferrite in the thickness direction (grain size in the rolling direction/grain size in the thickness direction). When the average aspect ratio is greater than 4.0, cold workability may deteriorate due to the ferritic structure elongated in the rolling direction. In addition, when the band structure elongated in the rolling direction remains in the thickness center of the hot annealed steel sheet, an uneven surface may be formed due to non-uniform transformation of the band structure during cold rolling, thereby deteriorating surface glossiness. Thus, the average aspect ratio is limited to 4.0 or less.
The other conditions, which are not particularly limited as described above in the method of manufacturing the ferritic stainless steel having excellent ridging properties and excellent surface quality, may comply with methods of manufacturing ferritic stainless steels well known in the art. Also, the hot annealed steel sheet may be processed by cold rolling and cold annealing to manufacture a cold rolled steel sheet.
Hereinafter, the present disclosure will be described in detail with reference to the following examples.

Examples

Molten steel having a composition shown in Table 1 was continuously cast to prepare a slab, and the slab was hot rolled by reheating. Then, the hot rolled steel sheet having an initial thickness of 3 to 7 mm was primarily cold rolled before conducting hot annealing. Table 1

C Si Mn P S Cr Ni Al N

Content (wt%) 0.049 0.18 0.58 0.02 0.002 16.18 0.25 0.094 0.028
The primary cold rolling was conducted using conventional cold rolling or asymmetric cold rolling with a reduction ratio of 20 to 50%. After the primarily cold rolled steel sheet was hot annealed and pickled, secondary cold rolling was performed with a reduction ratio of 50 to 85%, followed by cold annealing and pickling to prepare a sample.

The sample was processed and subjected to 15% tensile tests in directions of 0°, 45°, and 90° with respect to the rolling direction to measure r values (Lankford values). An r-bar value (r-bar=(r₀+r₉₀+2^∗r₄₅)/4) was calculated from the r values measured in respective directions (ro, r₄₅, and r₉₀). In addition, a ridging height was obtained by processing the sample for a 15% tensile test and measuring a surface roughness. Measurement results of the r-bar value and the ridging height (Wt) in accordance with changes in rolling conditions of the ferritic stainless steel according to examples and comparative examples below are shown in Table 2 below.

Table 2

Example	Primary cold rolling (Black Coil)		Hot annealing	Reduction ratio of secondary cold rolling (%) (once/twice)	r-bar	Wt (µm)
Example	Type	Reduction ratio (%)	Hot annealing	Reduction ratio of secondary cold rolling (%) (once/twice)	r-bar	Wt (µm)
Example 1	Normal rolling	43	Continuous annealing	50/60	1.76	10.4
Example 2		43		75	1.24	10.0
Example 3		46		70	1.27	11.1
Comparative Example 1		21		85	1.17	12.9
Comparative Example 2		26		78	1.12	12.3
Comparative Example 3		-		83	0.95	14.6
Comparative Example 4		-		73	0.88	13.5
Comparative Example 5		-		67	0.84	13.0
Comparative Example 6		-	Batch annealing	73	0.90	17.6
Comparative Example 7		-	Batch annealing	67	0.86	16.5
Example 4	Asymmetric rolling	43	Continuous annealing	50/60	1.55	8.4
Example 5		43		75	1.21	8.0
Example 6		46		70	1.24	9.1
Comparative Example 8		21		85	1.12	11.8
Comparative Example 9		26		78	1.05	11.5

The samples according to Comparative Examples 3 to 7 in which normal rolling was conducted had r-bar values 1 or less and high ridging heights 14 µm or greater. In the case of Comparative Examples 1 and 2 in which primary cold rolling was conducted after hot rolling and before hot annealing and the reduction ratio was less than 30%, the r-bar values were equal or less than 1.2 indicating poor formability. As shown in Examples 1 to 3, when the primary cold rolling was conducted before hot annealing and the reduction ratio was 30% or greater, the r-bar values were 1.2 or greater, achieving ridging heights 12 µm or less which is difficult to be distinguished by visual observation and does not deteriorate the appearance of a product.
The samples of Examples 4 to 6 were prepared in the same manner as in Examples 1 to 3, except that the primary cold rolling was asymmetric rolling instead of symmetric rolling, and the samples of Comparative Examples 8 and 9 were prepared in the same manner as in Comparative Examples 1 and 2, except that the primary cold rolling was asymmetric rolling instead of symmetric rolling.
In the case where asymmetric rolling was performed as the primary cold rolling, it was confirmed that the ridging height was reduced by about 20% or more, when compared with symmetric rolling. Particularly, in Examples 4 to 6, ridging heights 10 µm or less were achieved. Thus, it was confirmed that the band structure may be sufficiently refined into a microstructure by shear deformation during asymmetric rolling instead of symmetric rolling, resulting in improvement of ridging properties.
In the case of Comparative Examples 8 and 9 in which primary cold rolling was performed after hot rolling and before hot annealing, although asymmetric rolling was performed, the r-bar values were 1.2 or less indicating poor formability due to reduction ratios less than 30%.
That is, when asymmetric cold rolling was performed with a reduction ratio 30% or greater before conducting hot annealing as in Examples 4 to 6, r-bar values 1.2 or greater may be obtained and ridging heights 12 µm or less, which are difficult to be distinguished by visual observation and do not deteriorates the appearance of a product, may be achieved.
Meanwhile, average aspect ratios of hot annealed steel sheets manufactured according to a conventional method without performing cold rolling before conducting hot annealing and hot annealed steel sheets manufactured according to the present disclosure are shown in Table 3. Subsequently, surface microgroove area ratio of the cold annealed steel sheets having gone through cold rolling and cold annealing are shown in Table 3 below.
An average aspect ratio was measured as by obtaining an image of a microstructure of a cross-section of the hot annealed steel sheet parallel to the rolling direction using an optical microscope, measuring grain sizes of a band structure in the rolling direction and the thickness direction, and then getting the average aspect ratio of 5 crystal grains. FIG. 1 is a photograph of a microstructure of a cross-section of the sample according to Comparative Example 3 parallel to the rolling direction. As shown in FIG. 1, after measuring a length of the band structure in the rolling direction and a length of the band structure in the thickness direction in the photograph of the microstructure of the cross-section, an average aspect ratio was calculated.
The surface microgroove area ratio was evaluated by obtaining an image of the surface of the cold annealed steel sheet by using an optical microscope at a magnification of 50x with a maximized intensity of a light source and a long exposure, and measuring an area ratio using an Image Analyzer. Representative measurement results are shown in FIGS. 2 and 3.

FIG. 2 shows the surface of Example 2, and FIG. 3 shows the surface of Comparative Example 4. In the drawings, a dark portion is an area of a microgroove. It was confirmed that a surface microgroove area ratio of the sample according to Example 2 shown in FIG. 2 according to an embodiment was significantly reduced when compared with the sample according to Comparative Example 4 shown in FIG. 3.

Table 3

Example	Primary cold rolling (Black Coil)		Hot annealing	Reduction ratio of secondary cold rolling (%) (once/twice)	Average aspect ratio	Microgroove area ratio (%)
Example	Type	Reduction ratio (%)	Hot annealing	Reduction ratio of secondary cold rolling (%) (once/twice)	Average aspect ratio	Microgroove area ratio (%)
Example 2	Normal rolling	43	Continuous annealing	75	2.5	1.6
Example 3		46		70	2.8	1.8
Comparative Example 1		21		85	6.5	2.8
Comparative Example 3		-		83	17.0	3.3
Comparative Example 4		-		73	18.0	4.1
Example 5	Asymmetric rolling	43		75	2.1	1.2
Example 6	Asymmetric rolling	46		70	2.4	1.4

The sample according to Comparative Example 1 in which primary cold rolling was performed using normal rolling with a reduction ratio less than 30% had a high average aspect ratio 6 or greater. The average aspect ratios of the samples according to Comparative Examples 3 and 4 prepared according to a conventional method were about 3 times greater than that of the sample prepared according to Comparative Example 1 in which primary cold rolling was performed. On the contrary, in the case of Examples 2 and 3 in which normal rolling, as the primary cold rolling, was performed after hot rolling and before hot annealing with a reduction ratio 30% or greater and in the case of Examples 5 and 6 in which asymmetric rolling, as the primary cold rolling, was performed with a reduction ratio 30% or greater, the average aspects of the hot annealed steel sheets were maintained at very low levels 3 or lower.
In addition, while the cold annealed steel sheets according to Comparative Examples 1, 3, and 4 in which normal rolling, as the primary cold rolling, was performed had high surface microgroove area ratios 2.2% or greater. On the contrary, the cold annealed steel sheets according to Examples 2 and 3 in which primary cold rolling was performed after hot rolling and before hot annealing, and the cold annealed steel sheets according to Examples 5 and 6 in which the primary cold rolling was performed by an asymmetric rolling had low surface microgroove area ratios 1.8% or less.
That is, as shown in the results of Examples 2, 3, 5, and 6, as the average aspect ratio of the hot annealed steel sheet decreases, the surface microgroove area ratio of the cold annealed steel sheet decreases. Thus, when the average aspect ratio is 4.0 or less and the surface microgroove area ratio is 2.0% or less, a cold steel sheet having excellent surface quality may be obtained.
While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure.

INDUSTRIAL APPLICABILITY

The ferritic stainless steel according to embodiments of the present disclosure have excellent surface quality and glossiness to be applied to various kitchen appliances, parts of automobile exhaust systems, construction materials, household appliances, and the like.

Claims

A ferritic stainless steel having excellent ridging properties and excellent surface quality comprising, by wt%, 0.005 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of surfur (S), 10 to 30% of chromium (Cr), 0.005 to 0.1% of nitrogen (N), 0.005 to 0.2% of aluminum (Al), and the remainder of iron (Fe) and other impurities,
wherein a γ_max value represented by Formula 1 below is in the range of 20% to less than 50%, $420 \times C + 470 \times N + 10 \times Mn + 180 - 11.5 \times Cr - 11.5 \times Si - 52.0 \times Al$
wherein C, N, Mn, Cr, Si, and Al refer to amounts of respective elements (wt%).
The ferritic stainless steel according to claim 1, wherein the stainless steel has a surface microgroove area ratio of 2.0% or less.
The ferritic stainless steel according to claim 1, wherein the stainless steel has a ridging height of 12 µm or less.
The ferritic stainless steel according to claim 1, wherein the stainless steel has an r-bar value of 1.2 or greater.
A method of manufacturing a ferritic stainless steel having excellent ridging properties and excellent surface quality, the method comprising:
preparing a slab comprising, by wt%, 0.005 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of surfur (S), 10 to 30% of chromium (Cr), 0.005 to 0.1% of nitrogen (N), 0.005 to 0.2% of aluminum (Al), and the remainder of iron (Fe) and other impurities and having a γ_max value represented by Formula 1 below and satisfying 20% to less than 50%;

hot rolling the slab by reheating;

coiling the hot rolled steel sheet; and

cold rolling the coiled hot rolled steel sheet before conducting hot annealing:
$420 \times C + 470 \times N + 10 \times Mn + 180 - 11.5 \times Cr - 11.5 \times Si - 52.0 \times Al$
wherein C, N, Mn, Cr, Si, and Al refer to amounts of respective elements (wt%).
The method according to claim 5, wherein a coiling temperature is 750°C or higher in the coiling of the hot rolled steel sheet.
The method according to claim 5, wherein the cold rolling is performed by asymmetric cold rolling.
The method according to claim 5 or 7, wherein the cold rolling or the asymmetric cold rolling is performed with a reduction ratio of 30% or more.
The method according to claim 7, wherein the asymmetric cold rolling is performed under rolling conditions of a speed ratio between upper and lower rolling rolls (V_h/V_l) of 1.25 or greater and a rolling shape factor (l/d) of 1.7 or greater.
wherein
V_h: speed of fast roll,
V_l: speed of slow roll,
rolling shape factor: $l / d = 2 \sqrt{r (h_{0} - h)} / (h_{0} + h),$

I: length to which contact arc between roll and steel sheet in rolling roll bite is projected,
d: average thickness of the steel sheet, d=(ho+h)/2,
r: radius of rolling roll,
ho: initial thickness of steel sheet, and
h: final thickness of steel sheet.
The method according to claim 9, wherein a height of a stainless steel prepared by performing hot annealing, secondary cold rolling, and cold annealing after the asymmetric cold rolling is 10 µm or less.
The method according to claim 5, further comprising hot annealing after the cold rolling.
The method according to claim 11, wherein the hot annealing is performed in a temperature range of 550 to 950°C for 60 minutes or less.
The method according to claim 11, wherein an average aspect ratio of a structure of a thickness center of a cross-section of the hot annealed steel sheet is 4.0 or less after the hot annealing.