EP3556888A1 - Ferritischer edelstahl mit hervorragender ridging-eigenschaft und oberflächengüte und herstellungsverfahren dafür - Google Patents

Ferritischer edelstahl mit hervorragender ridging-eigenschaft und oberflächengüte und herstellungsverfahren dafür Download PDF

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EP3556888A1
EP3556888A1 EP17881883.7A EP17881883A EP3556888A1 EP 3556888 A1 EP3556888 A1 EP 3556888A1 EP 17881883 A EP17881883 A EP 17881883A EP 3556888 A1 EP3556888 A1 EP 3556888A1
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rolling
less
stainless steel
hot
cold rolling
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French (fr)
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EP3556888A4 (de
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Soo Ho Park
Kye Man Lee
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Posco Holdings Inc
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Posco Co Ltd
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/0421—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
    • C21D8/0426—Hot rolling
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0421—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
    • C21D8/0436—Cold rolling
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0447—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • 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.
  • 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.
  • 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.
  • 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.
  • 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 ⁇ max value represented by Formula 1 below is in the range of 20% to less than 50%, 420 ⁇ C + 470 ⁇ N + 10 ⁇ Mn + 180 ⁇ 11.5 ⁇ Cr ⁇ 11.5 ⁇ Si ⁇ 52.0 ⁇ Al wherein C, N, Mn, Cr, Si, and Al refer to amounts of respective elements (wt%).
  • the stainless steel may have a surface microgroove area ratio of 2.0% or less.
  • the stainless steel may have a ridging height of 12 ⁇ m or less.
  • 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 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 ⁇ 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 ⁇ C + 470 ⁇ N + 10 ⁇ Mn + 180 ⁇ 11.5 ⁇ Cr ⁇ 11.5 ⁇ Si ⁇ 52.0 ⁇ Al wherein C, N,
  • a coiling temperature may be 750°C or higher in the coiling of the hot rolled steel sheet.
  • the cold rolling may be performed by asymmetric cold rolling.
  • the cold rolling or the asymmetric cold rolling may be performed with a reduction ratio of 30% or more.
  • 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.
  • 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 ⁇ m or less.
  • the method may further include hot annealing after the cold rolling.
  • the hot annealing is performed in a temperature range of 550 to 950°C for 60 minutes or less.
  • 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.
  • 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.
  • a ridging height may be reduced during a forming process due to a high r value together with excellent ridging properties.
  • 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 ⁇ max value represented by Formula 1 below is in the range of 20% to less than 50%, 420 ⁇ C + 470 ⁇ N + 10 ⁇ Mn + 180 ⁇ 11.5 ⁇ Cr ⁇ 11.5 ⁇ Si ⁇ 52.0 ⁇ Al wherein C, N, Mn, Cr, Si, and Al refer to amounts of respective elements (wt%).
  • 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 ⁇ max value is in the range of 20% to less than 50%.
  • % 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.
  • the content of C is limited to 0.1% or less.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 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.
  • the content of Al is limited to 0.2% or less, preferably 0.15% or less.
  • a ⁇ max value is a well-known index of austenite stability corresponding to a maximum amount of austinite at high temperature.
  • the ⁇ max value is calculated using Formula 1 below.
  • the ⁇ max value is in the range of 20% to less than 50%.
  • the ⁇ 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 ⁇ max value, steel may be hardened or manufacturing costs may be increased thereby. Thus, the ⁇ max value needs to be less than less than 50%.
  • the ferritic stainless steel according to an embodiment of the present disclosure may have a ridging height of 12 ⁇ m or less and an r-bar value of 1.2 or greater.
  • 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.
  • a texture advantageous for formability is formed by performing cold rolling before hot annealing to promote recrystallization resulting from accumulation of strain energy.
  • a deformation state of a steel sheet during rolling may be expressed by two factors, shear deformation and planar deformation.
  • 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.
  • 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 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 ⁇ 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.
  • the cold rolling of the coiled hot rolled steel sheet conducted before hot annealing may be performed by asymmetric cold rolling.
  • 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.
  • V h /V l 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.
  • V h is a speed of a fast roll
  • 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 is a length to which a contact arc between a roll and a steel sheet in a roll bite is projected
  • r is a radius of the rolling roll
  • ho is an initial thickness of the steel sheet
  • 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.
  • the annealing may be performed at a temperature of 550°C or higher.
  • the annealing temperature is higher than 950°C or the annealing time exceeds 60 minutes, coarse crystal grains may deteriorate formability or ridging properties.
  • 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).
  • the average aspect ratio is greater than 4.0, cold workability may deteriorate due to the ferritic structure elongated in the rolling direction.
  • 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.
  • 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.
  • the hot annealed steel sheet may be processed by cold rolling and cold annealing to manufacture a cold rolled steel sheet.
  • 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).
  • 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.
  • Example 1 Primary cold rolling (Black Coil) Hot annealing Reduction ratio of secondary cold rolling (%) (once/twice) r-bar Wt ( ⁇ m) Type Reduction ratio (%)
  • 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 - 67 0.86 16.5
  • Example 4 Asymmetric rolling 43 Continuous annealing 50/60 1.55 8.4
  • 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 ⁇ m or greater.
  • the r-bar values were equal or less than 1.2 indicating poor formability.
  • the r-bar values were 1.2 or greater, achieving ridging heights 12 ⁇ m or less which is difficult to be distinguished by visual observation and does not deteriorate the appearance of a product.
  • Example 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.
  • the ridging height was reduced by about 20% or more, when compared with symmetric rolling. Particularly, in Examples 4 to 6, ridging heights 10 ⁇ m 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.
  • r-bar values 1.2 or greater may be obtained and ridging heights 12 ⁇ m or less, which are difficult to be distinguished by visual observation and do not deteriorates the appearance of a product, may be achieved.
  • 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
  • FIG. 3 shows the surface of Comparative Example 4.
  • 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 .
  • 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.
  • 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
  • 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.
  • the surface microgroove area ratio of the cold annealed steel sheet decreases.
  • 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.
  • 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.

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US20190316237A1 (en) 2019-10-17
KR20180068089A (ko) 2018-06-21
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KR101921595B1 (ko) 2018-11-26

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