EP4357478A1 - Tôle d'acier plaquée hautement résistante à la corrosion ayant une excellente résistance à la corrosion et une excellente qualité de surface, et son procédé de fabrication - Google Patents

Tôle d'acier plaquée hautement résistante à la corrosion ayant une excellente résistance à la corrosion et une excellente qualité de surface, et son procédé de fabrication Download PDF

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Publication number
EP4357478A1
EP4357478A1 EP22825232.6A EP22825232A EP4357478A1 EP 4357478 A1 EP4357478 A1 EP 4357478A1 EP 22825232 A EP22825232 A EP 22825232A EP 4357478 A1 EP4357478 A1 EP 4357478A1
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Prior art keywords
steel sheet
phase
plating layer
mgzn
single phase
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EP22825232.6A
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German (de)
English (en)
Inventor
Sung-Joo Kim
Il-Ryoung Sohn
Tae-Chul Kim
Kwang-Won Kim
Sang-Tae Han
Myung-Soo Kim
Yong-Kyun Cho
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Posco Holdings Inc
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Posco Co Ltd
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Publication of EP4357478A1 publication Critical patent/EP4357478A1/fr
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B21B1/30—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a non-continuous process
    • B21B1/32—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a non-continuous process in reversing single stand mills, e.g. with intermediate storage reels for accumulating work
    • B21B1/36—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a non-continuous process in reversing single stand mills, e.g. with intermediate storage reels for accumulating work by cold-rolling
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C23C2/29—Cooling or quenching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
    • C23C28/02—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
    • C23C28/02—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material
    • C23C28/023—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material only coatings of metal elements only
    • C23C28/025—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material only coatings of metal elements only with at least one zinc-based layer
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B21B2001/221—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length by cold-rolling

Definitions

  • the present invention relates to a highly corrosion-resistant plated steel sheet having excellent corrosion resistance and surface quality, and a manufacturing method therefor.
  • a zinc-based plated steel sheet When exposed to a corrosive environment, a zinc-based plated steel sheet has a characteristic of a sacrificial method in which zinc, which has a lower oxidation-reduction potential than iron, corrodes first, thereby suppressing corrosion of a steel material.
  • zinc in a plating layer oxidizes, dense corrosion products are formed on a surface of the steel material, thereby blocking the steel material from an oxidizing atmosphere, thereby improving corrosion resistance of the steel material. Thanks to these advantageous properties, the scope of application of the zinc-plated steel sheet has recently been expanded to steel sheets for construction materials, home appliances, and automobiles.
  • a representative example is a Zn-Mg-Al-based zinc alloy plated steel sheet in which Mg is additionally added to a Zn-Al plating composition.
  • the Zn-Mg-Al-based zinc alloy plated steel sheet is often processed and used as a common zinc-based steel sheet, but because the plated steel sheet contains a large amount of intermetallic compounds with high hardness in a plating layer, there may be a disadvantage in bendability may deteriorate, causing cracks in the plating layer during bending.
  • the corrosion resistance of the bending portion of a zinc alloy-plated steel sheet is usually caused by leaching of Mg and Al components in a moisture atmosphere, resulting in self-healing of the exposed portion of the base steel sheet.
  • a zinc-based plated steel sheet is often provided on an outside of a product, but the product is that a higher Mg content in the plating layer had a darker appearance, and surface quality was poor due to addition of surface damage factors caused by processing, so improvement in appearance quality was required.
  • Patent Document 1 Korean Publication No. 2010-0073819
  • An aspect of the present disclosure is to provide a plated steel sheet having excellent corrosion resistance in a flat plate portion as well as corrosion resistance in a bending portion and excellent appearance quality, and a method for manufacturing the same.
  • An object of the present disclosure is not limited to the above description.
  • the object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.
  • a plated steel sheet including:
  • a manufacturing method of a plated steel sheet including operations of:
  • t is a thickness of the steel sheet (mm)
  • A is an average cooling rate (°C/s) from a solidification initiation temperature to 375°C
  • B is an average cooling rate (°C/s) from 375°C to 340°C.
  • a plated steel sheet having excellent corrosion resistance of a flat plate as well as having corrosion resistance of a bending portion and excellent appearance quality, and a method for manufacturing the same may be provided.
  • Mg was added to improve corrosion resistance, but when Mg is added excessively, occurrence of floating dross in a plating bath increases, so that there was a problem that the dross is frequently removed. Therefore, an upper limit of Mg addition amount was limited to 3%. Accordingly, research was conducted to further improve corrosion resistance by increasing the amount of Mg added from 3%, but as the amount of Mg added increases, a large amount of intermetallic compounds with high hardness are generated, causing cracks in the plating layer during bending.
  • LDH Layered Double Hydroxide; (Zn,Mg) 6 Al 2 (OH) 16 (CO 3 ) ⁇ 4H 2 O)
  • LDH is formed as an initial corrosion product on a surface of the bending portion, and at the same time, LDH is uniformly distributed throughout the surface of the bending portion over time to shield a corrosion active area
  • a plated steel sheet includes a base steel sheet; a Zn-Mg-Al-based plating layer provided on at least one surface of the base steel sheet; and a Fe-Al-based inhibition layer provided between the base steel sheet and the Zn-Mg-Al-based plating layer.
  • the base steel sheet may be a Fe-based base steel sheet used as a base steel sheet for an ordinary zinc-based plated steel sheet, that is, a hot-rolled steel sheet or a cold-rolled steel sheet, but the present disclosure is not limited thereto.
  • the base steel sheet may be, for example, carbon steel, ultra-low carbon steel, or high manganese steel used as materials for construction, home appliances, and automobiles.
  • the base steel sheet may have a composition including, by weight: C: more than 0% to 0.18% or less, Si: more than 0% to 1.5% or less, Mn: 0.01 to 2.7%, P: more than 0% to 0.07% or less, S: more than 0% to 0.015% or less, Al: more than 0% to 0.5% or less, Nb: more than 0% to 0.06% or less, Cr: more than 0% to 1.1% or less, Ti: more than 0% to 0.06% or less, B: more than 0% to 0.03% or less, with a balance of Fe and other unavoidable impurities.
  • a Zn-Mg-Al-based plating layer formed of a Zn-Mg-Al-based alloy may be provided on at least one surface of the base steel sheet.
  • the plating layer may be formed on only one surface of the base steel sheet, or may be formed on both surfaces of the base steel sheet.
  • the Zn-Mg-Al-based plating layer refers to a plating layer containing Mg and Al and mainly containing Zn (that is, containing 50% or more of Zn) .
  • the thickness of the Zn-Mg-Al-based plating layer may be 5 to 100 pm, more preferably 5 to 90 um. If a thickness of the plating layer is less than 5um, the plating layer may become excessively thin locally due to errors resulting from a deviation in thickness of the plating layer, which may result in poor corrosion resistance. If the thickness of the plating layer exceeds 100 um, cooling of the hot-dip plating layer may be delayed, solidification defects such as flow patterns, for example, may occur on the surface of the plating layer, and productivity of the steel sheet may be reduced in order to solidify the plating layer.
  • an Fe-Al-based inhibition layer may be provided between the base steel sheet and the Zn-Mg-Al-based plating layer.
  • the Fe-Al-based inhibition layer is a layer mainly containing intermetallic compounds of Fe and Al, and examples of the intermetallic compounds of Fe and Al include FeAl, FeAl 3 , Fe 2 Al 5 , and the like.
  • some components derived from the plating layer, such as Zn and Mg may be further included, for example, in an amount of 40% or less.
  • the inhibition layer is a layer formed due to alloying of Fe diffused from the base steel sheet at the beginning of plating and plating bath components.
  • the inhibition layer may serve to improve adhesion between the base steel sheet and the plating layer, and at the same time prevent Fe diffusion from the base steel sheet to the plating layer.
  • the inhibition layer may be formed continuously between the base steel plate and the Zn-Mg-Al-based plating layer, or may be formed discontinuously. Except for the above-described description, information generally known in the art can be applied to the inhibition layer.
  • a thickness of the inhibition layer may be 0.02 to 2.5 um.
  • the inhibition layer may serve to secure corrosion resistance by preventing alloying, but because it brittles, the inhibition layer may affect processability, so the thickness thereof may be 2.5 um or less.
  • an upper limit of the thickness of the inhibition layer may be preferably 1.8 ⁇ m.
  • a lower limit of the thickness of the inhibition layer may be 0.05 um.
  • the thickness of the inhibition layer may mean a minimum thickness in a direction, perpendicular to an interface of the base steel sheet.
  • the Zn-Mg-Al-based plating layer may include, by weight percent, Mg: 4 to 6%, Al: 8.2 to 14.2%, with a balance of Zn, and other inevitable impurities.
  • Mg 4 to 6%
  • Al 8.2 to 14.2%
  • Mg 4% or more and 6% or less
  • Mg is an element serving to improve corrosion resistance of a plated steel material, and in the present disclosure, a Mg content in the plating layer is controlled to 4% or more to ensure the desired excellent corrosion resistance. Meanwhile, from a viewpoint of securing corrosion resistance, an effect of securing corrosion resistance improves as Mg is added, so an upper limit of the Mg content may not be particularly limited. However, as an example, when excessive Mg is added, dross may occur, so the Mg content can be controlled to 6% or less.
  • Mg was added at 1.0% or more in Zn-Mg-Al-based zinc alloy plating to secure corrosion resistance, but an upper limit of the Mg content was set to be 3.0% for commercialization.
  • an upper limit of the Mg content was set to be 3.0% for commercialization.
  • an upper limit of the Al content in the plating layer is preferably controlled to 14.2%, and more preferably to 14.0%.
  • Inevitable impurities may be all included, that may be unintentionally mixed in the manufacturing process of a normal hot-dip galvanized steel sheet, and the meaning may be easily understood by the person skilled in the art.
  • the Zn-Mg-Al-based plating layer may include a MgZn 2 phase and an Al single phase as a microstructure, and in addition thereto, various phases, such as an Al-Zn based binary eutectic phase, Zn-MgZn 2 -Al-based ternary eutectic phase, Zn single phase, and the like can also be included in the plating layer.
  • the MgZn 2 phase refers to a phase mainly composed of MgZn 2
  • the Al single phase refers to a phase mainly composed of Al, and specifically, a phase in which Zn is dissolved at less than 27% in atomic percentage, and a remainder thereof is composed of Al and other impurities. That is, in the Al single phase, in addition to the Al component, components such as Zn and Mg that can be included as plating layer components may be dissolved in solid solution, and in the present disclosure, the Al single phase refers to only a phase in which Zn is dissolved at less than 27 atomic%.
  • the Zn-MgZn 2 -Al-based ternary eutectic phase refers to a ternary eutectic phase in which the Zn phase, MgZn 2 phase, and Al phase are all mixed
  • the Al-Zn binary eutectic phase refers to an Al phase and a Zn phase arranged in an alternating lamellar or irregular mixed form.
  • the Al phase in the Al-Zn based binary eutectic phase and the Zn-MgZn 2 -Al-based ternary eutectic phase is not regarded as the Al single phase described above or the second Al single phase described later.
  • MgZn 2 in the Zn-MgZn 2 -Al-based ternary eutectic phase is not regarded as a MgZn 2 phase mainly composed of the above-described MgZn 2 .
  • the Zn-Mg-Al-based plating layer may additionally include a 'second Al single phase' that is distinguished from the Al single phase by a Zn solid solution ratio.
  • the second Al single phase refers to a single phase in which 27% or more and 60% or less (27 to 60%) of Zn is dissolved in atomic percentage, and a remainder thereof is composed of Al and other impurities.
  • a microstructure of the above-described Zn-Mg-Al-based plating layer may have a different distribution on a surface and cross-section thereof, and the microstructure on the surface and cross-section thereof may be confirmed using a scanning electron microscope (SEM) or transmission electron microscope (TEM) by enlarging magnification of the plating layer for each surface specimen or cross-section specimen.
  • SEM scanning electron microscope
  • TEM transmission electron microscope
  • the Zn-Mg-Al-based plating layer includes various phases depending on a composition and manufacturing conditions of the plating layer, but as a result of intensive studies to provide a plated steel sheet having excellent corrosion resistance in the bending portion and appearance quality, in addition to corrosion resistance of the conventional flat plate portion, the present inventors have discovered that when maintained in a corrosive environment (or in an atmospheric environment for a long time), uniform formation of LDH (Layered Double Hydroxide; (Zn,Mg) 6 Al 2 (OH) 16 (CO 3 ) ⁇ 4H 2 O) as an initial corrosion product is an important factor on a surface of the steel sheet.
  • LDH Layered Double Hydroxide
  • the present inventors have confirmed that it is related to microstructural characteristics on a surface of the Zn-Mg-Al-based plating layer (i.e., an external surface, not a surface of base iron), thereby completing the present invention.
  • a total area fraction of an Al single phase and the MgZn 2 phase is 45 to 60%, and an area ratio of the MgZn 2 phase to the Al single phase is 1.2 to 3.3.
  • the total area fraction of the Al single phase and the MgZn 2 phase and the area ratio of the MgZn 2 phase to the Al single phase are measured based on a surface specimen having an area of 24,000 ⁇ m 2 or more.
  • the Zn-Mg-Al-based plating layer includes a microstructure in which a MgZn 2 phase and an Al single phase are adjacent.
  • a form in which the MgZn 2 phase and the Al single phase are adjacent includes a case in which an Al single phase is completely contained within the MgZn 2 phase or a portion of the Al single phase is contained within the MgZn 2 phase, and additionally, a case in which an Al single phase is present to be in contact with the MgZn 2 phase.
  • the Zn-Mg-Al-based plating layer may include a Zn single phase and a Zn-MgZn 2 -Al-based ternary eutectic phase, which are common phases in a highly corrosion-resistant plated steel sheet.
  • a Zn single phase and a Zn-MgZn 2 -Al-based ternary eutectic phase are common phases in a highly corrosion-resistant plated steel sheet.
  • Al and Mg contents in the plating layer decreases, an amount of the Zn single phase and Zn-MgZn 2 -Al-based ternary eutectic phase generated in an entire plating layer tends to increase, and as the Al and Mg contents in the plating layer increases, an amount of the MgZn2 phase and the Al single phase generated tends to increase.
  • a MgZn 2 phase appears coarsely, and as a Mg content increases, an Al content should also increase simultaneously to suppress dross, accordingly, a coarse Al single phase also coexists. Accordingly, in order to secure the corrosion resistance of the bending portion described above, the present inventors have found that a total area fraction and area ratio of a MgZn 2 phase and an Al single phase adjacent to the MgZn 2 phase on a surface of the plating layer contribute to initial formation of LDH as a corrosion product when maintained under a corrosive environment (or under an atmospheric environment for a long time).
  • the MgZn 2 phase and the Al single phase are adjacent to each other on the surface of the plating layer by a certain amount or more.
  • the MgZn 2 phase and the Al single phase may serve to form a sacrificial projection cell between the MgZn 2 phase and the Al single phase to secure excellent corrosion resistance.
  • the corrosion resistance includes not only the corrosion resistance of the flat plate portion but also the corrosion resistance of the bending portion, and this corrosion resistance improves as the amount of MgZn 2 phase and the Al single phase present on the surface of the plating layer increases, compared to the inside of the plating layer.
  • each phase forming an anode (MgZn 2 ) and a cathode (Al) of the sacrificial projection cell is insufficient, so that the corrosion resistance of the bending portion may be insufficient, and light scattering due to a phase present on the surface is also insufficient, so that there may be a risk in which appearance quality may deteriorate.
  • the total area fraction of the MgZn 2 phase and the Al single phase exceeds 60%, a brittle MgZn 2 phase may be excessively formed, causing a problem of excessive cracking in the plating layer during processing.
  • an area ratio of the MgZn 2 phase to the Al single phase is less than 1.2, an amount of the MgZn 2 phase (anode) forming the sacrificial projection cell can be dissolved is small, causing a problem, disadvantageous for corrosion resistance. If the area ratio of the MgZn 2 phase to the Al single phase exceeds 3.3, a rate of cathodic reaction (oxygen reduction reaction) occurring in Al on the surface is limited by accepting electrons transmitted by dissolution of MgZn 2 , which may cause a problem, disadvantageous for corrosion resistance.
  • the corrosion resistance of the bending portion is determined by two mechanisms. First, a MgZn 2 phase and Al single phase present in the bending portion form an intact sacrificial projection cell, and corrosion products cover and obscure an exposed portion of the base steel sheet during bending. Second, it is a self-healing mechanism in which oxidation-friendly Mg and Al components are leached in a moisture atmosphere and move to the exposed portion of the base steel sheet in the bending portion to reform a plating layer. The greater an amount of Mg and Al components that are highly reactive with moisture which are present in a surface layer portion, the better the effect.
  • the sacrificial projection cell acting as the first mechanism, secures a large potential difference as a potential of MgZn 2 is -1.2V above a hydrogen reduction potential and a potential of Al is -0.7V above the hydrogen reduction potential, thereby acting as an anode and a cathode, respectively, which means forming a galvanic cell between the MgZn 2 phase and Al single phase microstructure, adjacent to each other.
  • the Zn-Mg-Al-based plating layer according to an aspect of the present disclosure may include, among phases mainly composed of Al, 1 an Al single phase with a Zn solid-solution ratio of less than 27 atomic%, and 2 a second Al single phase with a high Zn solid-solution ratio of 27 to 60%.
  • a phase that can maintain a high potential difference by existing adjacent to the MgZn 2 phase is the Al single phase (corresponding to 1) with a low Zn solid-solution ratio.
  • the number of second Al single phases present around the MgZn 2 phase increases, and accordingly, a potential difference between the anode and cathode of the above-described galvanic cell may be reduced, which may impair excellent corrosion resistance and sacrificial projection of the galvanic cell.
  • an area fraction of the second Al single phase may be 2 to 9%. If the area fraction of the second Al single phase exceeds 9%, a second Al single phase may be excessively formed around the MgZn 2 phase, which may reduce the potential difference between the galvanic cells and worsen corrosion resistance in the bending portion. Therefore, in the present disclosure, on the surface of the Zn-Mg-Al-based plating layer, an area fraction of the second Al single phase is controlled to 9% or less, and as an amount of the second Al single phase present on the surface decreases, an effect of improving the corrosion resistance of the bending portion improves, so a lower limit thereof may not be specifically limited. However, considering that the second Al single phase is inevitably formed in a temperature range at which the second Al single phase is formed during a cooling process, after hot-dip plating, the lower limit may be set to 2%.
  • the area fraction of the MgZn 2 phase may be 30 to 40%.
  • a part of the plating layer, primarily in contact with atmospheric and chloride environments is a surface, and the higher the ratio of the MgZn 2 phase acting as an anode in a sacrificial method, the better reactivity in the galvanic cell. Therefore, by promoting the formation of the above-described galvanic cell, an area fraction of the MgZn 2 phase on a surface of the plating layer may be set to 30% or more, to ensure corrosion resistance of the bending portion.
  • the corrosion resistance of the bending portion may be insufficient.
  • the plating layer may be brittle and cause cracks of the surface.
  • an area fraction of the Al single phase i.e., a phase in which, in atomic percentage, Zn is dissolved in less than 27%, and a balance thereof contains Al and other impurities
  • an area fraction of the Al single phase may be 15 to 20%.
  • the area fraction of the Al single phase is 15% or more, as described above, it may act as a cathode along with MgZn 2 , which acts as an anode in the galvanic cell, to help improve the corrosion resistance of the bending portion, and serve to maintain a skeleton of the MgZn 2 phase, so that the plating layer may contribute to a role as a physical protective barrier film.
  • the ratio of the Al single phase exceeds 20%, stability may deteriorate due to Al corrosion.
  • a total area fraction of the Zn single phase and a Zn-MgZn 2 -Al-based ternary eutectic phase on a surface of the Zn-Mg-Al-based plating layer may be 20 to 30%.
  • the Zn single phase and Zn-MgZn 2 -Al based ternary eutectic phase present on the surface of the Zn-Mg-Al plating layer contribute to formation of Simonkolleite or hydrozinsite rather than LDH in an initial stage of corrosion.
  • a presence ratio of the Zn single phase and the Zn-MgZn 2 -Al-based ternary eutectic phase on the surface of the Zn-Mg-Al-based plating layer among corrosion products formed on the surface at the initial stage of corrosion, the corrosion resistance of the bending portion may be improved by increasing a formation ratio of LDH, rather than a formation ratio of Simonkolleite or hydrozinsite. Therefore, a total area fraction of the Zn single phase and the Zn-MgZn 2 -Al-based ternary eutectic phase on a surface of the Zn-Mg-Al-based plating layer may be set to 20 to 30%.
  • the total area fraction of the Zn single phase and the Zn-MgZn 2 -Al-based ternary eutectic phase on the surface of the Zn-Mg-Al-based plating layer is less than 20%, formation of Simonkolleite or hydrozinsite, which is secondarily generated after the formation of LDH and helps improve corrosion resistance may be insufficient, so that there may be a risk of a problem with corrosion resistance.
  • an area fraction of the MgZn 2 phase is 20 to 40%, and an area fraction of the Al single phase may be 8 to 26%.
  • the characteristics of the plated steel sheet are related to the type and size of a crystal phase, and if the area fraction of the MgZn 2 phase is less than 20% or the area fraction of the Al single phase is less than 8%, the corrosion resistance of the plating layer may be weakened. Meanwhile, if the ratio of the MgZn 2 phase present in the plating layer exceeds 40%, it may be excessively brittle, which may have a side effect of excessive cracking in the plating layer occurring during processing. Based on the cross-section of the Zn-Mg-Al-based plating layer, the area fraction of the MgZn 2 phase and the Al single phase can be measured by observing an image taken by FE-SEM of a cross-sectional specimen of the plated steel sheet in the thickness direction.
  • the area fraction of the MgZn 2 phase and the Al single phase based on the cross-section of the Zn-Mg-Al-based plating layer described above is satisfied, and the corrosion resistance of the cross-sectional portion (cut-edge) of the steel sheet can be secured, but the area fraction of the MgZn 2 phase and the Al single phase, secured on the surface of the plating layer may be different. Therefore, the area fraction distribution of each phase on the surface of the plating layer may affect the degree of corrosion resistance of the bending portion during bending.
  • the present inventors have discovered that, even if the area fraction of the MgZn 2 phase and Al single phase described above is secured based on the cross-section of the plating layer in the thickness direction, securing a certain amount of more of the MgZn 2 phase and Al single phase on the surface of the plating layer is an important factor for securing corrosion resistance of the bending portion by promoting uniform formation of LDH on the surface of the plating layer in the initial stage of corrosion.
  • the present inventors have further discovered that, it is important to maintain a ratio of the total area fraction of the MgZn 2 phase and Al single phase in a center portion of the plating layer to the total area fraction of the MgZn 2 phase and Al single phase on the surface of the plating layer at an appropriate level.
  • a ratio (S1/C1) of a total area fraction (S1) of the MgZn 2 phase and Al single phase on the surface of the Zn-Mg-Al-based plating layer to a total area fraction (C1) of the MgZn 2 phase and Al single phase on a surface at any point in a region from 1/4t to 3/4t of the Zn-Mg-Al-based plating layer in the thickness direction may be in a range of 0.8 to 1.2.
  • S1/C1 is less than 0.8, a problem may occur in the corrosion resistance of the flat plate portion and the bending portion, due to a lack of microstructure forming LDH in the initial stage of corrosion in a surface layer portion of the plating layer, and if S1/C1 exceeds 1.2, a problem may occur in processability and corrosion resistance of bending portion, due to excessive coarsening of a brittle structure caused by the MgZn 2 phase in the surface layer portion of the plating layer.
  • the area fraction of the second Al single phase is 2 to 10%. If the value exceeds 10%, it may affect a structure of the surface layer portion, to adversely affect the corrosion resistance of the bending portion. In addition, considering that it passes through a temperature section in which a second Al single phase is formed, a lower limit thereof may be controlled to 2%.
  • a point at which a thickness of the plating layer is maximum is regarded as a total thickness t, wherein the region may refer to a region polished on a surface of the specimen to include any point in the region from 1/4t to 3/4t based on t.
  • the present inventors have conducted additional research and found that, after the uniform formation of LDH on the surface of the plating layer, a ratio of the Zn phase and a Zn-MgZn 2 -Al based ternary eutectic phase, which penetrates thereinto and promotes the formation of Simonkolleite and hydrozinsite, in the center portion to the surface thereof was also an important factor in further improving corrosion resistance.
  • a ratio (S2/C2) of a total area fraction (S2) of the Zn phase and the Zn-MgZn 2 -Al-based ternary eutectic phase on a surface of the Zn-Mg-Al-based plating layer to a total area fraction (C2) of the Zn phase and the Zn-MgZn 2 -Al-based ternary eutectic phase on a surface at any point in a region from 1/4t to 3/4t of the Zn-Mg-Al-based plating layer in a thickness direction may be in a range of 0.6 to 1.2.
  • S2/C2 is less than 0.6, problems with corrosion resistance may occur due to insufficient formation of Simonkolleite or hydrozinsite, which is generated secondarily after LDH formation in a surface layer portion of the plating layer, which helps improve corrosion resistance.
  • S2/C2 exceeds 1.2, there is a relative lack of an MgZn 2 phase and an Al single phase secured on a surface thereof, which may cause insufficient LDH formation on the surface, which may cause problems with corrosion resistance as described above in the present disclosure.
  • images taken using an SEM or EDS device on the surface of the Zn-Mg-Al-based plating layer may be distinguished by color and contrast differences for each microstructure, and each region may be calculated.
  • a fraction of elements dissolved in each phase depending on a contrast of the SEM image can be obtained using an EDS (Energy Dispersive Spectrometer) commonly known in the art.
  • EDS Electronic Dispersive Spectrometer
  • an Al-based phase excluding the MgZn 2 phase, Zn single phase, and Zn-MgZn 2 -Al-based ternary eutectic phase, which are clearly distinguished by color, brightness, and shape for each microstructure as shown in FIG.
  • region 1 represents an Al region, in which Al is observed to be 73%, Zn is 26%, and a balance thereof is less than 1%, in atomic percentage on average
  • a region 2 represents a second Al single phase, in which Al is observed to be 51%, Zn is 49%, and a balance thereof is less than 1%, in atomic percentage on average
  • region 3 represents a Al-Zn based binary eutectic phase, in which Al is observed to be 43%, Zn is 57%, and a balance thereof is less than 1%, in atomic percentage on average (in this case, the balance being Mg or other unavoidable impurities).
  • the Al single phase refers to a region 1 in which Zn is dissolved at less than 27% in atomic%
  • the second Al single phase refers to the region 2 in which Zn is dissolved at 27% or more to 60% or less in atomic!
  • the Al-Zn based binary eutectic phase refers to a region 3, and Fe and other components may be included as impurities in each phase.
  • microstructure labeling uses images derived under the above-described SEM measurement conditions using an automatic image generation software based on a super-pixel algorithm of RISA (microstructure phase fraction analysis software) of the Pohang Research Institute of Industrial Science and Technology (RIST).
  • the super-pixel algorithm is a mechanism measuring similarity by dividing an entire image into thousands or tens of thousands of regions (superpixels) and comparing superpixels with similar patterns or features, and calculating a histogram for a brightness value of a pixel, and then automatically selecting a superpixel when the similarity is greater than a pre-defined threshold. As an example of specifying a pre-defined threshold.
  • a boundary between the Al single phase and the second Al single phase in the image derived under the above-described SEM measurement conditions defines each phase in advance based on a Zn solid solution ratio in 27 atomic %, dissolved in an Al structure using EDS, so that histrogramming and structure distinguishing is possible for the brightness value of a soft phase.
  • the technical idea of the above-described RISA microstructure fraction analysis software
  • LDH may be formed on a surface of the plating layer before Simonkolleite and hydrozinsite under an atmospheric and chloride environment.
  • the above-described plating layer undergoes rapid nucleation and crystallization of LDH, a dense corrosion product, on a surface in an initial stage of the corrosion environment due to the MgZn 2 phase present in large quantities in the surface layer portion and the Al single phase, adjacent thereto. Thereafter, over time, it can be uniformly distributed throughout the surface to shield a corrosion active region, and uniform formation of Simonkolleite and hydrozinsite, which are corrosion products, secondarily formed, may be induced.
  • a LDH corrosion product formed in a surface layer portion of the plating layer may be formed within 6 hours in an atmospheric environment, and within 5 minutes in a chloride environment (i.e., as measured by ISO14993).
  • a time taken for red rust to occur in a chloride environment including salt spray and immersion environments may be 40 to 50 times longer in a flat plate portion; and 20 to 30 times in a 90° bending portion, compared to that of pure Zn plating of the same thickness.
  • an evaluation of the time taken for red rust to occur may be comparatively evaluated using a test method in accordance with ISO14993 using a salt spray test device (SST).
  • a step of first preparing a base steel sheet may be further included, and the type of the bases steel sheet is not particularly limited.
  • the base steel sheet may be a Fe-based steel sheet, which is used as a base steel sheet for ordinary hot-dip galvanized steel sheets, that is, a hot-rolled steel sheet or cold-rolled steel sheet, but the present disclosure is not limited thereto.
  • the base steel sheet may be, for example, carbon steel, ultra-low carbon steel, or high manganese steel used as materials for construction, home appliances, and automobiles, but the present disclosure is not limited thereto. In this case, the above description can be equally applied to the base steel sheet.
  • a plating bath containing, by wt%: Mg: 4 to 6%, Al: 8.2 to 14.2%, a balance of Zn, and other inevitable impurities.
  • a composite ingot containing predetermined Zn, Al, and Mg or a Zn-Mg and Zn-Al ingot containing individual components can be used.
  • the ingot is additionally dissolved and supplied.
  • a method of dissolving the ingot by directly immersing the same in a plating bath may be used, or dissolving the ingot in a separate pot and then replenishing molten metal in the plating bath may be used.
  • a temperature of the plating bath may be maintained at a temperature of 20 to 80°C higher than a solidification initiation temperature (Ts) in the equilibrium state.
  • Ts solidification initiation temperature
  • the solidification initiation temperature in the equilibrium state may be in a range of 390 to 460°C, or the temperature of the plating bath may be maintained in a range of 440 to 520°C. The higher the temperature of the plating bath, the more it is possible to secure fluidity and form a uniform composition within the plating bath, and to reduce an amount of floating dross generated.
  • the temperature of the plating bath is less than 20°C, compared to the solidification initiation temperature in the equilibrium state, the dissolution of the ingot is very slow and the viscosity of the plating bath is high, making it difficult to secure excellent plating layer surface quality.
  • the temperature of the plating bath exceeds 80°C, compared to the solidification initiation temperature in the equilibrium state, ash defects due to Zn evaporation may occur on the plating surface.
  • a step of cooling the hot-dip galvanized steel sheet using an inert gas at an average cooling rate of 2 to 12°C/s from the solidification initiation temperature to the solidification end temperature in the equilibrium state may be included. If the above-described average cooling rate is less than 2°C/s, the MgZn 2 structure develops too coarsely on the surface and a surface portion of the plating layer is brittle, so that occurrence of cracks may increase, which may be disadvantageous in ensuring uniform corrosion resistance and processibility.
  • a cooling rate may be controlled to satisfy the following Relations 1-1 and 1-2.
  • t is a thickness of the steel sheet (mm)
  • A is an average cooling rate (°C/s) from a solidification initiation temperature to 375°C
  • B is an average cooling rate (°C/s) from 375°C to 340°C.
  • an average cooling rate in each section according to a thickness of the steel sheet is controlled to satisfy the Relations 1-1 and 1-2, by dividing a first temperature section from a solidification initiation temperature to 375°C and a second temperature section from 375°C to 340°C.
  • an Al single phase adjacent to a MgZn 2 phase in the plating layer formed in the content range of Mg and Al according to the present disclosure is cooled to form a binary eutectic phase, which corresponds to a section from a solidification initiation temperature to a solidification end temperature for an Al single phase including, Zn dissolved at less than 27%, in atomic percentage %, and a remainder thereof consists of Al and other impurities.
  • a binary eutectic phase which corresponds to a section from a solidification initiation temperature to a solidification end temperature for an Al single phase including, Zn dissolved at less than 27%, in atomic percentage %, and a remainder thereof consists of Al and other impurities.
  • the 'Al single phase' is indicated differently from a 'second Al single phase' described later.
  • the second temperature section from 375°C to 340°C indicates a temperature section of forming the second Al single phase in which Zn is dissolved at 27% or more and 60% or less, in atomic percentage % (i.e., 27 to 60%) within the plating layer formed within a range of the contents of Mg and Al according to the present disclosure.
  • a step of performing pre-skin pass rolling treatment (SPM) by applying a roll reduction of 200 to 300 tons to the surface of the steel sheet using a BrightRoll with a surface roughness (Ra) of 0.2 to 0.4 um may be further included.
  • a surface shape of the base steel sheet may be controlled to be uniform, and the thickness of the hot-dip coating layer formed by the subsequent plating process is controlled to be uniform, and at the same time, making the base steel sheet smooth, formation sites of solidification nuclei can be minimized. That is, by contributing to smooth nucleation of a surface layer portion of the plating layer rather than formation nuclei therein, in a thickness direction during cooling, microstructure formation in the surface layer portion generated in the first temperature section may be promoted, which may contribute to lowering a ratio of the second Al single phase generated in the second temperature section.
  • the roll reduction of the pre-skin pass rolling treatment before hot-dip galvanizing can be set to 250 to 300 tons.
  • a step of heating the base steel sheet in a heating furnace with a dew point temperature of -60°C or higher and -15°C or lower, wherein a temperature of the base steel sheet is 20°C to 80°C higher than a temperature (Tb) of the plating bath to ensure plating wettability, in a last section of the heating furnace, may be included.
  • the dew point temperature of the heating furnace is to prevent oxidation of the surface of the base steel sheet, and to ensure plating adhesion, the temperature of the heating furnace may be set to -60°C or higher to -15°C.
  • cooling may be performed so that a ratio (De/Dc) of a damper opening rate (De) of an edge portion to a damper opening rate (Dc) of a center portion in a width direction of the hot-dip galvanized steel sheet satisfies 60 to 99%.
  • the 'width direction' of the steel sheet refers to a direction, perpendicular to a transport direction of the steel sheet, based on a surface excluding a thickness side surface of the hot-dip galvanized steel sheet (i.e., a surface where the thickness of the steel sheet is visible).
  • the damper opening rate is a value that refers to an opening degree of a damper controlling a flow rate of cooling gas to be sent from a cooling device to a base steel sheet.
  • a damper is installed so that a total cooling gas input or controlled to the cooling device can be divided into the center portion and the edge portion in the width direction of the base steel sheet and injected.
  • a boundary between the dampers may be divided into three sections according to the width of the base steel sheet, and a position thereof can be variably controlled so that a center is occupied by a center portion and two on an outer side thereof are occupied by an edge portion.
  • the present inventors have recognized that the edge portion in the width direction of the steel sheet has a larger area exposed to an external atmosphere than the center portion, so a rate at which a temperature of the steel sheet inevitably drops in a region corresponding to the edge portion is faster than in the center portion, and have found that uniform characteristics of the surface of the plating layer could be secured by artificially reducing the cooling rate in the edge portion. That is, the cooling gas incident on the center portion during the above-described cooling process naturally escapes from the center portion externally through the edge portion. However, since the edge portion receives the cooling gas after incident on the center portion in addition to the cooling gas incident on the edge portion, it may be overcooled compared to the center part and cause adverse effects.
  • the damper opening rate of the edge portion needs to be controlled to be lower than that of the center portion.
  • the ratio (De/Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion is less than 60%, the edge portion is cooled more slowly than the center portion, and if the ratio exceeds 99%, the edge portion may be overcooled compared to the center portion, which may be disadvantageous in realizing uniform cooling performance in the width direction of the steel sheet.
  • a microstructure of the surface of the plating layer in the edge portion and the center portion becomes uneven, and the structural characteristics of the Al single phase and MgZn 2 phase may not be secured on the surface of the plating layer, so there may be a risk that the corrosion resistance of the flat plate portion and the corrosion resistance of the bending portion may deteriorate.
  • cooling may be performed by changing the ratio (De/Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion, depending on the temperature section.
  • the cooling may be performed so that the ratio (De/Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion satisfies 60 to 70% from a solidification initiation temperature to 375°C (corresponding to the ⁇ first temperature section'), and 90 to 99% from 375°C to 340°C (corresponding to the 'second temperature section').
  • slow cooling may be uniformly performed in a width direction of the steel sheet from the solidification initiation temperature at which the above-described MgZn 2 -Al-based binary eutectic phase is formed to 375°C, so that corrosion resistance may be improved uniformly across the entire width.
  • a second Al single phase with a high Zn solid solution ratio to be as low as possible, from 375°C at which the above-described second Al single phase is formed to a solidification end temperature, it may be controlled so as not to affect a galvanic cell between microstructures formed of the MgZn 2 -Al-based binary eutectic phase, and as a result, not only the corrosion resistance of the flat plate portion, but also the corrosion resistance of the bending portion may be further improved.
  • a step of improving a surface and shape of a base steel sheet by performing a skin pass rolling (SPM) treatment to improve surface quality of a final product may be further included.
  • SPM skin pass rolling
  • the skin pass rolling treatment may be performed by applying a roll reduction of 50 to 300 tons to the surface of the steel sheet using a bright roll with a surface roughness (Ra) of 0.2 to 1.0 ⁇ m.
  • the surface roughness Ra of the bright roll is less than 0.2 um, the roughness of the roll is too low and a frictional force between the base steel sheet and the SPM roll is reduced, so that there may be a problem in which the base steel sheet is slipped, and if the surface roughness Ra of the bright roll exceeds 1.0 um, there may be a problem in which the microstructure of the surface of the plating layer is not preserved intact and excessive cracks occur.
  • the surface roughness Ra of the bright roll is in a range of 0.4 to 0.8 ⁇ m.
  • the roll reduction is less than 50 tons, a problem may occur in uniformizing the shape of the base steel sheet in the width direction, and if the roll reduction exceeds 300 tons, a problem in which the microstructure of the surface of the plating layer is not preserved intact due to an excessive pressing force, and excessive cracks occur even in a surface roughness range of the bright roll described above, may occur.
  • a pre-SMP treatment was performed on a base steel sheet, including by weight: C: 0.018%, Si: 0.01%, Mn: 0.2%, P: 0.009%, S: 0.005%, Al: 0.1%, Nb: 0.02%, Cr: 0.2%, Ti: 0.02%, B: 0.015%, with a balance of Fe and other inevitable impurities, using a bright roll with a surface roughness (Ra) of 0.2 um under a condition of 100 tons.
  • Ra surface roughness
  • the base steel sheet was heated to a temperature, which is 20°C higher than a plating bath temperature (Tb) in a heating furnace with a dew point temperature of -15°C, and then immersed in a plating bath having a composition illustrated in Table 1 below to obtain a hot-dip galvanized steel sheet.
  • the hot-dip galvanized steel sheet was cooled using one or more inert gases among N, Ar, and He in a portion of cooling sections to meet an average cooling rate (Vc) illustrated in Table 1 from a solidification initiation temperature to a solidification end temperature.
  • the average cooling rate for each temperature section was controlled as illustrated in Table 1 below, and at the same time, an average damper opening rate of an edge portion and a center portion of the steel sheet in a width direction was controlled as those illustrated in Table 2 below based on a surface of the hot-dip galvanized steel sheet.
  • a skin pass rolling (SPM) treatment was performed with a roll reduction of 50 to 150 tons using a dull roll with a surface roughness of 2 um to improve the characteristics and shape of the surface of the steel sheet.
  • a specimen of the above-described plated steel sheet was manufactured, a plating layer was dissolved in a hydrochloric acid solution, and the dissolved liquid was analyzed using a wet analysis (ICP) method to measure a composition of the plating layer, which was illustrated in Table 3 below.
  • ICP wet analysis
  • a cross-sectional specimen cut in a direction perpendicular to a rolling direction of the steel sheet was manufactured so that an interface between the plating layer and a base iron was observed, and then photographed with an SEM, so that it was confirmed that a base steel sheet; a Zn-Mg-Al-based plating layer; an Fe-Al-based inhibition layer was formed between the base steel sheet and the Zn-Mg-Al-based plating layer.
  • an Al single phase in which Zn is dissolved at less than 27 at% and a second Al single phase in which Zn is dissolved at 27 at% or more and 60% or less were identified by microstructure labeling using an SEM image.
  • the microstructure labeling is based on a super-pixel algorithm of RISA (microstructure phase fraction analysis software) of the Pohang Research Institute of Industrial Science and Technology (RIST), and is distinguished by color and contrast differences for each microstructure using automatic image generation software, and an area% thereof was quantified.
  • RISA microstructure phase fraction analysis software
  • RIST Pohang Research Institute of Industrial Science and Technology
  • Plating layer composition (Balance of Zn) [wt%] Surface of Zn-Mg-Al-based plating layer Mg Al Total area fraction of Al single phase and MgZn 2 phase [%] Area fraction of MgZn 2 phase [%] Area fraction of Al single phase [%] Area fraction of second Al single phase [%] Area ratio of MgZn 2 phase to Al single phase
  • Example 1 5.0 11.9 45.5 35 10.5 5.5 3.3
  • Example 4 5.0 11.0 52.2 35 17.2 6.8 2.0
  • Example 5 6.0 14.0 53 35.2 17.8 5 2.0
  • Comparative Example 2 4.2 11.5 41.5 23.1 18.4 12.3 1.3 Comparative Example 3 5.7 14.0 27.5 14.7 12.8 10 1.1 Comparative Example
  • a plated steel sheet was manufacture in the same manner as in Experimental Example 1 described above, except a ratio of a damper opening rate was changed as follows, according to a temperature section divided based on a surface temperature of a steel sheet. In this case, it was confirmed that a base steel sheet, an Fe-Al-based inhibition layer, and a Zn-Al-Mg-based plating layer were formed sequentially using the same analysis method as in Experimental Example 1. [Table 5] Reference No.
  • Example 7 A1 99 100 99% 98 100 98%
  • Example 8 A1 60 92 65% 91 94 97%
  • Example 9 A1 66 94 70% 95 97 98%
  • Example 10 B1 99 100 99% 91 92 99%
  • Example 11 B1 65 99 66% 90 100 90%
  • First temperature section* a section from a solidification initiation temperature to 375°C
  • Second temperature section* a section from 375°C to 340°C
  • a surface specimen having a size of 5, 400 ⁇ m 2 was collected in the same manner as in Experimental Example 1 described above, and an area fraction of the MgZn 2 phase and the Al phase in which Zn was dissolved at less than 27 at% in the MgZn 2 -Al based binary eutectic phase were measured, respectively, and was shown in Table 6 below.
  • an area fraction of the Zn phase and Zn-MgZn 2 -Al based ternary eutectic phase was measured for the above-described surface specimen.
  • Comparative Example 12 not meeting requirements in which a plating composition of the present disclosure, and a ratio (De/Dc) of a damper opening rate (De) of an edge portion to a damper opening rate (Dc) of the center portion is 60 to 70% from a solidification initiation temperature to 375°C, and is 90 to 99% from 375°C to 340°C, during a corrosion resistance evaluation experiment, Simonkolleite was first formed on a surface of the plated steel sheet, and LDH was formed on the surface only after 12 hours. For this reason, it was confirmed that corrosion resistance of a flat plate, corrosion resistance of a bending portion, and scattering reflectance of Comparative Example 12 were all inferior.
  • a specimen was manufactured in the same manner as in Experimental Example 1 described above, and then on a surface of the plating layer, an area fraction of a MgZn 2 phase and an Al single phase in which Zn was dissolved at less than 27 at% were measured, and an area fraction of a Zn single phase and a Zn-MgZn 2 -Al-based ternary eutectic phase were measured, respectively, and were shown in Tables 10 and 11 below.
  • surface polishing was performed using the same standard to prepare a specimen illustrating a surface having an area of 24,000 ⁇ m 2 at any point from 1/4t to 3/4t in a thickness direction of the plating layer.
  • the surface polishing was performed on a cold mounted specimen with the surface facing upwardly so that the surface could be observed in a depth direction.
  • Surface polishing was performed at a speed of approximately 2um/min under the conditions of load of 30N AND 105RPM, and forward direction using an automatic polisher and silica suspension.
  • Comparative Example 15 which met a plating composition and other manufacturing conditions of the present disclosure, but does not satisfy cooling conditions of Equation 1-2, although corrosion resistance of a flat plate was secured, corrosion resistance of a bending portion was inferior due to a side effect in which excessive brittle occurs due to an excessively formed MgZn 2 phase and cracks excessively occurred in a plating layer during processing.

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EP22825232.6A 2021-06-18 2022-06-10 Tôle d'acier plaquée hautement résistante à la corrosion ayant une excellente résistance à la corrosion et une excellente qualité de surface, et son procédé de fabrication Pending EP4357478A1 (fr)

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KR1020210079649A KR102529740B1 (ko) 2021-06-18 2021-06-18 내식성 및 표면 품질이 우수한 고내식 도금 강판 및 이의 제조방법
PCT/KR2022/008200 WO2022265307A1 (fr) 2021-06-18 2022-06-10 Tôle d'acier plaquée hautement résistante à la corrosion ayant une excellente résistance à la corrosion et une excellente qualité de surface, et son procédé de fabrication

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KR101079472B1 (ko) 2008-12-23 2011-11-03 주식회사 포스코 도금표면품질이 우수한 고망간강의 용융아연도금강판의 제조방법
KR101758529B1 (ko) * 2014-12-24 2017-07-17 주식회사 포스코 인산염 처리성과 스폿 용접성이 우수한 아연합금도금강판 및 그 제조방법
KR102010085B1 (ko) 2017-12-26 2019-08-12 주식회사 포스코 수퍼픽셀을 이용한 미세조직의 라벨링 이미지 생성방법 및 생성장치
KR102031466B1 (ko) * 2017-12-26 2019-10-11 주식회사 포스코 표면품질 및 내식성이 우수한 아연합금도금강재 및 그 제조방법
KR102384674B1 (ko) * 2019-09-24 2022-04-11 주식회사 포스코 내식성, 내골링성, 가공성 및 표면 품질이 우수한 도금 강판 및 이의 제조방법
KR102297298B1 (ko) * 2019-12-06 2021-09-03 주식회사 포스코 굽힘 가공성 및 내식성이 우수한 용융아연도금강판 및 이의 제조방법
US20230235438A1 (en) * 2020-06-19 2023-07-27 Posco Plated steel sheet having excellent corrosion resistance, workability and surface quality and method for manufacturing same

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CN117561347A (zh) 2024-02-13
WO2022265307A1 (fr) 2022-12-22
KR102529740B1 (ko) 2023-05-08
KR20220169450A (ko) 2022-12-27

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