EP4640911A1 - Plated steel sheet - Google Patents

Plated steel sheet

Info

Publication number
EP4640911A1
EP4640911A1 EP24753459.7A EP24753459A EP4640911A1 EP 4640911 A1 EP4640911 A1 EP 4640911A1 EP 24753459 A EP24753459 A EP 24753459A EP 4640911 A1 EP4640911 A1 EP 4640911A1
Authority
EP
European Patent Office
Prior art keywords
steel sheet
less
plating layer
element group
phase
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP24753459.7A
Other languages
German (de)
French (fr)
Other versions
EP4640911A4 (en
Inventor
Takuya MITSUNOBU
Masumi Hasegawa
Masaaki URANAKA
Hiroshi Takebayashi
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Publication of EP4640911A1 publication Critical patent/EP4640911A1/en
Publication of EP4640911A4 publication Critical patent/EP4640911A4/en
Pending legal-status Critical Current

Links

Classifications

    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • C22C18/04—Alloys based on zinc with aluminium as the next major constituent
    • 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
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • 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
    • C23C24/00—Coating starting from inorganic powder
    • C23C24/08—Coating starting from inorganic powder by application of heat or pressure and heat
    • 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
    • C23C24/00—Coating starting from inorganic powder
    • C23C24/08—Coating starting from inorganic powder by application of heat or pressure and heat
    • C23C24/082—Coating starting from inorganic powder by application of heat or pressure and heat without intermediate formation of a liquid in the layer
    • 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
    • C23C24/00—Coating starting from inorganic powder
    • C23C24/08—Coating starting from inorganic powder by application of heat or pressure and heat
    • C23C24/082—Coating starting from inorganic powder by application of heat or pressure and heat without intermediate formation of a liquid in the layer
    • C23C24/085—Coating with metallic material, i.e. metals or metal alloys, optionally comprising hard particles, e.g. oxides, carbides or nitrides
    • C23C24/087—Coating with metal alloys or metal elements only
    • 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
    • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process
    • 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
    • C23C4/00—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge
    • C23C4/04—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge characterised by the coating material
    • C23C4/06—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
    • C23C4/00—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge
    • C23C4/04—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge characterised by the coating material
    • C23C4/06—Metallic material
    • C23C4/08—Metallic material containing only metal elements
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese

Definitions

  • the present invention relates to a plated steel sheet.
  • the need for a high-strength steel sheet has increased from the viewpoint of reduction in weight in automobile and building material fields.
  • the high-strength steel sheet is subjected to various processes such as press working, bending, and the like in order to realize a desired shape, and is required to have high corrosion resistance. Therefore, the high-strength steel sheet is required to have strength, workability, and corrosion resistance in combination.
  • various plated steel sheets using the high-strength steel sheet as a plating substrate are often used.
  • Patent Document 1 proposes a technique of applying strain on the hot-dip Zn-Al-Mg-based plated steel sheet to introduce cracks into the plating layer and then performing a baking treatment on the plated steel sheet to decrease the hydrogen concentration in the steel.
  • Patent Document 1 Japanese Patent Application No. 2018-204065
  • the present invention has been made in consideration of the above viewpoint, and an object of the present invention is to provide a plated steel sheet which can further improve the hydrogen desorption while maintaining post-painting corrosion resistance.
  • the present inventors have focused attention on that the Zn-Al-Mg-based plating disclosed in Patent Document 1 is a relatively hard plating structure.
  • the present inventors then have obtained such an idea that if a structure which becomes a starting point for generating a crack can be introduced into the plating layer, a more suitable amount of cracks might be efficiently generated without paying excessive attention to the control of applying strain, leading to a possibility of further improving the hydrogen desorption.
  • the gist of the present invention completed based on that idea is as follows.
  • FIG. 1A and FIG. 1B are explanatory views schematically illustrating an example of the plated steel sheet according to this embodiment.
  • a plated steel sheet 1 has a steel sheet 11 being a base material, and a plating layer 13 located on at least a part of a surface of the steel sheet 11. Further, the plating layer 13 may be present on both surfaces of the steel sheet 11 as schematically illustrated in FIG. 1B instead of being present only on one surface of the steel sheet 11 as illustrated in FIG. 1A .
  • the dimension, component, structure, and mechanical property of the steel sheet 11 used as the base material of the plated steel sheet 1 according to this embodiment are not particularly limited.
  • various types of steel sheets can be used according to the mechanical strength (for example, tensile strength) or the like required for the plated steel sheet 1.
  • Examples of the steel sheet 11 are steel materials standardized by Japanese Industrial Standards (JIS) or the like, and include carbon steel, alloy steel, high-strength steel, and the like to be used for general structure and for machine structure.
  • the steel materials include cold-rolled steel material, hot-rolled steel material, hot-rolled steel sheet material for automobile structure, hot-rolled high tension steel sheet material for automotive processing, cold-rolled steel sheet material for automobile structure, cold-rolled high tension steel sheet material for automotive processing, high tension steel material generally called hot stamp material subjected to hardening during hot working, and the like.
  • the components of the steel materials are not particularly limited, but may contain one or two or more of Si, Mn, S, P, Al, N, Cr, Mo, Ni, Cu, Ca, Mg, Ce, Hf, La, Zr, and Sb, in addition to Fe and C.
  • One or two or more of these arbitrary additive elements may be appropriately selected in order to obtain the required material strength and formability, and their contents may also be appropriately adjusted.
  • high-strength steel having a tensile strength of 980 MPa or higher (a so-called high-strength steel at a 980 MPa class or higher) among the above high-strength steels because the robustness of a product to be manufactured can be further improved.
  • the tensile strength of the steel sheet 11 can be measured by a publicly-known method. As an example, it is only necessary to produce a test piece prescribed in JIS Z 2241:2011 from a part of the steel sheet whose tensile strength is desired to be measured, and measure the tensile strength of the obtained test piece by a prescribed method.
  • the thickness of the steel sheet 11 is not particularly limited and appropriately set according to the mechanical strength or the like required for the plated steel sheet 1.
  • the plating layer 13 is provided on the surface of the steel sheet 11 as schematically illustrated in FIG. 1A and FIG. 1B , and is more preferably provided over the entire surface of the steel sheet 11. In the following, a chemical composition of the plating layer 13 will be first explained in detail.
  • the chemical composition of the plating layer 13 according to this embodiment has, in an aspect, a chemical composition containing, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, and Fe: 0.01 to 15.00%, with the balance composed of Zn and impurities.
  • the chemical composition of the plating layer 13 has, in another aspect, a chemical composition containing, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, and Fe: 0.01 to 15.00%, and further containing one or two or more selected from the group consisting of the following element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the balance composed of Zn and impurities.
  • the contents of Al, Mg, and Fe are within the above ranges and the total of the contents of these Al, Mg, Fe, and element group A to element group G is less than 100 mass%, with the balance composed of Zn and impurities.
  • the plating layer 13 is a plating layer having a chemical composition containing, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, and Fe: 0.01 to 15.00%, and selectively further containing one or two or more selected from the group consisting of the element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the balance composed Zn and impurities.
  • Al is an element necessary to constitute a main phase (Zn-Al-Mg-based alloy phase) of the plating layer 13 according to this embodiment, and is contained in a predetermined content or more to ensure corrosion resistance of a region to be a welding heat-affected portion and corrosion resistance of a region to be a non-welded portion after welding, as a plated steel sheet. If an Al content in the plating layer 13 is less than 0.50 mass%, the above corrosion resistance of the regions to be the welding heat-affected portion and the non-welded portion cannot be ensured. Therefore, in the plating layer 13 according to this embodiment, the Al content is 0.50 mass% or more. The Al content is preferably 1.00 mass% or more, and more preferably 1.50 mass% or more. The Al content falling within the above range can ensure the corrosion resistance of the plated steel sheet 1.
  • the Al content in the plating layer 13 exceeds 5.00 mass%, a coagulation behavior from the liquid phase to the solid phase of the plating layer changes, thus making it difficult to crystallize an ⁇ phase, so that a dendrite structure mainly containing Al is formed. If the dendrite structure mainly containing Al is formed, the corrosion of the surroundings is promoted, so that the corrosion resistance of the plated steel sheet 1 cannot be ensured. In addition, the interface between the ⁇ phase being a crack originating source and a eutectic composition decreases because of a lack of the ⁇ phase, causing a lack of cracks generated during processing. Therefore, in the plating layer 13 according to this embodiment, the Al content is 5.00 mass% or less. The Al content is preferably 4.00 mass% or less, and more preferably 3.00 mass% or less.
  • Mg is an element necessary to constitute the main phase (Zn-Al-Mg-based alloy phase) of the plating layer 13 according to this embodiment, and is contained in a predetermined content or more to ensure corrosion resistance of the region to be the welding heat-affected portion and corrosion resistance of the region to be the non-welded portion, as the plated steel sheet. Therefore, in the plating layer 13 according to this embodiment, an Mg content is 0.50 mass% or more.
  • the Mg content is preferably 1.50 mass% or more, and more preferably 2.00 mass% or more.
  • the Mg content falling within the above range makes it possible to ensure the corrosion resistance of the plated steel sheet 1. If the Mg content exceeds 3.00%, an ⁇ phase becomes difficult to precipitate from the ⁇ phase. Since the ⁇ phase in which Al is solid-dissolved is hard, the deformation cannot be concentrated on the ⁇ phase during processing, causing a decrease in the number of cracks. Therefore, the upper limit of the Mg content is 3.00%.
  • the Mg content in the plating layer 13 is 0.50 mass% or more.
  • the Mg content is preferably 1.50 mass% or more, and more preferably 2.00 mass% or more. The Mg content falling within the above range makes it possible to ensure the corrosion resistance of the plated steel sheet 1.
  • Elements constituting the steel sheet 11, which is the base material, may be sometimes mixed into the plating layer 13.
  • the elements constituting the steel sheet 11 are easily mixed into the plating layer 13 due to interdiffusion of the elements by a solid-liquid reaction between the steel sheet 11 and the plating layer 13. Due to the mixing of the elements, a certain amount of Fe is contained in the plating layer 13, and its content is generally 0.01 mass% or more.
  • an Fe content in the plating layer 13 is preferably 0.20 mass% or more.
  • Fe may be intentionally added to a plating bath to be used when manufacturing the plating layer 13 within a range that does not impair the effect of the present invention.
  • a high-melting point intermetallic compound of Fe and Al is formed in the plating bath, and the high-melting point intermetallic compound adheres, as dross, to the plating layer to significantly degrade an appearance quality, which is undesirable.
  • the Fe content in the plating bath is adjusted, so that the Fe content in the plating layer 13 is 15.00 mass% or less.
  • the Fe content in the plating layer 13 is more preferably 10.00 mass% or less.
  • the balance of Al, Mg, and Fe is composed of Zn and impurities.
  • Zn is an element necessary to constitute the main phase (Zn-Al-Mg-based alloy phase) of the plating layer 13 according to this embodiment, and is an important element to improve the corrosion resistance of the plated steel sheet. Further, the plating layer 13 containing the above Al, Mg, and Fe in the above ranges and further containing Zn makes it possible to ensure the corrosion resistance as the plated steel sheet.
  • At least any of the following elements belonging to the element group B to element group E is contained in the plating layer 13 according to this embodiment, it is preferable that at least any of the following elements belonging to the element group B to element group E is contained in the following content range and a total content of 5.0000 mass% or less.
  • the total content of the elements belonging to the element group B to the element group E is preferably 1.0000 mass% or less, and more preferably 0.2000 mass% or less.
  • the element group A which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment will be explained. At least any of the following elements in the element group A is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
  • element group A one or two selected from the group consisting of Si: more than 0% and 2.00% or less and Ca: more than 0% and 2.00% or less
  • Si is an element capable of suppressing excessive growth of a Fe-Al-based intermetallic compound phase that is formed at an interface between the plating layer and the steel sheet and further improving the adhesiveness between the plating layer and the steel sheet.
  • an Si content is preferably 0.05 mass% or more and more preferably 0.10 mass% or more to suppress the excessive growth of the Fe-Al-based intermetallic compound phase.
  • the Si content in the plating layer 13 is preferably 2.00 mass%. Further, if the Si content in the plating bath for manufacturing the plating layer 13 is too large, the viscosity of the plating bath may increase more than necessary to decrease the plating operability. Therefore, the Si content in the plating bath is adjusted from the viewpoint of the plating operability so that the Si content in the plating layer 13 is preferably 1.00 mass% or less and more preferably 0.50 mass% or less.
  • the plating layer 13 according to this embodiment does not contain Ca is conceivable, and therefore the lower limit of the Ca content is 0 mass%.
  • the plating layer 13 when Ca is contained in the plating layer 13, it forms intermetallic compound phases with Al and Zn.
  • Ca when Si is contained together with Ca in the plating layer 13, Ca forms an intermetallic compound phase with Si.
  • These intermetallic compound phases have high melting points and stable structures, and therefore can suppress liquid metal embrittlement (LME) cracking during welding of the plated steel sheet.
  • LME liquid metal embrittlement
  • the Ca content in the plating layer 13 is more preferably 0.05 mass% or more.
  • the Ca content in the plating layer 13 exceeds 2.00 mass%, the corrosion resistance of the plated steel sheet may decrease. From this viewpoint, the Ca content in the plating layer 13 is 2.00 mass% or less.
  • the Ca content in the plating layer 13 is preferably 1.50 mass% or less, and more preferably 1.00 mass% or less.
  • element group B which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment. At least any of the following elements in the element group B is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
  • element group B one or two or more selected from the group consisting of Sb: more than 0% and 0.5000% or less, Pb: more than 0% and 0.5000% or less, and Sr: more than 0% and 0.5000% or less
  • the plating layer 13 according to this embodiment does not contain Sb, Pb, and Sr is conceivable, and therefore the lower limits of the contents of these elements are 0 mass%.
  • the plating layer 13 when at least any of Sb, Pb, and Sr is contained in the plating layer 13, spangles are formed on the surface of the plating layer 13, making it possible to improve metallic luster. Therefore, from the viewpoint of improving the design of the plated steel sheet, at least any of Sb, Pb, and Sr is preferably contained in the plating layer 13.
  • the design improvement effect is exhibited when the content of at least any of Sb, Pb, and Sr is 0.0500 mass% or more. Therefore, when at least any of Sb, Pb, and Sr is contained in the plating layer 13, the contents of these elements are each independently preferably 0.0500 mass% or more.
  • the contents of Sb, Pb, and Sr in the plating layer 13 are each independently 0.5000 mass% or less.
  • the contents of Sb, Pb, and Sr are each independently preferably 0.2000 mass% or less.
  • the element group C which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment will be explained.
  • At least any of the following elements in the element group C is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
  • the plating layer 13 according to this embodiment does not contain Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is conceivable, and therefore the lower limits of the contents of these elements are 0 mass%.
  • these elements are incorporated into an Al-Fe alloy phase generated by welding when the plated steel sheet is welded, so that the corrosion resistance of a welded portion to be formed can be improved.
  • the effect of improving the corrosion resistance of the welded portion is exhibited when the content of at least any of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V in the plating layer 13 is 0.0050 mass% or more. Therefore, when at least any of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is contained in the plating layer 13, the contents of these elements are each independently preferably 0.0050 mass% or more.
  • the contents of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V in the plating layer 13 are each independently 1.0000 mass% or less.
  • the contents of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V are each independently preferably 0.2000 mass% or less.
  • the plating layer 13 according to this embodiment does not contain Mo is conceivable, and therefore the lower limit of an Mo content is 0 mass%.
  • Mo when Mo is contained in the plating layer 13, the corrosion resistance can be further improved.
  • the effect of improving the corrosion resistance is exhibited when the Mo content is 0.0100 mass% or more. Therefore, when Mo is contained, its content is preferably 0.0100 mass% or more.
  • the Mo content is 1.0000 mass% or less.
  • the Mo content is preferably 0.0500 mass% or less.
  • element group D which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment.
  • the following elements in the element group D are elements which can be contained in the plating layer 13 in place of a part of the balance Zn.
  • element group D one or two or more selected from the group consisting of Sn: more than 0% and 1.0000% or less, In: more than 0% and 1.0000% or less, and Bi: more than 0% and 1.0000% or less
  • Sn, In, and Bi are elements that increase an Mg dissolution rate when the plating layer 13 containing them is placed in a corrosive environment.
  • Mg dissolution rate increases, Mg ions are supplied to portions where the steel sheet 11 is exposed, so that the corrosion resistance is improved.
  • the contents of Sn, In, and Bi are each independently preferably 0.0050 mass% or more.
  • excessive addition of Sn, In, and Bi may excessively promote the Mg dissolution rate to decrease the corrosion resistance of the plated steel sheet.
  • the increase in the Mg dissolution rate becomes prominent when any of the contents of Sn, In, and Bi exceeds 1.0000 mass%, and therefore the contents of Sn, In, and Bi are each independently 1.0000 mass% or less.
  • the contents of Sn, In, and Bi are each independently preferably 0.2000 mass% or less.
  • element group E which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment. At least any of the following elements in the element group E is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
  • element group E one or two or more selected from the group consisting of Zr: more than 0% and 1.0000% or less, Ag: more than 0% and 1.0000% or less, and Li: more than 0% and 1.0000% or less
  • the plating layer 13 according to this embodiment does not contain Zr, Ag, and Li is conceivable, and therefore the lower limits of the contents of these elements are 0 mass%.
  • the plating operability can be improved.
  • the effect of improving the platability is exhibited when the content of at least any of Zr, Ag, and Li in the plating layer 13 is 0.0100 mass% or more. Therefore, when at least any of Zr, Ag, and Li is contained, the contents of these elements are each independently preferably 0.0100 mass% or more.
  • each content of at least any of Zr, Ag, and Li is independently 1.0000 mass% or less.
  • Each content of at least any of Zr, Ag, and Li is independently preferably 0.1000 mass% or less.
  • element group F which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment. At least any of the following elements in the element group F is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
  • element group F one or two or more selected from the group consisting of La: more than 0% and 0.5000% or less, Ce: more than 0% and 0.5000% or less, and Y: more than 0% and 0.5000% or less
  • the plating layer 13 according to this embodiment does not contain La, Ce, and Y is conceivable, and therefore the lower limits of the contents of these elements are 0 mass%.
  • La, Ce, and Y are elements that exhibit almost the same effect as Ca and further suppress the blowhole formation during welding. This is due to the fact that an atomic radius of each element is close to the atomic radius of Ca.
  • these elements are contained in the plating layer 13, they are substituted at Ca positions. Therefore, these elements are detected at the same positions as Ca by EDS.
  • the oxides of these elements are detected at the same positions as CaO.
  • the effect of suppressing the blowhole formation during welding is exhibited when the contents of these elements are each independently 0.0100 mass% or more. Therefore, when at least any of Zr, Ag, and Li is contained, the contents of these elements are each independently preferably 0.0100 mass% or more.
  • the contents of La, Ce, and Y in the plating layer 13 are each independently more preferably 0.0500 mass% or more.
  • the La, Ce, and Y contents in the plating bath for manufacturing the plating layer 13 are adjusted from the viewpoint of the plating operability, so that the contents of La, Ce, and Y are each independently 0.5000 mass% or less.
  • the contents of La, Ce, and Y are each independently preferably 0.1000 mass% or less.
  • element group G which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment will be explained.
  • the following element in the element group G is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
  • element group G B: more than 0% and 0.5000% or less
  • the plating layer 13 according to this embodiment does not contain B is conceivable, and therefore the lower limit of its content is 0 mass%.
  • B when B is contained in the plating layer 13, there is an effect of suppressing the LME. This is presumably because when B is contained in the plating layer 13, it combines with at least any of Zn, Al, Mg, and Ca to form various intermetallic compound phases. Further, the presence of B in the plating layer 13 is considered to produce the effect of suppressing the LME of the steel sheet 11 because B diffuses from the plating layer 13 into the steel sheet 11 to strengthen the grain boundary. Furthermore, it is presumed that since the various intermetallic compounds formed concerning B have extremely high melting points, they also act in suppressing the Zn evaporation during welding. These improvement effects are exhibited when 0.0500 mass% or more of B is contained. Therefore, when B is contained, the content of B is preferably 0.0500 mass% or more.
  • a plating melting point rapidly increases to decrease the plating operability, failing to manufacture a plated steel sheet excellent in plating property.
  • the decrease in plating operability becomes prominent when the content of B exceeds 0.50 mass%, so that the content of B is 0.5000 mass% or less.
  • the content of B is preferably 0.1000 mass% or less.
  • the chemical components of the above plating layer 13 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry).
  • ICP-AES Inductively Coupled Plasma Atomic Emission Spectrometry
  • ICP-MS Inductively Coupled Plasma Mass Spectrometry
  • ICP-AES shall be used in the case of analyzing the chemical component up to 0.1 mass% unit
  • ICP-MS shall be used in the case of analyzing the chemical component in a trace amount of less than 0.1 mass%.
  • the plated steel sheet is immersed in a 10% HCl aqueous solution with an inhibitor added for about 1 minute to peel off the plating layer part, thereby preparing the solution in which the plating layer is dissolved.
  • the obtained solution can be analyzed by ICP-AES or ICP-MS to obtain chemical components as an overall average of the plating layer.
  • the deposition amount of the plating layer 13 as explained above is not particularly specified and is preferably, for example, about 15 to 250 g/m 2 per one side of the steel sheet.
  • the plated steel sheet 1 according to this embodiment can exhibit sufficient corrosion resistance.
  • the deposition amount of the plating layer 13 is measured as follows. First, a sample having a size of 30 mm ⁇ 30 mm in plan view is cut out from the plated steel sheet and the mass of the sample is measured in advance. Note that when cutting out the sample, it is cut out entirely in the thickness direction. A tape seal is bonded to one side of the sample to prevent the plating layer on the one side from dissolving in the next process. The sample is then immersed in a 10% HCl aqueous solution with an inhibitor added to peel off the plating layer by pickling, and the mass of the sample after pickling is measured. From a change in mass of the sample between before and after the pickling, the deposition amount of the plating layer 13 per side can be decided.
  • the plating layer 13 has the above chemical composition. Further, the plating layer 13 is formed through a manufacturing method as will be explained in detail below, and thereby contains metal phases such as an ⁇ phase, an ⁇ -Zn phase, a MgZn 2 phase, an ⁇ / ⁇ /MgZn 2 ternary eutectic phase, and an ⁇ /MgZn 2 binary eutectic phase, and an intermetallic compound phase. Further, the plating layer 13 can contain intermetallic compound such as an Al-Si-Ca phase, an Al-Si-Ca-Fe phase, a Mg 2 Si phase, a Mg 2 Sn phase, and so on in addition to the above phases depending on elements which the plating layer 13 can further contain.
  • the plating layer 13 according to this embodiment has the above metal structures and thereby exhibits the property that is excellent in corrosion resistance.
  • part of the above ⁇ phase and ⁇ -Zn phase exist in a specific state as will be explained below in detail, and come to function as a starting point of crack generation.
  • which phases the plating layer 13 according to this embodiment has can be identified by observing the surface of the plating layer 13 under a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • a pre-treatment such as polishing does not need to be performed before the SEM observation.
  • polishing or the like may be performed to remove the chemical treatment film or the like.
  • the polishing is performed such that 80% or more of the thickness of the plating layer remains, and the surface is brought into a mirror finish state, then observed under the SEM, and regarded as a surface structure.
  • FIG. 2 and FIG. 3 are schematic views for explaining an ⁇ precipitated ⁇ phase possessed by the plating layer 13 of the plated steel sheet 1 according to this embodiment.
  • the surface structure of the plating layer 13 of the plated steel sheet 1 according to this embodiment has the ⁇ precipitated ⁇ phase (code 101 in FIG. 2 ) as will be explained below, and its average area ratio is 5 to 95%. Further, the remaining portion of the ⁇ precipitated ⁇ phase 101 is a hard structure 103 composed of the MgZn 2 phase, the ⁇ / ⁇ /MgZn 2 ternary eutectic phase, the ⁇ /MgZn 2 binary eutectic phase, and the like. Note that the plating layer 13 according to this embodiment contains 0.50 mass% or more of Mg as the chemical composition of the plating, so that the above hard structure 103 is necessarily formed from the chemical composition.
  • the ⁇ precipitated ⁇ phase 101 is a metal structure in which Al supersaturated and solid-dissolved in a parent phase (hereinafter, referred to as an " ⁇ parent phase 113") composed of the ⁇ -Zn phase precipitates as an ⁇ phase 111 and softened as schematically illustrated in FIG. 3 .
  • a parent phase hereinafter, referred to as an " ⁇ parent phase 113”
  • the ⁇ phase 111 exists as a black matter
  • the ⁇ parent phase 113 exists as a white matter. Therefore, the ⁇ precipitated ⁇ phase 101 is recognized as a structure in which black particles are dispersed in a white phase at the SEM observation by the backscattered electron image.
  • the ⁇ precipitated ⁇ phase 101 exists in the hard structure 103 to cause nonuniformity in hardness as the plating layer 13.
  • the strain accompanying the processing or the like is applied to the plating layer 13 in a state where the relatively soft metal structure exists in the hard metal structure, the applied deformation concentrates on the ⁇ precipitated ⁇ phase 101 being the soft metal structure.
  • the crack 105 is generated starting from the ⁇ precipitated ⁇ phase 101 as schematically illustrated in FIG. 2 .
  • the position of an end portion of the crack 105 generated in the hard structure 103 often reached the interface between the plating layer 13 and the steel sheet 11 within a range of the above adhesion amount of the plating layer 13.
  • hydrogen existing in the steel sheet 11 is released to the outside of the system (namely, outside air) via the crack 105. Further, even if the crack 105 does not reach the interface between the plating layer 13 and the steel sheet 11, hydrogen in the steal reached the position of the end portion of the crack 105 is then released to the outside of the system along the crack 105.
  • the average area ratio of the ⁇ precipitated ⁇ phase 101 falls in a range of 5 to 95% as explained above.
  • the average area ratio of the ⁇ precipitated ⁇ phase 101 falling in the above range introduces a suitable abundance of cracks 105 into the plating layer 13, for example, even in a range of the strain to be applied when processing the plated steel sheet into various shapes.
  • the average area ratio of the ⁇ precipitated ⁇ phase 101 is preferably 8% or more, and more preferably 15% or more.
  • the average area ratio of the ⁇ precipitated ⁇ phase 101 exceeds 95%, a soft ⁇ precipitated ⁇ phase undesirably occupies almost all the plating layer, so that the plating layer exhibits ductility as a whole and the ⁇ precipitated ⁇ phase comes not to function as a crack generation site.
  • the average area ratio of the ⁇ precipitated ⁇ phase 101 is 95% or less, the ⁇ precipitated ⁇ phase 101 is made to function as the crack generation site of the plating layer while maintaining the corrosion resistance, thereby making it possible to improve the hydrogen desorption as the plated steel sheet 1.
  • the average area ratio of the ⁇ precipitated ⁇ phase 101 is preferably 70% or less, more preferably 40% or less, and furthermore preferably 30% or less.
  • an arbitrary position on the surface of the plating layer 13 is observed under the SEM, and an element mapping is acquired using the SEM-EPMA.
  • the obtained element mapping is binarized using a binarization function of a commercially available image analysis application, and a region corresponding to the ⁇ precipitated ⁇ phase 101 is identified and its area ratio is calculated.
  • a region of 120 ⁇ m ⁇ 100 ⁇ m (corresponding to a magnification of about 1000 times) in plan view at the arbitrary position on the surface of the plating layer 13 is observed under the SEM, and a point analysis is executed by the SEM-EPMA.
  • the above region of 120 ⁇ m ⁇ 100 ⁇ m is observed with an acceleration voltage: 15.0 kV, an irradiation current: 4.999 ⁇ 10 -8 A, an irradiation time: 50 milliseconds, and a magnification of 1000 times.
  • the backscattered electron image in the focused range is acquired under these conditions, and then the point analysis for each of the metal structures only needs to be performed for three points using the contrast in the backscattered electron image.
  • a phase satisfying Al: 20 to 99 atom%, Zn: 0.5 to 80 atom%, Mg: 0 to 5 atom%, and a total of Al and Zn of 70 atom% or more in this point analysis can be determined as the ⁇ phase. Further, a phase having a content of Zn of 98 atom% or more and a total of contents of other elements of 2 atom% or less is determined as the ⁇ -Zn phase.
  • a phase having a content of each of Mg and Zn of 10 atom% or more and a total content of Mg and Zn of 85 atom% or more is determined as the hard structure 103 composed of the MgZn 2 phase, the ⁇ / ⁇ /MgZn 2 ternary eutectic phase, the ⁇ /MgZn 2 binary eutectic phase, and the like.
  • Mg is not contained in the ⁇ -Zn phase, whereas the hard structure 103 containing Mg exists around the ⁇ -Zn phase. Therefore, a contour line representing a boundary between the metal structure containing Mg and the metal structure not containing Mg can be identified by focusing attention on the distribution of Mg elements. The thus-identified metal structure not containing Mg surrounded by the contour line can be determined as the ⁇ -Zn phase.
  • the ⁇ -Zn phase and the hard structure 103 therearound can be easily distinguished by visual recognition by those skilled in the art.
  • the contour line of ⁇ -Zn is identified as above in a field of view during the SEM observation, and the contour line is manually drawn on various image analysis applications (for example, ImageJ and the like). Thereafter, the image is binarized in the image analysis application, whereby whether or not a black precipitate corresponding to the ⁇ phase exists in the ⁇ -Zn phase can be easily determined.
  • the threshold of binarization only needs to be set to 200 as a Minimum value, for example, in Brightness/Contrast.
  • the ⁇ phase is visually recognized as the black precipitate (namely, black particle), whereas the ⁇ -Zn phase is visually recognized as the white parent phase. Accordingly, the ⁇ precipitated ⁇ phase 101 to be focused on in this embodiment is observed as a phase in which the black particles are dispersed in the white parent phase as schematically illustrated in FIG. 3 . Note that in FIG. 3 , the contour line indicated with a broken line corresponds to the contour line of the above ⁇ -Zn phase.
  • the phase in which the black particles are dispersed in the white parent phase is identified by the above observation using the backscattered electron image, and then the point analysis by the SEM-EPMA is executed. Attention is focused on one phase in which the black particles are dispersed in the white parent phase, and the point analysis by the SEM-EPMA is executed on an arbitrary size of 5 ⁇ m ⁇ 5 ⁇ m including both the black particles and the white parent phase as schematically illustrated in FIG. 3 .
  • a region in a range where the content of Zn is 90 atom% or more and the content of Al is 0.05 to 10.00 atom% can be determined as the ⁇ phase 111 in the ⁇ parent phase 113.
  • the black phases existing around a portion determined as the ⁇ phase only need to be collectively determined as the ⁇ phase.
  • the region of the ⁇ precipitated ⁇ phase 101 in the region of the region of 120 ⁇ m ⁇ 100 ⁇ m in plan view at the arbitrary position on the surface of the plating layer 13 can be identified.
  • the area ratio of the identified region only needs to be calculated using various image analysis applications (for example, ImageJ and the like). Specifically, the area of the region corresponding to the ⁇ precipitated ⁇ phase 101 is calculated by the image analysis application, and then the obtained area of the ⁇ precipitated ⁇ phase 101 is divided by the area of the entire field of view, whereby the area ratio of the ⁇ precipitated ⁇ phase 101 in the field of view focused on can be obtained.
  • the above measurement and calculation processing is executed at arbitrary five points, and an average value of the five area ratios obtained is calculated.
  • the thus-obtained average value is regarded as the average area ratio of the ⁇ precipitated ⁇ phase 101.
  • the total sum of the lengths of the cracks 105 existing in the above region is 50 ⁇ m or more.
  • the plated steel sheet 1 When the total sum of the lengths of the cracks 105 existing in the above region becomes 50 ⁇ m or more, the plated steel sheet 1 according to this embodiment becomes to exhibit excellent hydrogen desorption.
  • the total sum of the lengths of the cracks 105 existing in the above region is preferably 150 ⁇ m or more, and more preferably 500 ⁇ m or more.
  • a larger total sum of the lengths of the cracks 105 is better, and its upper limit value is not particularly specified and is substantially about 1000 ⁇ m. Note that even if the total sum of the lengths of the cracks 105 is large, Mg contained in the plating layer 13 exhibits corrosion protection, so that the corrosion resistance as the plated steel sheet 1 can be maintained.
  • the total sum of the lengths of the cracks 105 is measured using a length measurement application or the like belonging to the SEM.
  • the above measurement processing is performed at arbitrary five points, and an average of the obtained five measured values is calculated.
  • the thus-obtained average value is regarded as the total sum of the lengths of the cracks 105 in the region of 130 ⁇ m ⁇ 100 ⁇ m.
  • the plated steel sheet 1 according to this embodiment has been explained above in detail referring to FIG. 1A to FIG. 3 .
  • the plated steel sheet 1 according to this embodiment as explained above can be preferably used, for example, as a material for automobile undercarriage parts.
  • the plated steel sheet 1 according to this embodiment may further have one or two or more layers of various types of films on the plating layer 13.
  • the film include a chromate film, a phosphate film, a chromate-free film, an organic resin film, and so on.
  • the plated steel sheet 1 according to this embodiment is manufactured by using the steel sheet 11 as explained above as a base material, forming the plating layer 13 on the surface of the steel sheet 11, and then applying strain to the steel sheet 11 on which the plating layer 13 is formed by various methods.
  • a thermal spraying method, a cold spraying method, a sputtering method, a vapor deposition method, an electroplating method, and the like can be applied in addition to a hot-dip plating method.
  • the hot-dip plating method is the most preferable in terms of cost in order to form a plating layer with a thickness generally used in automobiles and the like.
  • a specific heat treatment process as will be explained below is performed. This forms the ⁇ precipitated ⁇ phase 101 in the plating layer 13. Thereafter, strain is applied to the plated steel sheet subjected to the heat treatment process, by various methods to create the crack 105 starting from the ⁇ precipitated ⁇ phase 101, so that the plated steel sheet 1 according to this embodiment can be obtained.
  • the steel sheet 11 to be used as the base material is rolled by a Sendzimir method into a desired thickness, and then coiled and installed on a hot-dip plating line.
  • the steel sheet On the hot-dip plating line, the steel sheet is continuously passed while being unrolled from the coil. Thereafter, the steel sheet is subjected to a heat reduction treatment by an annealing facility installed on the line at 800°C in an N 2 -5% H 2 gas atmosphere under an environment with an oxygen concentration of 20 ppm or less where oxidation is less likely to occur, then air-cooled with an N 2 gas down to about a bath temperature of the plating bath at a subsequent stage + 20°C, and then immersed in the plating bath.
  • a heat reduction treatment by an annealing facility installed on the line at 800°C in an N 2 -5% H 2 gas atmosphere under an environment with an oxygen concentration of 20 ppm or less where oxidation is less likely to occur, then air-cooled with an N 2 gas down to about a bath temperature of the plating bath at a subsequent stage + 20°C, and then immersed in the plating bath.
  • a plating alloy in a molten state having the chemical components as above is prepared in the plating bath in advance.
  • the bath temperature of the plating bath is set to equal to or higher than the melting point of the plating alloy (for example, about 400 to 500°C).
  • the material of the plating alloy it is preferable to prepare it using pure metal (purity of 99% or more) as the alloy material.
  • a predetermined amount of the alloy metal is mixed so as to obtain the composition of the plating layer as explained above, and completely melted into an alloy using a high-frequency induction furnace, arc furnace, or the like in vacuum or inert gas replacement state. Furthermore, the alloy mixed with the predetermined components (composition of the above plating layer) is melted in air, and the resulting melt is used as the plating bath.
  • the steel sheet is immersed in the plating bath as explained above, and then pulled up at a predetermined speed.
  • a plating weight is controlled by, for example, an N 2 wiping gas so that the plating layer 13 to be formed has a desired thickness.
  • general plating operation conditions may be applied, and no special facilities or conditions are required.
  • first cooling process and second cooling process are performed on the plating alloy in a molten state located on the steel sheet to make the plating alloy in the molten state into the plating layer 13 and to grow the ⁇ precipitated ⁇ phase 101 in the plating layer 13.
  • first cooling process and the second cooling process will be explained in detail.
  • the first cooling process is a cooling process performed when the temperature of the plating alloy is in a range of the bath temperature or lower and 240°C or higher.
  • the plated steel sheet in the above temperature range is rapidly cooled at an average cooling rate of 15.0 °C/sec or more.
  • the concentration of Al solid-dissolved in the ⁇ phase decreases and the driving force for the ⁇ phase precipitation in the ⁇ phase thereafter is insufficient, resulting in difficulty in forming the ⁇ precipitated ⁇ phase.
  • the hot-dip plating method is employed in the plating process, the first cooling process is performed immediately after the steel sheet is pulled up from the plating bath. This causes Al to solid-dissolve in the ⁇ -Zn phase during coagulation.
  • the average cooling rate is preferably 25.0 °C/sec or more.
  • the upper limit value of the average cooling rate is not particularly specified and, for example, about 90.0 °C/sec is a substantial upper limit.
  • a second cooling process is performed when the temperature of the plating alloy (plating layer) falls within a range of lower than 240°C and 70°C or higher.
  • the plated steel sheet falling within the above temperature range is gradually cooled at an average cooling rate of 1.0 °C/sec or less.
  • the average cooling rate is more than 1.0 °C/sec, the time when the ⁇ phase precipitates from the ⁇ phase cannot be ensured, resulting in difficulty in forming the ⁇ precipitated ⁇ phase.
  • Al solid-dissolved in the ⁇ -Zn phase precipitates as the ⁇ phase in the first cooling process to cause the ⁇ -Zn phase to soften and become the ⁇ precipitated ⁇ phase 101 and cause the hard structure 103 to exist around the ⁇ precipitated ⁇ phase 101.
  • the average cooling rate is preferably 0.5 °C/sec or less.
  • the plating layer is subjected to a two-stage cooling process of the rapid cooling in the temperature range of the bath temperature or lower and 240°C or higher and the gradual cooling in the temperature range of lower than 240°C and 70°C or higher, whereby the ⁇ precipitated ⁇ phase 101 can be formed in the plating layer 13 at a desired average area ratio.
  • cooling state from 70°C to room temperature is not particularly specified, and the cooling down to room temperature may be performed by various methods.
  • an interval from the end of the first cooling process to the start of the second cooling process is preferably within 3 seconds, and the second cooling process is preferably started immediately after the end of the first cooling process. If the interval from the end of the first cooling process to the start of the second cooling process exceeds 3 seconds, an unintended cooling process occurs to fail to generate the desired ⁇ precipitated ⁇ phase 101.
  • the desired ⁇ precipitated ⁇ phase 101 cannot be realized.
  • the ⁇ precipitated ⁇ phase 101 can be generated in the plating layer 13 at a desired average area ratio and the soft structure can be appropriately introduced into the plating layer 13.
  • an alloying thermal treatment process for example, a thermal treatment process involving heating to an ultimate sheet temperature of about 480 to 550°C
  • an alloying thermal treatment process which is often performed generally in the manufacture of an alloyed hot-dip galvanized steel sheet
  • the state of the ⁇ precipitated ⁇ phase 101 controlled by the first cooling process and the second cooling process is disrupted, resulting in a failure to obtain the hydrogen desorption as focused on in this embodiment. From this viewpoint, it is important not to perform the thermal treatment process after the second cooling process.
  • N 2 gas cooling mist cooling, water submergence, and the like
  • gases high in heat removal effect such as a He gas and a hydrogen gas may be used, in addition to the N 2 gas, as the cooling gas.
  • a contact-type thermocouple K-type
  • the average temperature of the entire plating layer can be always monitored.
  • the temperature of the entire plating layer at that point in time under the manufacturing conditions can be monitored almost accurately. This makes it possible to precisely control the cooling treatments in the first cooling process and the second cooling process.
  • the surface temperature of the plating layer may also be measured by a non-contact radiation thermometer, although not as accurate as the contact type.
  • a relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer may be found by a simulation that performs heat conduction analysis. Specifically, the surface temperature of the plating layer and the average temperature of the entire plating layer are found based on various manufacturing conditions such as the preheating temperature of the steel sheet, the temperature of the hot-dip plating bath, the pulling-up speed of the steel sheet from the plating bath, the sheet thickness of the steel sheet, the layer thickness of the plating layer, an amount of heat exchange between the plating layer and the manufacturing facility, and a heat release amount of the plating layer. Then, using the obtained results, the relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer may be found.
  • strain is applied to the plated steel sheet obtained as above. This generates cracks in the plating layer 13 having the ⁇ precipitated ⁇ phase 101.
  • a method of applying the strain is not particularly limited, and bending and stretching deformation processing by a tension leveler, rolling processing by a skin pass roller, cold press processing when processing the plated steel sheet into a desired shape, or the like only needs to be performed.
  • a total elongation percentage of 0.2% or more to the plated steel sheet.
  • a total elongation percentage R TOTAL (unit: %) is a value decided by Expression (101) below.
  • L 0 is a length (unit: m) in a sheet-passing direction in an arbitrary sheet-passing direction section X of the plated steel sheet before subjected to the processing for applying strain
  • L 1 is a length in the sheet-passing direction of a portion derived from the arbitrary sheet-passing direction section X of the plated steel sheet after subjected to the processing for applying strain.
  • the total elongation percentage is more preferably 0.6% or more.
  • R TOTAL % L 1 ⁇ L 0 / L 0 ⁇ 100
  • the upper limit value of the total elongation percentage R TOTAL is not particularly specified, and about 1.5% is substantially the upper limit.
  • the processing for applying strain can be executed at arbitrary timing, and may be executed without a pause after the two-stage cooling process, or may be executed after a certain amount of time passes after the end of the two-stage cooling process.
  • the ⁇ precipitated ⁇ phase 101 is generated at the desired average area ratio in the plating layer 13 by being subjected to the above two-stage cooling process. Therefore, the crack 105 can be generated in the plating layer 13 by executing the processing for applying strain at arbitrary timing if only after the two-stage cooling process.
  • a treatment for further forming one layer or two or more layers of various types of films may be performed after the above second cooling process.
  • the treatment include a chromate treatment, a phosphate treatment, a chromate-free treatment, an organic resin film formation treatment, and so on.
  • Examples of the chromate treatment include an electrolytic chromate treatment for forming a chromate film by electrolysis, a reactive chromate treatment for forming a film using a reaction with a material and then washing off excessive treatment solution, and a coating-type chromate treatment for forming a film by applying a treatment solution and drying it without water washing, and so on, and any of the chromate treatments may be employed.
  • electrolytic chromate treatment examples include electrolytic chromate treatments using, for example, chromic acid, silica sol, resins (phosphoric acid resin, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine-modified epoxy resin, and the like), and hard silica.
  • Examples of the phosphate treatment include a zinc phosphate treatment, a calcium zinc phosphate treatment, a manganese phosphate treatment, and so on.
  • the chromate-free treatment is particularly preferable because it does not place a burden on the environment.
  • the chromate-free treatment include an electrolytic chromate-free treatment for forming a chromate-free film by electrolysis, a reactive chromate-free treatment for forming a film using a reaction with a raw material and then washing off excessive treatment solution, a coating-type chromate-free treatment for forming a film by applying a treatment solution and drying it without water washing, and so on, and any of the chromate-free treatments may be employed.
  • the organic resin to be used in the organic resin film formation treatment is not limited to a specific resin and, for example, various resins such as polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, and modified versions of these resins can be used.
  • the modified versions here refer to resins in which reactive functional groups contained in structures of these resins are made to react with other compounds (for example, monomers, crosslinking agents, or the like) containing, in the structures, functional groups which can react with the functional groups.
  • organic resin one of the above-mentioned organic resins may be used alone, or a mixture of two or more organic resins (not modified) may be used. Further, one or two or more organic resins obtained by modifying at least one other organic resin in the presence of at least one organic resin may be used by mixture. In addition, an organic resin that has been made aqueous by dissolving or dispersing it in water may be used. Furthermore, various kinds of color pigments and rust preventive pigments may be contained in the organic resin films.
  • the plated steel sheet according to the present invention will be concretely explained while illustrating examples and comparative examples. Note that the examples illustrated below are merely examples of the plated steel sheet according to the present invention, and the plated steel sheet according to the present invention is not limited to the examples illustrated below.
  • cold-rolled steel sheets a to e each manufactured by Nippon Steel Corporation
  • the chemical compositions of the cold-rolled steel sheets are as follows, with the balance of Fe and impurities.
  • a JIS13B test piece was sampled from an arbitrary position of the cold-rolled steel sheet based on JIS Z2201:1998, and its tensile strength was measured by a commercially available tensile tester.
  • the tensile strengths of the cold-rolled steel sheets a to e were 590 MPa (cold-rolled steel sheet a), 980 MPa (cold-rolled steel sheet b), 1180 MPa (cold-rolled steel sheet c), 1470 MPa (cold-rolled steel sheet d), and 2500 MPa (cold-rolled steel sheet e).
  • the above plating substrates were cut into a size of 100 mm ⁇ 200 mm, and then subjected to plating by a batch-type hot-dip plating test apparatus manufactured in-house, whereby a plurality of the plated steel sheets having the compositions of the plating layers in Table 1 were produced based on respective levels.
  • the sheet temperatures were measured using thermocouples spot-welded to the centers of the plating substrates.
  • the surfaces of the plating substrates were subjected to a heat-reduction treatment at 800°C in an N 2 -5% H 2 gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less before immersion in the plating baths. After the heat-reduction treatment, the plating substrates were air-cooled with an N 2 gas and immersed in the plating baths at the bath temperatures listed in Table 1 for about 3 seconds after the immersed sheet temperatures reached the bath temperatures + 20°C.
  • the steel sheets were pulled up at a speed of 20 to 200 mm/sec. At pulling up, the plating weights were controlled by an N 2 wiping gas. After pulling up the steel sheets from the plating baths, the steel sheets were cooled from the plating bath temperatures to room temperature under the conditions listed in Table 1.
  • each plated steel sheet after the strain was applied was observed under the SEM according to the method explained above, and the average area ratio of the ⁇ precipitated ⁇ phase and the total sum of the lengths of the cracks existing in the arbitrary region of 130 ⁇ m ⁇ 100 ⁇ m were calculated.
  • composition of the plating layer in each plated steel sheet was measured by immersing the sample cut into 30 mm ⁇ 30 mm in a 10% HCl aqueous solution with an inhibitor added to peel off the plating layer by pickling, and then performing ICP analysis of elements dissolved into the aqueous solution.
  • a sample having a size of 50 mm ⁇ 30 mm was taken from each plated steel sheet after the strain was applied.
  • the obtained sample was intentionally charged with hydrogen while the plating layer was still present thereon.
  • the hydrogen amount of the sample immediately after the hydrogen charge was measured by heating it from room temperature up to 250°C at a heating rate of 100 °C/h in an N 2 atmosphere and analyzing the amount of hydrogen released accompanying the temperature increase by gas chromatography (CHROMATOGRAPH G2800 manufactured by J-SCIENCE LAB CO., Ltd.).
  • the sample after the measurement was left to stand for 24 hours in a thermohygrostat at 25°C and RH of 30%, and then a diffusible hydrogen amount was measured by the same thermal desorption method.
  • the hydrogen desorption was evaluated based on a percentage (unit: %) obtained by dividing the diffusible hydrogen amount of the sample after left stand for 24 hours by the hydrogen amount of the sample immediately after the hydrogen charge.
  • the evaluation criteria are as follows, and Grade "A" or higher was determined to be acceptable.
  • a sample having a size of 50 mm ⁇ 100 mm was taken, and its post-painting corrosion resistance was evaluated. More specifically, the obtained sample was subjected to a Zn phosphoric acid treatment (SD5350 system: Nipponpaint Industrial Coatings Co., LTD. standard). Thereafter, the sample was subjected to electrodeposition coating (PN110 Power Nix Gray: Nipponpaint Industrial Coatings Co., LTD. standard) to have a thickness of 20 ⁇ m, and subjected to baking at a baking temperature of 150°C for 20 minutes. After the baking, a cut reaching the base steel was introduced at the center of the sample.
  • PN110 Power Nix Gray Nipponpaint Industrial Coatings Co., LTD. standard
  • the obtained paint film swelling width was evaluated based on the following evaluation criteria.
  • the evaluation criteria are as follows, and Grade "A" or higher was determined to be acceptable. Note that when the grade of the paint film swelling width is A or higher, the plated steel sheet to be focused on can be said to have excellent post-painting corrosion resistance.

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Abstract

To further improve the hydrogen desorption while maintaining the post-painting corrosion resistance.
A plated steel sheet according to the present invention includes on a surface of a steel sheet, a plating layer having a chemical composition containing, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, Fe: 0.01 to 15.00%, and selectively further containing one or two or more selected from the group consisting of an element group A, element group B, element group C, element group D, element group E, element group F, and element group G with a balance composed of Zn and impurities, wherein: in a surface structure in plan view of the plating layer, an average area ratio of an α precipitated η phase being a metal structure in which an α phase is precipitated in an η parent phase is 5 to 95%; and when a region of 130 µm × 100 µm of a surface of the plating layer is observed under an electron microscope, a total sum of lengths of cracks existing in the region is 50 µm or more.

Description

    TECHNICAL FIELD
  • The present invention relates to a plated steel sheet.
    • BACKGROUND ART
  • In recent years, the need for a high-strength steel sheet has increased from the viewpoint of reduction in weight in automobile and building material fields. The high-strength steel sheet is subjected to various processes such as press working, bending, and the like in order to realize a desired shape, and is required to have high corrosion resistance. Therefore, the high-strength steel sheet is required to have strength, workability, and corrosion resistance in combination. To realize the high corrosion resistance, various plated steel sheets using the high-strength steel sheet as a plating substrate are often used.
  • In the case of using the high-strength steel sheet as the plating substrate, it is known that so-called hydrogen embrittlement is likely to occur due to hydrogen inevitably entering the steel on a plating line. In particular, in a hot-dip Zn-Al-Mg-based plated steel sheet, the hydrogen embrittlement may occur even in the case of using a steel sheet having a relatively low strength such as 590 Ma class as the plating substrate, and therefore various countermeasures against the hydrogen embrittlement are conventionally proposed.
  • For example, Patent Document 1 below proposes a technique of applying strain on the hot-dip Zn-Al-Mg-based plated steel sheet to introduce cracks into the plating layer and then performing a baking treatment on the plated steel sheet to decrease the hydrogen concentration in the steel.
    • PRIOR ART DOCUMENT PATENT DOCUMENT
  • Patent Document 1: Japanese Patent Application No. 2018-204065
    • SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION
  • In the examination of the technique proposed in Patent Document 1 by the present inventors, it is important to strike a balance between the realization of the amount of introducing cracks for facilitating hydrogen desorption and the decrease in ductility of the steel sheet occurring due to the introduction of excessive cracks. The present inventors have found that it is necessary to control the amount of cracks to be introduced by paying attention to the application of strain when introducing cracks in Patent Document 1 in order to satisfy the tradeoff relationship and there is still room for improvement in terms of hydrogen desorption.
  • The present invention has been made in consideration of the above viewpoint, and an object of the present invention is to provide a plated steel sheet which can further improve the hydrogen desorption while maintaining post-painting corrosion resistance.
    • MEANS FOR SOLVING THE PROBLEMS
  • The present inventors have focused attention on that the Zn-Al-Mg-based plating disclosed in Patent Document 1 is a relatively hard plating structure. The present inventors then have obtained such an idea that if a structure which becomes a starting point for generating a crack can be introduced into the plating layer, a more suitable amount of cracks might be efficiently generated without paying excessive attention to the control of applying strain, leading to a possibility of further improving the hydrogen desorption.
  • The gist of the present invention completed based on that idea is as follows.
    1. (1) A plated steel sheet including on a surface of a steel sheet, a plating layer having a chemical composition containing, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, Fe: 0.01 to 15.00%, and selectively further containing one or two or more selected from the group consisting of following element group A, element group B, element group C, element group D, element group E, element group F, and element group G with a balance composed of Zn and impurities, wherein: in a surface structure in plan view of a surface of the plating layer, an average area ratio of an α precipitated η phase being a metal structure in which an α phase is precipitated in an η parent phase is 5 to 95%; and when a region of 130 µm × 100 µm of the surface of the plating layer is observed under an electron microscope, a total sum of lengths of cracks existing in the region is 50 µm or more.
      • [Element group A]: one or two selected from the group consisting of Si: more than 0% and 2.00% or less and Ca: more than 0% and 2.00% or less
      • [Element group B]: one or two or more selected from the group consisting of Sb: more than 0% and 0.5000% or less, Pb: more than 0% and 0.5000% or less, and Sr: more than 0% and 0.5000% or less
      • [Element group C]: one or two or more selected from the group consisting of Cu: more than 0% and 1.0000% or less, Ti: more than 0% and 1.0000% or less, Cr: more than 0% and 1.0000% or less, Nb: more than 0% and 1.0000% or less, Ni: more than 0% and 1.0000% or less, Mn: more than 0% and 1.0000% or less, Mo: more than 0% and 1.0000% or less, Co: more than 0% and 1.0000% or less, and V: more than 0% and 1.0000% or less
      • [Element group D]: one or two or more selected from the group consisting of Sn: more than 0% and 1.0000% or less, In: more than 0% and 1.0000% or less, and Bi: more than 0% and 1.0000% or less
      • [Element group E]: one or two or more selected from the group consisting of Zr: more than 0% and 1.0000% or less, Ag: more than 0% and 1.0000% or less, and Li: more than 0% and 1.0000% or less
      • [Element group F]: one or two or more selected from the group consisting of La: more than 0% and 0.5000% or less, Ce: more than 0% and 0.5000% or less, and Y: more than 0% and 0.5000% or less
      • [Element group G]: B: more than 0% and 0.5000% or less
    2. (2) The plated steel sheet according to (1) having a chemical composition containing the element group A.
    3. (3) The plated steel sheet according to (1) having a chemical composition containing the element group B.
    4. (4) The plated steel sheet according to (1) having a chemical composition containing the element group C.
    5. (5) The plated steel sheet according to (1) having a chemical composition containing the element group D.
    6. (6) The plated steel sheet according to (1) having a chemical composition containing the element group E.
    7. (7) The plated steel sheet according to (1) having a chemical composition containing the element group F.
    8. (8) The plated steel sheet according to (1) having a chemical composition containing the element group G.
    9. (9) The plated steel sheet according to any one of (1) to (8), wherein the plating layer contains 1.00 to 5.00 mass% of Al and 1.00 to 3.00 mass% of Mg.
    10. (10) The plated steel sheet according to (1), wherein a tensile strength of the steel sheet is 980 MPa or higher.
    11. (11) The plated steel sheet according to (1), wherein a tensile strength of the steel sheet is 1180 MPa or higher.
    12. (12) The plated steel sheet according to (1), wherein the average area ratio of the α precipitated η phase is 5 to 70%.
    13. (13) The plated steel sheet according to (1), wherein the average area ratio of the α precipitated η phase is 5 to 40%.
    EFFECT OF THE INVENTION
  • As explained above, according to the present invention, it is possible to further improve the hydrogen desorption while maintaining the post-painting corrosion resistance in a Zn-Al-Mg-based plated steel sheet.
    • BRIEF DESCRIPTION OF THE DRAWINGS
    • [FIG. 1A] FIG. 1A is an explanatory view schematically illustrating a configuration of a plated steel sheet according to an embodiment of the present invention.
    • [FIG. 1B] FIG. 1B is an explanatory view schematically illustrating the configuration of the plated steel sheet according to the embodiment.
    • [FIG. 2] FIG. 2 is a schematic view for explaining a plating layer of the plated steel sheet according to the embodiment.
    • [FIG. 3] FIG. 3 is a schematic view for explaining the plating layer of the plated steel sheet according to the embodiment.
    EMBODIMENTS FOR CARRYING OUT THE INVENTION
  • Preferred embodiments of the present invention will be explained in detail below with reference to the accompanying drawings. Note that, in the description and drawings, the same codes denote components having substantially the same functional configurations to omit duplicate explanations thereof.
    • (Regarding a plated steel sheet)
  • First, an overall configuration of a plated steel sheet according to an embodiment of the present invention will be explained with reference to FIG. 1A and FIG. 1B. FIG. 1A and FIG. 1B are explanatory views schematically illustrating an example of the plated steel sheet according to this embodiment.
  • As schematically illustrated in FIG. 1A, a plated steel sheet 1 according to this embodiment has a steel sheet 11 being a base material, and a plating layer 13 located on at least a part of a surface of the steel sheet 11. Further, the plating layer 13 may be present on both surfaces of the steel sheet 11 as schematically illustrated in FIG. 1B instead of being present only on one surface of the steel sheet 11 as illustrated in FIG. 1A.
    • <Regarding the steel sheet 11>
  • The dimension, component, structure, and mechanical property of the steel sheet 11 used as the base material of the plated steel sheet 1 according to this embodiment are not particularly limited. For example, various types of steel sheets can be used according to the mechanical strength (for example, tensile strength) or the like required for the plated steel sheet 1. Examples of the steel sheet 11 are steel materials standardized by Japanese Industrial Standards (JIS) or the like, and include carbon steel, alloy steel, high-strength steel, and the like to be used for general structure and for machine structure. Concrete examples of the steel materials include cold-rolled steel material, hot-rolled steel material, hot-rolled steel sheet material for automobile structure, hot-rolled high tension steel sheet material for automotive processing, cold-rolled steel sheet material for automobile structure, cold-rolled high tension steel sheet material for automotive processing, high tension steel material generally called hot stamp material subjected to hardening during hot working, and the like. The components of the steel materials are not particularly limited, but may contain one or two or more of Si, Mn, S, P, Al, N, Cr, Mo, Ni, Cu, Ca, Mg, Ce, Hf, La, Zr, and Sb, in addition to Fe and C. One or two or more of these arbitrary additive elements may be appropriately selected in order to obtain the required material strength and formability, and their contents may also be appropriately adjusted.
  • It is more preferable to use, for example, high-strength steel having a tensile strength of 980 MPa or higher (a so-called high-strength steel at a 980 MPa class or higher) among the above high-strength steels because the robustness of a product to be manufactured can be further improved. Here, the tensile strength of the steel sheet 11 can be measured by a publicly-known method. As an example, it is only necessary to produce a test piece prescribed in JIS Z 2241:2011 from a part of the steel sheet whose tensile strength is desired to be measured, and measure the tensile strength of the obtained test piece by a prescribed method.
  • Besides, the thickness of the steel sheet 11 is not particularly limited and appropriately set according to the mechanical strength or the like required for the plated steel sheet 1.
    • <Regarding the plating layer 13>
  • The plating layer 13 is provided on the surface of the steel sheet 11 as schematically illustrated in FIG. 1A and FIG. 1B, and is more preferably provided over the entire surface of the steel sheet 11. In the following, a chemical composition of the plating layer 13 will be first explained in detail.
    • ◊ Regarding the chemical composition of the plating layer 13
  • The chemical composition of the plating layer 13 according to this embodiment has, in an aspect, a chemical composition containing, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, and Fe: 0.01 to 15.00%, with the balance composed of Zn and impurities.
  • Further, the chemical composition of the plating layer 13 according to this embodiment has, in another aspect, a chemical composition containing, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, and Fe: 0.01 to 15.00%, and further containing one or two or more selected from the group consisting of the following element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the balance composed of Zn and impurities. In other words, in the chemical composition of the plating layer 13 according to this embodiment, the contents of Al, Mg, and Fe are within the above ranges and the total of the contents of these Al, Mg, Fe, and element group A to element group G is less than 100 mass%, with the balance composed of Zn and impurities.
    • [Element group A]: one or two selected from the group consisting of Si: more than 0% and 2.00% or less and Ca: more than 0% and 2.00% or less
    • [Element group B]: one or two or more selected from the group consisting of Sb: more than 0% and 0.5000% or less, Pb: more than 0% and 0.5000% or less, and Sr: more than 0% and 0.5000% or less
    • [Element group C]: one or two or more selected from the group consisting of Cu: more than 0% and 1.0000% or less, Ti: more than 0% and 1.0000% or less, Cr: more than 0% and 1.0000% or less, Nb: more than 0% and 1.0000% or less, Ni: more than 0% and 1.0000% or less, Mn: more than 0% and 1.0000% or less, Mo: more than 0% and 1.0000% or less, Co: more than 0% and 1.0000% or less, and V: more than 0% and 1.0000% or less
    • [Element group D]: one or two or more selected from the group consisting of Sn: more than 0% and 1.0000% or less, In: more than 0% and 1.0000% or less, and Bi: more than 0% and 1.0000% or less
    • [Element group E]: one or two or more selected from the group consisting of Zr: more than 0% and 1.0000% or less, Ag: more than 0% and 1.0000% or less, and Li: more than 0% and 1.0000% or less
    • [Element group F]: one or two or more selected from the group consisting of La: more than 0% and 0.5000% or less, Ce: more than 0% and 0.5000% or less, and Y: more than 0% and 0.5000% or less
    • [Element group G]: B: more than 0% and 0.5000% or less
  • As explained above, the plating layer 13 according to this embodiment is a plating layer having a chemical composition containing, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, and Fe: 0.01 to 15.00%, and selectively further containing one or two or more selected from the group consisting of the element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the balance composed Zn and impurities.
    • [Al: 0.50 to 5.00 mass%]
  • Al is an element necessary to constitute a main phase (Zn-Al-Mg-based alloy phase) of the plating layer 13 according to this embodiment, and is contained in a predetermined content or more to ensure corrosion resistance of a region to be a welding heat-affected portion and corrosion resistance of a region to be a non-welded portion after welding, as a plated steel sheet. If an Al content in the plating layer 13 is less than 0.50 mass%, the above corrosion resistance of the regions to be the welding heat-affected portion and the non-welded portion cannot be ensured. Therefore, in the plating layer 13 according to this embodiment, the Al content is 0.50 mass% or more. The Al content is preferably 1.00 mass% or more, and more preferably 1.50 mass% or more. The Al content falling within the above range can ensure the corrosion resistance of the plated steel sheet 1.
  • On the other hand, if the Al content in the plating layer 13 exceeds 5.00 mass%, a coagulation behavior from the liquid phase to the solid phase of the plating layer changes, thus making it difficult to crystallize an η phase, so that a dendrite structure mainly containing Al is formed. If the dendrite structure mainly containing Al is formed, the corrosion of the surroundings is promoted, so that the corrosion resistance of the plated steel sheet 1 cannot be ensured. In addition, the interface between the η phase being a crack originating source and a eutectic composition decreases because of a lack of the η phase, causing a lack of cracks generated during processing. Therefore, in the plating layer 13 according to this embodiment, the Al content is 5.00 mass% or less. The Al content is preferably 4.00 mass% or less, and more preferably 3.00 mass% or less.
    • [Mg: 0.50 to 3.00 mass%]
  • Mg is an element necessary to constitute the main phase (Zn-Al-Mg-based alloy phase) of the plating layer 13 according to this embodiment, and is contained in a predetermined content or more to ensure corrosion resistance of the region to be the welding heat-affected portion and corrosion resistance of the region to be the non-welded portion, as the plated steel sheet. Therefore, in the plating layer 13 according to this embodiment, an Mg content is 0.50 mass% or more. The Mg content is preferably 1.50 mass% or more, and more preferably 2.00 mass% or more. The Mg content falling within the above range makes it possible to ensure the corrosion resistance of the plated steel sheet 1. If the Mg content exceeds 3.00%, an α phase becomes difficult to precipitate from the η phase. Since the η phase in which Al is solid-dissolved is hard, the deformation cannot be concentrated on the η phase during processing, causing a decrease in the number of cracks. Therefore, the upper limit of the Mg content is 3.00%.
  • On the other hand, if the Mg content in the plating layer 13 is less than 0.50 mass%, the effect of improving the corrosion resistance by the modification of a corrosion product is insufficient, so that the corrosion resistance of the plated steel sheet 1 cannot be ensured. Further, when the Mg content is insufficient, the entire plating layer 13 is softened and the deformation cannot be concentrated on the η phase during processing, causing a decrease in the number of cracks. Therefore, in the plating layer 13 according to this embodiment, the Mg content is 0.50 mass% or more. The Mg content is preferably 1.50 mass% or more, and more preferably 2.00 mass% or more. The Mg content falling within the above range makes it possible to ensure the corrosion resistance of the plated steel sheet 1.
    • [Fe: 0.01 to 15.00 mass%]
  • Elements constituting the steel sheet 11, which is the base material, may be sometimes mixed into the plating layer 13. Especially, in a hot-dip plating method, the elements constituting the steel sheet 11 are easily mixed into the plating layer 13 due to interdiffusion of the elements by a solid-liquid reaction between the steel sheet 11 and the plating layer 13. Due to the mixing of the elements, a certain amount of Fe is contained in the plating layer 13, and its content is generally 0.01 mass% or more. When the above interdiffusion is promoted, adhesiveness between the steel sheet 11 and the plating layer 13 is improved. From the viewpoint of improving the adhesiveness between the steel sheet 11 and the plating layer 13, an Fe content in the plating layer 13 is preferably 0.20 mass% or more.
  • Further, Fe may be intentionally added to a plating bath to be used when manufacturing the plating layer 13 within a range that does not impair the effect of the present invention. However, when the Fe content in the plating layer 13 is 15.00 mass% or more, a high-melting point intermetallic compound of Fe and Al is formed in the plating bath, and the high-melting point intermetallic compound adheres, as dross, to the plating layer to significantly degrade an appearance quality, which is undesirable. From this viewpoint, the Fe content in the plating bath is adjusted, so that the Fe content in the plating layer 13 is 15.00 mass% or less. The Fe content in the plating layer 13 is more preferably 10.00 mass% or less.
  • In the plating layer 13, the balance of Al, Mg, and Fe is composed of Zn and impurities.
  • Zn is an element necessary to constitute the main phase (Zn-Al-Mg-based alloy phase) of the plating layer 13 according to this embodiment, and is an important element to improve the corrosion resistance of the plated steel sheet. Further, the plating layer 13 containing the above Al, Mg, and Fe in the above ranges and further containing Zn makes it possible to ensure the corrosion resistance as the plated steel sheet.
  • Next, the element group A to the element group E which the chemical composition of the plating layer 13 according to another aspect of this embodiment can have will be explained in detail.
  • Note that in the case where at least any of the following elements belonging to the element group B to element group E is contained in the plating layer 13 according to this embodiment, it is preferable that at least any of the following elements belonging to the element group B to element group E is contained in the following content range and a total content of 5.0000 mass% or less.
  • Setting the total content of the elements belonging to the element group B to the element group E to 5.0000 mass% or less makes it possible to enjoy the effects exhibited by the addition of the elements as explained in detail below without impairing effects each other. The total content of the elements belonging to the element group B to the element group E is preferably 1.0000 mass% or less, and more preferably 0.2000 mass% or less.
  • ◊ The element group A
  • The element group A which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment will be explained. At least any of the following elements in the element group A is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
    [Element group A]: one or two selected from the group consisting of Si: more than 0% and 2.00% or less and Ca: more than 0% and 2.00% or less
    • [Si: 0 to 2.00 mass%]
  • A case where the plating layer 13 according to this embodiment does not contain Si is conceivable, and therefore the lower limit of Si content is 0 mass%. On the other hand, Si is an element capable of suppressing excessive growth of a Fe-Al-based intermetallic compound phase that is formed at an interface between the plating layer and the steel sheet and further improving the adhesiveness between the plating layer and the steel sheet. In the case where Si is contained in the plating layer 13, an Si content is preferably 0.05 mass% or more and more preferably 0.10 mass% or more to suppress the excessive growth of the Fe-Al-based intermetallic compound phase.
  • On the other hand, if the Si content exceeds 2.00 mass%, Si excessively forms an intermetallic compound phase for a high melting point with Mg and possibly inhibits the formation of an Al-Mg oxide film having a Zn evaporation suppression effect, making it difficult to suppress Zn evaporation when the plated steel sheet is welded. Therefore, the Si content in the plating layer 13 is preferably 2.00 mass%. Further, if the Si content in the plating bath for manufacturing the plating layer 13 is too large, the viscosity of the plating bath may increase more than necessary to decrease the plating operability. Therefore, the Si content in the plating bath is adjusted from the viewpoint of the plating operability so that the Si content in the plating layer 13 is preferably 1.00 mass% or less and more preferably 0.50 mass% or less.
    • [Ca: 0 to 2.00 mass%]
  • A case where the plating layer 13 according to this embodiment does not contain Ca is conceivable, and therefore the lower limit of the Ca content is 0 mass%. On the other hand, when Ca is contained in the plating layer 13, it forms intermetallic compound phases with Al and Zn. Furthermore, when Si is contained together with Ca in the plating layer 13, Ca forms an intermetallic compound phase with Si. These intermetallic compound phases have high melting points and stable structures, and therefore can suppress liquid metal embrittlement (LME) cracking during welding of the plated steel sheet. In the case where Ca is contained in the plating layer 13, the effect of suppressing the LME during welding can be exhibited by setting the Ca content to 0.01 mass% or more. The Ca content in the plating layer 13 is more preferably 0.05 mass% or more.
  • On the other hand, if the Ca content in the plating layer 13 exceeds 2.00 mass%, the corrosion resistance of the plated steel sheet may decrease. From this viewpoint, the Ca content in the plating layer 13 is 2.00 mass% or less. The Ca content in the plating layer 13 is preferably 1.50 mass% or less, and more preferably 1.00 mass% or less.
    • ◊ The element group B
  • Next, the element group B which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment will be explained. At least any of the following elements in the element group B is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
    [Element group B]: one or two or more selected from the group consisting of Sb: more than 0% and 0.5000% or less, Pb: more than 0% and 0.5000% or less, and Sr: more than 0% and 0.5000% or less
    • [Sb: 0 to 0.5000 mass%]
    • [Pb: 0 to 0.5000 mass%]
    • [Sr: 0 to 0.5000 mass%]
  • A case where the plating layer 13 according to this embodiment does not contain Sb, Pb, and Sr is conceivable, and therefore the lower limits of the contents of these elements are 0 mass%. On the other hand, when at least any of Sb, Pb, and Sr is contained in the plating layer 13, spangles are formed on the surface of the plating layer 13, making it possible to improve metallic luster. Therefore, from the viewpoint of improving the design of the plated steel sheet, at least any of Sb, Pb, and Sr is preferably contained in the plating layer 13. The design improvement effect is exhibited when the content of at least any of Sb, Pb, and Sr is 0.0500 mass% or more. Therefore, when at least any of Sb, Pb, and Sr is contained in the plating layer 13, the contents of these elements are each independently preferably 0.0500 mass% or more.
  • On the other hand, when forming the plating layer 13 in which any of the contents of Sb, Pb, and Sr exceeds 0.5000 mass%, the amount of dross generated in the plating bath used to form the plating layer 13 increases, so that a plated steel sheet with good plating properties cannot be manufactured. Therefore, the contents of Sb, Pb, and Sr in the plating layer 13 are each independently 0.5000 mass% or less. The contents of Sb, Pb, and Sr are each independently preferably 0.2000 mass% or less.
    • ◊ The element group C
  • Next, the element group C which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment will be explained. At least any of the following elements in the element group C is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
    [Element group C]: one or two or more selected from the group consisting of Cu: more than 0% and 1.0000% or less, Ti: more than 0% and 1.0000% or less, Cr: more than 0% and 1.0000% or less, Nb: more than 0% and 1.0000% or less, Ni: more than 0% and 1.0000% or less, Mn: more than 0% and 1.0000% or less, Mo: more than 0% and 1.0000% or less, Co: more than 0% and 1.0000% or less, and V: more than 0% and 1.0000% or less
    • [Cu: 0 to 1.0000 mass%]
    • [Ti: 0 to 1.0000 mass%]
    • [Cr: 0 to 1.0000 mass%]
    • [Nb: 0 to 1.0000 mass%]
    • [Ni: 0 to 1.0000 mass%]
    • [Mn: 0 to 1.0000 mass%]
    • [Co: 0 to 1.0000 mass%]
    • [V: 0 to 1.0000 mass%]
  • A case where the plating layer 13 according to this embodiment does not contain Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is conceivable, and therefore the lower limits of the contents of these elements are 0 mass%. On the other hand, when at least any of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is contained in the plating layer 13, these elements are incorporated into an Al-Fe alloy phase generated by welding when the plated steel sheet is welded, so that the corrosion resistance of a welded portion to be formed can be improved. The effect of improving the corrosion resistance of the welded portion is exhibited when the content of at least any of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V in the plating layer 13 is 0.0050 mass% or more. Therefore, when at least any of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is contained in the plating layer 13, the contents of these elements are each independently preferably 0.0050 mass% or more.
  • On the other hand, when forming the plating layer 13 in which any of the contents of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V exceeds 1.0000 mass%, these elements form various intermetallic compound phases in the plating bath for forming the plating layer 13, thus causing an increase in viscosity of the plating bath to fail to manufacture a plated steel sheet with good plating properties. Therefore, the contents of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V in the plating layer 13 are each independently 1.0000 mass% or less. The contents of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V are each independently preferably 0.2000 mass% or less.
    • [Mo: 0 to 1.0000 mass%]
  • A case where the plating layer 13 according to this embodiment does not contain Mo is conceivable, and therefore the lower limit of an Mo content is 0 mass%. On the other hand, when Mo is contained in the plating layer 13, the corrosion resistance can be further improved. The effect of improving the corrosion resistance is exhibited when the Mo content is 0.0100 mass% or more. Therefore, when Mo is contained, its content is preferably 0.0100 mass% or more.
  • On the other hand, when forming the plating layer 13 in which the Mo content exceeds 1.0000 mass%, it undesirably causes a large amount of dross generated in the plating bath to be used. Therefore, the Mo content is 1.0000 mass% or less. The Mo content is preferably 0.0500 mass% or less.
    • ◊ The element group D
  • Next, the element group D which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment will be explained. The following elements in the element group D are elements which can be contained in the plating layer 13 in place of a part of the balance Zn.
    [Element group D]: one or two or more selected from the group consisting of Sn: more than 0% and 1.0000% or less, In: more than 0% and 1.0000% or less, and Bi: more than 0% and 1.0000% or less
    • [Sn: 0 to 1.0000 mass%]
    • [In: 0 to 1.0000 mass%]
    • [Bi: 0 to 1.0000 mass%]
  • A case where the plating layer 13 according to this embodiment does not contain Sn, In, and Bi is conceivable, and therefore the lower limit of Sn content is 0 mass%. On the other hand, Sn, In, and Bi are elements that increase an Mg dissolution rate when the plating layer 13 containing them is placed in a corrosive environment. When the Mg dissolution rate increases, Mg ions are supplied to portions where the steel sheet 11 is exposed, so that the corrosion resistance is improved. From this viewpoint, when Sn, In, and Bi are contained, the contents of Sn, In, and Bi are each independently preferably 0.0050 mass% or more. On the other hand, excessive addition of Sn, In, and Bi may excessively promote the Mg dissolution rate to decrease the corrosion resistance of the plated steel sheet. The increase in the Mg dissolution rate becomes prominent when any of the contents of Sn, In, and Bi exceeds 1.0000 mass%, and therefore the contents of Sn, In, and Bi are each independently 1.0000 mass% or less. The contents of Sn, In, and Bi are each independently preferably 0.2000 mass% or less.
    • ◊ The element group E
  • Next, the element group E which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment will be explained. At least any of the following elements in the element group E is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
    [Element group E]: one or two or more selected from the group consisting of Zr: more than 0% and 1.0000% or less, Ag: more than 0% and 1.0000% or less, and Li: more than 0% and 1.0000% or less
    • [Zr: 0 to 1.0000 mass%]
    • [Ag: 0 to 1.0000 mass%]
    • [Li: 0 to 1.0000 mass%]
  • A case where the plating layer 13 according to this embodiment does not contain Zr, Ag, and Li is conceivable, and therefore the lower limits of the contents of these elements are 0 mass%. On the other hand, when at least any of Zr, Ag, and Li is contained in the plating layer 13, the plating operability can be improved. The effect of improving the platability is exhibited when the content of at least any of Zr, Ag, and Li in the plating layer 13 is 0.0100 mass% or more. Therefore, when at least any of Zr, Ag, and Li is contained, the contents of these elements are each independently preferably 0.0100 mass% or more.
  • On the other hand, when forming the plating layer 13 in which any of the contents of Zr, Ag, and Li exceeds 1.0000 mass%, a large amount of dross is likely to be generated in the plating bath to be used for forming the plating layer 13. Therefore, each content of at least any of Zr, Ag, and Li is independently 1.0000 mass% or less. Each content of at least any of Zr, Ag, and Li is independently preferably 0.1000 mass% or less.
    • ◊ The element group F
  • Next, the element group F which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment will be explained. At least any of the following elements in the element group F is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
    [Element group F]: one or two or more selected from the group consisting of La: more than 0% and 0.5000% or less, Ce: more than 0% and 0.5000% or less, and Y: more than 0% and 0.5000% or less
    • [La: 0 to 0.5000 mass%]
    • [Ce: 0 to 0.5000 mass%]
    • [Y: 0 to 0.5000 mass%]
  • A case where the plating layer 13 according to this embodiment does not contain La, Ce, and Y is conceivable, and therefore the lower limits of the contents of these elements are 0 mass%. On the other hand, La, Ce, and Y are elements that exhibit almost the same effect as Ca and further suppress the blowhole formation during welding. This is due to the fact that an atomic radius of each element is close to the atomic radius of Ca. When these elements are contained in the plating layer 13, they are substituted at Ca positions. Therefore, these elements are detected at the same positions as Ca by EDS. Besides, also when these elements become oxides after the welding of the plated steel sheet, the oxides of these elements are detected at the same positions as CaO.
  • The effect of suppressing the blowhole formation during welding is exhibited when the contents of these elements are each independently 0.0100 mass% or more. Therefore, when at least any of Zr, Ag, and Li is contained, the contents of these elements are each independently preferably 0.0100 mass% or more. The contents of La, Ce, and Y in the plating layer 13 are each independently more preferably 0.0500 mass% or more.
  • On the other hand, if the La, Ce, and Y contents in the plating bath for manufacturing the plating layer 13 are too high, the viscosity of the plating bath may increase more than necessary to decrease the plating operability. Therefore, the La, Ce, and Y contents in the plating bath are adjusted from the viewpoint of the plating operability, so that the contents of La, Ce, and Y are each independently 0.5000 mass% or less. The contents of La, Ce, and Y are each independently preferably 0.1000 mass% or less.
    • ◊ The element group G
  • Next, the element group G which the plating layer 13 can contain in another aspect of the plating layer 13 according to this embodiment will be explained. The following element in the element group G is an element which can be contained in the plating layer 13 in place of a part of the balance Zn.
    [Element group G]: B: more than 0% and 0.5000% or less
    • [B: 0 to 0.5000 mass%]
  • A case where the plating layer 13 according to this embodiment does not contain B is conceivable, and therefore the lower limit of its content is 0 mass%. On the other hand, when B is contained in the plating layer 13, there is an effect of suppressing the LME. This is presumably because when B is contained in the plating layer 13, it combines with at least any of Zn, Al, Mg, and Ca to form various intermetallic compound phases. Further, the presence of B in the plating layer 13 is considered to produce the effect of suppressing the LME of the steel sheet 11 because B diffuses from the plating layer 13 into the steel sheet 11 to strengthen the grain boundary. Furthermore, it is presumed that since the various intermetallic compounds formed concerning B have extremely high melting points, they also act in suppressing the Zn evaporation during welding. These improvement effects are exhibited when 0.0500 mass% or more of B is contained. Therefore, when B is contained, the content of B is preferably 0.0500 mass% or more.
  • On the other hand, if B is excessively contained in the plating bath so that B is contained in the plating layer 13, a plating melting point rapidly increases to decrease the plating operability, failing to manufacture a plated steel sheet excellent in plating property. The decrease in plating operability becomes prominent when the content of B exceeds 0.50 mass%, so that the content of B is 0.5000 mass% or less. The content of B is preferably 0.1000 mass% or less.
    • [Measurement method of the chemical components]
  • The chemical components of the above plating layer 13 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry). Note that ICP-AES shall be used in the case of analyzing the chemical component up to 0.1 mass% unit, and ICP-MS shall be used in the case of analyzing the chemical component in a trace amount of less than 0.1 mass%. The plated steel sheet is immersed in a 10% HCl aqueous solution with an inhibitor added for about 1 minute to peel off the plating layer part, thereby preparing the solution in which the plating layer is dissolved. The obtained solution can be analyzed by ICP-AES or ICP-MS to obtain chemical components as an overall average of the plating layer.
    • ◊ Regarding the deposition amount of the plating layer 13
  • The deposition amount of the plating layer 13 as explained above is not particularly specified and is preferably, for example, about 15 to 250 g/m2 per one side of the steel sheet. When the deposition amount of the plating layer 13 falls within the above range, the plated steel sheet 1 according to this embodiment can exhibit sufficient corrosion resistance.
  • Note that the deposition amount of the plating layer 13 is measured as follows. First, a sample having a size of 30 mm × 30 mm in plan view is cut out from the plated steel sheet and the mass of the sample is measured in advance. Note that when cutting out the sample, it is cut out entirely in the thickness direction. A tape seal is bonded to one side of the sample to prevent the plating layer on the one side from dissolving in the next process. The sample is then immersed in a 10% HCl aqueous solution with an inhibitor added to peel off the plating layer by pickling, and the mass of the sample after pickling is measured. From a change in mass of the sample between before and after the pickling, the deposition amount of the plating layer 13 per side can be decided.
    • ◊ Regarding the metal structure of the plating layer 13
  • Next, the metal structure of the plating layer 13 having the chemical composition explained above will be explained.
  • The plating layer 13 according to this embodiment has the above chemical composition. Further, the plating layer 13 is formed through a manufacturing method as will be explained in detail below, and thereby contains metal phases such as an α phase, an η-Zn phase, a MgZn2 phase, an η/α/MgZn2 ternary eutectic phase, and an η/MgZn2 binary eutectic phase, and an intermetallic compound phase. Further, the plating layer 13 can contain intermetallic compound such as an Al-Si-Ca phase, an Al-Si-Ca-Fe phase, a Mg2Si phase, a Mg2Sn phase, and so on in addition to the above phases depending on elements which the plating layer 13 can further contain. The plating layer 13 according to this embodiment has the above metal structures and thereby exhibits the property that is excellent in corrosion resistance.
  • In addition, in the plating layer 13 according to this embodiment, part of the above α phase and η-Zn phase exist in a specific state as will be explained below in detail, and come to function as a starting point of crack generation. Thus, in the plating layer 13 according to this embodiment, it is possible to efficiently cause a more suitable amount of cracks without paying excessive attention to the control of applying strain, and to further improve the hydrogen desorption in the plated steel sheet 1.
  • Here, which phases the plating layer 13 according to this embodiment has can be identified by observing the surface of the plating layer 13 under a scanning electron microscope (SEM). In other words, it is possible to observe solidified structures in the surface of the plating layer 13 under the SEM and identify which phases the plating layer 13 has from a point analysis result by a SEM-EPMA (Electron Probe Micro Analyzer) and the form of a crystal phase in a backscattered electron image in an observation field of view. In this event, for example, a pre-treatment such as polishing does not need to be performed before the SEM observation. However, if a chemical treatment film or the like has been applied, polishing or the like may be performed to remove the chemical treatment film or the like. In the case of performing polishing, the polishing is performed such that 80% or more of the thickness of the plating layer remains, and the surface is brought into a mirror finish state, then observed under the SEM, and regarded as a surface structure.
  • Next, the specific state exhibited by some α phase and η-Zn phase in the plating layer 13 according to this embodiment will be explained in detail with reference to FIG. 2 and FIG. 3. FIG. 2 and FIG. 3 are schematic views for explaining an α precipitated η phase possessed by the plating layer 13 of the plated steel sheet 1 according to this embodiment.
  • Attention is focused on the case of observing a surface (X-Y plane in FIG. 2) of the plating layer 13 according to this embodiment from a surface normal direction (Z-axis positive direction side in FIG. 2) under the electron microscope (SEM).
  • In this observation, the surface structure of the plating layer 13 of the plated steel sheet 1 according to this embodiment has the α precipitated η phase (code 101 in FIG. 2) as will be explained below, and its average area ratio is 5 to 95%. Further, the remaining portion of the α precipitated η phase 101 is a hard structure 103 composed of the MgZn2 phase, the η/α/MgZn2 ternary eutectic phase, the η/MgZn2 binary eutectic phase, and the like. Note that the plating layer 13 according to this embodiment contains 0.50 mass% or more of Mg as the chemical composition of the plating, so that the above hard structure 103 is necessarily formed from the chemical composition.
  • The α precipitated η phase 101 is a metal structure in which Al supersaturated and solid-dissolved in a parent phase (hereinafter, referred to as an "η parent phase 113") composed of the η-Zn phase precipitates as an α phase 111 and softened as schematically illustrated in FIG. 3. When focusing attention on the backscattered electron image at the SEM observation, the α phase 111 exists as a black matter, and the η parent phase 113 exists as a white matter. Therefore, the α precipitated η phase 101 is recognized as a structure in which black particles are dispersed in a white phase at the SEM observation by the backscattered electron image.
  • The α precipitated η phase 101 exists in the hard structure 103 to cause nonuniformity in hardness as the plating layer 13. When the strain accompanying the processing or the like is applied to the plating layer 13 in a state where the relatively soft metal structure exists in the hard metal structure, the applied deformation concentrates on the α precipitated η phase 101 being the soft metal structure. As a result, in the plating layer 13 according to this embodiment, the crack 105 is generated starting from the α precipitated η phase 101 as schematically illustrated in FIG. 2.
  • As a result of the inspection by the present inventors, the position of an end portion of the crack 105 generated in the hard structure 103 often reached the interface between the plating layer 13 and the steel sheet 11 within a range of the above adhesion amount of the plating layer 13. In the plated steel sheet 1 according to this embodiment, hydrogen existing in the steel sheet 11 is released to the outside of the system (namely, outside air) via the crack 105. Further, even if the crack 105 does not reach the interface between the plating layer 13 and the steel sheet 11, hydrogen in the steal reached the position of the end portion of the crack 105 is then released to the outside of the system along the crack 105.
  • Further, in the surface structure of the plating layer 13 according to this embodiment, the average area ratio of the α precipitated η phase 101 falls in a range of 5 to 95% as explained above. The average area ratio of the α precipitated η phase 101 falling in the above range introduces a suitable abundance of cracks 105 into the plating layer 13, for example, even in a range of the strain to be applied when processing the plated steel sheet into various shapes. As a result, in the plated steel sheet 1 according to this embodiment, it is possible to further improve the hydrogen desorption while maintaining the corrosion resistance.
  • When the average area ratio is less than 5%, the abundance of the α precipitated η phase 101 is too small, failing to introduce a suitable amount of cracks 105 and to improve the hydrogen desorption as the plated steel sheet 1. When the average area ratio becomes 5% or more, it is possible to improve the hydrogen desorption as the plated steel sheet 1 while maintaining the corrosion resistance. The average area ratio of the α precipitated η phase 101 is preferably 8% or more, and more preferably 15% or more.
  • On the other hand, when the average area ratio of the α precipitated η phase 101 exceeds 95%, a soft α precipitated η phase undesirably occupies almost all the plating layer, so that the plating layer exhibits ductility as a whole and the α precipitated η phase comes not to function as a crack generation site. When the average area ratio of the α precipitated η phase 101 is 95% or less, the α precipitated η phase 101 is made to function as the crack generation site of the plating layer while maintaining the corrosion resistance, thereby making it possible to improve the hydrogen desorption as the plated steel sheet 1. The average area ratio of the α precipitated η phase 101 is preferably 70% or less, more preferably 40% or less, and furthermore preferably 30% or less.
    • ◊ The calculation method of the average area ratio of the α precipitated η phase 101 in the plating layer 13
  • Here, the above average area ratio is measured as follows.
  • Specifically, an arbitrary position on the surface of the plating layer 13 is observed under the SEM, and an element mapping is acquired using the SEM-EPMA. The obtained element mapping is binarized using a binarization function of a commercially available image analysis application, and a region corresponding to the α precipitated η phase 101 is identified and its area ratio is calculated.
  • More specifically, a region of 120 µm × 100 µm (corresponding to a magnification of about 1000 times) in plan view at the arbitrary position on the surface of the plating layer 13 is observed under the SEM, and a point analysis is executed by the SEM-EPMA.
  • Specifically, the above region of 120 µm × 100 µm is observed with an acceleration voltage: 15.0 kV, an irradiation current: 4.999 × 10-8 A, an irradiation time: 50 milliseconds, and a magnification of 1000 times. The backscattered electron image in the focused range is acquired under these conditions, and then the point analysis for each of the metal structures only needs to be performed for three points using the contrast in the backscattered electron image.
  • A phase satisfying Al: 20 to 99 atom%, Zn: 0.5 to 80 atom%, Mg: 0 to 5 atom%, and a total of Al and Zn of 70 atom% or more in this point analysis can be determined as the α phase. Further, a phase having a content of Zn of 98 atom% or more and a total of contents of other elements of 2 atom% or less is determined as the η-Zn phase.
  • Note that a phase having a content of each of Mg and Zn of 10 atom% or more and a total content of Mg and Zn of 85 atom% or more is determined as the hard structure 103 composed of the MgZn2 phase, the η/α/MgZn2 ternary eutectic phase, the η/MgZn2 binary eutectic phase, and the like.
  • Mg is not contained in the η-Zn phase, whereas the hard structure 103 containing Mg exists around the η-Zn phase. Therefore, a contour line representing a boundary between the metal structure containing Mg and the metal structure not containing Mg can be identified by focusing attention on the distribution of Mg elements. The thus-identified metal structure not containing Mg surrounded by the contour line can be determined as the η-Zn phase. Here, the η-Zn phase and the hard structure 103 therearound can be easily distinguished by visual recognition by those skilled in the art.
  • The contour line of η-Zn is identified as above in a field of view during the SEM observation, and the contour line is manually drawn on various image analysis applications (for example, ImageJ and the like). Thereafter, the image is binarized in the image analysis application, whereby whether or not a black precipitate corresponding to the α phase exists in the η-Zn phase can be easily determined. In this event, the threshold of binarization only needs to be set to 200 as a Minimum value, for example, in Brightness/Contrast.
  • Next, for the η-Zn phase identified as above, whether the α phase exists therein is determined. As explained above, during the observation using the backscattered electron image, the α phase is visually recognized as the black precipitate (namely, black particle), whereas the η-Zn phase is visually recognized as the white parent phase. Accordingly, the α precipitated η phase 101 to be focused on in this embodiment is observed as a phase in which the black particles are dispersed in the white parent phase as schematically illustrated in FIG. 3. Note that in FIG. 3, the contour line indicated with a broken line corresponds to the contour line of the above η-Zn phase.
  • Therefore, the phase in which the black particles are dispersed in the white parent phase is identified by the above observation using the backscattered electron image, and then the point analysis by the SEM-EPMA is executed. Attention is focused on one phase in which the black particles are dispersed in the white parent phase, and the point analysis by the SEM-EPMA is executed on an arbitrary size of 5 µm × 5 µm including both the black particles and the white parent phase as schematically illustrated in FIG. 3. In the result of the point analysis, a region in a range where the content of Zn is 90 atom% or more and the content of Al is 0.05 to 10.00 atom% can be determined as the α phase 111 in the η parent phase 113. In this event, the black phases existing around a portion determined as the α phase only need to be collectively determined as the α phase.
  • By the above method, the region of the α precipitated η phase 101 in the region of the region of 120 µm × 100 µm in plan view at the arbitrary position on the surface of the plating layer 13 can be identified. In addition, the area ratio of the identified region only needs to be calculated using various image analysis applications (for example, ImageJ and the like). Specifically, the area of the region corresponding to the α precipitated η phase 101 is calculated by the image analysis application, and then the obtained area of the α precipitated η phase 101 is divided by the area of the entire field of view, whereby the area ratio of the α precipitated η phase 101 in the field of view focused on can be obtained.
  • The above measurement and calculation processing is executed at arbitrary five points, and an average value of the five area ratios obtained is calculated. The thus-obtained average value is regarded as the average area ratio of the α precipitated η phase 101.
  • Note that, in the case of focusing attention on the plated steel sheet 1 which has been already processed into a product by welding processing, bonding processing, and the like, it is only necessary to focus attention on a flat portion not subjected to the processing and separated, for example, by 50 mm or more from each of the end portion of the product and the welding heat-affected portion or a bonding site, and execute the above observation for an arbitrary region of 120 µm × 100 µm. Further, in the case where various paint films exist on the surface of the plating layer 13, it is only necessary to remove the existing paint films using a paint film release agent.
    • ◊ The length of the crack 105 in the plating layer 13
  • Here, when focusing attention on a region separated, for example, by 50 mm or more from the end portion of the plated steel sheet 1 and observing a region having a size of 130 µm × 100 µm in plan view at an arbitrary position on the surface of plating layer 13 according to this embodiment under the electron microscope (more specifically, SEM), the total sum of the lengths of the cracks 105 existing in the above region is 50 µm or more.
  • When the total sum of the lengths of the cracks 105 existing in the above region becomes 50 µm or more, the plated steel sheet 1 according to this embodiment becomes to exhibit excellent hydrogen desorption. The total sum of the lengths of the cracks 105 existing in the above region is preferably 150 µm or more, and more preferably 500 µm or more.
  • On the other hand, a larger total sum of the lengths of the cracks 105 is better, and its upper limit value is not particularly specified and is substantially about 1000 µm. Note that even if the total sum of the lengths of the cracks 105 is large, Mg contained in the plating layer 13 exhibits corrosion protection, so that the corrosion resistance as the plated steel sheet 1 can be maintained.
  • Here, when measuring the total sum of the lengths of the cracks 105, it is only necessary to execute the observation under the SEM as in the observation method of the metal structure of the plating layer 13. At the observation, while focusing attention on an arbitrary region having a size of 130 µm × 100 µm, the total sum of the lengths of the cracks 105 existing in the region is measured using a length measurement application or the like belonging to the SEM. The above measurement processing is performed at arbitrary five points, and an average of the obtained five measured values is calculated. The thus-obtained average value is regarded as the total sum of the lengths of the cracks 105 in the region of 130 µm × 100 µm.
  • The plated steel sheet 1 according to this embodiment has been explained above in detail referring to FIG. 1A to FIG. 3. The plated steel sheet 1 according to this embodiment as explained above can be preferably used, for example, as a material for automobile undercarriage parts.
  • Note that the plated steel sheet 1 according to this embodiment may further have one or two or more layers of various types of films on the plating layer 13. Examples of the film include a chromate film, a phosphate film, a chromate-free film, an organic resin film, and so on.
    • (Regarding a manufacturing method for the plated steel sheet)
  • Next, an example of a manufacturing method for the plated steel sheet as explained above will be explained.
  • The plated steel sheet 1 according to this embodiment is manufactured by using the steel sheet 11 as explained above as a base material, forming the plating layer 13 on the surface of the steel sheet 11, and then applying strain to the steel sheet 11 on which the plating layer 13 is formed by various methods.
  • For the formation of the plating layer 13, a thermal spraying method, a cold spraying method, a sputtering method, a vapor deposition method, an electroplating method, and the like can be applied in addition to a hot-dip plating method. However, the hot-dip plating method is the most preferable in terms of cost in order to form a plating layer with a thickness generally used in automobiles and the like.
  • For the plated steel sheet obtained by the above method (steel sheet 11 having the plating layer 13), a specific heat treatment process as will be explained below is performed. This forms the α precipitated η phase 101 in the plating layer 13. Thereafter, strain is applied to the plated steel sheet subjected to the heat treatment process, by various methods to create the crack 105 starting from the α precipitated η phase 101, so that the plated steel sheet 1 according to this embodiment can be obtained.
  • Hereinafter, an example of the manufacturing method for obtaining the plated steel sheet 1 according to this embodiment using the hot-dip plating method will be explained in detail.
  • In the manufacturing process of the plated steel sheet 1, first, the steel sheet 11 to be used as the base material is rolled by a Sendzimir method into a desired thickness, and then coiled and installed on a hot-dip plating line.
  • On the hot-dip plating line, the steel sheet is continuously passed while being unrolled from the coil. Thereafter, the steel sheet is subjected to a heat reduction treatment by an annealing facility installed on the line at 800°C in an N2-5% H2 gas atmosphere under an environment with an oxygen concentration of 20 ppm or less where oxidation is less likely to occur, then air-cooled with an N2 gas down to about a bath temperature of the plating bath at a subsequent stage + 20°C, and then immersed in the plating bath.
  • Here, a plating alloy in a molten state having the chemical components as above is prepared in the plating bath in advance. The bath temperature of the plating bath is set to equal to or higher than the melting point of the plating alloy (for example, about 400 to 500°C).
  • When producing the material of the plating alloy, it is preferable to prepare it using pure metal (purity of 99% or more) as the alloy material. First, a predetermined amount of the alloy metal is mixed so as to obtain the composition of the plating layer as explained above, and completely melted into an alloy using a high-frequency induction furnace, arc furnace, or the like in vacuum or inert gas replacement state. Furthermore, the alloy mixed with the predetermined components (composition of the above plating layer) is melted in air, and the resulting melt is used as the plating bath.
  • Note that there is no particular restriction on using the pure metal in the production of the plating alloy as explained above, and existing Zn alloy, Mg alloy, and Al alloy may be melted and used. In this case, there is no problem as long as a predetermined composition alloy with less impurities is used.
  • The steel sheet is immersed in the plating bath as explained above, and then pulled up at a predetermined speed. In this event, a plating weight is controlled by, for example, an N2 wiping gas so that the plating layer 13 to be formed has a desired thickness. Here, for conditions other than the bath temperature, general plating operation conditions may be applied, and no special facilities or conditions are required.
  • Next, following first cooling process and second cooling process are performed on the plating alloy in a molten state located on the steel sheet to make the plating alloy in the molten state into the plating layer 13 and to grow the α precipitated η phase 101 in the plating layer 13. Hereinafter, the first cooling process and the second cooling process will be explained in detail.
  • The first cooling process is a cooling process performed when the temperature of the plating alloy is in a range of the bath temperature or lower and 240°C or higher. In the first cooling process, the plated steel sheet in the above temperature range is rapidly cooled at an average cooling rate of 15.0 °C/sec or more. Here, in the case where the average cooling rate is less than 15.0 °C/sec, the concentration of Al solid-dissolved in the η phase decreases and the driving force for the α phase precipitation in the η phase thereafter is insufficient, resulting in difficulty in forming the α precipitated η phase. Note that when the hot-dip plating method is employed in the plating process, the first cooling process is performed immediately after the steel sheet is pulled up from the plating bath. This causes Al to solid-dissolve in the η-Zn phase during coagulation.
  • Here, the average cooling rate is preferably 25.0 °C/sec or more. Further, the upper limit value of the average cooling rate is not particularly specified and, for example, about 90.0 °C/sec is a substantial upper limit.
  • Then, a second cooling process is performed when the temperature of the plating alloy (plating layer) falls within a range of lower than 240°C and 70°C or higher. In the second cooling process, the plated steel sheet falling within the above temperature range is gradually cooled at an average cooling rate of 1.0 °C/sec or less. Here, when the average cooling rate is more than 1.0 °C/sec, the time when the α phase precipitates from the η phase cannot be ensured, resulting in difficulty in forming the α precipitated η phase. Thus, Al solid-dissolved in the η-Zn phase precipitates as the α phase in the first cooling process to cause the η-Zn phase to soften and become the α precipitated η phase 101 and cause the hard structure 103 to exist around the α precipitated η phase 101. Here, the average cooling rate is preferably 0.5 °C/sec or less.
  • As explained above, the plating layer is subjected to a two-stage cooling process of the rapid cooling in the temperature range of the bath temperature or lower and 240°C or higher and the gradual cooling in the temperature range of lower than 240°C and 70°C or higher, whereby the α precipitated η phase 101 can be formed in the plating layer 13 at a desired average area ratio.
  • Note that the cooling state from 70°C to room temperature is not particularly specified, and the cooling down to room temperature may be performed by various methods.
  • Here, an interval from the end of the first cooling process to the start of the second cooling process is preferably within 3 seconds, and the second cooling process is preferably started immediately after the end of the first cooling process. If the interval from the end of the first cooling process to the start of the second cooling process exceeds 3 seconds, an unintended cooling process occurs to fail to generate the desired α precipitated η phase 101.
  • Note that when any one of the above first cooling process and second cooling process is not performed, the desired α precipitated η phase 101 cannot be realized. By performing both of the above first cooling process and second cooling process, the α precipitated η phase 101 can be generated in the plating layer 13 at a desired average area ratio and the soft structure can be appropriately introduced into the plating layer 13.
  • Further, when an alloying thermal treatment process (for example, a thermal treatment process involving heating to an ultimate sheet temperature of about 480 to 550°C) which is often performed generally in the manufacture of an alloyed hot-dip galvanized steel sheet is applied after the above second cooling process, the state of the α precipitated η phase 101 controlled by the first cooling process and the second cooling process is disrupted, resulting in a failure to obtain the hydrogen desorption as focused on in this embodiment. From this viewpoint, it is important not to perform the thermal treatment process after the second cooling process.
  • Here, in the above cooling treatment, generally known methods such as N2 gas cooling, mist cooling, water submergence, and the like can be applied. Further, gases high in heat removal effect such as a He gas and a hydrogen gas may be used, in addition to the N2 gas, as the cooling gas.
  • Note that as a substantial measurement method of the temperature of the plating layer, for example, a contact-type thermocouple (K-type) only needs to be used. By attaching the contact-type thermocouple to the steel sheet as the base material, the average temperature of the entire plating layer can be always monitored. Besides, by mechanically controlling various speeds and thicknesses and standardizing various operating conditions such as a preheating temperature of the steel sheet and the temperature of the plating bath, the temperature of the entire plating layer at that point in time under the manufacturing conditions can be monitored almost accurately. This makes it possible to precisely control the cooling treatments in the first cooling process and the second cooling process. Note that the surface temperature of the plating layer may also be measured by a non-contact radiation thermometer, although not as accurate as the contact type.
  • Further, a relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer may be found by a simulation that performs heat conduction analysis. Specifically, the surface temperature of the plating layer and the average temperature of the entire plating layer are found based on various manufacturing conditions such as the preheating temperature of the steel sheet, the temperature of the hot-dip plating bath, the pulling-up speed of the steel sheet from the plating bath, the sheet thickness of the steel sheet, the layer thickness of the plating layer, an amount of heat exchange between the plating layer and the manufacturing facility, and a heat release amount of the plating layer. Then, using the obtained results, the relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer may be found. This makes it possible to estimate the average temperature of the entire plating layer at that point in time under the manufacturing conditions by actually measuring the surface temperature of the plating layer during the manufacture of the plated steel sheet. As a result, the cooling treatments in the first cooling process and the second cooling process can be precisely controlled.
  • Next, strain is applied to the plated steel sheet obtained as above. This generates cracks in the plating layer 13 having the α precipitated η phase 101. Here, a method of applying the strain is not particularly limited, and bending and stretching deformation processing by a tension leveler, rolling processing by a skin pass roller, cold press processing when processing the plated steel sheet into a desired shape, or the like only needs to be performed.
  • Here, it is preferable, in the processing for applying strain, to apply strain of a total elongation percentage of 0.2% or more to the plated steel sheet. By applying the strain corresponding to the total elongation percentage, it is possible to set the total sum of the lengths of the cracks existing in an arbitrary region of 130 µm × 100 µm on the surface of the plating layer 13 when observed under the electron microscope, to 50 µm or more. Here, a total elongation percentage RTOTAL (unit: %) is a value decided by Expression (101) below. Note that in Expression (101) below, L0 is a length (unit: m) in a sheet-passing direction in an arbitrary sheet-passing direction section X of the plated steel sheet before subjected to the processing for applying strain, and L1 is a length in the sheet-passing direction of a portion derived from the arbitrary sheet-passing direction section X of the plated steel sheet after subjected to the processing for applying strain. Note that the total elongation percentage is more preferably 0.6% or more. R TOTAL % = L 1 − L 0 / L 0 × 100
  • Further, the upper limit value of the total elongation percentage RTOTAL is not particularly specified, and about 1.5% is substantially the upper limit.
  • Note that the processing for applying strain can be executed at arbitrary timing, and may be executed without a pause after the two-stage cooling process, or may be executed after a certain amount of time passes after the end of the two-stage cooling process. In the plated steel sheet according to this embodiment, the α precipitated η phase 101 is generated at the desired average area ratio in the plating layer 13 by being subjected to the above two-stage cooling process. Therefore, the crack 105 can be generated in the plating layer 13 by executing the processing for applying strain at arbitrary timing if only after the two-stage cooling process.
  • One example of the manufacturing method for the plated steel sheet according to this embodiment has been concretely explained above.
  • Note that in the manufacturing method for the plated steel sheet according to this embodiment, a treatment for further forming one layer or two or more layers of various types of films may be performed after the above second cooling process. Examples of the treatment include a chromate treatment, a phosphate treatment, a chromate-free treatment, an organic resin film formation treatment, and so on.
  • Examples of the chromate treatment include an electrolytic chromate treatment for forming a chromate film by electrolysis, a reactive chromate treatment for forming a film using a reaction with a material and then washing off excessive treatment solution, and a coating-type chromate treatment for forming a film by applying a treatment solution and drying it without water washing, and so on, and any of the chromate treatments may be employed.
  • Examples of the electrolytic chromate treatment include electrolytic chromate treatments using, for example, chromic acid, silica sol, resins (phosphoric acid resin, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine-modified epoxy resin, and the like), and hard silica.
  • Examples of the phosphate treatment include a zinc phosphate treatment, a calcium zinc phosphate treatment, a manganese phosphate treatment, and so on.
  • The chromate-free treatment is particularly preferable because it does not place a burden on the environment. Examples of the chromate-free treatment include an electrolytic chromate-free treatment for forming a chromate-free film by electrolysis, a reactive chromate-free treatment for forming a film using a reaction with a raw material and then washing off excessive treatment solution, a coating-type chromate-free treatment for forming a film by applying a treatment solution and drying it without water washing, and so on, and any of the chromate-free treatments may be employed.
  • Further, the organic resin to be used in the organic resin film formation treatment is not limited to a specific resin and, for example, various resins such as polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, and modified versions of these resins can be used. The modified versions here refer to resins in which reactive functional groups contained in structures of these resins are made to react with other compounds (for example, monomers, crosslinking agents, or the like) containing, in the structures, functional groups which can react with the functional groups.
  • As the organic resin, one of the above-mentioned organic resins may be used alone, or a mixture of two or more organic resins (not modified) may be used. Further, one or two or more organic resins obtained by modifying at least one other organic resin in the presence of at least one organic resin may be used by mixture. In addition, an organic resin that has been made aqueous by dissolving or dispersing it in water may be used. Furthermore, various kinds of color pigments and rust preventive pigments may be contained in the organic resin films.
    • EXAMPLES
  • Hereinafter, the plated steel sheet according to the present invention will be concretely explained while illustrating examples and comparative examples. Note that the examples illustrated below are merely examples of the plated steel sheet according to the present invention, and the plated steel sheet according to the present invention is not limited to the examples illustrated below.
  • For plating substrates, cold-rolled steel sheets a to e (each manufactured by Nippon Steel Corporation) each having a sheet thickness of 1.6 mm were used. The chemical compositions of the cold-rolled steel sheets are as follows, with the balance of Fe and impurities.
    • Cold-rolled steel sheet
    1. a: 0.04 mass% C-0.40 mass% Si-2.20 mass% Mn
    2. b: 0.09 mass% C-0.40 mass% Si-2.20 mass% Mn
    3. c: 0.20 mass% C-0.80 mass% Si-2.20 mass% Mn
    4. d: 0.25 mass% C-0.40 mass% Si-2.40 mass% Mn
    5. e: 0.35 mass% C-0.70 mass% Si-2.40 mass% Mn
  • For each of the cold-rolled steel sheets a to e, a JIS13B test piece was sampled from an arbitrary position of the cold-rolled steel sheet based on JIS Z2201:1998, and its tensile strength was measured by a commercially available tensile tester. As a result, the tensile strengths of the cold-rolled steel sheets a to e were 590 MPa (cold-rolled steel sheet a), 980 MPa (cold-rolled steel sheet b), 1180 MPa (cold-rolled steel sheet c), 1470 MPa (cold-rolled steel sheet d), and 2500 MPa (cold-rolled steel sheet e).
  • The above plating substrates were cut into a size of 100 mm × 200 mm, and then subjected to plating by a batch-type hot-dip plating test apparatus manufactured in-house, whereby a plurality of the plated steel sheets having the compositions of the plating layers in Table 1 were produced based on respective levels. The sheet temperatures were measured using thermocouples spot-welded to the centers of the plating substrates. The surfaces of the plating substrates were subjected to a heat-reduction treatment at 800°C in an N2-5% H2 gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less before immersion in the plating baths. After the heat-reduction treatment, the plating substrates were air-cooled with an N2 gas and immersed in the plating baths at the bath temperatures listed in Table 1 for about 3 seconds after the immersed sheet temperatures reached the bath temperatures + 20°C.
  • After the immersion in the plating baths, the steel sheets were pulled up at a speed of 20 to 200 mm/sec. At pulling up, the plating weights were controlled by an N2 wiping gas. After pulling up the steel sheets from the plating baths, the steel sheets were cooled from the plating bath temperatures to room temperature under the conditions listed in Table 1.
  • From each of the obtained plated steel sheets, a sample having a size of 200 mm × 80 mm was taken, and strain was applied to the plating layer by rolling the sample to have a total elongation percentage listed in Table 1 below using a skin pass roller.
  • The surface of each plated steel sheet after the strain was applied was observed under the SEM according to the method explained above, and the average area ratio of the α precipitated η phase and the total sum of the lengths of the cracks existing in the arbitrary region of 130 µm × 100 µm were calculated.
  • Note that the composition of the plating layer in each plated steel sheet was measured by immersing the sample cut into 30 mm × 30 mm in a 10% HCl aqueous solution with an inhibitor added to peel off the plating layer by pickling, and then performing ICP analysis of elements dissolved into the aqueous solution.
  • For the obtained plated steel sheet, the states of hydrogen desorption and paint film swelling were evaluated. Each evaluation method is as follows.
    • <Hydrogen desorption>
  • A sample having a size of 50 mm × 30 mm was taken from each plated steel sheet after the strain was applied. The obtained sample was intentionally charged with hydrogen while the plating layer was still present thereon. The hydrogen amount of the sample immediately after the hydrogen charge was measured by heating it from room temperature up to 250°C at a heating rate of 100 °C/h in an N2 atmosphere and analyzing the amount of hydrogen released accompanying the temperature increase by gas chromatography (CHROMATOGRAPH G2800 manufactured by J-SCIENCE LAB CO., Ltd.). The sample after the measurement was left to stand for 24 hours in a thermohygrostat at 25°C and RH of 30%, and then a diffusible hydrogen amount was measured by the same thermal desorption method.
    • [Hydrogen charge conditions]
    • Electrolytic solution: 3.0% NH4SCN + 33.0% NaCl aqueous solution
    • Current density: 1.0 mA/cm2
    • Charge time: 18 hours
    • Solution temperature: r.t. (about 25°C)
    [Thermal desorption method]
    • Measurement temperature: room temperature to 250°C
    • Heating rate: 100 °C/h
    • Measurement interval: 5 min
  • The hydrogen desorption was evaluated based on a percentage (unit: %) obtained by dividing the diffusible hydrogen amount of the sample after left stand for 24 hours by the hydrogen amount of the sample immediately after the hydrogen charge. The evaluation criteria are as follows, and Grade "A" or higher was determined to be acceptable.
    • [Evaluation Criteria]
  • Grade
    • AAA: less than 5%
    • AA: 5% or more and 20% or less
    • A: more than 20% and 50% or less
    • B: more than 50%
    <Post-painting corrosion resistance>
  • From each plated steel sheet, a sample having a size of 50 mm × 100 mm was taken, and its post-painting corrosion resistance was evaluated. More specifically, the obtained sample was subjected to a Zn phosphoric acid treatment (SD5350 system: Nipponpaint Industrial Coatings Co., LTD. standard). Thereafter, the sample was subjected to electrodeposition coating (PN110 Power Nix Gray: Nipponpaint Industrial Coatings Co., LTD. standard) to have a thickness of 20 µm, and subjected to baking at a baking temperature of 150°C for 20 minutes. After the baking, a cut reaching the base steel was introduced at the center of the sample. Thereafter, a combined cyclic corrosion test according to JASO (M609-91) was performed 180 cycles, and the paint film swelling width was measured. The obtained paint film swelling width was evaluated based on the following evaluation criteria. The evaluation criteria are as follows, and Grade "A" or higher was determined to be acceptable. Note that when the grade of the paint film swelling width is A or higher, the plated steel sheet to be focused on can be said to have excellent post-painting corrosion resistance.
    • [Evaluation Criteria]
  • Grade
    • AAA: paint film swelling width of 2 mm or less
    • AA: paint film swelling width of more than 2 mm and 3 mm or less
    • A: paint film swelling width of more than 3 mm and 4 mm or less
    • B: paint film swelling width of more than 4 mm
  • As is clear from Table 1 above, the plated steel sheets corresponding to examples of the present invention exhibited excellent hydrogen desorption, whereas the plated steel sheets corresponding to comparative examples of the present invention resulted to be inferior in hydrogen desorption.
  • Preferred embodiments of the present invention have been explained above in detail with reference to the attached drawings, but the present invention is not limited to the embodiments. It should be understood that various changes and modifications are readily apparent to those skilled in the art who have the common general knowledge in the technical field to which the present invention pertains, within the scope of the technical spirit as set forth in claims, and they should also be covered by the technical scope of the present invention.
  • The embodiments disclosed herein are examples in all respects and should not be considered to be restrictive. Various omissions, substitutions, and changes may be made in the embodiments without departing from the scope of the attached claims, and the configuration and the spirit belonging to the technical field of the present invention as will be described. For example, configuration requirements of the above embodiments can be arbitrarily combined as long as the effects thereof are not impaired. From an arbitrary combination, the operations and effects about the configuration requirements relating to the combination can be obtained as a matter of course, and other operations and other effects apparent to those skilled in the art are obtained from the description herein.
  • Besides, the effects explained herein are merely explanatory or illustrative in all respects and not restrictive. In other words, the technique relating to the present invention can offer other effects apparent to those skilled in the art from the description herein in addition to or in place of the above effects.
  • Note that the following configurations also belong to the technical scope of the present invention.
    1. (1) A plated steel sheet including
      • on a surface of a steel sheet, a plating layer having a chemical composition containing, by mass%,
      • Al: 0.50 to 5.00%,
      • Mg: 0.50 to 3.00%,
      • Fe: 0.01 to 15.00%, and
      • selectively further containing one or two or more selected from the group consisting of following element group A, element group B, element group C, element group D, element group E, element group F, and element group G with a balance composed of Zn and impurities, wherein:
        • in a surface structure in plan view of a surface of the plating layer, an average area ratio of an α precipitated η phase being a metal structure in which an α phase is precipitated in an η parent phase is 5 to 95%; and
        • when a region of 130 µm × 100 µm of the surface of the plating layer is observed under an electron microscope, a total sum of lengths of cracks existing in the region is 50 µm or more.
          • [Element group A]: one or two selected from the group consisting of Si: more than 0% and 2.00% or less and Ca: more than 0% and 2.00% or less
          • [Element group B]: one or two or more selected from the group consisting of Sb: more than 0% and 0.5000% or less, Pb: more than 0% and 0.5000% or less, and Sr: more than 0% and 0.5000% or less
          • [Element group C]: one or two or more selected from the group consisting of Cu: more than 0% and 1.0000% or less, Ti: more than 0% and 1.0000% or less, Cr: more than 0% and 1.0000% or less, Nb: more than 0% and 1.0000% or less, Ni: more than 0% and 1.0000% or less, Mn: more than 0% and 1.0000% or less, Mo: more than 0% and 1.0000% or less, Co: more than 0% and 1.0000% or less, and V: more than 0% and 1.0000% or less
          • [Element group D]: one or two or more selected from the group consisting of Sn: more than 0% and 1.0000% or less, In: more than 0% and 1.0000% or less, and Bi: more than 0% and 1.0000% or less
          • [Element group E]: one or two or more selected from the group consisting of Zr: more than 0% and 1.0000% or less, Ag: more than 0% and 1.0000% or less, and Li: more than 0% and 1.0000% or less
          • [Element group F]: one or two or more selected from the group consisting of La: more than 0% and 0.5000% or less, Ce: more than 0% and 0.5000% or less, and Y: more than 0% and 0.5000% or less
          • [Element group G]: B: more than 0% and 0.5000% or less
    2. (2) The plated steel sheet according to (1) having a chemical composition containing the element group A.
    3. (3) The plated steel sheet according to (1) having a chemical composition containing the element group B.
    4. (4) The plated steel sheet according to (1) having a chemical composition containing the element group C.
    5. (5) The plated steel sheet according to (1) having a chemical composition containing the element group D.
    6. (6) The plated steel sheet according to (1) having a chemical composition containing the element group E.
    7. (7) The plated steel sheet according to (1) having a chemical composition containing the element group F.
    8. (8) The plated steel sheet according to (1) having a chemical composition containing the element group G.
    9. (9) The plated steel sheet according to any one of (1) to (8), wherein
      the plating layer contains 1.00 to 5.00 mass% of Al and 1.00 to 3.00 mass% of Mg.
    10. (10) The plated steel sheet according to any one of (1) to (9), wherein
      a tensile strength of the steel sheet is 980 MPa or higher.
    11. (11) The plated steel sheet according to any one of (1) to (10), wherein
      a tensile strength of the steel sheet is 1180 MPa or higher.
    12. (12) The plated steel sheet according to any one of (1) to (11), wherein
      the average area ratio of the α precipitated η phase is 5 to 70%.
    13. (13) The plated steel sheet according to any one of (1) to (12), wherein
      the average area ratio of the α precipitated η phase is 5 to 40%.
    EXPLANATION OF CODES
    • 1 plated steel sheet
    • 11 steel sheet
    • 13 plating layer
    • 101 α precipitated η phase
    • 103 hard structure
    • 105 crack
    • 111 α phase
    • 113 η parent phase

Claims (13)

  1. A plated steel sheet comprising
    on a surface of a steel sheet, a plating layer having a chemical composition containing, by mass%,
    Al: 0.50 to 5.00%,
    Mg: 0.50 to 3.00%,
    Fe: 0.01 to 15.00%, and
    selectively further containing one or two or more selected from the group consisting of following element group A, element group B, element group C, element group D, element group E, element group F, and element group G with a balance composed of Zn and impurities, wherein:
    in a surface structure in plan view of a surface of the plating layer, an average area ratio of an α precipitated η phase being a metal structure in which an α phase is precipitated in an η parent phase is 5 to 95%; and
    when a region of 130 µm × 100 µm of the surface of the plating layer is observed under an electron microscope, a total sum of lengths of cracks existing in the region is 50 µm or more.
    [Element group A]: one or two selected from the group consisting of Si: more than 0% and 2.00% or less and Ca: more than 0% and 2.00% or less
    [Element group B]: one or two or more selected from the group consisting of Sb: more than 0% and 0.5000% or less, Pb: more than 0% and 0.5000% or less, and Sr: more than 0% and 0.5000% or less
    [Element group C]: one or two or more selected from the group consisting of Cu: more than 0% and 1.0000% or less, Ti: more than 0% and 1.0000% or less, Cr: more than 0% and 1.0000% or less, Nb: more than 0% and 1.0000% or less, Ni: more than 0% and 1.0000% or less, Mn: more than 0% and 1.0000% or less, Mo: more than 0% and 1.0000% or less, Co: more than 0% and 1.0000% or less, and V: more than 0% and 1.0000% or less
    [Element group D]: one or two or more selected from the group consisting of Sn: more than 0% and 1.0000% or less, In: more than 0% and 1.0000% or less, and Bi: more than 0% and 1.0000% or less
    [Element group E]: one or two or more selected from the group consisting of Zr: more than 0% and 1.0000% or less, Ag: more than 0% and 1.0000% or less, and Li: more than 0% and 1.0000% or less
    [Element group F]: one or two or more selected from the group consisting of La: more than 0% and 0.5000% or less, Ce: more than 0% and 0.5000% or less, and Y: more than 0% and 0.5000% or less
    [Element group G]: B: more than 0% and 0.5000% or less
  2. The plated steel sheet according to claim 1 having a chemical composition containing the element group A.
  3. The plated steel sheet according to claim 1 having a chemical composition containing the element group B.
  4. The plated steel sheet according to claim 1 having a chemical composition containing the element group C.
  5. The plated steel sheet according to claim 1 having a chemical composition containing the element group D.
  6. The plated steel sheet according to claim 1 having a chemical composition containing the element group E.
  7. The plated steel sheet according to claim 1 having a chemical composition containing the element group F.
  8. The plated steel sheet according to claim 1 having a chemical composition containing the element group G.
  9. The plated steel sheet according to any one of claims 1 to 8, wherein the plating layer contains 1.00 to 5.00 mass% of Al and 1.00 to 3.00 mass% of Mg.
  10. The plated steel sheet according to claim 1, wherein
    a tensile strength of the steel sheet is 980 MPa or higher.
  11. The plated steel sheet according to claim 1, wherein
    a tensile strength of the steel sheet is 1180 MPa or higher.
  12. The plated steel sheet according to claim 1, wherein
    the average area ratio of the α precipitated η phase is 5 to 70%.
  13. The plated steel sheet according to claim 1, wherein
    the average area ratio of the α precipitated η phase is 5 to 40%.
EP24753459.7A 2023-02-09 2024-02-09 CLAD STEEL SHEET Pending EP4640911A4 (en)

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