WO2018169084A1 - めっき鋼板 - Google Patents
めっき鋼板 Download PDFInfo
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- WO2018169084A1 WO2018169084A1 PCT/JP2018/010627 JP2018010627W WO2018169084A1 WO 2018169084 A1 WO2018169084 A1 WO 2018169084A1 JP 2018010627 W JP2018010627 W JP 2018010627W WO 2018169084 A1 WO2018169084 A1 WO 2018169084A1
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
- B32B15/012—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of aluminium or an aluminium alloy
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
- B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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- 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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C21/00—Alloys based on aluminium
- C22C21/10—Alloys based on aluminium with zinc as the next major constituent
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—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
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—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
- C23C2/12—Aluminium or alloys based thereon
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/34—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
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
- C23C28/02—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
- C23C28/02—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material
- C23C28/021—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material including at least one metal alloy layer
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
- C23C28/02—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material
- C23C28/023—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material only coatings of metal elements only
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
- C23C28/02—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material
- C23C28/023—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material only coatings of metal elements only
- C23C28/025—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material only coatings of metal elements only with at least one zinc-based layer
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12785—Group IIB metal-base component
- Y10T428/12792—Zn-base component
- Y10T428/12799—Next to Fe-base component [e.g., galvanized]
Definitions
- This disclosure relates to a plated steel sheet.
- Alloyed hot-dip galvanized steel sheet is plated with improved weldability and post-coating corrosion resistance by applying hot-dip alloying to the steel sheet and then diffusing Fe from the steel sheet (underlying steel sheet) in the plating layer. It is a steel plate.
- the plated steel sheet shown in Patent Document 1 is typically used as a plated steel sheet for automobiles in Japan.
- a plated steel sheet for automobiles is used in a state of being formed into a complicated shape from a plate shape, and is often used for press forming.
- the plated layer becomes hard due to diffusion of Fe from the base steel sheet. For this reason, the plating layer is easily peeled off, and there are also peculiar problems such as powdering or flaking that are not found in a hot dip galvanized steel sheet having a soft plating layer.
- a plated steel sheet provided with a hard plated layer is easily damaged by external pressure, and once a crack is generated, the crack propagates to the interface between the plated layer and the ground iron (steel sheet). Therefore, it is regarded as a problem that the plating layer is peeled off starting from the interface with the ground iron (steel plate) and is likely to drop off.
- the coating and the plating layer are peeled off at the same time due to the collision (chipping) of pebbles caused by the hopping of the traveling vehicle, and the ground iron (steel sheet) is exposed Prone.
- the galvannealed steel sheet contains Fe in the plating layer. Therefore, when such chipping occurs, reddish brown rust is readily generated due to corrosion of the plating layer, causing a problem on the appearance of the automobile.
- a galvanized steel sheet for automobiles that does not contain Fe in the plating layer
- a galvanized steel sheet is mainly used in North America, Europe, and the like.
- hot dip galvanized steel sheet that has not been alloyed is less likely to cause chipping.
- the plating layer does not contain Fe, red rust at the initial stage of corrosion hardly occurs.
- the plating layer is easily corroded under the coating film, and the coating film is lifted (swells). Therefore, the hot dip galvanized steel sheet is by no means suitable as a plated steel sheet for automobiles.
- a method for increasing the corrosion resistance of hot-dip Zn-based plating a method of containing Al in the Zn plating layer can be mentioned.
- a molten Al—Zn-based plated steel sheet has been widely put into practical use as a highly corrosion-resistant plated steel sheet.
- the plated layer of such a molten Al—Zn-based steel sheet is composed of a dendritic ⁇ -Al phase (dendritic structure) crystallized first from the molten state, and a Zn phase and an Al phase formed in the gap between the dendritic structures. It is formed from a structure (interdendrite-like structure).
- the dendrite-like structure is passivated, and the interdendrite-like structure has a higher Zn concentration than the dendrite-like structure. Therefore, corrosion concentrates on the interdendrite structure. As a result, the corrosion progresses in an worm-like manner in the interdendritic structure, and the corrosion path is complicated. Therefore, it becomes difficult for corrosion to easily reach the ground iron (steel plate). As a result, when the hot-dip Al—Zn-based plated steel sheet is used as a bare material that has not been coated, it has superior corrosion resistance compared to a hot-dip galvanized steel sheet having the same plating layer thickness.
- the plated steel sheet When such a molten Al—Zn-based plated steel sheet is used as an automobile outer panel, the plated steel sheet is supplied to an automobile manufacturer or the like in a state where it has been subjected to plating in a continuous hot-dip plating facility. Therefore, the plated steel sheet is generally processed into a panel part shape and then subjected to a general automobile coating including chemical conversion treatment, electrodeposition coating, intermediate coating and top coating.
- Patent Document 3 discloses a hot-dip Zn—Al—Mg-based plated steel sheet in which a Zn / Al / MgZn 2 ternary eutectic structure containing an Mg compound such as MgZn 2 is formed in a plating layer to improve corrosion resistance. It is disclosed. By containing Mg, the sacrificial anticorrosive property of the plating layer is improved, and the anticorrosive effect of the base iron is considered to be improved.
- Patent Document 4 discloses a molten Al—Zn-plated steel sheet containing Sn or In and having improved post-coating corrosion resistance by destroying the passivity of the dendritic structure.
- Patent Documents 5 and 6 describe a molten Al-Zn alloy-plated steel sheet containing Mg and Sn in a composite manner. Patent Documents 5 and 6 describe that a molten Al—Zn alloy-plated steel sheet is excellent in post-coating corrosion resistance and / or workability.
- Patent Document 1 Japanese Patent Application Laid-Open No. 2003-253416
- Patent Document 2 Japanese Patent Publication No. Sho 46-7161
- Patent Document 3 Japanese Patent Application Laid-Open No. 2001-329383
- Patent Document 4 Japanese International Publication No. 2014 / Patent Document No. 155944
- Patent Document 5 Japanese Patent Application Laid-Open No. 2015-214747
- Patent Document 6 Japanese Patent Application Laid-Open No. 2002-180225
- the outer panel using the molten Al—Zn-plated steel sheet described in Patent Document 2 is incompatible with the dendrite-like structure described above when the coating film and the plating layer are damaged (when the base iron is exposed). Due to the unique phase structure of the plating layer consisting of two structures, the dendrite structure, the preferential dissolution of Zn (selective corrosion of the interdendrite structure) occurs at the interface between the coating film and the plating layer. appear. This progresses deeper into the paint-sound part and causes large swelling of the paint film. As a result, it is known that there is a problem that sufficient corrosion resistance (corrosion resistance after painting) cannot be ensured. Furthermore, the interdendrite-like structure has a lower hardness than the dendrite-like structure.
- the MgZn 2 phase contained in the plated layer of the hot-dip Zn—Al—Mg based steel sheet described in Patent Document 3 is brittle. Therefore, when subjected to this plated steel sheet to the processing, there is a risk that results in a number of cracks starting from the Zn / Al / MgZn 2 ternary eutectic structure. When the crack occurs, the ground iron is exposed, so there is still a problem that the corrosion resistance after painting of the processed part cannot be secured.
- the molten Al—Zn-based plated steel sheet described in Patent Document 4 does not contain Mg in the plating layer, and no attempt is made to reduce the corrosion rate of the plating layer itself. Therefore, it is considered that the corrosion resistance after painting is not satisfactory as a plated steel sheet for automobiles.
- the problem to be solved by one aspect of the present disclosure is to provide a plated steel sheet having excellent post-painting corrosion resistance and workability.
- the means for solving the above problems include the following aspects.
- the plating layer is mass%, Al: 15% -60% Mg: 0.5% to 8.0% Sn: 0.5% to 20.0% Si: 0.05% to 1.50% Bi: 0% to 5.0%
- Ca 0% to 2.0%
- Y 0% to 0.5%
- La 0% to 0.5% Ce: 0% to 0.5%
- Cr 0% to 0.25%
- Nb 0% to 0.25%
- Cu 0% to 0.25%
- Sr 0% to 0.5%
- Sb 0% to 0.5%
- Pb 0% to 0.5%
- B 0% to 0.5%
- the balance has a chemical composition consisting of Zn and impurities
- the plating layer has a granular Mg 2
- ⁇ 5> The plated steel sheet according to any one of ⁇ 1> to ⁇ 4>, wherein the area fraction of the granular Mg 2 Sn phase-containing structure is 20% to 60%.
- ⁇ 6> The plated steel sheet according to any one of ⁇ 1> to ⁇ 5>, wherein the area fraction of the granular Mg 2 Sn phase-containing structure is 30% to 60%.
- ⁇ 7> The plated steel sheet according to any one of ⁇ 1> to ⁇ 6>, wherein the area fraction of the structure containing the solid solution of Zn and Al is 35% to 95%.
- ⁇ 8> The plated steel sheet according to any one of ⁇ 1> to ⁇ 7>, wherein the plating layer has a block MgZn 2 phase having an equivalent circular diameter of 1 ⁇ m or more in an area fraction of 0% to 20%.
- ⁇ 9> The plated steel sheet according to any one of ⁇ 1> to ⁇ 8>, wherein the plated layer has a block MgZn 2 phase having an equivalent circular diameter of 1 ⁇ m or more in an area fraction of 0% to 5%.
- ⁇ 10> The plated steel sheet according to any one of ⁇ 1> to ⁇ 9>, wherein the plated layer has a massive Zn phase having an equivalent circular diameter of 2 ⁇ m or more in an area fraction of 0% to 20%.
- ⁇ 11> The plated steel sheet according to any one of ⁇ 1> to ⁇ 10>, wherein the plated layer has a bulk Zn phase having an equivalent circular diameter of 2 ⁇ m or more in an area fraction of 0% to 10%.
- ⁇ 12> The plating according to any one of ⁇ 1> to ⁇ 11>, further including an interface alloy layer made of an Al—Fe-based intermetallic compound having a thickness of 100 nm to 1.5 ⁇ m between the steel plate and the plating layer. steel sheet.
- a plated steel sheet having excellent post-painting corrosion resistance and workability can be provided.
- BSE image reflection electron image
- region A in FIG. No. of an Example. 3 is a reflected electron image (BSE image) of an SEM obtained by photographing the plated layer of the plated steel sheet according to 31 at a magnification of 2000 times.
- No. of an Example. 32 is a SEM reflected electron image (BSE image) obtained by photographing a plated layer of the plated steel sheet according to No. 32 at a magnification of 2000 times.
- % display of content of each element of chemical composition means “mass%”.
- a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
- a numerical range in the case where “over” or “less than” is added to the numerical values described before and after “to” means a range not including these numerical values as a lower limit value or an upper limit value.
- the element content of the chemical composition may be expressed as an element amount (for example, Zn amount, Mg amount, etc.) or an element concentration (for example, Zn concentration, Mg concentration, etc.).
- the “corrosion resistance after painting” indicates a property that the plating layer itself is hardly corroded.
- “Sacrificial anticorrosion” refers to a property that suppresses corrosion of exposed parts of the iron base (for example, the cut end face part of the plated steel sheet, the cracked part of the plating layer during processing, and the part where the iron part is exposed due to peeling of the plating layer).
- “Equivalent circle diameter” means that when the cross section of the plating layer (cross section cut along the thickness direction of the plating layer) is observed, the region defined by the contour of the phase is regarded as a circle having the same area. The diameter of the circle.
- “C direction” indicates a direction perpendicular to the rolling direction of the steel sheet.
- the “L direction” indicates a direction parallel to the rolling direction of the steel sheet.
- the plated steel sheet of the present disclosure includes a steel sheet and a plating layer provided on at least a part of the surface of the steel sheet.
- the plating layer has a predetermined chemical composition.
- the plating layer has a granular Mg 2 Sn phase-containing structure having an area fraction of 5 to 65% and a structure containing a solid solution of Zn and Al (hereinafter also referred to as “dendritic structure” for convenience).
- the granular Mg 2 Sn phase-containing structure is a structure composed of a Zn phase and a granular Mg 2 Sn phase having a crystal grain size of less than 1 ⁇ m dispersed in the Zn phase.
- the plated steel sheet of the present disclosure is a plated steel sheet having excellent post-coating corrosion resistance and workability due to the above configuration.
- the plated steel sheet of this indication was discovered based on the following knowledge.
- the inventors examined post-coating corrosion resistance and workability suitable for plated steel sheets for automobile use, building material use, and the like. As a result, the following knowledge was obtained.
- the Mg-based intermetallic compound is a brittle phase
- the Mg 2 Sn phase has plastic deformability compared to the MgZn 2 phase.
- the Mg 2 Sn phase serves as a supply source of Mg ions in a corrosive environment, and Mg ions cause corrosion products to form an insulating film, so that under-coating corrosion in a painted state is suppressed.
- the presence of a predetermined amount of a granular Mg 2 Sn phase-containing structure composed of a Zn phase and a granular Mg 2 Sn phase dispersed in the Zn phase and having a crystal grain size of less than 1 ⁇ m in an area fraction allows for post-coating corrosion resistance. And processability are both increased. Specifically, when the area fraction of the granular Mg 2 Sn phase-containing structure is 5% or more, it becomes higher in workability and post-coating corrosion resistance than a commercially available plated steel sheet.
- the plated steel sheet of the present disclosure was found to be a plated steel sheet having excellent post-painting corrosion resistance and workability.
- the plated steel sheet of this indication has the granular layer dispersion structure which expresses plastic deformation ability in a plating layer, it is excellent also in chipping resistance, and it can also realize the life extension of the plated steel sheet after painting.
- the plated steel sheet of the present disclosure includes a predetermined amount of Al in the plating layer and has a dendrite-like structure that increases the melting point of the plating layer. Therefore, it has excellent seizure resistance and can suppress adhesion of the plating layer to the press mold during press molding. That is, the plated steel sheet of the present disclosure can achieve both excellent post-coating corrosion resistance and press formability.
- a plated steel sheet excellent in corrosion resistance and workability after coating of the plating layer is suitable for automotive applications.
- steel plate There are no particular limitations on the steel plate used as the plating base plate, and various steel plates such as Al killed steel, extremely low carbon steel, high carbon steel, various high strength steels, Ni and Cr-containing steels can be used. There is no particular limitation on the pretreatment processing of the steel plate such as steel making method, steel strength, hot rolling method, pickling method, cold rolling method and the like.
- the chemical composition (C, Si, etc.) of the steel plate is not particularly limited. It has not been confirmed that elements such as Ni, Mn, Cr, Mo, Ti, and B contained in the steel plate affect the plating layer.
- the chemical composition of the plated steel sheet includes Al, Mg, Sn, and Si as essential elements, with the balance being Zn and impurities.
- the chemical composition of the plated steel sheet is Bi, In, Ca, Y, La, Ce, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Sr, Sb, Pb, and B as an optional element. You may contain. That is, an arbitrary element may not be contained.
- content of each element of a plating layer means the average content of each element contained in the whole plating layer.
- Al 15% -60%
- Al is an essential element for improving the corrosion resistance and seizure property after coating of the plating layer.
- Most of Al exists as an Al phase in the dendritic structure in the plating layer.
- the dendritic structure to be described later is not passivated due to the Sn inclusion effect, and is in a state that does not cause a decrease in corrosion resistance after coating.
- the melting point of the plating layer is low, there is a problem that the metal in the plating layer is baked on the press die.
- the area fraction of the dendritic structure which is a structure having a high melting point, increases.
- the Al concentration necessary for ensuring the area fraction of the dendrite-like structure capable of exhibiting sufficient seizure resistance is 15% or more. Therefore, the lower limit of the Al concentration is set to 15%. A preferable Al concentration is 20% or more.
- the Al concentration exceeds 60% an “interfacial alloy layer made of an Al—Fe intermetallic compound” formed at the interface between the plating layer and the ground iron, which will be described later, grows excessively and the workability is impaired. Therefore, the upper limit of Al concentration is set to 60%.
- a preferable Al concentration is 40% or less.
- Mg is an essential element for forming a granular Mg 2 Sn phase-containing structure in the plating layer and imparting post-coating corrosion resistance and workability to the plating layer.
- Mg exists as an Mg-based intermetallic compound in the plating layer, and is eluted into the corrosive environment as Mg ions in the corrosive environment.
- Mg ions form a Zn-based corrosion product into an insulating film and rust into a barrier film. Thereby, the penetration
- Most of Mg is contained in the granular Mg 2 Sn phase-containing structure.
- the Mg concentration necessary for improving the corrosion resistance and workability after coating is 0.5%. Therefore, the lower limit of Mg concentration is 0.5%.
- a preferable Mg concentration is 1.0% or more.
- the Mg concentration exceeds 8.0% a massive MgZn 2 phase described later is excessively generated, and the workability is impaired. Therefore, the upper limit of Mg concentration is set to 8.0%. From the viewpoint of suppressing the formation of massive MgZn 2 phase that impairs workability, the preferred Mg concentration is 3.0% or less. A more preferable Mg concentration is 2.0% or less.
- Sn is an essential element for forming a granular Mg 2 Sn phase-containing structure together with Mg in the plating layer and imparting post-coating corrosion resistance and workability to the plating layer. Further, Sn is an element having an effect of suppressing the formation of massive MgZn 2 phase together with the Zn / Al / MgZn 2 ternary eutectic structure. Therefore, Sn is also an element that improves the corrosion resistance and workability after coating on the plating layer.
- the lower limit of the Sn concentration is 0.5%.
- the preferable Sn concentration is 0.1% or more. .
- a more preferable Sn concentration is 1.5% or more.
- the upper limit of Sn concentration is 20.0%.
- the preferable Sn concentration is 7.5% or less.
- a more preferable Sn concentration is 5.0% or less.
- Si 0.05% to 1.50%
- Si is an element that suppresses the reactivity between Zn and Al contained in the plating bath and the Fe element of the plating original plate. That is, Si controls the reactivity between the plating layer and the ground iron, thereby affecting the adhesion and workability of the plating layer.
- Interfacial alloy layer composed of an Al—Fe intermetallic compound (particularly Fe 2 Al 5 is an essential element for controlling the formation behavior of the interface alloy layer containing or consisting of 5. The minimum Si concentration necessary for suppressing the interface alloy layer is 0.05%.
- the Si concentration is less than 0.05%, the interfacial alloy layer grows immediately after the plating original plate is immersed in the plating bath, it becomes difficult to impart ductility to the plating layer, and the workability tends to decrease. Therefore, the lower limit of the Si concentration is 0.05%. A preferable Si concentration is 0.2% or more. On the other hand, when the Si concentration exceeds 1.50%, a potential noble Si phase remains in the plating layer and functions as a cathode portion in corrosion. As a result, the corrosion resistance after painting is reduced. Therefore, the upper limit of the Si concentration is set to 1.50%. A preferable Si concentration is 1.0% or less. Si may exist as an Mg 2 Si phase that is an intermetallic compound with Mg in the plating layer. However, if the area fraction of the Mg 2 Si phase is 5% or less, the performance is not affected. Not give.
- Bi 0% to 5.0%
- the lower limit of Bi concentration should be more than 0% (preferably 0.1% or more, more preferably 3.0 or more).
- the upper limit of Bi concentration is 5.0% or less (preferably 0.5% or less, more preferably 0.1% or less).
- In is an element that contributes to improving workability. Therefore, the lower limit of the In concentration is preferably over 0% (preferably 0.1% or more, more preferably 3.0 or more). On the other hand, when the In concentration increases, the corrosion resistance after coating tends to deteriorate. Therefore, the upper limit of the In concentration is 2.0% or less (preferably 0.3% or less).
- Ca 0% to 3.0%
- Ca is an element capable of adjusting the optimum Mg elution amount for imparting post-coating corrosion resistance and sacrificial corrosion resistance. Therefore, the lower limit of the Ca concentration is preferably over 0% (preferably 0.05% or more). On the other hand, if the Ca concentration increases, the corrosion resistance and workability after coating tend to deteriorate. Therefore, the upper limit of the Ca concentration is 3.00 or less (preferably 1.0% or less).
- Y is an element that contributes to improving workability. Therefore, the lower limit of the Y concentration is preferably over 0% (preferably 0.1% or more). On the other hand, when the Y concentration increases, the corrosion resistance after painting tends to deteriorate. Therefore, the upper limit of the Y concentration is 0.5% or less (preferably 0.3% or less).
- La and Ce are elements that contribute to workability improvement. Therefore, the lower limits of the La concentration and the Ce concentration are each preferably over 0% (preferably 0.1% or more). On the other hand, when the La concentration and Ce concentration increase, the corrosion resistance after coating tends to deteriorate. Therefore, the upper limits of La concentration and Ce concentration are 0.5% or less (preferably 0.3% or less), respectively.
- Cr, Ti, Ni, Co, V, Nb, Cu and Mn 0% to 0.25%
- Cr, Ti, Ni, Co, V, Nb, Cu, and Mn are elements that contribute to workability improvement. Therefore, the lower limit of the concentration of Cr, Ti, Ni, Co, V, Nb, Cu and Mn is preferably such that the Si concentration exceeds 0 (preferably 0.05% or more, more preferably 0.1% or more).
- the concentration of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn increases, the corrosion resistance after coating tends to deteriorate. Therefore, the upper limit of the concentration of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn is 0.25% or less, respectively.
- Sr, Sb, Pb and B 0% to 0.5%
- Sr, Sb, Pb and B are elements that contribute to the improvement of workability. Therefore, the lower limit of the concentration of Sr, Sb, Pb and B is preferably such that the Si concentration exceeds 0 (preferably 0.05% or more, more preferably 0.1% or more).
- the upper limit of the concentration of Sr, Sb, Pb and B is 0.5% or less (preferably 0.1% or less).
- Zn and impurities The balance of the chemical composition of the plating layer is Zn and impurities.
- Zn is contained in the plating layer at a certain concentration or more in order to appropriately ensure the sacrificial anticorrosive ability, post-coating corrosion resistance, and coating base treatment property of the plating layer. From these viewpoints, Al and Zn occupy most of the chemical composition of the plating layer.
- An impurity refers to a component contained in a raw material or a component mixed in a manufacturing process and not intentionally included. For example, a trace amount of components such as Fe may be mixed as impurities into the plating layer due to mutual atomic diffusion between the ground iron (steel plate) and the plating bath.
- the plating layer when a plating layer is formed by a hot dipping method, the plating layer may contain a certain Fe concentration as an impurity. It has been confirmed that when the Fe concentration is up to 3.0%, the performance is not adversely affected even if it is contained in the plating layer.
- the Mg content is preferably 0.5% to 3.0% and the Sn content is preferably 1.0% to 7.5%.
- the Mg concentration and Sn concentration are in the above ranges, the post-coating corrosion resistance and workability are further improved.
- the Al content is 20% to 60%
- the Mg content is 1.0% to 2.0%
- the Sn content is 1.0% to 5.0%
- the Si content is preferably 0.05% to 1.0%.
- the Al concentration, Mg concentration, Sn concentration and Si concentration are in the above ranges, the post-coating corrosion resistance and workability are further improved.
- seizure resistance is further improved.
- the plating layer has a granular Mg 2 Sn phase-containing structure and a dendrite-like structure (a structure containing a solid solution of Zn and Al).
- the plated layer has a structure other than the granular Mg 2 Sn phase-containing structure, a massive MgZn 2 phase having an equivalent circular diameter of 1 ⁇ m or more, a massive Zn phase having an equivalent circular diameter of 2 ⁇ m or more, and a Zn / Al / MgZn 2 ternary eutectic structure. And so on.
- FIG. 1 shows a reflected electron image (BSE image) of an SEM obtained by photographing an example of the plated layer of the plated steel sheet of the present disclosure at a magnification of 2000 times.
- the plated steel sheet has, for example, a plating layer 1, a steel sheet 2, and an interfacial alloy layer 3 made of an Al—Fe intermetallic compound between the plating layer 1 and the steel sheet 2.
- the structure of the plating layer 1 is mainly composed of a granular Mg 2 Sn phase-containing structure 4 and a dendrite-like structure 5.
- FIG. 2 which is an enlarged view of the region A in FIG.
- the granular Mg 2 Sn phase-containing structure 4 has a granular Mg 2 Sn phase 8 having a crystal grain size of less than 1 ⁇ m dispersed in a Zn phase 7.
- the dendrite-like structure 5 corresponds to a region shown in black surrounded by the region together with a region shown in gray. The difference in color between the two regions is due to the difference in Al concentration.
- the dendrite-like structure 5 having a low Al concentration is a region indicated by a gray color
- the dendrite-like structure 5 indicated by a gray color having a high Al concentration is a region indicated by a black color.
- the massive MgZn 2 phase 6 In addition to the granular Mg 2 Sn phase-containing structure 4 and the dendrite-like structure 5, the massive MgZn 2 phase 6 (see FIGS. 1 and 2), the massive Zn phase 10 (see FIG. 3), It may have a Zn / Al / MgZn 2 ternary eutectic structure 9 (see FIG. 3).
- the granular Mg 2 Sn phase-containing structure is composed of a Zn phase and a granular Mg 2 Sn phase having a crystal grain size of less than 1 ⁇ m dispersed in the Zn phase. That is, in the granular Mg2Sn phase-containing structure, the granular Mg 2 Sn phase is contained (that is, included) in the Zn phase.
- the crystal grain size of the granular Mg 2 Sn phase is an equivalent circular diameter.
- the granular Mg 2 Sn phase containing tissues are particulate Mg 2 Sn phase than the crystal grain size 1 ⁇ m are dispersed by the number density of 1 to 25 / [mu] m 2 tissue Zn phase. If the crystal grain size of the granular Mg 2 Sn phase is less than 1 ⁇ m, the stress applied to the boundary between the granular Mg 2 Sn phase and the Zn phase can be suppressed. Thus while preventing the granular Mg 2 Sn phase is a starting point of cracking, the plastic deformability of the granular Mg 2 Sn phase can be maintained sufficiently.
- the average composition of the entire granular Mg 2 Sn phase-containing structure is not particularly limited.
- the Mg concentration is 1 to 10% by mass
- the Sn concentration is 1 to 25% by mass
- the Al concentration is 1 to 8% by mass
- the composition of the entire granular Mg 2 Sn phase-containing structure may include the above-mentioned optional elements that can be included in the chemical composition of the plating layer.
- the particulate phase of the intermetallic compound corresponding to the following (1) to (5) is also regarded as the particulate Mg 2 Sn phase.
- Mg 2 Sn with interstitial solid solution of elements such as Si (2) Mg 9 Sn 5 in which Mg 2 Sn phase is transformed (3) Substituted Mg 2 Sn and Mg 9 Sn 5 (Mg 2 Sn and Mg) in which at least one of Bi, In, Cr, Ti, Ni, Co, V, Nb, Cu, and Mn is substituted in part of Sn 9 Substitute for Sn 5 ) (4) Ca in a part of Mg, Y, La and substituted Mg 2 Sn and Mg 9 Sn which at least one is substituted for Ce 5 (Mg 2 Sn and Mg 9 Sn 5 of substitutions) (5) A part of Mg is substituted with at least one of Ca, Y, La and Ce, and a part of Sn is Bi, In, Cr, Ti, Ni, Co, V, N
- Granular Mg 2 Sn phase containing tissue, as described above, but containing Mg 2 Sn phase is brittle Mg intermetallic compounds, Mg 2 Sn phase has a plastic deformability than the MgZn 2 phase.
- Mg 2 Sn phase serves as a supply source of Mg ions in a corrosive environment, and Mg ions cause corrosion products to form an insulating film, so that under-coating corrosion in a painted state is suppressed.
- the particulate Mg 2 Sn phase containing tissue effect of improving corrosion resistance after painting and workability due is greater the higher the particulate Mg 2 Sn phase containing tissue area fraction of that present in the plating layer.
- the lower limit of the area fraction of the granular Mg 2 Sn phase-containing structure is 5%.
- the area fraction of the granular Mg 2 Sn phase-containing structure is preferably 20% or more, more preferably 30% or more.
- the higher the area fraction of the granular Mg 2 Sn phase-containing structure the greater the effect of improving post-coating corrosion resistance and workability.
- the area fraction of the granular Mg 2 Sn phase-containing structure that can be produced is 65% at maximum due to production restrictions. Therefore, the upper limit of the area fraction of the granular Mg 2 Sn phase-containing structure is set to 65%.
- the area fraction of the granular Mg 2 Sn phase-containing structure is preferably 60% or less. That is, the area fraction of the granular Mg 2 Sn phase-containing structure is 5 to 65%.
- the area fraction of the granular Mg 2 Sn phase-containing structure is preferably 20 to 60%, more preferably 30 to 60%.
- the dendrite-like structure is a structure containing a solid solution of Zn and Al.
- the dendrite-like structure is a structure in which an Al phase and a Zn phase are finely separated and has an Al concentration of 15 to 85% and a Zn concentration of 15 to 85%. Therefore, the dendrite-like structure is a structure that essentially has composition deformability and can contribute to improving the workability of the plating layer. It is also a structure that contributes to improving seizure properties.
- the area fraction of the dendritic structure is preferably 35% or more. From the viewpoint of imparting better workability to the plating layer, the area fraction of the dendritic structure is more preferably 40% or more. On the other hand, from the viewpoint of production, the upper limit of the dendritic structure is preferably 95%. From the viewpoint of improving post-coating corrosion resistance and workability by the granular phase dispersed phase, the dendritic structure is preferably 80% or less, more preferably 70% or less. That is, the area fraction of the dendritic structure is preferably 35 to 95%, more preferably 35 or 40 to 80%, and still more preferably 35 or 40 to 70%.
- the massive Zn phase is an agglomerated Zn phase present in an irregular shape in the plating layer and having an equivalent circular diameter of 2 ⁇ m or more.
- the upper limit of the equivalent circular diameter of the massive Zn phase is not particularly limited, but is, for example, 10 ⁇ m or less.
- the area fraction of the bulk Zn phase is most preferably 0% (that is, the bulk Zn phase is most preferably not included). That is, the area fraction of the bulk Zn phase is preferably 0 to 20%, more preferably 0 to 10%, and still more preferably 0%.
- the massive MgZn 2 phase exists in an irregular shape in the plating layer, and is a massive Zn phase having an equivalent circular diameter of 2 ⁇ m or more.
- the upper limit of the equivalent circular diameter of the massive MgZn 2 phase is not particularly limited, but is, for example, 10 ⁇ m or less.
- the massive MgZn 2 phase is a brittle phase and easily becomes a starting point of cracks during processing. And corrosion accelerates
- the area fraction of the massive MgZn 2 phase is preferably 20% or less from the viewpoint of ensuring post-coating corrosion resistance and workability. From the viewpoint of securing sufficient post-coating corrosion resistance and workability, the area fraction of the massive MgZn 2 phase is more preferably 5% or less.
- the area fraction of the massive MgZn 2 phase is most preferably 0% (that is, it is most preferred not to contain the massive MgZn 2 phase). That is, the area fraction of the massive MgZn 2 phase is preferably 0 to 20%, more preferably 0 to 5%, and even more preferably 0%.
- the Zn / Al / MgZn 2 ternary eutectic structure is a structure composed of an Al phase, a Zn phase, and an MgZn phase. Since the shape of each phase changes depending on the component composition, the shape is indefinite. However, since the eutectic structure is an isothermal transformation and element movement during solidification is suppressed, each phase forms a complicated shape, and usually each phase precipitates finely (see FIG. 5).
- each phase has a large Zn phase and forms an island shape, and then the MgZn phase is large and fills the gap of the Zn phase, and the Al phase is dispersed in the form of spots between the MgZn 2 phases. I often take it.
- the constituent phases do not change, but what precipitates in an island shape may be an MgZn 2 phase, an Al phase or an MgZn 2 phase, and the positional relationship is a component immediately before solidification. Depends on change. A method for specifying the ternary eutectic structure will be described later.
- the Zn / Al / MgZn 2 ternary eutectic structure is prone to corrosion, and the brittle MgZn 2 phase in the ternary eutectic structure tends to be the starting point of cracks during processing. And corrosion accelerates
- the area fraction of the Zn / Al / MgZn 2 ternary eutectic structure is preferably 3% or less. Included in order to ensure sufficient corrosion resistance after painting and workability, the area fraction of the Zn / Al / MgZn 2 ternary eutectic structure is most preferably 0% (i.e. Zn / Al / MgZn 2 ternary eutectic structure is Most preferably not.) That is, the area fraction of the Zn / Al / MgZn 2 ternary eutectic structure is preferably 0 to 3%, and most preferably 0%.
- the thickness of the plating layer is, for example, about 100 ⁇ m or less. Since the thickness of the entire plating layer depends on the plating conditions, the upper limit and the lower limit of the thickness of the entire plating layer are not particularly limited. For example, the thickness of the entire plating layer is related to the viscosity and specific gravity of the plating bath in a normal hot dipping method. Further, the basis weight is adjusted by the drawing speed of the steel plate (plating original plate) and the strength of wiping. Therefore, the minimum of the thickness of the whole plating layer is about 2 micrometers, for example. On the other hand, the thickness of the plating layer that can be produced by the hot dipping method is approximately 95 ⁇ m due to the weight and uniformity of the plated metal. Therefore, the thickness of the plating layer is preferably 2 to 95 ⁇ m. ⁇
- the plated steel sheet of the present disclosure may further include an interface alloy layer made of an Al—Fe intermetallic compound between the steel sheet and the plating layer.
- an interfacial alloy layer composed of an Al—Fe intermetallic compound of 3 ⁇ m or less is generally formed between the plating layer and the steel sheet.
- the interface alloy layer may not be formed.
- the interfacial alloy layer preferably has a thickness of 100 nm or more in order to ensure adhesion between the ground iron (steel plate) and the plating layer.
- the Al—Fe-based intermetallic compound constituting the interface alloy layer is a brittle intermetallic compound, if the thickness of the interface alloy layer exceeds 1.5 ⁇ m, chipping resistance may be lowered. Therefore, when the plated steel sheet of the present disclosure has an interface alloy layer, the thickness of the interface alloy layer is preferably 100 nm to 1.5 ⁇ m.
- the interfacial alloy layer is in a state where Si is dissolved, and also has a role of suppressing the alloying reaction between the plating layer and the ground iron.
- the interface alloy layer made of an Al—Fe-based intermetallic compound is a layer in which the Al 5 Fe phase is the main phase.
- the Al—Fe alloy layer is formed by mutual atomic diffusion of the base iron (steel plate) and the plating bath.
- the interface alloy layer may partially contain a small amount of an AlFe phase, an Al 3 Fe phase, an Al 5 Fe 2 phase, or the like.
- the interface alloy layer may also contain various elements such as Zn and Si, which are components of the plating layer.
- Si when Si is taken into the interface alloy layer, an Al—Fe—Si intermetallic compound is formed in the interface alloy layer.
- the interface alloy layer may contain a pre-plating component (for example, Ni etc.), when various pre-plated steel plates are used for the plating base plate.
- a pre-plating component for example, Ni
- an Al—Fe—Ni intermetallic compound is formed in the interface alloy layer.
- the interface alloy layer made of an Al—Fe-based intermetallic compound is a layer that includes the alloy layers of the various aspects described above, in addition to the alloy layer mainly composed of the Al 5 Fe phase.
- the plated steel material of the present disclosure can be obtained by forming a plating layer on the surface (that is, one side or both sides) of the plating original plate by a hot dipping method.
- the Sendzimir method, the pre-plating method, etc. can be applied to the method for manufacturing the plated steel sheet of the present disclosure.
- Ni may be contained in the “interfacial alloy layer made of an Al—Fe-based intermetallic compound” that may be generated when the plating layer is heated.
- a pure metal or an alloy is prepared so as to be in the range of the chemical composition of the plating layer, and melted at 450 to 650 ° C. to build the bath. Then, the plating original plate whose surface is sufficiently reduced is immersed in a plating bath maintained at a predetermined bath temperature after being built, and then cooled, and then cooled to form a plating layer on the surface of the plating original plate (steel plate). be able to.
- wiping with N 2 gas is performed immediately after pulling up the plating original plate from the plating bath.
- the cooling rate in the temperature range from 320 ° C. immediately after pulling up the plating original plate from the plating bath to 320 ° C. is set to 10 ° C./s or more, and the cooling rate in the temperature range from 320 ° C. to 280 ° C. is 6 ° C. / S or less.
- the SEM reflected electron image (BSE image) of the cross section of the plated layer of the plated steel sheet shown in FIG. 1 has a cooling rate of 10 ° C./s from 320 ° C. to 280 ° C. in the temperature range from the plating bath temperature to 320 ° C.
- BSE image SEM reflection electron image
- the cooling rate in the temperature range from immediately after pulling up the plating base plate from the plating bath (that is, the plating bath temperature) to 320 ° C. is 10 ° C. or more, and the cooling rate in the temperature range from 320 ° C. to 280 ° C. is 6 ° C./s or less.
- the Sn concentration is not appropriate, a sufficient amount of the granular Mg 2 Sn phase-containing structure 4 may not be formed.
- the granular Mg 2 Sn phase-containing structure 4 is not formed in the plating layer 1, and instead, the Zn / Al / MgZn 2 ternary together with the dendritic structure 5.
- a eutectic structure 9 is formed.
- the cooling rate is not changed within the above range between the temperature range from immediately after pulling up the plating base plate from the plating bath (that is, the plating bath temperature) to 320 ° C and the temperature range from 320 ° C to 280 ° C, sufficient A certain amount of the granular Mg 2 Sn phase-containing structure 4 may not be formed.
- the granular Mg 2 Sn phase-containing structure 4 is not formed in the plating layer 1, and instead, a plate shape is formed in the Zn phase.
- a structure 11 in which the Mg 2 Sn phase is mixed is formed.
- the cooling rate A in the temperature range from 320 ° C. to immediately after pulling up the plating original plate from the plating bath (that is, the plating bath temperature) is set to 10 ° C. or more, from 320 ° C. to 280 ° C.
- the cooling rate B in the temperature range exceeds 6 ° C./s, it is difficult to obtain sufficient time for the Mg 2 Sn phase to be spheroidized.
- the cooling rate A is set to less than 10 ° C./s and the cooling rate B is set to 6 ° C./s or less, the solidification behavior that originally proceeds to non-equilibrium approaches the equilibrium state, and the nucleation of the spheroidized Mg 2 Sn phase can be performed. hard. As a result, it is considered that a plate-like Mg 2 Sn phase is formed.
- the chemical component of the plating layer is measured by the following method. First, a calibration curve for quantitative analysis of each element is obtained by GDS (High Frequency Glow Discharge Spectroscopy). Thereafter, the chemical component in the depth direction of the target plating layer is measured. Specifically, GDS (High Frequency Glow Discharge Spectroscopy) is performed on a standard sample such as each element pure metal plate, and a calibration curve indicating the relationship between the element intensity plot and each element concentration is obtained in advance. On the other hand, several 30 mm squares are sampled from the sample of the plated steel sheet to be measured and used as a GDS sample.
- GDS High Frequency Glow Discharge Spectroscopy
- Argon ion sputtering is performed from the surface layer of the plating layer, and an element strength plot in the depth direction is obtained. Using the calibration curve, the element concentration is converted from the obtained intensity plot.
- the chemical composition analysis by GDS is performed at 10 or more locations with an analysis area of ⁇ 4 mm or more and a sputtering rate of 0.04 to 0.1 ⁇ m / sec. And each element concentration of a chemical composition is taken as the average value of the element concentration in each place. However, at each GDS analysis point, in order to remove the influence of the outermost oxide layer, the component plot of the depth of the surface layer of 1 ⁇ m is ignored, and the average value of each element concentration of the depth of 1 to 10 ⁇ m (5 ⁇ m width) Is adopted.
- the area fraction of the plated layer structure (excluding the Zn / Al / MgZn 2 ternary eutectic structure) is measured by the following method.
- An FE-SEM equipped with an EDS (energy dispersive X-ray analyzer) is used to measure the area fraction of the structure of the plating layer.
- a sample piece having a cross section of 25 mm in the C direction ⁇ 15 mm in the L direction is cut from the plated steel sheet.
- the obtained sample piece is embedded in a resin, and CP (cross session polisher) processing is performed on the cross section of the plating layer to be measured.
- CP cross session polisher
- an SEM reflected electron image and an element mapping image by EDS of the cross section of the plating layer are prepared.
- the reflected electron image of SEM and the element mapping image by EDS have a magnification of 5000 times and the size of the visual field: 50 ⁇ m long ⁇ 200 ⁇ m wide.
- the region of each tissue is identified based on the reflected electron image of SEM and the element mapping image by EDS.
- the three values of gray scale brightness, hue, and contrast value indicated by each structure in the plating layer are determined. Since the three values of brightness, hue, and contrast value indicated by each structure reflect the atomic number of the element contained in each structure, the amount of Al with a small atomic number and the phase with a large content of Mg are usually black. The phase with a large amount of Zn tends to exhibit a white color.
- tissue of a plating layer is taken as the average value of the area fraction of each structure
- the area fraction of the granular Mg 2 Sn phase-containing structure is a Zn phase in which a granular Mg 2 Sn phase having a crystal grain size of less than 1 ⁇ m is confirmed at a number density of 1 to 25 particles / ⁇ m 2 in the Zn phase region. It is the area fraction of the Zn phase including the granular Mg 2 Sn phase.
- the area fraction of the dendritic structure is an area fraction of a region occupied by a solid solution of Zn and Al (structure showing an Al concentration of 15 to 85% and a Zn concentration of 15 to 85%).
- Area fraction of bulk MgZn 2 phase is the area fraction of the equivalent circular diameter 1 ⁇ m or more MgZn 2 phase.
- the area fraction of the bulk Zn phase is the area fraction of a Zn phase having an equivalent circular diameter of 2 ⁇ m or more.
- the average crystal grain size of the granular Mg 2 Sn phase is measured as follows. The SEM reflected electron image of the cross section of the plating layer with a magnification of 10,000 times and a visual field size of 10 ⁇ m ⁇ 10 ⁇ m was observed, and the top five of the granular Mg 2 Sn phases with a crystal grain size of less than 1 ⁇ m confirmed in the visual field A granular Mg 2 Sn phase having a crystal grain size of is selected. Then, this operation is performed for five visual fields, and the arithmetic average of the total 25 crystal grain sizes is defined as the average crystal grain size of the granular Mg 2 Sn phase having a crystal grain size of less than 1 ⁇ m.
- the number density of the granular Mg 2 Sn phase was 10,000 times, and the SEM reflected electron image of the cross section of the plating layer with a field size of 12 ⁇ m ⁇ 12 ⁇ m was observed, and an arbitrary 12 ⁇ m ⁇ 12 ⁇ m granular Mg 2 Sn phase The number of granular Mg 2 Sn phases having a grain size of less than 1 ⁇ m present in the contained structure is counted, and the number of granular Mg 2 Sn phases per unit area ( ⁇ m 2 ) is calculated.
- the number density of the granular Mg 2 Sn phase will, in any five visual fields of the cross section (cross section cut in the plating layer thickness direction) of the plating layer, the number density of the granular Mg 2 Sn phase obtained by the operation Average value.
- Identification and area fraction of Zn / Al / MgZn 2 ternary eutectic structure in the coating layer is measured by the following method.
- a structure in which three phases of an Al phase, a Zn phase, and a MgZn 2 phase are co-crystallized is identified from the reflected electron image of the SEM by the same method as the measurement of the area fraction of each structure in the plating layer.
- a part of the tissue is observed in a rectangular field having a magnification of 30000 times and a size of 3 ⁇ m ⁇ 4 ⁇ m (diagonal line is 5 ⁇ m) (see FIG. 5).
- the diagonal line crosses the Zn phase 5 times or more per Mg, and the MgZn 2 phase or Al phase spreading around the Zn phase 5 times or more.
- the ternary eutectic structure is determined. This determination is based on a standard that “a structure in which each of the three phases is finely dispersed” peculiar to the ternary eutectic structure.
- the structure is in a 1 ⁇ m square lattice. When one or more of each phase is contained in the lattice, it is determined as a ternary eutectic structure.
- the above operation was repeated on the same SEM reflected electron image (magnification 5000 times, size of field of view: 50 ⁇ m ⁇ 200 ⁇ m) as in the measurement of the area fraction of each structure in the plating layer. While confirming the continuity of the eutectic structure, grasp the outline (region) of the ternary eutectic structure. Then, the area fraction of the ternary eutectic structure in the plating layer in the grasped SEM reflected electron image is obtained.
- the area fraction of the ternary eutectic structure is the area fraction of the ternary eutectic structure obtained by the above operation in at least five fields of view of the arbitrary cross section of the plating layer (cross section cut in the thickness direction of the plating layer). Average value.
- the average equivalent circular diameter of the massive MgZn 2 phase and the massive Zn phase is measured by the following method.
- each phase having the top five equivalent circular diameters is selected. Then, this operation is performed for five visual fields, and the arithmetic average of a total of 25 equivalent circular diameters is defined as the average equivalent circular diameter of the massive MgZn 2 phase and the massive Zn phase.
- the thickness of the interface alloy layer made of the Al—Fe intermetallic compound is measured as follows. When the area fraction of the tissue is measured, the reflected electron image of the SEM in which each tissue is identified (magnification 5000 times, size of field of view: length 50 ⁇ m ⁇ width 200 ⁇ m, provided that the field of view of the interface alloy layer is visible. ), The thickness is measured at any five locations of the identified interface alloy layer. And the arithmetic average of five places is made into the thickness of an interface alloy layer.
- a film may be formed on the plated layer in the plated steel sheet of the present disclosure.
- the coating can form one layer or two or more layers.
- Examples of the type of film directly above the plating layer include a chromate film, a phosphate film, and a chromate-free film.
- the chromate treatment, phosphate treatment, and chromate-free treatment for forming these films can be performed by known methods.
- electrolytic chromate treatment which forms a chromate film by electrolysis, a film is formed by utilizing the reaction with the material, and then the reactive chromate treatment, in which excess treatment liquid is washed away, is applied to the substrate.
- a coating-type chromate treatment in which a film is formed by drying without washing with water. Any processing may be adopted.
- Electrolytic chromate treatment uses chromic acid, silica sol, resin (phosphoric acid, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine-modified epoxy resin, etc.), and electrolysis using hard silica.
- a chromate treatment can be exemplified.
- Examples of the phosphate treatment include zinc phosphate treatment, zinc calcium phosphate treatment, and manganese phosphate treatment.
- Chromate-free treatment is particularly suitable without any environmental impact.
- electrolytic chromate-free treatment that forms a chromate-free coating by electrolysis
- reaction-type chromate-free treatment that removes excess treatment liquid
- treatment solution that forms a film using the reaction with the material.
- coating-type chromate-free treatment in which a film is formed by applying to an object and drying without washing with water. Any processing may be adopted.
- organic resin films may be provided on the film immediately above the plating layer.
- the organic resin is not limited to a specific type, and examples thereof include polyester resins, polyurethane resins, epoxy resins, acrylic resins, polyolefin resins, and modified products of these resins.
- the modified product is obtained by reacting a reactive functional group contained in the structure of these resins with another compound (such as a monomer or a crosslinking agent) containing a functional group capable of reacting with the functional group in the structure. Refers to resin.
- organic resin one or two or more organic resins (unmodified) may be mixed and used, or in the presence of at least one organic resin, at least one other One or two or more organic resins obtained by modifying the organic resin may be used.
- the organic resin film may contain an arbitrary colored pigment or rust preventive pigment. What was made water-based by melt
- a plating bath whose components were adjusted so that the chemical composition of the plating layer was the chemical composition shown in Table 1 was constructed.
- the plating bath temperature was 465 to 593 ° C. shown in Table 1 depending on the composition.
- a hot rolled steel sheet (carbon concentration: 0.2%) having a thickness of 0.8 mm was used as a plating original sheet.
- the original plate was cut into 100 mm ⁇ 200 mm, and then plated with an in-house batch-type hot dipping test apparatus. The plate temperature was monitored using a thermocouple spot welded to the center of the plating plate.
- OK the case where the formula (1) which is the composition balance of Mg and Sn disclosed in the present disclosure is satisfied is described as OK, and the case where the formula (1) is not satisfied is described as NG.
- the plating layer was cooled from the plating bath temperature to room temperature under the conditions shown in Table 1 to produce a plated steel plate.
- a commercially available hot-dip galvanized steel sheet No. 103 in Table 1
- an alloyed galvanized steel sheet No. 104 in Table 1
- an electrogalvanized steel sheet No. 105 in Table 1
- the bending workability of the plating layer was evaluated as follows. A test piece of 30 mm in the C direction ⁇ 60 mm (L) in the L direction was cut from the obtained plated steel sheet. The test piece was bent 180 ° in the C direction (1T bend), and the top of the processed portion of the plating layer was observed with an SEM, and the number of cracks present on the top (1.6 mm ⁇ 30 mm) was counted. A test piece in which three test pieces with the same plate thickness are sandwiched inside, and a test piece in which five test pieces with the same thickness are sandwiched inside are bent 180 ° in the C direction (3T bending and 5T bending), respectively. Similarly, the number of cracks was counted.
- the evaluation criteria are “A” when the number of existing cracks is 0 and crackless, “B” when the number of average cracks is 1 to 20, and “B” when the number of average cracks is 21 to 100. “C” and the case where the average number of cracks was 101 or more were defined as “D”.
- the chipping resistance of the plating layer was performed as follows. In the same manner as the evaluation of the corrosion resistance after coating, a test piece in which the surface of the plating layer was subjected to electrodeposition coating was prepared. Further, intermediate coating, top coating, and clear coating were further performed on the surface of the electrodeposition coating film of this test piece, and each coating film was formed so that the film thickness as a whole was 40 ⁇ m. Using a gravel tester (manufactured by Suga Test Instruments Co., Ltd.), an angle of 90 degrees is applied to the coating film of a test piece that is 100 g of No. 7 crushed stone and cooled to ⁇ 20 ° C.
- the peeling part of the plating layer in a collision part was exposed using the adhesive tape, the diameter of the peeled part was measured, and five pieces were selected from those having a large peeling diameter, and the average value was defined as the average peeling diameter.
- the chipping resistance was evaluated based on the average peel diameter. The smaller the average peel diameter, the better the chipping resistance. Evaluation criteria are “A” when the average peel diameter is less than 1.0 mm, “B” when the average peel diameter is 1.0 mm or more and less than 1.5 mm, and the average peel diameter is 1.5 mm or more and less than 3.0. In the case of “C”, the case where the average peel diameter was 3.0 mm or more was designated as “D”.
- the seizure resistance of the plating layer was carried out as follows. Two test pieces each having a C direction of 80 mm ⁇ L direction of 350 mm were collected from the obtained plated steel sheet. Two test pieces were subjected to a draw bead process using a jig simulating a die and a bead, and a slide having a length of 150 mm or more was generated between the plating formation layer forming surface of the test piece and the die shoulder and bead part. .
- the local radii of the die shoulder and bead portion of the jig used in the test were 2 mmR and 5 mmR, the die pressing pressure was 60 kNm 2 , and the draw bead processing drawing speed was 2 m / min.
- the lubricating oil on the surface of the test piece was 10 mg / m 2 coated on both sides with (550S Nippon Parkerizing Co., Ltd.).
- lubricating oil (550S: Nihon Parkerizing company make) 0.5g / m ⁇ 2 > was apply
- the evaluation criteria are “A” when the plating layer is not baked on the die and the bead visually, “B” when the plating layer is baked on the die and the bead visually, and “D” and the bead are visually checked. The case where the seizure of the plating layer was remarkable was defined as “D”.
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Abstract
Description
例えば、自動車の外板に合金化溶融亜鉛めっき鋼板を用いた場合、走行車の石跳による小石の衝突(チッピング)によって、塗装とめっき層とが同時に剥離し、地鉄(鋼板)がむき出しになりやすい。そのため、合金化をしない軟質のめっき層を備えるめっき鋼板よりも腐食が激しくなることがある。
さらに、防錆の観点からは、合金化溶融亜鉛めっき鋼板は、めっき層中にFeを含有する。そのため、このようなチッピングが発生すると、めっき層の腐食により、直ちに赤褐色の錆が発生しやすく、自動車外観上に問題を引き起こす。
結果として、腐食はインターデンドライト状組織を虫食い状に進行し、腐食進行経路が複雑になる。そのため、腐食が容易に地鉄(鋼板)に到達しにくくなる。
これにより、溶融Al-Zn系めっき鋼板は、塗装がなされていない裸材として使用される場合には、めっき層の厚みが同一の溶融亜鉛めっき鋼板に比べ優れた耐食性を有する。
特許文献5及び6では、溶融Al-Zn系合金めっき鋼板が塗装後耐食性及び/又は加工性に優れると記載されている。
特許文献2:日本国特公昭46-7161号公報
特許文献3:日本国特開2001-329383号公報
特許文献4:日本国国際公開第2014/155944号
特許文献5:日本国特開2015-214747号公報
特許文献6:日本国特開2002-180225号公報
さらに、インターデンドライト状組織は、デンドライト状組織に比べて、硬度が低い。そのため、インターデンドライト状組織とデンドライト状組織との硬度差に起因し、プレス加工時には、変形がインターデンドライト状組織に集中する。その結果、めっき層に地鉄へ到達する亀裂を生じることが知られている。地鉄が露出した亀裂近傍部では腐食が促進されるため、加工部の塗装後耐食性は満足されていなかった。
この場合には、得られる溶融Al-Zn系合金めっき鋼板は、加工性に劣り、プレス加工時にめっき層に亀裂を生じる。そのため、加工部には亀裂を起点とした腐食が進行すると考えられる。
これらの溶融Al-Zn系合金めっき鋼板も自動車用めっき鋼板として満足いく加工性と塗装後耐食性を有していないと推察される。
鋼板と、前記鋼板の表面の少なくとも一部に設けられためっき層と、を有するめっき鋼板であって、
前記めっき層が、質量%で、
Al:15%~60%
Mg:0.5%~8.0%
Sn:0.5%~20.0%
Si:0.05%~1.50%
Bi:0%~5.0%、
In:0%~2.0%、
Ca:0%~3.0%、
Y :0%~0.5%、
La:0%~0.5%、
Ce:0%~0.5%、
Cr:0%~0.25%、
Ti:0%~0.25%、
Ni:0%~0.25%、
Co:0%~0.25%、
V :0%~0.25%、
Nb:0%~0.25%、
Cu:0%~0.25%、
Mn:0%~0.25%、
Sr:0%~0.5%、
Sb:0%~0.5%、
Pb:0%~0.5%、
B :0%~0.5%、
を含有し、残部がZn及び不純物からなる化学組成を有し、
かつ、前記めっき層が、面積分率5~65%の粒状Mg2Sn相含有組織と、ZnおよびAlの固溶体を含む組織と、を有し、
前記粒状Mg2Sn相含有組織が、Zn相と、前記Zn相中に分散された結晶粒径1μm未満の粒状Mg2Sn相と、からなる組織である、めっき鋼板。
<2>
質量%で、前記Mgの含有量が0.5%~3.0%、前記Snの含有量が1.0%~7.5%である前記<1>に記載のめっき鋼板。
<3>
質量%で、前記Alの含有量が20%~60%、前記Mgの含有量が1.0%~2.0%、前記Snの含有量が1.0%~5.0%、及び前記Siの含有量が0.05%~1.0%である前記<1>又は<2>に記載のめっき鋼板。
<4>
前記Snの含有量及び前記Mgの含有量が、下記式(1)を満たす前記<1>~<3>のいずれか1項に記載のめっき鋼板。
Mg≦Sn≦2.5×Mg ・・・式(1)
式(1)中、元素記号は、質量%での各元素の含有量を示す。
<5>
前記粒状Mg2Sn相含有組織の面積分率が、20%~60%である前記<1>~<4>のいずれか1項に記載のめっき鋼板。
<6>
前記粒状Mg2Sn相含有組織の面積分率が、30%~60%である前記<1>~<5>のいずれか1項に記載のめっき鋼板。
<7>
前記ZnおよびAlの固溶体を含む組織の面積分率が、35%~95%である前記<1>~<6>のいずれか1項に記載のめっき鋼板。
<8>
前記めっき層が、相当円直径1μm以上の塊状MgZn2相を面積分率で0%~20%有する前記<1>~<7>のいずれか1項に記載のめっき鋼板。
<9>
前記めっき層が、相当円直径1μm以上の塊状MgZn2相を面積分率で0%~5%有する前記<1>~<8>のいずれか1項に記載のめっき鋼板。
<10>
前記めっき層が、相当円直径2μm以上の塊状Zn相を面積分率で0%~20%有する前記<1>~<9>のいずれか1項に記載のめっき鋼板。
<11>
前記めっき層が、相当円直径2μm以上の塊状Zn相を面積分率で0%~10%有する前記<1>~<10>のいずれか1項に記載のめっき鋼板。
<12>
前記鋼板と前記めっき層との間に、厚み100nm~1.5μmのAl-Fe系金属間化合物からなる界面合金層をさらに有する前記<1>~<11>のいずれか1項に記載のめっき鋼板。
なお、本開示において、化学組成の各元素の含有量の「%」表示は、「質量%」を意味する。
「~」を用いて表される数値範囲は、「~」の前後に記載される数値を下限値及び上限値として含む範囲を意味する。
「~」の前後に記載される数値に「超」または「未満」が付されている場合の数値範囲は、これら数値を下限値または上限値として含まない範囲を意味する。
化学組成の元素の含有量は、元素量(例えば、Zn量、Mg量等)又は元素濃度(例えば、Zn濃度、Mg濃度等)と表記することがある。
「塗装後耐食性」とは、めっき層自体の腐食し難い性質を示す。
「犠牲防食性」とは、地鉄むき出し部(例えばめっき鋼板の切断端面部、加工時のめっき層割れ部、およびめっき層の剥離により、地鉄が露出する箇所)の腐食を抑制する性質を示す。
「相当円直径」とは、めっき層断面(めっき層の厚さ方向に沿って切断した断面)を観察したとき、相の輪郭によって画定される領域を、同じ面積を有する円とみなしたときの円の直径である。
「C方向」とは、鋼板の圧延方向に垂直な方向を示す。
「L方向」とは、鋼板の圧延方向と平行な方向を示す。
めっき層は、所定の化学組成を有する。また、めっき層は、面積分率5~65%の粒状Mg2Sn相含有組織と、ZnおよびAlの固溶体を含む組織(以下、便宜上、「デンドライト状組織」とも称する)と、を有する。
そして、粒状Mg2Sn相含有組織は、Zn相と、前記Zn相中に分散された結晶粒径1μm未満の粒状Mg2Sn相と、からなる組織である。
Mg系金属間化合物は、脆性な相であるものの、Mg2Sn相はMgZn2相に比べると塑性変形能を有する。このMg2Sn相を、塑性変形能を有するZn相中に微細分散した組織とすることで、組織全体として塑性変形能を発現し、加工性の向上に寄与する。加えて、Mg2Sn相は、腐食環境下においてMgイオンの供給源となり、Mgイオンは腐食生成物を絶縁被膜化させるため、塗装した状態での塗膜下腐食が抑制される。
本開示のめっき鋼板は、めっき層に所定量のAlを含み、めっき層の融点を高めるデンドライト状組織を有する。そのため、耐焼付き性にも優れ、プレス成形時にめっき層のプレス金型への凝着を抑制できる。つまり、本開示のめっき鋼板は、優れた塗装後耐食性とプレス成形性を両立することが可能である。
めっき原板となる鋼板は、特に限定はなく、Alキルド鋼、極低炭素鋼、高炭素鋼、各種高張力鋼、Ni、Cr含有鋼等の各種鋼板が使用可能である。製鋼方法、鋼の強度、熱間圧延方法、酸洗方法、冷延方法等の鋼板の前処理加工についても特に制限がない。
まず、めっき層の化学組成について説明する。
めっき鋼板の化学組成は、Al、Mg、Sn、及びSiを必須元素として含み、残部がZn及び不純物からなる。めっき鋼板の化学組成は、Bi、In、Ca、Y、La、Ce、Cr、Ti、Ni、Co、V、Nb、Cu、Mn、Sr、Sb、PbおよびBの少なくとも1種を任意元素として含有してもよい。つまり、任意元素は含有しなくてもよい。
なお、めっき層の各元素の含有量は、めっき層全体に含まれる各元素の平均含有量を意味する。
Alは、めっき層の塗装後耐食性と焼き付き性を向上させるために必須な元素である。Alのほとんどはめっき層中において、デンドライト状組織の内部にてAl相として存在する。
後述するデンドライト状組織は、Snの含有効果により不動態化しておらず、塗装後耐食性を低下させる要因とならない状態となっている。一方、めっき層の融点が低い場合はめっき層中の金属がプレス金型に焼付くことが問題となる。しかし、Al濃度が高いほど、融点が高い組織であるデンドライト状組織の面積分率が増加する。そのため、結果としてプレス成形時にめっき層のプレス金型への凝着を抑制すること(つまり、耐焼付き性を向上させること)が可能となる。
十分な耐焼付き性を発現できるデンドライト状組織の面積分率を確保するために必要なAl濃度は15%以上である。そのため、Al濃度の下限を15%とする。好ましいAl濃度は20%以上である。
一方、Al濃度が60%を超えると、後述するめっき層と地鉄の界面に形成される「Al-Fe系金属間化合物からなる界面合金層」が過剰に成長し、加工性を損なう。そのため、Al濃度の上限を60%とする。好ましいAl濃度は40%以下である。
Mgは、粒状Mg2Sn相含有組織をめっき層中に形成させ、めっき層に塗装後耐食性と加工性を付与するために必須な元素である。Mgは、めっき層中でMg系金属間化合物として存在し、腐食環境下でMgイオンとして腐食環境中に溶出する。Mgイオンは、Zn系の腐食生成物を絶縁被膜化し、錆をバリア被膜化する。それにより、めっき層中及び塗膜下への腐食因子の侵入を抑制し、塗装後耐食性向上に寄与することができる。Mgのほとんどは粒状Mg2Sn相含有組織に含有される。粒状Mg2Sn相含有組織の形成により、塗装後耐食性と加工性との両方が向上する。塗装後耐食性と加工性を向上させるために必要なMg濃度は0.5%である。そのため、Mg濃度の下限を0.5%とする。好ましいMg濃度は1.0%以上である。
一方、Mg濃度が8.0%を超えた場合、後述する塊状MgZn2相が過剰に生成し、加工性を損なう。そのため、Mg濃度の上限を8.0%とする。加工性を損なう塊状MgZn2相の生成を抑える観点から、好ましいMg濃度は3.0%以下とする。より好ましいMg濃度は2.0%以下である。
Snは、Mgと共に粒状Mg2Sn相含有組織をめっき層中に形成させ、めっき層に塗装後耐食性と加工性を付与するために必須な元素である。また、Snは、Zn/Al/MgZn2三元共晶組織と共に、塊状MgZn2相の生成も抑制する効果を有する元素である。
そのため、Snは、めっき層に塗装後耐食性と加工性を高める元素でもある。
Sn濃度が少ないと、粒状Mg2Sn相含有組織が形成され難くなる一方で、Zn/Al/MgZn2三元共晶組織および塊状MgZn2相の生成量が増加し、めっき層の塗装後耐食性および加工性が低下する傾向となる。そのため、Sn濃度の下限は、0.5%とする。粒状Mg2Sn相含有組織を十分に形成し、Zn/Al/MgZn2三元共晶組織および塊状MgZn2相の生成を十分に抑制する観点から、好ましいSn濃度は0.1%以上とする。より好ましいSn濃度は1.5%以上である。
一方、Sn濃度の過剰であると、余剰分のSnが電位的に貴なSn相として晶出し、塗装後耐食性を低下させる。そのため、Sn濃度の上限は、20.0%とする。塗装後耐食性を向上する観点から、好ましいSn濃度は7.5%以下とする。より好ましいSn濃度は、5.0%以下である。
Siは、めっき浴中に含有されると、めっき浴中に含有されるZn及びAlと、めっき原板のFe元素との反応性を抑制する元素である。すなわち、Siは、めっき層と地鉄の反応性を制御することによって、めっき層の密着性及び加工性に影響を与える「Al-Fe系金属間化合物からなる界面合金層(特に、Fe2Al5を含むか又はそれからなる界面合金層)の形成挙動を制御するために必須な元素である。界面合金層の抑制に必要な最低限のSi濃度は、0.05%である。
Si濃度が0.05%未満であると、めっき原板をめっき浴に浸漬直後に界面合金層が成長し、めっき層の延性付与は難しくなり、加工性が低下する傾向となる。そのため、Si濃度の下限は、0.05%とする。好ましいSi濃度は0.2%以上である。
一方、Si濃度が1.50%を超えると、めっき層中へ電位的に貴なSi相が残存し、腐食におけるカソード部として働く。その結果、塗装後耐食性の低下につながる。そのため、Si濃度の上限は、1.50%とする。好ましいSi濃度は1.0%以下である。
なお、Siは、めっき層中にてMgとの金属間化合物であるMg2Si相として存在する場合もあるが、Mg2Si相の面積分率が5%以下であれば、性能になんら影響を与えない。
Biは、加工性向上に寄与する元素である。よって、Bi濃度の下限は、0%超え(好ましくは0.1%以上、より好ましくは3.0以上)がよい。
一方、Bi濃度が過剰に増加すると、塗装後耐食性が劣化する傾向となる。よって、Bi濃度の上限は5.0%以下(好ましくは0.5%以下、より好ましくは0.1%以下)とする。
Inは、加工性向上に寄与する元素である。よって、In濃度の下限は、0%超え(好ましくは0.1%以上、より好ましくは3.0以上)がよい。
一方、In濃度が増加すると、塗装後耐食性が劣化する傾向となる。よって、In濃度の上限は2.0%以下(好ましくは0.3%以下)とする。
Caは、塗装後耐食性及び犠牲防食性を付与するのに最適なMg溶出量を調整することができる元素である。よって、Ca濃度の下限は、0%超え(好ましくは0.05%以上)がよい。
一方、Ca濃度が増加すると、塗装後耐食性および加工性が劣化する傾向となる。よって、Ca濃度の上限は3.00以下(好ましくは1.0%以下)とする。
Yは、加工性向上に寄与する元素である。よって、Y濃度の下限は、0%超え(好ましくは0.1%以上)がよい。
一方、Y濃度が増加すると、塗装後耐食性が劣化する傾向となる。よって、Y濃度の上限は0.5%以下(好ましくは0.3%以下)とする。
LaおよびCeは、加工性向上に寄与する元素である。よって、La濃度およびCe濃度の下限は、各々、0%超え(好ましくは0.1%以上)がよい。
一方、La濃度およびCe濃度が増加すると、塗装後耐食性が劣化する傾向となる。よって、La濃度およびCe濃度の上限は、各々、0.5%以下(好ましくは0.3%以下)とする。
Cr、Ti、Ni、Co、V、Nb、CuおよびMnは、加工性向上に寄与する元素である。よって、Cr、Ti、Ni、Co、V、Nb、CuおよびMnの濃度の下限は、各々、Si濃度は0超え(好ましくは0.05%以上、より好ましくは0.1%以上)がよい。
一方、Cr、Ti、Ni、Co、V、Nb、CuおよびMnの濃度が増加すると、塗装後耐食性が劣化する傾向となる。よって、Cr、Ti、Ni、Co、V、Nb、CuおよびMnの濃度の上限は、各々、0.25%以下とする。
Sr、Sb、PbおよびBは、加工性向上に寄与する元素である。よって、Sr、Sb、PbおよびBの濃度の下限は、各々、Si濃度は0超え(好ましくは0.05%以上、より好ましくは0.1%以上)がよい。
一方、Sr、Sb、PbおよびBの濃度が増加すると、塗装後耐食性が劣化する傾向となる。よって、Sr、Sb、PbおよびBの濃度の上限は、各々、0.5%以下(好ましくは0.1%以下)とする。
めっき層の化学組成の残部は、Zn及び不純物である。
Znは、めっき層の犠牲防食能、塗装後耐食性、塗装下地処理性を適正に確保するために、一定濃度以上、めっき層に含有される。これら観点から、めっき層の化学組成は、AlとZnで大半を占める。
不純物は、原材料に含まれる成分、または、製造の工程で混入する成分であって、意図的に含有させたものではない成分を指す。例えば、めっき層には、地鉄(鋼板)とめっき浴との相互の原子拡散によって、不純物として、Fe等の成分も微量混入することがある。
めっき層の化学組成において、Mgの含有量が0.5%~3.0%、Snの含有量が1.0%~7.5%であることが好ましい。Mg濃度およびSn濃度を上記範囲にすると、塗装後耐食性および加工性がさらに向上する。
特に、めっき層の化学組成において、Alの含有量が20%~60%、Mgの含有量が1.0%~2.0%、Snの含有量が1.0%~5.0%、及びSiの含有量が0.05%~1.0%であることが好ましい。Al濃度、Mg濃度、Sn濃度およびSi濃度を上記範囲にすると、塗装後耐食性および加工性がさらに向上する。また、耐焼付き性もさらに向上する。
塗装後耐食性および加工性がさらに向上させるためには、粒状Mg2Sn相含有組織を十分に形成し、Zn/Al/MgZn2三元共晶組織および塊状MgZn2相の生成を十分抑制することが好ましい。
そのためには、Snの含有量及びMgの含有量は、下記式(1)を満たすことが好ましく、下記式(2)を満たすことがより好ましい。
Mg≦Sn≦2.5×Mg ・・・式(1)
1.5×Mg≦Sn≦2.0×Mg ・・・式(2)
式(1)~式(2)中、元素記号は、質量%での各元素の含有量を示す
一方、Sn濃度が式(1)を満たさず、Mgに対してSnが過剰な場合、電位的に貴なSn相が晶出し、塗装後耐食性が低くなる傾向がある。
めっき層は、粒状Mg2Sn相含有組織と、デンドライト状組織(ZnおよびAlの固溶体を含む組織)と、を有する。
そして、めっき層は、粒状Mg2Sn相含有組織以外の組織として、相当円直径1μm以上の塊状MgZn2相、相当円直径2μm以上の塊状Zn相、Zn/Al/MgZn2三元共晶組織等を有する場合がある。
図1に示すように、めっき鋼板は、例えば、めっき層1と、鋼板2と、めっき層1および鋼板2の間に、Al-Fe系金属間化合物からなる界面合金層3と、を有している。
めっき層1の組織は、主として、粒状Mg2Sn相含有組織4とデンドライト状組織5と、から構成されている。また、図1中の領域Aの拡大図である図2に示すように、粒状Mg2Sn相含有組織4は、Zn相7中に結晶粒径1μm未満の粒状Mg2Sn相8が分散された組織となっている。
図1中、デンドライト状組織5は、グレー色で示された領域と共に、当該領域囲まれた黒色示された領域が該当する。両領域の色の違いは、Al濃度の違いによる。具体的には、Al濃度が低いデンドライト状組織5がグレー色で示された領域であり、Al濃度が高いグレー色で示されたデンドライト状組織5が黒色で示された領域である。
なお、めっき層1中には、粒状Mg2Sn相含有組織4及びデンドライト状組織5以外にも、塊状MgZn2相6(図1及び図2参照)、塊状Zn相10(図3参照)、Zn/Al/MgZn2三元共晶組織9(図3参照)を有する場合がある。
粒状Mg2Sn相含有組織は、Zn相と、前記Zn相中に分散された結晶粒径1μm未満の粒状Mg2Sn相と、からなる。つまり、粒状Mg2Sn相含有組織において、粒状Mg2Sn相は、Zn相中に含有(つまり内包)されている。
なお、粒状Mg2Sn相の結晶粒径は、相当円直径である。
粒状Mg2Sn相の結晶粒径が1μm未満であれば、粒状Mg2Sn相とZn相の境界にかかる応力を抑えることができる。このため粒状Mg2Sn相が亀裂発生の起点となることを防ぎつつ、粒状Mg2Sn相の塑性変形能を十分に保つことができる。一方、粒状Mg2Sn相の結晶粒径が1μmを超えると粒状Mg2Sn相とZn相の境界にかかる応力が増加して粒状Mg2Sn相が亀裂発生の起点となるおそれがあるため好ましくない。
粒状Mg2Sn相含有組織全体の平均組成は、特に限定されないが、例えば、Mg濃度が1~10質量%、Sn濃度が1~25質量%、Al濃度が1~8質量%であり、残部がZn及び約2質量%未満の不純物からなる。粒状Mg2Sn相含有組織全体の組成には、めっき層の化学組成に含まれ得る上記任意元素も含むことがある。
(1)Si等の元素を侵入型固溶したMg2Sn
(2)Mg2Sn相が変態したMg9Sn5
(3)Snの一部にBi、In、Cr、Ti、Ni、Co、V、Nb、Cu、及びMnの少なくとも1種が置換した置換Mg2SnおよびMg9Sn5(Mg2SnおよびMg9Sn5の置換体)
(4)Mgの一部にCa、Y、La及びCeの少なくとも1種が置換した置換Mg2SnおよびMg9Sn5(Mg2SnおよびMg9Sn5の置換体)
(5)Mgの一部にCa,Y,La及びCeの少なくとも1種が置換し、かつSnの一部にBi、In、Cr、Ti、Ni、Co、V、Nb、Cu、及びMnの少なくとも1種が置換した置換Mg2SnおよびMg9Sn5(Mg2SnおよびMg9Sn5の置換体)
この粒状Mg2Sn相含有組織による塗装後耐食性および加工性の向上効果は、めっき層中に存在する粒状Mg2Sn相含有組織の面積分率が高いほど大きくなる。
一方、上述のとおり、粒状Mg2Sn相含有組織の面積分率は高いほど、塗装後耐食性及び加工性の向上効果が大きくなる。性能面からの上限値に制約はないが、製造の制約上、生成可能な粒状Mg2Sn相含有組織の面積分率は最大で65%である。そのため、粒状Mg2Sn相含有組織の面積分率の上限は、65%とする。安定的な製造の観点から、粒状Mg2Sn相含有組織の面積分率は、好ましくは60%以下である。
つまり、粒状Mg2Sn相含有組織の面積分率は、5~65%とする。そして、粒状Mg2Sn相含有組織の面積分率は、好ましは20~60%であり、より好ましくは30~60%である。
デンドライト状組織は、ZnおよびAlの固溶体を含む組織である。具体的には、デンドライト状組織は、Al相とZn相と微細に分離した組織であって、Al濃度15~85%、Zn濃度15~85%を示す組織である。そのため、デンドライト状組織は、本質的に組成変形能を有し、めっき層の加工性向上に寄与し得る組織である。また、焼付き性向上に寄与する組織でもある。
つまり、デンドライト状組織の面積分率は、好ましくは35~95%であり、より好ましくは35又は40~80%であり、さらに好ましくは35又は40~70%である。
塊状Zn相は、めっき層中に不定形で存在し、相当円直径で2μm以上の塊状のZn相である。塊状Zn相の相当円直径の上限は、特に制限はないが、例えば、10μm以下である。
塊状Zn相の面積分率が高い程、耐焼付き性及び耐食性が低下する傾向にある。そのため、耐焼付き性及び耐食性を確保する観点から、塊状Zn相の面積分率は20%以下が好ましい。十分な耐焼付き性と耐食性を確保する観点から、塊状Zn相の面積分率は10%以下がより好ましい。塊状Zn相の面積分率は0%が最も好ましい(すなわち塊状Zn相は含まないことが最も好ましい。)
つまり、塊状Zn相の面積分率は、好ましくは0~20%であり、より好ましくは0~10%であり、さらに好ましくは0%である。
塊状MgZn2相は、めっき層中に不定形で存在し、相当円直径で2μm以上の塊状のZn相である。塊状MgZn2相の相当円直径の上限は、特に制限はないが、例えば、10μm以下である。
塊状MgZn2相は、脆性な相で、加工時に亀裂の起点となり易い。そして、当該亀裂の近傍部では腐食が促進し、加工部の塗装後耐食性を低下させる原因となることがある。塊状MgZn2相の面積分率が高い程、塗装後耐食性及び加工性が低下する傾向にある。そのため、塗装後耐食性及び加工性を確保する観点から、塊状MgZn2相の面積分率は20%以下が好ましい。十分な塗装後耐食性及び加工性を確保する観点から、塊状MgZn2相の面積分率は5%以下がより好ましい。塊状MgZn2相の面積分率は0%が最も好ましい(すなわち塊状MgZn2相は含まないことが最も好ましい。)
つまり、塊状MgZn2相の面積分率は、好ましくは0~20%であり、より好ましくは0~5%であり、さらに好ましくは0%である。
Zn/Al/MgZn2三元共晶組織は、Al相、Zn相およびMgZn相からなる組織である。それぞれ相の形状は、成分組成によって大きさが変化するために、形状は不定形である。しかし、共晶組織は、定温変態で、凝固時の元素移動が抑制されることから、各々の相が入り組んだ形状を形成し、通常、各相は微細に析出する(図5参照)。
通常、それぞれの各相は、Zn相が大きく、島状を形成し、次いで、MgZn相が大きく、Zn相の隙間を満たし、Al相は、MgZn2相の間に斑点状に分散する構成をとることが多い。なお、成分組成によっては、構成する相は、変化しないが、島状に析出するものが、MgZn2相になる場合、Al相またはMgZn2相になる場合もあり、位置関係が凝固直前の成分変化に依存する。
なお、三元共晶組織の特定方法については後述する。
つまり、Zn/Al/MgZn2三元共晶組織の面積分率は、好ましくは0~3%であり、最も好ましくは0%である。
よって、めっき層の厚みは、2~95μmとすることがよい。・
本開示のめっき鋼板は、鋼板と前記めっき層との間にAl-Fe系金属間化合物からなる界面合金層をさらに有してもよい。通常、めっき層と鋼板の間には、一般的に3μm以下のAl-Fe系金属間化合物からなる界面合金層が形成する。ただし、めっき層の形成条件によっては、界面合金層は形成されない場合もある。
界面合金層は、地鉄(鋼板)とめっき層の密着性を確保するために、100nm以上の厚みを有することが好ましい。一方、界面合金層を構成するAl-Fe系金属間化合物は、脆性な金属間化合物であるため、界面合金層の厚みが1.5μmを超えると耐チッピング性を低下させる場合がある。
したがって、本開示のめっき鋼板が界面合金層を有する場合、界面合金層の厚みは100nm~1.5μmであることが好ましい。
なお、界面合金層はSiを固溶した状態にあり、めっき層と地鉄の合金化反応を抑制する役割も有する。
さらに、界面合金層は、めっき原板に各種プレめっき鋼板を使用した場合、プレめっき成分(例えばNi等)を含むことがある。プレめっき成分(例えばNi等)が界面合金層に取り込まれると、界面合金層中でAl-Fe-Ni金属間化合物を形成する。
本開示のめっき鋼材は、めっき原板の表面(つまり、片面又は両面)に溶融めっき法によりめっき層を形成することで得られる。
そして、表面が十分に還元されためっき原板を、建浴後、所定の浴温に保持しためっき浴に浸漬し、引き上げた後、冷却して、めっき原板(鋼板)の表面にめっき層を形成することができる。なお、めっき層の付着量を制御するためには、例えば、めっき浴からめっき原板を引き上げ直後にN2ガスによるワイピングを実施する。
図1に示す本開示めっき鋼板のめっき層の断面のSEMの反射電子像(BSE像)は、めっき浴温から320℃までの温度域の冷却速度を10℃/s、320℃から280℃までの温度域の冷却速度を6℃/sとして作製しためっき鋼板のめっき層の断面のSEMの反射電子像(BSE像)である。
図1に示すように、上記冷却条件とすると、めっき層中に、粒状Mg2Sn相含有組織4とデンドライト状組織5とを有する組織が形成できる。
例えば、図4に示すように、冷却速度を上記範囲で変更しない冷却速度条件では、めっき層1中に粒状Mg2Sn相含有組織4が形成されず、その代わりに、Zn相中に板状Mg2Sn相が混在した組織11が形成される。
この組織11の詳細な形成機構は明らかでないが、めっき浴からめっき原板を引き上げ直後(つまりめっき浴温)から320℃までの温度域の冷却速度Aを10℃以上とし、320℃から280℃までの温度域の冷却速度Bが6℃/s超えとした場合、Mg2Sn相が球状化するために十分な時間が得られ難い。冷却速度Aを10℃/s未満とし、冷却速度Bを6℃/s以下とした場合、本来非平衡に進行する凝固挙動が平衡状態に近づき、球状化するMg2Sn相の核生成ができ難い。その結果として板状のMg2Sn相が形成すると考えられる。
まず、GDS(高周波グロー放電分光分析)で各元素の定量分析の検量線を得る。その後、対象とするめっき層の深さ方向の化学成分を測定する。
具体的には、各元素純金属板等の標準試料に対してGDS(高周波グロー放電分光分析)を実施し、あらかじめ元素強度プロットと各元素濃度との関係を示す検量線を得る。
一方、測定対象のめっき鋼板のサンプルから30mm角を数枚採取し、GDS用サンプルとする。めっき層の表層よりアルゴンイオンスパッタを実施し、深さ方向の元素強度プロットを得る。検量線により、得られた強度プロットから元素濃度に換算する。
GDSによる化学組成の分析は、分析面積をφ4mm以上、スパッタ速度を0.04~0.1μm/秒の範囲として、10箇所以上測定する。そして、化学組成の各元素濃度は、各々の場所における元素濃度の平均値とする。
ただし、各々のGDS分析点において、最表層の酸化層の影響を除去するために、表層1μmの深さの成分プロットを無視し、深さ1~10μm(5μm幅)の各元素濃度の平均値を採用する。
めっき層の組織の面積分率の測定には、EDS(エネルギー分散型X線分析装置)を搭載したFE-SEMを使用する。
SEMの反射電子像及びEDSによる元素マッピング像に基づいて、各組織の領域を同定する。
デンドライト状組織の面積分率は、ZnおよびAlの固溶体(Al濃度15~85%、Zn濃度15~85%を示す組織)が占める領域の面積分率である。
塊状MgZn2相の面積分率は、相当円直径1μm以上のMgZn2相の面積分率である。
塊状Zn相の面積分率は、相当円直径2μm以上のZn相の面積分率である。
そして、三元共晶組織の面積分率は、めっき層の任意の断面(めっき層厚み方向に切断した断面)の少なくとも5視野において、上記操作により求めた三元共晶組織の面積分率の平均値とする。
上記組織の面積分率を測定するとき、各組織を同定したSEMの反射電子像において、同定された各相のうち、上位5個の相当円直径を持つ各相を選択する。そして、この操作を5視野分行い、計25個の相当円直径の算術平均を、塊状MgZn2相、及び塊状Zn相の平均相当円直径とする。
上記組織の面積分率を測定するとき、各組織を同定したSEMの反射電子像(倍率5000倍、視野の大きさ:縦50μm×横200μm、ただし、界面合金層が視認される視野とする。)において、同定された界面合金層の任意の5箇所について、厚さを測定する。そして、5箇所の算術平均を界面合金層の厚さとする。
本開示のめっき鋼板には、めっき層上に皮膜を形成してもよい。皮膜は、1層または2層以上を形成することができる。めっき層直上の皮膜の種類としては、例えば、クロメート皮膜、りん酸塩皮膜、クロメートフリー皮膜が挙げられる。これら皮膜を形成する、クロメート処理、りん酸塩処理、クロメートフリー処理は既知の方法で行うことができる。
た後、めっき浴に約3秒浸漬した。めっき浴浸漬後、引上速度100mm/秒で引上げた。引き抜き時、N2ワイピングガスでめっき付着量調整を行った。
なお、市販の溶融亜鉛めっき鋼板(表1中のNo.103)、合金化亜鉛めっき鋼板(表1中のNo.104)、及び電気亜鉛めっき鋼板(表1中のNo.105)も準備した。
得られためっき鋼板のめっき層の下記組織の面積分率を、既述の方法に従って測定した。
・粒状Mg2Sn相含有組織(表中「粒状相含有組織」と表記)
・デンドライト状組織
・Zn/Al/MgZn2三元共晶組織
・相当円直径1μm以上の塊状MgZn2相
・相当円直径2μm以上の塊状Zn相
・Zn相中に板状Mg2Sn相が混在した組織(表中、「Zn相+板状Mg2Sn相」と表記)
・Sn相
・Si相
・Mg2Si相
・上記組織以外の金属間化合物相(表中「その他相」と表記」)
得られためっき鋼板のめっき層の下記組織の平均相当円直径を、既述の方法に従って測定した。ただし、表1中、平均相当円直径を「相当円直径」と表記する。
・相当円直径1μm以上の塊状MgZn2相
・相当円直径2μm以上の塊状Zn相
・Sn相
・Si相
・Mg2Si相
得られためっき鋼板の界面合金層の厚さを、既述の方法に従って測定した。
表1に示すNo.29のSEMの反射電子像(BSE像)を得た。表1に示すNo.29のSEMの反射電子像(BSE像)を図1及び図2に示す。図1から明らかなように、めっき層1は、主として粒状Mg2Sn相含有組織4及びデンドライト状組織5から構成されていた。そして、図2に示す粒状Mg2Sn相含有組織4中に形成された粒状Mg2Sn相8の平均結晶粒径と数密度を調べた。
同様に、他のサンプルについても、粒状Mg2Sn相の平均結晶粒径と数密度を調べた。その結果、粒状Mg2Sn相含有組織中に形成された粒状Mg2Sn相の平均結晶粒径と数密度の代表的な数値は下表2の通りであった。
めっき層の曲げ加工性の評価は、次の通り実施した。
得られためっき鋼板から、C方向30mm×L方向60mm(L)の試験片を切断した。試験片をC方向に180°曲げ(1T曲げ)して、めっき層の加工部の頂上をSEM観察し、頂上部(1.6mm×30mm)に存在するクラック数を数えた。
同板厚の試験片を内側に3枚挟んだ試験片、同板厚の試験片を内側に5枚挟んだ試験片を、各々C方向に180°曲げ(3T曲げおよび5T曲げ)して、同様に、クラック数を数えた。
そして、各めっき鋼板を少なくとも3サンプル作製し、存在するクラックの平均値を算出し、曲げ加工性を評価した。平均のクラック数が少ない方が、塑性変形能に優れ、曲げ加工性が良好と評価できる。
評価基準は、存在する平均クラック数が0本でありクラックレスの場合を「A」、平均クラック数が1~20本の場合を「B」、平均クラック数が21~100本の場合を「C」、平均クラック数が101本以上の場合を「D」とした。
めっき層の塗装後耐食性の評価は、次の通り実施した。
得られためっき鋼板から、C方向50mm×L方向100mmの試験片を採取した。試験片のめっき層表面に、Znりん酸処理(SD5350システム:日本ペイント・インダストリアルコーディング社製規格)を施した。
次に、Znりん酸処理した試験片のめっき層表面に、電着塗装(PN110パワーニクスグレー-:日本ペイント・インダストリアルコーディング社製規格)により、厚さ20μmの塗膜を形成し、焼き付け温度150℃、20分で塗膜の焼き付けを行って、電着塗膜を形成した。
次に、試験片の電着塗膜に対して地鉄(鋼板)に到達するクロスカット傷(40×√2 2本)を入れた。
得られた試験片を、JASO(M609-91)に従った複合サイクル腐食試験に供した。そして、30、60、90、150サイクルの各経過後のクロスカット周囲8箇所の最大膨れ幅を測定し、平均値を求めた。
この膨れ幅により塗装後耐食性を評価した。評価基準は、JASO(M609-91)のサイクル数が30、60、90、150サイクルの時点で、クロスカット傷からの膨れ幅が1mm以下の場合は「A」、1mm超え~2mm以下の場合は「B」、2mm超え~4mm以下の場合は「C」、赤錆が発生した場合は「D」とした。
めっき層の耐チッピング性は、次の通り実施した。
塗装後耐食性の評価と同様にして、めっき層表面に電着塗装を施した試験片を準備した。この試験片の電着塗膜表面に、さらに中塗り塗装、上塗り塗装、クリヤー塗装を実施し、全体としての膜厚が40μmになるよう各塗膜を形成した。
グラベロ試験機(スガ試験機株式会社製)を用いて、7号砕石100gを30cmの距離から3.0kg/cm2の空気圧で、-20℃に冷却した試験片の塗膜に90度の角度で衝突させた。その後、粘着テープを用いて衝突部分におけるめっき層の剥離部を露出させ、剥離した部分の径を測定し、剥離径の大きいものから5個を選んでその平均値を平均剥離径とした。
この平均剥離径により、耐チッピング性を評価した。平均剥離径が小さい程、耐チッピング性に優れることを意味する。
評価基準は、平均剥離径が1.0mm未満の場合を「A」、平均剥離径が1.0mm以上1.5mm未満の場合を「B」、平均剥離径が1.5mm以上3.0未満の場合を「C」、平均剥離径が3.0mm以上の場合を「D」とした。
めっき層の耐焼付き性は、次の通り実施した。
得られためっき鋼板から、C方向80mm×L方向350mmの試験片をそれぞれ2枚ずつ採取した。2枚の試験片にダイスとビードを模した冶具を用いてドロービード加工を施し、試験片のめっき形成層形成面とダイス肩およびビード部との間で長さ150mm以上の摺動を発生させた。なお、上記試験に用いた冶具のダイス肩およびビード部の局率半径はそれぞれ2mmRおよび5mmR、ダイスの押付け圧力は60kNm2、ドロービード加工の引き抜き速度は2m/minとした。また、試験時には、試験片表面に潤滑油(550S:日本パーカライジング社製)を両面で10mg/m2塗布した。
評価基準は、目視においてダイスとビードにめっき層が焼付いていない場合を「A」、目視においてダイスとビードにめっき層が焼付いているものの軽微な場合を場合を「B」、目視においてダイスとビードにめっき層が焼付きが顕著な場合を「D」とした。
1 めっき層
2 鋼板
3 界面合金層
4 粒状Mg2Sn相含有組織
5 ZnおよびAlの固溶体を含む組織(デンドライト状組織)
6 塊状MgZn2相
7 Zn相
8 粒状Mg2Sn相
9 Zn/Al/MgZn2三元共晶組織
10 塊状Zn相
11 Zn相中に板状Mg2Sn相が混在した組織
20:Zn/Al/MgZn2三元共晶組織のZn相
21:Zn/Al/MgZn2三元共晶組織のMgZn2相
22:Zn/Al/MgZn2三元共晶組織のAl相
本明細書に記載された全ての文献、特許出願、および技術規格は、個々の文献、特許出願、および技術規格が参照により取り込まれることが具体的かつ個々に記された場合と同程度に、本明細書中に参照により取り込まれる。
Claims (12)
- 鋼板と、前記鋼板の表面の少なくとも一部に設けられためっき層と、を有するめっき鋼板であって、
前記めっき層が、質量%で、
Al:15%~60%
Mg:0.5%~8.0%
Sn:0.5%~20.0%
Si:0.05%~1.50%
Bi:0%~5.0%、
In:0%~2.0%、
Ca:0%~3.0%、
Y :0%~0.5%、
La:0%~0.5%、
Ce:0%~0.5%、
Cr:0%~0.25%、
Ti:0%~0.25%、
Ni:0%~0.25%、
Co:0%~0.25%、
V :0%~0.25%、
Nb:0%~0.25%、
Cu:0%~0.25%、
Mn:0%~0.25%、
Sr:0%~0.5%、
Sb:0%~0.5%、
Pb:0%~0.5%、
B :0%~0.5%、
を含有し、残部がZn及び不純物からなる化学組成を有し、
かつ、前記めっき層が、面積分率5~65%の粒状Mg2Sn相含有組織と、ZnおよびAlの固溶体を含む組織と、を有し、
前記粒状Mg2Sn相含有組織が、Zn相と、前記Zn相中に分散された結晶粒径1μm未満の粒状Mg2Sn相と、からなる組織である、めっき鋼板。 - 質量%で、前記Mgの含有量が0.5%~3.0%、前記Snの含有量が1.0%~7.5%である請求項1に記載のめっき鋼板。
- 質量%で、前記Alの含有量が20%~60%、前記Mgの含有量が1.0%~2.0%、前記Snの含有量が1.0%~5.0%、及び前記Siの含有量が0.05%~1.0%である請求項1又は請求項2に記載のめっき鋼板。
- 前記Snの含有量及び前記Mgの含有量が、下記式(1)を満たす請求項1~請求項3のいずれか1項に記載のめっき鋼板。
Mg≦Sn≦2.5×Mg ・・・式(1)
式(1)中、元素記号は、質量%での各元素の含有量を示す。 - 前記粒状Mg2Sn相含有組織の面積分率が、20%~60%である請求項1~請求項4のいずれか1項に記載のめっき鋼板。
- 前記粒状Mg2Sn相含有組織の面積分率が、30%~60%である請求項1~請求項5のいずれか1項に記載のめっき鋼板。
- 前記ZnおよびAlの固溶体を含む組織の面積分率が、35%~95%である請求項1~請求項6のいずれか1項に記載のめっき鋼板。
- 前記めっき層が、相当円直径1μm以上の塊状MgZn2相を面積分率で0%~20%有する請求項1~請求項7のいずれか1項に記載のめっき鋼板。
- 前記めっき層が、相当円直径1μm以上の塊状MgZn2相を面積分率で0%~5%有する請求項1~請求項8のいずれか1項に記載のめっき鋼板。
- 前記めっき層が、相当円直径2μm以上の塊状Zn相を面積分率で0%~20%有する請求項1~請求項9のいずれか1項に記載のめっき鋼板。
- 前記めっき層が、相当円直径2μm以上の塊状Zn相を面積分率で0%~10%有する請求項1~請求項10のいずれか1項に記載のめっき鋼板。
- 前記鋼板と前記めっき層との間に、厚み100nm~1.5μmのAl-Fe系金属間化合物からなる界面合金層をさらに有する請求項1~請求項11のいずれか1項に記載のめっき鋼板。
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WO2021095442A1 (ja) * | 2019-11-14 | 2021-05-20 | 日本製鉄株式会社 | めっき鋼材 |
JP2021172834A (ja) * | 2020-04-21 | 2021-11-01 | 日本製鉄株式会社 | めっき線及びその製造方法 |
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JP2023504496A (ja) * | 2019-12-06 | 2023-02-03 | ポスコホールディングス インコーポレーティッド | 曲げ加工性及び耐食性に優れた溶融亜鉛めっき鋼板及びその製造方法 |
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HUE063660T2 (hu) | 2024-01-28 |
KR102266075B1 (ko) | 2021-06-18 |
AU2018235734B2 (en) | 2020-08-27 |
US20210079498A1 (en) | 2021-03-18 |
JP6428975B1 (ja) | 2018-11-28 |
AU2018235734A1 (en) | 2019-10-17 |
ES2958769T3 (es) | 2024-02-14 |
BR112019018900A2 (pt) | 2020-05-19 |
CN110431249B (zh) | 2022-03-08 |
EP3597786A1 (en) | 2020-01-22 |
KR20190120284A (ko) | 2019-10-23 |
MX2019010927A (es) | 2019-10-21 |
PH12019502094A1 (en) | 2020-03-09 |
SG11201908424QA (en) | 2019-10-30 |
EP3597786C0 (en) | 2023-08-09 |
TWI666341B (zh) | 2019-07-21 |
US11236409B2 (en) | 2022-02-01 |
JPWO2018169084A1 (ja) | 2019-03-22 |
CN110431249A (zh) | 2019-11-08 |
EP3597786A4 (en) | 2020-08-12 |
PL3597786T3 (pl) | 2023-09-25 |
TW201837207A (zh) | 2018-10-16 |
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