WO2016059741A1 - 高強度溶融亜鉛めっき鋼板 - Google Patents

高強度溶融亜鉛めっき鋼板 Download PDF

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Publication number
WO2016059741A1
WO2016059741A1 PCT/JP2015/004136 JP2015004136W WO2016059741A1 WO 2016059741 A1 WO2016059741 A1 WO 2016059741A1 JP 2015004136 W JP2015004136 W JP 2015004136W WO 2016059741 A1 WO2016059741 A1 WO 2016059741A1
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WIPO (PCT)
Prior art keywords
less
steel sheet
hot
dip galvanized
plating
Prior art date
Application number
PCT/JP2015/004136
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English (en)
French (fr)
Inventor
善継 鈴木
Original Assignee
Ｊｆｅスチール株式会社
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Filing date
Publication date
Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to US15/519,348 priority Critical patent/US9963771B2/en
Priority to EP15850575.0A priority patent/EP3178960B1/en
Priority to KR1020177010251A priority patent/KR101897054B1/ko
Priority to JP2015559355A priority patent/JP5907323B1/ja
Priority to MX2017004854A priority patent/MX370648B/es
Priority to CN201580055851.9A priority patent/CN106795612B/zh
Publication of WO2016059741A1 publication Critical patent/WO2016059741A1/ja

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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
    • 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
    • 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
    • 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
    • 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
    • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • 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
    • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • B32B15/043—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material of metal
    • 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
    • B32B15/18—Layered products comprising a layer of metal comprising iron or steel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
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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
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    • 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
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    • 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
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    • C23C2/40—Plates; Strips
    • C—CHEMISTRY; METALLURGY
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    • C22C18/04—Alloys based on zinc with aluminium as the next major constituent
    • 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
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Definitions

  • the present invention relates to a high-strength hot-dip galvanized steel sheet used for an inner plate of an automobile.
  • so-called high-strength steel sheets used as strength members are required to have strict workability and rust prevention of the processed parts, and therefore, excellent plating quality is required for the processed parts.
  • Patent Document 1 discloses a method for producing a hot-dip galvanized steel sheet having excellent slidability at the time of press work that regulates the amount of Al in the plating layer and the amount of Al at the interface between the plating layer and the steel sheet.
  • the plating quality and plating appearance such as the impact resistance adhesion and the corrosion resistance after coating have not yet been sufficiently considered, and further improvements have been demanded.
  • the present invention has been made in view of such circumstances, and an object thereof is to provide a high-strength hot-dip galvanized steel sheet excellent in plating quality and plating appearance of a processed part.
  • the present inventors do not simply perform hot dip galvanizing treatment as in the prior art, but (1) form an FeAl intermetallic compound with a predetermined property at the plating layer / steel plate interface, (2) In addition to controlling the solidification structure of plating, (3) controlling the texture of the surface, and (4) controlling the state of internal oxidation in the surface layer of the steel sheet, thereby improving the plating quality and plating appearance of the pressed parts.
  • the present inventors have found that an excellent high-strength hot-dip galvanized steel sheet can be provided, and have completed the present invention.
  • the present invention has been made based on the above findings, and the gist thereof is as follows.
  • the high strength molten steel sheet means a steel sheet having a tensile strength (TS) of 590 to 690 MPa.
  • a high-strength hot-dip galvanized steel sheet that is excellent in plating quality and plating appearance of a processed part.
  • the high-strength hot-dip galvanized steel sheet of the present invention has a hot-dip galvanized layer having a specific structure on a cold-rolled steel sheet having a specific structure.
  • an intermetallic compound having a specific configuration is formed between the cold-rolled steel sheet and the hot-dip galvanized layer.
  • the high-strength hot-dip galvanized steel sheet of the present invention is excellent in the plating quality and plating appearance of the processed part. More specifically, the high-strength hot-dip galvanized steel sheet of the present invention is excellent in plating adhesion, post-coating corrosion resistance, and plating appearance during an impact resistance test of a 60 ° bent portion.
  • the cold-rolled steel sheet constituting the high-strength hot-dip galvanized steel sheet of the present invention is C: 0.06% or more and 0.09% or less, Si: 0.30% or less, Mn: 1.7% or more and 2.3% or less.
  • the reasons for limiting the components of this cold-rolled steel sheet will be described.
  • “%” of each component in the steel sheet of the present invention indicates “mass%” unless otherwise specified.
  • C is one of the important basic components of steel.
  • the volume fraction of the austenite ( ⁇ ) phase when heated in the ( ⁇ (ferrite) + ⁇ (austenite)) region and thus martensite after transformation.
  • ⁇ austenite
  • Mechanical properties such as strength are greatly influenced by the martensite fraction and the hardness of the martensite phase. If the C content is less than 0.06%, the martensite phase is difficult to be generated. On the other hand, if it exceeds 0.09%, the spot weldability deteriorates, so the C content is 0.06% or more and 0.09%. The following.
  • Si 0.30% or less
  • Si is an element that improves the workability such as elongation by reducing the amount of solid solution C in the ferrite ( ⁇ ) phase. However, if the Si content exceeds 0.30%, the plating quality is impaired. The upper limit of the content is 0.30%.
  • Mn 1.7% to 2.3%
  • Mn is an important element as a basic component because it concentrates in the austenite ( ⁇ ) phase and promotes martensitic transformation.
  • the Mn content is less than 1.7%, the effect is not obtained.
  • the Mn content exceeds 2.3%, spot weldability and plating quality are remarkably impaired. Therefore, the Mn content is 1.7% or more and 2.3%. The following.
  • P 0.001% to 0.020%
  • P is an element effective in achieving high strength at a low cost.
  • the P content is set to 0.001% or more.
  • the P content exceeds 0.020%, the spot weldability is remarkably impaired, so the upper limit of the P content is 0.020%.
  • S not only causes hot cracking during hot rolling, but also induces breakage in the nugget of the spot weld, so it is desirable to reduce it as much as possible. Therefore, in the present invention, the S content is suppressed to 0.010% or less.
  • Mo 0.05% or more and 0.30% or less
  • Mo is an important element for obtaining a composite structure of ferrite and martensite without impairing the plating quality, and at least the Mo content needs to be 0.05%.
  • the Mo content is 0.30% as the upper limit.
  • N 0.005% or less
  • Al 0.01% or more and 0.10% or less
  • Al is an effective element that fixes N as AlN as a deoxidizer in the steelmaking process and causes aging deterioration, and in order to fully exhibit the effect, the Al content is 0.01% or more. To do. On the other hand, if the Al content exceeds 0.10%, the production cost increases, so the Al content must be suppressed to 0.10% or less.
  • the balance consists of Fe and inevitable impurities.
  • the cold-rolled steel sheet constituting the high-strength hot-dip galvanized steel sheet of the present invention described above has a specific structure and physical properties, and details thereof will be described below.
  • the area ratio of martensite mainly controls the C content in steel to 0.06% or more and 0.09% or less, and the heating temperature during annealing (the annealing temperature, which means the highest steel sheet temperature) Can be adjusted to 730 ° C. or higher and 880 ° C. or lower.
  • the area ratio of ferrite is 50% or more.
  • the area ratio of this ferrite is preferably 60% or more.
  • the area ratio of ferrite can be adjusted mainly by controlling the C content in the steel to 0.06% or more and 0.09% or less.
  • the area ratio of ferrite is the ratio of the area of the ferrite phase in the observation area
  • the area ratio of martensite is the ratio of the area of the martensite phase in the observation area.
  • the actual ferrite area ratio and martensite area ratio can be calculated as follows. That is, after the plate thickness direction cross section of the obtained steel plate is polished, it is corroded with 3% nital (alcohol solution containing 3% nitric acid). Then, the vicinity of the 1 ⁇ 4 position in the plate thickness direction is observed at a magnification of about 1500 times using an SEM (scanning electron microscope), and the obtained image is analyzed using general image analysis software. The area ratio can be obtained. In the obtained gray scale image, ferrite is gray (underlying structure), and martensite can be identified as a white structure.
  • the amount of internal oxidation of the surface layer of the steel sheet on the surface of the cold-rolled steel sheet is 0.05 g / m 2 or less per side.
  • oxidizable elements such as Si, Mn, Al, and P contained in the steel sheet are oxidized in a hot rolling process, an annealing process in a CGL (continuous hot dip galvanizing line), or the like.
  • the coiling temperature at the time of hot rolling should not be increased excessively or in the annealing atmosphere in CGL It is necessary not to raise the dew point excessively.
  • winding-up temperature does not exceed 700 degreeC for the descalability improvement.
  • the base metal surface layer part on the surface of the cold-rolled steel sheet refers to a surface layer part in contact with the hot dip galvanized layer, and can be said to be a steel sheet surface layer part directly under the plated layer after removing the plated layer.
  • the surface iron surface layer portion in which the internal oxidation amount is defined refers to a range from the interface between the hot dip galvanized layer and the steel plate to 50 ⁇ m in the thickness direction of the steel plate.
  • This amount of internal oxidation can be obtained by measuring the amount of oxygen in the steel after the plating layer is removed.
  • the method for removing the plating layer is not particularly limited, but any removal by acid or alkali is possible. However, care should be taken not to remove the base iron and to oxidize the surface after the removal by using an inhibitor (base iron dissolution inhibitor) together.
  • the plating layer can be removed with 195 cc of a 20 mass% NaOH-10 mass% triethanolamine aqueous solution and 7 cc of a 35 mass% H 2 O 2 aqueous solution.
  • the plating layer can be removed with a dilute HCl solution containing an inhibitor.
  • the amount of oxide in steel is measured, for example, by “impulse furnace melting-infrared absorption method”.
  • the high-strength hot-dip galvanized steel sheet of the present invention improves the adhesion at the impact resistance test of the processed part by containing an intermetallic compound at the interface between the plating layer and the steel sheet.
  • This intermetallic compound contains 0.12 g / m 2 or more and 0.22 g / m 2 or less of Al, and contains Fe 2 Al 5 having an average particle size of less than 1.0 ⁇ m.
  • the FeAl intermetallic compound can be formed at the interface between the plating layer and the steel sheet with a fine and dense property.
  • Al in order for Al to be less than 0.12 g / m 2 in the intermetallic compound, it is necessary to reduce the Al concentration in the hot dip zinc bath of plating, and when this Al concentration is too low, Dross deposits and the appearance deteriorates.
  • Al concentration in the hot dip zinc bath of plating in order to make Al more than 0.22 g / m 2 in the intermetallic compound, it is necessary to increase the Al concentration in the hot dip zinc bath of plating, and if this Al concentration is too high, the plating layer A large amount of an oxide film of Al is formed on the surface and spot weldability is deteriorated.
  • Average particle size of Fe 2 Al 5 is less than 1.0 ⁇ m
  • the average particle diameter of Fe 2 Al 5 is set to less than 1.0 ⁇ m.
  • the amount of the intermetallic compound produced depends on the activity of Al in the molten zinc bath, it can be increased mainly by increasing the Al concentration. However, if there is too much Al, the impact resistance characteristics deteriorate as described above, so it is necessary to control to an appropriate amount.
  • the average particle diameter of this Fe 2 Al 5 can be adjusted by controlling the Al concentration in the plating bath.
  • the average particle diameter is not particularly limited, but can be measured by a measuring method set to a predetermined magnification using a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • the hot-dip galvanized layer constituting the high-strength hot-dip galvanized steel sheet of the present invention contains Zn and 0.3% ⁇ Al% ⁇ 0.6%.
  • Al% indicates the Al content (mass%) of the hot-dip galvanized layer.
  • the hot-dip galvanized layer can also contain Pb, Sb, Mg, Ni, Mn, Si, Ti, Cr, Sr, and Ca.
  • Ra of hot dip galvanized layer surface 0.8 ⁇ m or more and 1.6 ⁇ m or less
  • surface roughness Ra of the hot dip galvanized layer surface is less than 0.8 ⁇ m, oil is not retained during pressing, and press formability during processing is poor.
  • Ra is set to 0.8 ⁇ m or more and 1.6 ⁇ m or less.
  • ⁇ Ra is adjusted by using a dull roll that has been processed with high roughness in the skin pass process to ensure an appropriate amount.
  • shot dull EDT (Electron discharged texture), EBT (Electron beam texturing), or a scratch dull processed dull roll is used.
  • said Ra is not specifically limited, Based on JISB0601 (2001), it can measure with a roughness meter.
  • Zinc base bottom surface orientation ratio: Zn (002) / (004) is 60% or more and 90% or less
  • the glossiness (G value) is set to 550 or more and 750 or less
  • the rate Zn (002) / (004) is 60% or more and 90% or less.
  • the solidification structure of the above plating mainly when the Al concentration in the bath reaches a predetermined value, an Fe-Al intermetallic compound is formed at the interface between the plating phase and the steel sheet, and the solidification structure of zinc develops soundly. If the Al concentration is too high, the solidified structure is formed in a dendritic shape, so that the surface is uneven and the glossiness (G value) is reduced to less than 550. Conversely, when the Al concentration is low, the formation of the Fe—Al intermetallic compound at the interface is suppressed, and at the same time, the Fe—Zn alloy layer develops.
  • the solidification structure is refined and smoothed, whereby the glossiness (G value) increases more than necessary and exceeds 750.
  • the glossiness (G value) is less than 550, it corresponds to adding excess Al to the zinc bath, and the spot weldability is deteriorated.
  • the glossiness (G value) exceeds 750, it corresponds to a case where the amount of Al added to the zinc bath is small, and Fe is eluted to cause surface defects due to dross.
  • the glossiness (G value) is not particularly limited, but can be measured with a glossometer based on JIS Z 8741 (1997).
  • the desired surface texture can be ensured by controlling the texture with a skin pass or the like.
  • the zinc base bottom surface orientation ratio Zn (002) / (004) is less than 60%, since the crystal size when zinc solidifies immediately after plating becomes small when the orientation of the zinc crystal is relatively random, It is too smooth and oil is not retained on the steel sheet during pressing, resulting in poor formability. If the orientation ratio of the basal plane of the zinc base exceeds 90%, the orientation of the basal plane of the Zn crystal is too high and the crystal grains are likely to grow. As a result, the dendritic arm develops. Deteriorate.
  • the zinc base bottom surface orientation ratio Zn (002) / (004) is not particularly limited, but can be obtained by measuring the X-ray diffraction intensity.
  • Zn has an hcp structure and is usually easily oriented on the basal plane, but the measurement of the zinc basal plane orientation ratio represented by formula (1) shows how much the crystals are randomly oriented.
  • the gloss, crystal size, and surface roughness are affected by the degree of orientation of the solidified structure, so accurate control of the zinc base orientation ratio is not only the surface properties of high-strength hot-dip galvanized steel sheets, but also press workability. It is extremely important when controlling.
  • the orientation is adjusted by securing an appropriate amount of the Fe—Al alloy layer at the plating phase / steel plate interface and suppressing the formation of the Fe—Zn alloy layer that serves as a precipitation nucleus of the zinc solidified structure.
  • the tensile strength (TS) is set to 590 MPa or more and 690 MPa or less in order to mainly secure the strength of the inner plate.
  • a high-strength hot-dip galvanized steel sheet having a tensile strength (TS) of 590 MPa or more and 690 MPa or less can be obtained by passing the CGL under appropriate annealing conditions for the material of the steel sheet component in the present invention.
  • the annealing temperature is preferably about 800 to 850 ° C. because annealing in a two-phase region is necessary.
  • a high-strength hot-dip galvanized steel sheet can be produced by the following method. First, steel having the above component composition is made into a slab by continuous casting, the slab is heated, and scale removal and rough rolling are performed. Next, after cooling, finish rolling, cooling, winding, pickling, and cold rolling are performed. Next, the steel sheet is annealed and hot-dip galvanized in a continuous hot-dip galvanizing facility.
  • the heating time, heating temperature, rough rolling conditions, cooling conditions, finish rolling conditions, winding conditions, etc. when heating the slab can be appropriately set based on common technical knowledge.
  • the annealing conditions of the steel sheet affect the yield stress of the high-strength hot-dip galvanized steel sheet.
  • the heating temperature during annealing (the annealing temperature, which means the highest steel sheet temperature) is set to 730 ° C. or higher and 880 ° C. or lower, preferably 800 It sets to 850 degreeC or more.
  • the annealing atmosphere may be adjusted as appropriate, but in the present invention, it is preferable to adjust the dew point to 0 ° C. or lower. Exceeding 0 ° C. is not preferable because the surface of the furnace body tends to become brittle.
  • the hydrogen concentration in the annealing atmosphere is preferably 1 vol% or more and 50 vol% or less. If the hydrogen concentration is 1 vol% or more, it is preferable because the surface of the steel sheet is activated, and if the hydrogen concentration exceeds 50 vol%, it is not preferable because it is economically disadvantageous.
  • Normally containing N 2 is other than hydrogen. Inevitable components include CO 2 , CO, O 2 and the like.
  • the intrusion plate temperature which is the temperature of the steel plate when the annealed steel plate enters the plating bath, is not particularly limited, but is preferably a plating bath temperature (bath temperature) of ⁇ 20 ° C. or higher and a bath temperature of + 20 ° C. or lower. If the infiltration plate temperature is in the above range, the change in bath temperature is small, and it is easy to perform desired hot dip galvanization continuously.
  • the Al content in the hot-dip galvanized layer and the Al content in the intermetallic compound tend to decrease by increasing the bath temperature. Moreover, the glossiness of the surface of the hot dip galvanized layer tends to increase as the bath temperature is increased.
  • the composition of the plating bath into which the annealed steel sheet enters is not limited as long as it contains Al in addition to Zn, and may contain other components as necessary.
  • the concentration of Al in the plating bath is not particularly limited, but is preferably 0.16% by mass or more and 0.25% by mass or less. It is preferable that the Al concentration is in the above range because an Fe—Al alloy phase is formed and the Fe—Zn alloy phase is suppressed.
  • the glossiness can be adjusted by the Al concentration in the plating bath. When the Al concentration in the plating bath is lowered, Fe—Zn crystals are formed slightly at the interface instead of Fe—Al, and they become Zn solidification nucleation sites, so that a large number of zinc crystals are generated, and the zinc crystal orientation is random.
  • a more preferable Al concentration is 0.19% by mass or more and 0.22% by mass or less. Since the Al concentration also affects the Al content in the hot-dip galvanized layer and the Al content in the intermetallic compound, it is preferable to determine the Al concentration in consideration of these contents.
  • the temperature of the plating bath is not particularly limited, but is preferably 430 ° C. or higher and 470 ° C. or lower.
  • a bath temperature of 430 ° C. or higher is preferred for the reason that the zinc bath dissolves stably without solidification, and a bath temperature of 470 ° C. or lower is preferred for the reason that Fe elution is small and dross defects are reduced.
  • a more preferable range of the bath temperature is 450 ° C. or higher and 465 ° C. or lower.
  • the immersion time when the steel sheet is immersed in the plating bath is not particularly limited, but is preferably 0.1 second or more and 5 seconds or less. When the immersion time is in the above range, a desired hot dip galvanized layer is easily formed on the surface of the steel plate.
  • the amount of plating adhesion is adjusted by gas jet wiping or the like.
  • the plating adhesion amount is not particularly limited, but is preferably in the range of 20 g / m 2 or more and 120 g / m 2 or less. If it is less than 20 g / m 2 , it may be difficult to ensure corrosion resistance. On the other hand, if it exceeds 120 g / m 2 , the plating peel resistance may deteriorate.
  • temper rolling is performed.
  • the type of roll used for the SK treatment is not particularly limited, and an Electro-Discharge Texture roll (EDT roll), an Electron Beam Texture roll (EBT roll), a shotdal roll, a topochrome roll, or the like can be used.
  • the rolling reduction rate during SK treatment is not particularly limited, but is preferably 0.7 to 0.9%. If the SK rolling reduction is in the above range, the surface roughness can be easily adjusted to the above preferable range. Further, if it is outside the above range, the press workability may be deteriorated without a dull eye that holds the lubricating oil, and the yield strength may also be lowered.
  • the cooling rate after the steel sheet is lifted from the plating bath is preferably ⁇ 5 ° C./second or more and ⁇ 30 ° C./second or less.
  • the high-strength hot-dip galvanized steel sheet of the present invention is preferably used for applications in which a coating film is formed on the surface of a hot-dip galvanized layer because it has excellent post-painting corrosion resistance after press working.
  • the high-strength hot-dip galvanized steel sheet of the present invention is excellent in plating adhesion even when applied to applications requiring strict workability, and does not significantly reduce corrosion resistance and mechanical properties. Examples of applications in which strict processability is required and a coating film is formed include automotive steel plates such as automobile outer plates and inner plates.
  • the formation method of a coating film is not specifically limited, After performing a chemical conversion treatment to the surface of a hot-dip galvanized layer and forming a chemical conversion film, it is preferable to form a coating film on this chemical conversion film.
  • Either a coating type or a reaction type can be used as the chemical conversion treatment liquid.
  • the component contained in a chemical conversion liquid is not specifically limited, either a chromate processing liquid may be used and a chromium free chemical conversion liquid may be used.
  • the chemical conversion film may be a single layer or a multilayer.
  • the coating method for forming the coating film is not particularly limited, but examples of the coating method include electrodeposition coating, roll coater coating, curtain flow coating, and spray coating.
  • the coating method include electrodeposition coating, roll coater coating, curtain flow coating, and spray coating.
  • means such as hot air drying, infrared heating, induction heating and the like can be used.
  • the steel sheet as shown in Table 1 was removed by pickling the hot-rolled steel sheet produced by winding at a coiling temperature of 650 ° C. or less, and cold rolled at a cold rolling reduction of 50%.
  • a cold-rolled material having a thickness of 1.2 mm or 2.3 mm was produced.
  • the line speed (LS) was 60 mpm or 100 mpm.
  • the bath temperature and the Al concentration in the bath were appropriately changed.
  • temper rolling SK treatment
  • the amount of adhesion was 55 g / m 2 per side. The results are shown in Table 2.
  • the obtained high-strength hot-dip galvanized steel sheet was first visually judged as good ( ⁇ ) when there was no poor appearance such as uneven plating as the appearance (plating appearance), and poor ( ⁇ ) when there was.
  • the Zn crystal orientation on the (002) plane and the (004) plane on X-ray were measured, and the zinc base surface orientation ratio on the surface of the hot-dip galvanized layer Zn (002) / (004) was measured.
  • the Al content of the hot-dip galvanized layer was peeled off with dilute hydrochloric acid containing an inhibitor, and quantified by ICP emission analysis.
  • the FeAl-enriched layer amount (total mass of the Fe 2 Al 5 alloy layer) was determined by peeling the galvanized layer with fuming nitric acid and quantifying the FeAl-enriched layer amount as Al by ICP emission analysis.
  • the average particle diameter of Fe 2 Al 5 constituting the intermetallic compound was measured by observing it at a magnification of 5000 using a scanning electron microscope (SEM).
  • composition of the intermetallic compound was Fe 2 Al 5 was determined by thin film X-ray diffraction.
  • the amount of internal oxidation was obtained by measuring the amount of oxygen in the steel after the plating layer was removed.
  • the amount of oxide in steel was measured by “impulse furnace melting-infrared absorption method”.
  • the steel of the sample obtained by mechanically polishing the front and back surface portions of the sample from which the plating layer has been removed 100 ⁇ m or more. Separately measure the amount of oxygen in the medium, and subtract from the amount of oxygen in the sample with the plating layer removed to calculate the amount of increase in oxidation only on the surface layer, and convert it to the amount per unit area to obtain the value of internal oxidation It was.
  • the cross section in the plate thickness direction of the obtained steel plate was polished and then corroded with 3% nital (3% nitric acid alcohol solution). Then, the vicinity of the 1 ⁇ 4 position in the plate thickness direction is observed at a magnification of about 1500 times using an SEM (scanning electron microscope), and the obtained image is analyzed using image analysis software, and the area ratio of each phase. Asked. In the obtained gray scale image, ferrite was gray (underlying structure), and martensite was identified as a white structure.
  • An impact resistance test was performed in which a punch with a hit diameter of 5/8 inch was dropped from a height of 1 m, and peeling with a Nichiban tape was observed. Those with peeling were marked with x, and those without peeling were marked with ⁇ .
  • a JIS No. 5 tensile test piece is taken from the sample in a direction 90 ° with respect to the rolling direction, and a tensile test is performed at a constant crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241 to obtain a tensile strength (TS (MPa)).
  • TS tensile strength
  • the high-strength hot-dip galvanized steel sheet of the present invention had very good characteristics despite the press work, no plating peeling, and excellent impact resistance adhesion. Moreover, the corrosion resistance after coating was also good. The plating appearance was also good.

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Abstract

　加工部のめっき品質およびめっき外観に優れた高強度溶融亜鉛めっき鋼板を提供する。　特定の組成からなり、マルテンサイト面積率が７％以上２５％未満、フェライト面積率が５０％以上の組織を有し、地鉄表層部の内部酸化量が片面当たり０．０５ｇ／ｍ２以下である冷延鋼板と、該冷延鋼板上に形成され、０．１２ｇ／ｍ２以上０．２２ｇ／ｍ２以下のＡｌを含み、かつ平均粒径１．０μｍ未満のＦｅ２Ａｌ５を含む金属間化合物と、該金属間化合物上に形成され、０．３％≦Ａｌ％≦０．６％（Ａｌ％は、溶融亜鉛めっき層のＡｌの含有量（質量％）を示す。）を含有し、表面粗さＲａが０．８μｍ以上１．６μｍ以下であり、光沢度（Ｇ値）が５５０以上７５０以下であり、亜鉛基底面配向率Ｚｎ（００２）／（００４）が６０％以上９０％以下である溶融亜鉛めっき層と、を有するようにする。

Description

高強度溶融亜鉛めっき鋼板
　本発明は、自動車の内板に使用される高強度溶融亜鉛めっき鋼板に関するものである。
　近年、自動車、家電、建材等の分野において素材鋼板に防錆性を付与した表面処理鋼板、中でも安価に製造でき、かつ防錆性に優れた溶融亜鉛めっき鋼板が使用されている。特に欧米の自動車メーカーは、めっき厚を簡単に増やせる溶融亜鉛めっき鋼板の適用により防錆性能を向上させることを考えており、経済成長著しい東アジア地区において大きな自動車用鋼板の需要が見込める状況である。
　良好な加工性が厳しく求められる自動車用鋼板の場合、プレス加工後における耐衝撃密着性や加工後の塗装後耐食性といっためっき品質が良好でなければ耐久性が維持できない。従来、十分なめっき品質を有する溶融亜鉛めっき鋼板を提供することは出来なかった。
　また、特に強度部材として使用されるいわゆる高強度鋼板についても厳しい加工性と加工部の防錆性が要求されるため、加工部に優れためっき品質が求められる。
　特許文献１ではめっき層中Ａｌ量、めっき層と鋼板の界面のＡｌ量を規定するプレス加工時の摺動性に優れた溶融亜鉛めっき鋼板の製造方法が開示されている。しかしながら、特許文献１に記載の技術では、加工部の耐衝撃密着性や塗装後耐食性といっためっき品質およびめっき外観については、まだ十分には考慮されておらず、更なる改良が求められていた。
特開２００４－３１５９６５号公報
　本発明はかかる事情に鑑みてなされたものであって、加工部のめっき品質およびめっき外観に優れた高強度溶融亜鉛めっき鋼板を提供することを目的とする。
　本発明者らは、鋭意検討の結果、従来技術の様に単に溶融亜鉛めっき処理するのではなく、（１）ＦｅＡｌ金属間化合物をめっき層／鋼板界面に所定の性状で形成させ、（２）めっきの凝固組織を制御すると共に、（３）表面のテクスチャーを制御し、かつ（４）鋼板表層部における内部酸化の状態を制御することで、プレス加工された加工部のめっき品質およびめっき外観に優れた高強度溶融亜鉛めっき鋼板を提供できることを見出し、本発明の完成に至った。本発明は、以上の知見に基づきなされたものであり、その要旨は以下の通りである。
［１］質量％で、Ｃ：０．０６％以上０．０９％以下、Ｓｉ：０．３０％以下、Ｍｎ：１．７％以上２．３％以下、Ｐ：０．００１％以上０．０２０％以下、Ｓ：０．０１０％以下、Ｍｏ：０．０５％以上０．３０％以下、Ｎ：０．００５％以下、Ａｌ：０．０１％以上０．１０％以下を含有し、残部がＦｅ及び不可避不純物の組成からなり、
　マルテンサイト面積率が７％以上２５％未満、フェライト面積率が５０％以上の組織を有し、地鉄表層部の内部酸化量が片面当たり０．０５ｇ／ｍ２以下である冷延鋼板と、
　該冷延鋼板上に形成された０．１２ｇ／ｍ２以上０．２２ｇ／ｍ２以下のＡｌを含み、かつ平均粒径１．０μｍ未満のＦｅ２Ａｌ５を含む金属間化合物と、
　該金属間化合物上に形成され、０．３％≦Ａｌ％≦０．６％（Ａｌ％は、溶融亜鉛めっき層のＡｌの含有量（質量％）を示す。）を含有し、表面粗さＲａが０．８μｍ以上１．６μｍ以下であり、光沢度（Ｇ値）が５５０以上７５０以下であり、以下の式（１）で規定される亜鉛基底面配向率Ｚｎ（００２）／（００４）が６０％以上９０％以下である溶融亜鉛めっき層と、を有する高強度溶融亜鉛めっき鋼板。
Figure JPOXMLDOC01-appb-M000001
  
　なお、本発明において、高強度溶融鋼板とは、引張強度（ＴＳ）が５９０～６９０ＭＰａの鋼板のことを指す。
　本発明によれば、加工部のめっき品質およびめっき外観に優れた高強度溶融亜鉛めっき鋼板が提供される。
　本発明の高強度溶融亜鉛めっき鋼板は、特定の構成の冷延鋼板上に、特定の構成の溶融亜鉛めっき層を有する。また、この冷延鋼板と溶融亜鉛めっき層間には特定の構成の金属間化合物が形成される。これにより、本発明の高強度溶融亜鉛めっき鋼板は、加工部のめっき品質およびめっき外観に優れる。より具体的には、本発明の高強度溶融亜鉛めっき鋼板は、６０°曲げ加工部の耐衝撃性試験時のめっき密着性と塗装後耐食性およびめっき外観に優れる。
　以下、本発明について具体的に説明する。
　＜冷延鋼板＞
　本発明の高強度溶融亜鉛めっき鋼板を構成する冷延鋼板は、Ｃ：０．０６％以上０．０９％以下、Ｓｉ：０．３０％以下、Ｍｎ：１．７％以上２．３％以下、Ｐ：０．００１％以上０．０２０％以下、Ｓ：０．０１０％以下、Ｍｏ：０．０５％以上０．３０％以下、Ｎ：０．００５％以下、Ａｌ：０．０１％以上０．１０％以下を含有し、残部がＦｅ及び不可避不純物の組成からなる。ここでは、まず、この冷延鋼板の成分限定理由について説明する。なお、本発明の鋼板中の各成分の「％」は、特に説明のない限り、「質量％」を示す。
　［Ｃ：０．０６％以上０．０９％以下］
　Ｃは鋼の重要な基本成分の一つであり、とくに本発明では、（α（フェライト）＋γ（オーステナイト））域に加熱したときのオーステナイト（γ）相の体積率、ひいては変態後のマルテンサイトの量に影響するために重要な元素である。そして、強度などの機械的特性は、このマルテンサイト分率とマルテンサイト相の硬度によって大きく左右される。Ｃ含有量が、０．０６％未満ではマルテンサイト相が生成しにくく、一方、０．０９％超ではスポット溶接性が劣化することから、Ｃ含有量は、０．０６％以上０．０９％以下とする。
　［Ｓｉ：０．３０％以下］
　Ｓｉはフェライト（α）相中の固溶Ｃ量を減少させることにより、伸びなどの加工性を向上させる元素であるが、０．３０％超のＳｉ量の含有はめっき品質を損ねるので、Ｓｉ含有量は上限を０．３０％とする。
　［Ｍｎ：１．７％以上２．３％以下］
　Ｍｎは、本発明においてはオーステナイト（γ）相に濃化し、マルテンサイト変態を促す効果があり、基本成分として重要な元素である。ただし、Ｍｎ含有量が１．７％未満ではその効果がなく、一方、２．３％を超えるとスポット溶接性およびめっき品質を著しく損なうので、Ｍｎ含有量は１．７％以上２．３％以下とする。
　［Ｐ：０．００１％以上０．０２０％以下］
　Ｐは、高強度化を安価に達成するうえで有効な元素であり、高強度化を実現するために、Ｐ含有量は、０．００１％以上とする。一方、Ｐを０．０２０％超えて含有するとスポット溶接性を著しく損なうので、Ｐ含有量の上限を０．０２０％とする。
　［Ｓ：０．０１０％以下］
　Ｓは熱延時の熱間割れを引き起こすもとになるほか、スポット溶接部のナゲット内破断を誘発するので、極力低減することが望ましい。よって、本発明では、Ｓ含有量は０．０１０％以下に抑制する。
　［Ｍｏ：０．０５％以上０．３０％以下］
　Ｍｏはめっき品質を損なうことなく、フェライト＋マルテンサイトの複合組織を得るのに重要な元素であり、少なくともＭｏ含有量を０．０５％とすることは必要である。しかし、０．３０％を超えてＭｏを含有しても、さらなる効果が少なく、製造コストの上昇を招くので、Ｍｏ含有量は０．３０％を上限とする。
　［Ｎ：０．００５％以下］
　Ｎは時効劣化をもたらすほか、降伏点（降伏比）の上昇、降伏伸びの発生を招くことから、Ｎ含有量は０．００５％以下に抑制する必要がある。
　［Ａｌ：０．０１％以上０．１０％以下］
　Ａｌは製鋼工程での脱酸剤として、また時効劣化を引き起こすＮをＡｌＮとして固定する有効な元素であり、その効果を十分に発揮するためにも、Ａｌ含有量は、０．０１％以上とする。一方、０．１０％を超えてＡｌを含有すると製造コストの上昇を招くので、Ａｌ含有量は０．１０％以下に抑える必要がある。
　本発明の鋼板は、残部は、Ｆｅ及び不可避不純物からなる。
　以上説明した本発明の高強度溶融亜鉛めっき鋼板を構成する冷延鋼板は、特定の組織及び物性を有し、以下でその詳細について説明する。
　（マルテンサイトの面積率：７％以上２５％未満）
　本発明の高強度溶融亜鉛めっき鋼板を構成する冷延鋼板では、マルテンサイトの面積率が７％未満になると、降伏比ＹＲが顕著に上昇する。一方、マルテンサイトの面積率が２５％以上になると、局部延性が低下するために全伸びＥＬが低下する。したがって、本発明の高強度溶融亜鉛めっき鋼板を構成する冷延鋼板では、マルテンサイトの面積率は、７％以上２５％未満とする。このマルテンサイトの面積率は、好ましくは７％以上２２％以下、より好ましくは７％以上２０％以下である。マルテンサイトの面積率は、主に鋼中のＣ含有量を０．０６％以上０．０９％以下に制御することと共に、焼鈍の際の加熱温度（焼鈍温度であり、鋼板最高到達温度を意味する。）を７３０℃以上８８０℃以下に設定することで調整することができる。
　（フェライトの面積率：５０％以上）
　本発明の高強度溶融亜鉛めっき鋼板を構成する冷延鋼板では、フェライトの面積率が５０％未満になると、全伸びＥＬの低下が顕著になる。したがって、フェライトの面積率は、５０％以上とする。このフェライトの面積率は、好ましくは６０％以上である。フェライトの面積率は、主に鋼中のＣ含有量を０．０６％以上０．０９％以下に制御することで調整することができる。
　ここで、フェライトの面積率は、観察面積内に占めるフェライト相の面積の割合であり、マルテンサイトの面積率は、観察面積内に占めるマルテンサイト相の面積の割合である。実際のフェライトの面積率およびマルテンサイトの面積率は、次のようにして算出することができる。すなわち、得られた鋼板の板厚方向断面を研磨した後、３％のナイタール（硝酸３％のアルコール液）で腐食する。そして、板厚方向の１／４位置付近をＳＥＭ（走査型電子顕微鏡）を用いて１５００倍程度の倍率で観察し、得られた画像を一般的な画像解析ソフトを用いて解析し、各相の面積率を求めることができる。得られたグレースケールの画像において、フェライトは灰色（下地組織）であり、マルテンサイトは白色を呈する組織として判別できる。
　（冷延鋼板の表面における地鉄表層部の内部酸化量が片面当たり０．０５ｇ／ｍ２以下）
　鋼板表層部（地鉄表層部）における内部酸化の状態を制御することで、高強度溶融亜鉛めっき鋼板における、プレス加工された加工部の耐衝撃性試験時のめっき密着性と塗装後耐食性を優れたものとすることができる。また、スポット溶接性も優れたものとすることができる。良好なめっき密着性の確保のためには、冷延鋼板の表面における地鉄表層部の内部酸化量が片面当たり０．０５ｇ／ｍ２以下であることが必要である。
　内部酸化は鋼板が含有するＳｉ、Ｍｎ、Ａｌ、Ｐ等の易酸化性元素が熱延工程やＣＧＬ（連続溶融亜鉛めっきライン）での焼鈍工程等で酸化されることで起こる。冷延鋼板の表面における地鉄表層部の内部酸化量を片面当たり０．０５ｇ／ｍ２以下にするためには、熱延時の巻き取り温度を過剰に上げないことや、ＣＧＬでの焼鈍雰囲気中の露点を過剰に上げないことが必要である。露点の適正範囲としては、ロール表面が酸化してロールが劣化することを避けるため、０℃を超えないことが好ましい。また、巻き取り温度は、脱スケール性向上のため、７００℃を超えないことが好ましい。
　なお、上記の冷延鋼板の表面における地鉄表層部とは、溶融亜鉛めっき層と接する表層部のことを指し、めっき層除去後のめっき層直下の鋼板表層部とも言える。また、この内部酸化量が規定される地鉄表層部とは、溶融亜鉛めっき層と鋼板の界面から鋼板の厚み方向に５０μｍまでの範囲を指す。
　上記の内部酸化量が片面当たり０．０５ｇ／ｍ２を超えると、加工部における粒界が脆化し、加工後のめっき密着性が劣化し、さらに溶接性も劣化する。
　この内部酸化量は、めっき層除去後の地鉄鋼中酸素量を測定することで得られる。めっき層の除去方法は特に問わないが、酸、アルカリによる除去のいずれでも可能である。但し、インヒビター（地鉄溶解抑制剤）の併用などにより、地鉄を除去しないことと、除去後の表面が酸化しないように注意する。一例として、２０質量％ＮａＯＨ－１０質量％トリエタノールアミン水溶液１９５ｃｃ＋３５質量％Ｈ２Ｏ２水溶液７ｃｃでめっき層を除去することが可能である。他にもインヒビターを含有する希ＨＣｌ溶液でもめっき層を除去することが可能である。
　鋼中酸化物量は、例えば「インパルス炉溶融－赤外線吸収法」で測定する。但し、めっき層直下の内部酸化量を見積もるには、母材自体が含有する酸素量を差し引く必要があるため、同様にめっき層を除去した試料の表裏の表層部を１００μｍ以上機械研磨した試料についての鋼中酸素量を別途測定し、めっき層を除去したままの試料の酸素量から差し引くことで、表層部のみの酸化増量を算出し、単位面積あたりの量に換算して値を得る。
　＜金属間化合物＞
　次に、前述した冷延鋼板上に形成される金属間化合物の構成について説明する。
　［Ａｌ：０．１２ｇ／ｍ２以上０．２２ｇ／ｍ２以下］
　本発明の高強度溶融亜鉛めっき鋼板は、めっき層と鋼板との界面に金属間化合物を含有することで、加工部の耐衝撃性試験時の密着性を向上させる。この金属間化合物は、０．１２ｇ／ｍ２以上０．２２ｇ／ｍ２以下のＡｌを有し、かつ平均粒径１．０μｍ未満のＦｅ２Ａｌ５を含有する。これにより、本発明の高強度溶融亜鉛めっき鋼板では、ＦｅＡｌ金属間化合物をめっき層と鋼板の界面に微細かつ緻密な性状で形成させることができる。
　ここで、Ａｌが、金属間化合物中０．１２ｇ／ｍ２未満であるようにするためには、めっきの溶融亜鉛浴中のＡｌ濃度を低くする必要があり、このＡｌ濃度が低過ぎると、ドロスが析出して外観性が劣化する。一方、金属間化合物中、Ａｌが０．２２ｇ／ｍ２を超えるようにするためには、めっきの溶融亜鉛浴中のＡｌ濃度を高くする必要があり、このＡｌ濃度が高過ぎると、めっき層表面にＡｌの酸化皮膜が多量に形成されてスポット溶接性が劣化する。
　（Ｆｅ２Ａｌ５の平均粒径１．０μｍ未満）
　Ｆｅ２Ａｌ５の平均粒径が１．０μｍ以上の場合は硬質のＦｅＡｌ金属間化合物が過剰成長している結果であるため、本発明の高強度溶融亜鉛めっき鋼板の耐衝撃特性が劣化する。そのため、Ｆｅ２Ａｌ５の平均粒径は１．０μｍ未満にする。
　この金属間化合物の生成量は溶融亜鉛浴中のＡｌの活量に依存するため、主にはＡｌ濃度を増加させることにより増加させることができる。但し、Ａｌが多すぎると前述のように耐衝撃特性が劣化するため適正な量に制御することが必要である。
　このＦｅ２Ａｌ５の平均粒径は、めっき浴中のＡｌ濃度を制御することにより調整することができる。
　なお、上記の平均粒径は、特に限定されないが、走査型電子顕微鏡（ＳＥＭ）を用い、所定の倍率に設定した測定方法により測定することができる。
　＜溶融亜鉛めっき層＞
　次に、冷延鋼板の表面に溶融亜鉛めっき処理を行うことによって形成され、前述した金属間化合物上に形成される溶融亜鉛めっき層の構成について説明する。
　［Ｚｎおよび０．３％≦Ａｌ％≦０．６％］
　本発明の高強度溶融亜鉛めっき鋼板を構成する溶融亜鉛めっき層は、Ｚｎおよび０．３％≦Ａｌ％≦０．６％を含有する。ここで、Ａｌ％は、溶融亜鉛めっき層のＡｌの含有量（質量％）を示す。Ａｌが０．３％未満であるようにするためには、めっきの溶融亜鉛浴中のＡｌ濃度を低くする必要があり、このＡｌ濃度が低過ぎると、Ｆｅの溶出があるため、ドロスが析出して外観性が劣化する。Ａｌが０．６％超えであると、めっき層表面にＡｌの酸化皮膜が多量に形成されてスポット溶接性が劣化する。また、溶融亜鉛めっき層は、Ｐｂ、Ｓｂ、Ｍｇ、Ｎｉ、Ｍｎ、Ｓｉ、Ｔｉ、Ｃｒ、Ｓｒ、Ｃａを含有することもできる。
　（溶融亜鉛めっき層表面の表面粗さＲａ：０．８μｍ以上１．６μｍ以下）
　溶融亜鉛めっき層表面の表面粗さＲａが０．８μｍ未満であると、プレス時に油が保持されずに加工時のプレス成形性が劣る。一方、Ｒａが１．６μｍ超えであると、塗装後鮮映性や密着性が劣る。そのため、Ｒａは、０．８μｍ以上１．６μｍ以下とする。
　Ｒａの調整はスキンパス処理において、高粗度加工を施したダルロールを用いることで適正量を確保する。ダル調整方法では、ショットダル、ＥＤＴ（Ｅｌｅｃｔｒｏｎ　ｄｉｓｃｈａｒｇｅｄ　ｔｅｘｔｕｒｉｎｇ）、ＥＢＴ（Ｅｌｅｃｔｒｏｎ　ｂｅａｍ　ｔｅｘｔｕｒｉｎｇ）、スクラッチダル加工ダルロールを用いる。なお、上記のＲａは、特に限定されないが、ＪＩＳ　Ｂ０６０１（２００１年）に基づいて粗度計により測定することができる。
　（溶融亜鉛めっき層表面の光沢度（Ｇ値）：５５０以上７５０以下）かつ（亜鉛基底面配向率：Ｚｎ（００２）／（００４）が６０％以上９０％以下）
　本発明の高強度溶融亜鉛めっき鋼板では、めっきの凝固組織を制御するために、まず、光沢度（Ｇ値）を５５０以上７５０以下とし、以下の式（１）で規定される亜鉛基底面配向率Ｚｎ（００２）／（００４）を６０％以上９０％以下とする。亜鉛基底面配向率Ｚｎ（００２）／（００４）を６０％以上９０％以下とすることにより、ｈｃｐ（ｈｅｘａｇｏｎａｌ　ｃｌｏｓｅ－ｐａｃｋｅｄ：六方最密充填構造）の亜鉛を凝固する際に基底面に配向し易くすることができる。
Figure JPOXMLDOC01-appb-M000002
  
　上記のめっきの凝固組織については、主には浴中のＡｌ濃度が所定の値になるとめっき相と鋼板界面にＦｅ－Ａｌ金属間化合物が生成し、亜鉛の凝固組織が健全に発達する。そして、Ａｌ濃度が高過ぎると、凝固組織はデンドライド状に形成されるため、表面に凹凸が多くなり、光沢度（Ｇ値）が低下し、５５０未満となる。逆に、Ａｌ濃度が低いと界面のＦｅ－Ａｌ金属間化合物の形成が抑制されると同時にＦｅ－Ｚｎ合金層が発達する。これにより、Ｚｎ凝固核の基点を増やすため凝固組織が微細化し平滑化することで光沢度（Ｇ値）が必要以上に増加し、７５０を超える。光沢度（Ｇ値）が５５０未満である場合は過剰なＡｌを亜鉛浴中に添加することに相当し、スポット溶接性が劣化することとなる。また、光沢度（Ｇ値）が７５０を超えると、Ａｌを亜鉛浴中に添加する量が少ない場合に相当し、Ｆｅが溶出してドロスによる表面欠陥を引き起こすこととなる。
　なお、上記の光沢度（Ｇ値）は、特に限定されないが、ＪＩＳ　Ｚ　８７４１（１９９７年）に基づいて光沢度計により測定することができる。所望の表面性状は、スキンパス等でテクスチャーを制御することで確保することができる。
　また、亜鉛基底面配向率Ｚｎ（００２）／（００４）が６０％未満であると、亜鉛結晶の配向が比較的ランダムの場合、めっき直後に亜鉛が凝固する際の結晶サイズが細かくなるため、平滑すぎてプレス時に鋼板に油が保持されずに成形性に劣る。亜鉛基底面配向率が９０％超えだとＺｎ結晶の基底面の配向が高すぎて結晶粒が成長しやすく、結果としてデンドライドアームが発達するため、塗装後鮮映性が劣るだけでなく耐食性も劣化する。なお、上記の亜鉛基底面配向率Ｚｎ（００２）／（００４）は、特に限定されないが、Ｘ線回折強度の測定により得ることができる。
　Ｚｎはｈｃｐ構造をとり通常は基底面に配向し易いが、式（１）で示されている亜鉛基底面配向率の測定により、どの程度結晶がランダムに配向したかが分かる。この凝固組織の配向程度によって光沢、結晶サイズ、表面での粗度が影響されるため、本亜鉛基底配向率を正確に制御することは高強度溶融亜鉛めっき鋼板の表面性状だけでなくプレス加工性を制御する際に極めて重要である。配向性は、前述のようにめっき相／鋼板界面のＦｅ－Ａｌ合金層を適正量確保し、亜鉛凝固組織の析出核となるＦｅ－Ｚｎ合金層の形成を抑制することで調整する。
　（引張強度（ＴＳ）：５９０ＭＰａ以上６９０ＭＰａ以下）
　本発明の高強度溶融亜鉛めっき鋼板では、引張強度（ＴＳ）は主に内板の強度を確保するため、５９０ＭＰａ以上６９０ＭＰａ以下とする。このような引張強度（ＴＳ）が５９０ＭＰａ以上６９０ＭＰａ以下の高強度溶融亜鉛めっき鋼板は、本発明での鋼板成分の材料について適正な焼鈍条件でＣＧＬ通板することで得ることが出来る。焼鈍温度としては、２相域での焼鈍が必要であるため、８００～８５０℃程度の温度とすることが好ましい。
　［加工部の耐衝撃性試験時のめっき密着性（耐衝撃密着性）と塗装後耐食性］
　自動車のサイドメンバーなどの加工部が自動車の衝突時の衝撃による加工を模擬するため、９０°曲げした部分を対象とする。この部分をいわゆるデュポン試験器で耐衝撃性を調査し、テープ剥離して評価する。塗装後耐食性はこの曲げた部分について化成処理・電着塗装を実施し、クロスカットを施しＳＳＴ試験で膨れ腐食幅で評価する。上述のように界面のＦｅ－Ａｌ金属間化合物の量が必要以上に確保出来ていれば、良好なめっき密着性（耐衝撃密着性）を確保出来る。
　＜高強度溶融亜鉛めっき鋼板の製造方法＞
　続いて、高強度溶融亜鉛めっき鋼板の製造方法について説明する。例えば、以下の方法で高強度溶融亜鉛めっき鋼板を製造可能である。先ず、上記のような成分組成を有する鋼を連続鋳造によりスラブとし、該スラブを加熱し、スケール除去および粗圧延を施す。次いで、冷却した後、仕上げ圧延し、冷却し、巻取り、次いで、酸洗、冷間圧延を行う。次いで、連続式溶融亜鉛めっき設備において、鋼板の焼鈍および溶融亜鉛めっき処理を行う。
　スラブを加熱する際の加熱時間、加熱温度、粗圧延の条件、冷却条件、仕上げ圧延の条件、巻取りの条件等は、技術常識に基づいて適宜設定可能である。ただし、本発明において地鉄表層部での内部酸化量を上記の範囲に調整するためには、仕上げ圧延（熱間圧延）の条件や巻取り温度を調整することが好ましい。
　また、鋼板の焼鈍の条件は、高強度溶融亜鉛めっき鋼板の降伏応力に影響を与える。本発明においては、引張強度を上記範囲に設定するために、焼鈍の際の加熱温度（焼鈍温度であり、鋼板最高到達温度を意味する）を７３０℃以上８８０℃以下に設定し、好ましくは８００℃以上８５０℃以下に設定する。
　また、焼鈍雰囲気の調整も適宜行えばよいが、本発明においては露点を０℃以下に調整することが好ましい。０℃超えにすると炉体表面が脆化しやすいという理由で好ましくない。
　また、焼鈍雰囲気中の水素濃度は１ｖｏｌ％以上５０ｖｏｌ％以下であることが好ましい。水素濃度が１ｖｏｌ％以上であれば鋼板表面を活性化するという理由で好ましく、水素濃度が５０ｖｏｌ％超えであれば経済的に不利という理由で好ましくない。なお、水素以外はＮ２を通常含有する。不可避的に含有される成分としてはＣＯ２、ＣＯ、Ｏ２等が挙げられる。
　本発明においては、溶融亜鉛めっき層のＡｌ含有量を制御し、鋼板と溶融亜鉛めっき層との間に金属間化合物を存在させるために、溶融亜鉛めっき処理の条件を調整する必要がある。また、溶融亜鉛めっき層の表面状態（表面粗さＲａ、光沢度（Ｇ値）、亜鉛基底面配向率）を所望の状態にするためにも、溶融亜鉛めっき処理の条件を調整する必要がある。以下、溶融亜鉛めっき処理の条件について説明する。
　焼鈍後の鋼板がめっき浴に浸入する際の鋼板の温度である浸入板温は、特に限定されないが、めっき浴の温度（浴温）－２０℃以上浴温＋２０℃以下であることが好ましい。浸入板温が上記範囲にあれば、浴温の変化が小さく、所望の溶融亜鉛めっき処理を連続して行いやすい。溶融亜鉛めっき層中のＡｌ含有量、金属間化合物中のＡｌ含有量は、浴温を上げることで、低下する傾向にある。また、溶融亜鉛めっき層の表面の光沢度は浴温を上げると上昇する傾向にある。
　焼鈍後の鋼板が浸入するめっき浴の組成はＺｎ以外にＡｌを含むものであればよく、必要に応じて他の成分が含まれていてもよい。めっき浴中のＡｌの濃度は特に限定されないが、０．１６質量％以上０．２５質量％以下であることが好ましい。Ａｌの濃度が上記範囲にあればＦｅ－Ａｌ合金相が形成されてＦｅ－Ｚｎ合金相が抑制されるために好ましい。光沢度はめっき浴中のＡｌ濃度により調整可能である。めっき浴中のＡｌ濃度が低くなると界面にＦｅ－ＡｌではなくＦｅ－Ｚｎ結晶が僅かに形成され、それがＺｎ凝固核発生サイトとなることで多数の亜鉛結晶が生成し、亜鉛結晶配向がランダム化することで配向率が低下する傾向にある。その結果、Ａｌ濃度が低いほど、デンドライド状のＺｎ結晶成長が抑制されて、表面の凹凸が低減して平滑化するため、光沢度が上昇する。より好ましいＡｌの濃度は０．１９質量％以上０．２２質量％以下である。なお、Ａｌ濃度は、溶融亜鉛めっき層中のＡｌ含有量、金属間化合物中のＡｌ含有量にも影響を与えるため、これらの含有量も考慮してＡｌ濃度を決定することが好ましい。
　また、めっき浴の温度（浴温）は特に限定されないが、４３０℃以上４７０℃以下が好ましい。浴温が４３０℃以上であれば亜鉛浴が凝固せずに安定的に溶解するという理由で好ましく、浴温が４７０℃以下であればＦｅ溶出が少なくドロス欠陥が低減するという理由で好ましい。より好ましい浴温の範囲は４５０℃以上４６５℃以下である。
　鋼板をめっき浴への浸漬させる際の浸漬時間は特に限定されないが、０．１秒以上５秒以下であることが好ましい。浸漬時間が上記範囲にあることで、鋼板の表面に所望の溶融亜鉛めっき層を形成しやすい。
　鋼板をめっき浴から引き上げた直後にガスジェットワイピング等でめっき付着量を調整する。本発明においてめっき付着量は特に限定されないが、２０ｇ／ｍ２以上１２０ｇ／ｍ２以下の範囲であることが好ましい。２０ｇ／ｍ２未満では耐食性の確保が困難になる場合がある。一方、１２０ｇ／ｍ２を超えると耐めっき剥離性が劣化する場合がある。
　上記のようにしてめっき付着量を調整後、調質圧延（ＳＫ処理）を行う。ＳＫ処理に用いるロールの種類は特に限定されず、Ｅｌｅｃｔｒｏ－Ｄｉｓｃｈａｒｇｅ　Ｔｅｘｔｕｒｅロール（ＥＤＴロール）、Ｅｌｅｃｔｒｏｎ　Ｂｅａｍ　Ｔｅｘｔｕｒｅロール（ＥＢＴロール）、ショットダルロール、トポクロムロール等を使用可能である。
　ＳＫ処理の際の圧下率（ＳＫ圧下率（％））も特に限定されないが、０．７～０．９％であることが好ましい。ＳＫ圧下率が上記範囲にあれば、表面粗さを上記好ましい範囲に調整しやすい。また、上記範囲外であると、潤滑油を保持するダル目がつかずにプレス加工性が低下する場合があり、また、降伏強度も低下する場合がある。
　鋼板をめっき浴から引き上げた後の冷却速度は、－５℃／秒以上－３０℃／秒以下であることが好ましい。
　以上の通り、本発明の高強度溶融亜鉛めっき鋼板を説明したが、以下では本発明の高強度溶融亜鉛めっき鋼板の使用について説明する。
　本発明の高強度溶融亜鉛めっき鋼板は、プレス加工後の塗装後耐食性に優れるため、溶融亜鉛めっき層の表面に塗膜が形成される用途に使用されることが好ましい。また、本発明の高強度溶融亜鉛めっき鋼板は、厳しい加工性が要求される用途に適用してもめっき密着性に優れ、耐食性や機械特性も大幅に低下することは無い。厳しい加工性が要求され且つ塗膜が形成される用途としては、自動車の外板、内板等の自動車用鋼板が挙げられる。塗膜の形成方法は特に限定されないが、溶融亜鉛めっき層の表面に化成処理を施し、化成皮膜を形成した後、この化成皮膜上に塗膜を形成することが好ましい。
　化成処理液としては、塗布型、反応型のいずれも使用可能である。また、化成処理液に含まれる成分も特に限定されず、クロメート処理液を使用してもよいし、クロムフリー化成処理液を使用してもよい。また、化成皮膜は単層であってもよいし、複層であってもよい。
　塗膜を形成するための塗装方法は特に限定しないが、塗装方法としては電着塗装、ロールコーター塗装、カーテンフロー塗装、スプレー塗装等が挙げられる。また、塗料を乾燥させるために、熱風乾燥、赤外線加熱、誘導加熱等の手段を用いることができる。
　以下、本発明を、実施例に基づいて具体的に説明する。なお、本発明は以下の実施例に限定されない。
　表１に示すような鋼組成の、巻き取り温度６５０℃以下で巻き取って製造した熱延鋼板の黒皮スケールを酸洗で除去して、冷延圧下率５０％で冷間圧延して板厚１．２ｍｍまたは２．３ｍｍの冷延素材を製造した。その後、表面を脱脂処理して表２に示す条件で、焼鈍、溶融亜鉛めっき処理を行った。ライン速度（ＬＳ）は６０ｍｐｍまたは１００ｍｐｍとした。浴温、浴中Ａｌ濃度は適宜変更した。めっき付着量を調整した後、調質圧延（ＳＫ処理）を行い、ＳＫ処理に用いるロールはＥＤＴ加工ロールを使用し適宜圧下率を変更した。付着量は片面当たり５５ｇ／ｍ２とした。結果を表２に示す。
Figure JPOXMLDOC01-appb-T000001
  
　得られた高強度溶融亜鉛めっき鋼板について、まず外観性（めっき外観）として、めっきムラなどの外観不良がない場合は良好（○）、ある場合には不良（×）と目視で判定した。
　また、ＪＩＳ　Ｚ　８７４１（１９９７年）に基づいて、光沢度計で６０度鏡面光沢度（Ｇ値）を測定した。
　Ｘ線回折装置でθ－２θスキャン方式を用い、Ｘ線で（００２）面のＺｎ結晶配向性及び（００４）面のＺｎ結晶配向性を測定し、溶融亜鉛めっき層表面の亜鉛基底面配向率Ｚｎ（００２）／（００４）を測定した。
　ＪＩＳ　Ｂ０６０１（２００１年）に基づいて、粗度計で溶融亜鉛めっき層表面の表面粗さＲａを測定した。
　溶融亜鉛めっき層のＡｌ含有量は、インヒビター入りの希塩酸で剥離し、ＩＣＰ発光分析法で定量した。
　金属間化合物の組成として、ＦｅＡｌ富化層量（Ｆｅ２Ａｌ５合金層の総質量）は発煙硝酸で亜鉛めっき層を剥離し、ＩＣＰ発光分析法でＦｅＡｌ富化層量をＡｌとして定量した。
　金属間化合物を構成するＦｅ２Ａｌ５の平均粒径は、走査型電子顕微鏡（ＳＥＭ）を用い、５０００倍で観察して測定した。
　金属間化合物の組成は薄膜Ｘ線回折でＦｅ２Ａｌ５であるかどうかについて判定した。
　内部酸化量はめっき層除去後の地鉄鋼中酸素量を測定することで得た。鋼中酸化物量は、「インパルス炉溶融－赤外線吸収法」で測定した。めっき層直下の内部酸化量を見積もるには、母材自体が含有する酸素量を差し引く必要があるため、同様にめっき層を除去した試料の表裏の表層部を１００μｍ以上機械研磨した試料についての鋼中酸素量を別途測定し、めっき層を除去したままの試料の酸素量から差し引くことで、表層部のみの酸化増量を算出し、単位面積あたりの量に換算して内部酸化量の値を得た。
　マルテンサイト面積率及びフェライト面積率の測定としては、まず、得られた鋼板の板厚方向断面を研磨した後、３％のナイタール（硝酸３％のアルコール液）で腐食した。そして、板厚方向の１／４位置付近をＳＥＭ（走査型電子顕微鏡）を用いて１５００倍程度の倍率で観察し、得られた画像を画像解析ソフトを用いて解析し、各相の面積率を求めた。得られたグレースケールの画像において、フェライトは灰色（下地組織）であり、マルテンサイトは白色を呈する組織として判別した。
　加工部の耐衝撃密着性は、長さ８０ｍｍ×幅３０ｍｍのサンプルを長さ方向４０ｍｍの位置で、曲げＲ＝１．５ｍｍで６０°曲げした部分について、曲げ部の外側凸部の部分に１８４３ｇで撃芯径　５／８ｉｎｃｈのポンチを高さ１ｍから落下させる耐衝撃性試験を実施し、ニチバンテープで剥離してめっき剥離をみた。剥離があるものを×、無いものを○とした。
　　○：密着良好
　　×：密着不良
　また同じ加工処理を施した部分を化成処理、電着塗装、中塗り、上塗りの総合塗装を実施し、塗装後耐食性を調査した。ＪＩＳ　Ｚ　２３７１（２０００年）に基づく塩水噴霧試験を１０日間行い、曲げ加工部外側における顕著な膨れ有無を評価した。
良好（○）：膨れ無し
不良（×）：膨れ有り
　試料から圧延方向に対して９０°方向にＪＩＳ５号引張試験片を採取し、ＪＩＳ　Ｚ　２２４１の規定に準拠してクロスヘッド速度１０ｍｍ／ｍｉｎ一定で引張試験を行い、引張強度（ＴＳ（ＭＰａ））を測定した。
Figure JPOXMLDOC01-appb-T000002
　表２から明らかなように、本発明の高強度溶融亜鉛めっき鋼板はプレス加工したにもかかわらず特性が極めて良好でありめっき剥離が起こらず、耐衝撃密着性に優れていた。また、塗装後耐食性も良好であった。また、めっき外観も良好であった。
 

Claims (1)

  1. 　質量％で、Ｃ：０．０６％以上０．０９％以下、Ｓｉ：０．３０％以下、Ｍｎ：１．７％以上２．３％以下、Ｐ：０．００１％以上０．０２０％以下、Ｓ：０．０１０％以下、Ｍｏ：０．０５％以上０．３０％以下、Ｎ：０．００５％以下、Ａｌ：０．０１％以上０．１０％以下を含有し、残部がＦｅ及び不可避不純物の組成からなり、
    　マルテンサイト面積率が７％以上２５％未満、フェライト面積率が５０％以上の組織を有し、
    　地鉄表層部の内部酸化量が片面当たり０．０５ｇ／ｍ２以下である冷延鋼板と、
    　該冷延鋼板上に形成され、０．１２ｇ／ｍ２以上０．２２ｇ／ｍ２以下のＡｌを含み、かつ平均粒径１．０μｍ未満のＦｅ２Ａｌ５を含む金属間化合物と、
    　該金属間化合物上に形成され、
    　０．３％≦Ａｌ％≦０．６％（Ａｌ％は、溶融亜鉛めっき層のＡｌの含有量（質量％）を示す。）を含有し、
    　表面粗さＲａが０．８μｍ以上１．６μｍ以下であり、
    　光沢度（Ｇ値）が５５０以上７５０以下であり、
    　以下の式（１）で規定される亜鉛基底面配向率Ｚｎ（００２）／（００４）が６０％以上９０％以下である溶融亜鉛めっき層と、
    を有する高強度溶融亜鉛めっき鋼板。
    Figure JPOXMLDOC01-appb-M000003
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