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WO2018139191A1 - めっき密着性に優れた高強度溶融めっき鋼板およびその製造方法 - Google Patents

めっき密着性に優れた高強度溶融めっき鋼板およびその製造方法 Download PDF

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Publication number
WO2018139191A1
WO2018139191A1 PCT/JP2018/000247 JP2018000247W WO2018139191A1 WO 2018139191 A1 WO2018139191 A1 WO 2018139191A1 JP 2018000247 W JP2018000247 W JP 2018000247W WO 2018139191 A1 WO2018139191 A1 WO 2018139191A1
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Prior art keywords
steel sheet
hot
less
rolling
plating
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PCT/JP2018/000247
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English (en)
French (fr)
Inventor
正貴 木庭
祐介 伏脇
長滝　康伸
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Ｊｆｅスチール株式会社
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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 JP2018524845A priority Critical patent/JP6436268B1/ja
Priority to MX2019008776A priority patent/MX2019008776A/es
Priority to KR1020197021210A priority patent/KR102303592B1/ko
Priority to CN201880008066.1A priority patent/CN110249069A/zh
Priority to EP18745297.4A priority patent/EP3575432B1/en
Priority to US16/480,428 priority patent/US11148395B2/en
Priority to KR1020217022086A priority patent/KR102359573B1/ko
Publication of WO2018139191A1 publication Critical patent/WO2018139191A1/ja

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    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
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    • 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
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    • 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
    • C21D8/0273—Final recrystallisation annealing
    • 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.]
    • Y10T428/12736—Al-base component
    • Y10T428/1275—Next to Group VIII or IB metal-base component
    • Y10T428/12757—Fe
    • 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]
    • 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
    • Y10T428/12861—Group VIII or IB metal-base component
    • Y10T428/12951—Fe-base component
    • 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
    • Y10T428/12861—Group VIII or IB metal-base component
    • Y10T428/12951—Fe-base component
    • Y10T428/12958—Next to Fe-base component
    • 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
    • Y10T428/12861—Group VIII or IB metal-base component
    • Y10T428/12951—Fe-base component
    • Y10T428/12958—Next to Fe-base component
    • Y10T428/12965—Both containing 0.01-1.7% carbon [i.e., steel]
    • 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
    • Y10T428/12861—Group VIII or IB metal-base component
    • Y10T428/12951—Fe-base component
    • Y10T428/12972—Containing 0.01-1.7% carbon [i.e., steel]

Definitions

  • the present invention relates to a high-strength hot-dip galvanized steel sheet excellent in plating adhesion, which uses a high-strength steel sheet containing Si and Mn as a base material, and a method for producing the same.
  • a hot dip galvanized steel sheet is manufactured by using a steel sheet obtained by hot rolling or cold rolling a slab as a base material, recrystallizing the base steel sheet in a CGL annealing furnace, and then performing a hot dip coating process. . Further, the alloyed hot-dip galvanized steel sheet is manufactured by further alloying after hot-dip plating.
  • hot dip plated steel sheets used for the above applications are extremely important for plating adhesion that can withstand severe processing such as hole expansion.
  • hot-plated steel sheets containing Si and Mn which are easily oxidizable elements, have defects such as unplating and non-alloyed treatment because Si and Mn oxides are formed on the steel surface during recrystallization annealing. Not only is it easy, but the plating adhesion is impaired.
  • Patent Document 1 proposes a method of improving powdering resistance by applying Ni plating before heating a steel plate and then performing hot dipping treatment.
  • Patent Document 2 proposes a method for improving the adhesion of a plating film in which the surface of a base material is ground, heated to 600 ° C. or higher in a reducing atmosphere, cooled, hot-dip plated, and then alloyed. .
  • Patent Document 3 a hot rolling steel sheet or an annealed cold rolled steel sheet is subjected to a light reduction with a rolling reduction of 1.0 to 20%, and a low temperature heat treatment is performed at 520 to 650 ° C. for 5 seconds or more.
  • Patent Document 1 requires a step of performing Ni plating on the surface of the base material before heating.
  • the method proposed in Patent Document 2 requires a grinding process.
  • both Patent Document 1 and Patent Document 2 require processing that is costly and troublesome, and thus there is a problem in that productivity is reduced.
  • JP 2010-196146 A Japanese Patent Laid-Open No. 10-81948 JP 2002-317257 A
  • 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 adhesion and a method for producing the same.
  • the present inventors diligently studied to solve the above problems. As a result, it has been found that a high-strength hot-dip galvanized steel sheet having excellent plating adhesion can be obtained by including a peeled steel in the plating layer.
  • the present invention is based on the above findings, and features are as follows.
  • component composition in mass%, C: 0.02% to 0.30%, Si: 0.01% to 2.0%, Mn: 0.2% to 3.0%, P: 0.08% or less, S: 0.02% or less, Al: 0.001% or more and 0.40% or less, with the balance being plated on one side on a steel plate made of Fe and inevitable impurities It has a plating layer having an amount of 30 to 90 g / m 2 , and the plating layer contains 0.3 to 1.5 g / m 2 of peeled iron. Strength hot-dip galvanized steel sheet.
  • a component composition further, by mass, Ti: 0.01% to 0.40%, Nb: 0.001% to 0.200%, V: 0.001% to 0.500%
  • Mo 0.01% or more and 0.50% or less
  • W 0.001% or more and 0.200% or less
  • B One or more of 0.0003% or more and 0.01% or less are contained.
  • a steel slab having the composition described in [1] or [2] above is hot-rolled, rolled at a rolling reduction of 1 to 10%, pickled, and then rolled at a rolling reduction of 0.3 to
  • the high strength hot dip galvanized steel sheet of the present invention is a steel sheet having a tensile strength (TS) of 590 MPa or more, and a hot-rolled steel sheet or a cold-rolled steel sheet is used as a base material, and galvanizing treatment is performed. Anything that performs processing is included.
  • the plating includes plating such as Zn plating, Zn—Al plating, and Al plating.
  • a high-strength hot-dip galvanized steel sheet having excellent plating adhesion can be obtained. Since it has high corrosion resistance even after processing, it is effective for the production of members having complicated molding, and the industrial effect obtained by the present invention is great.
  • FIG. 1 is a view showing a state of a peeled iron of the galvannealed steel sheet.
  • the unit of the content of each element of the steel component composition and the unit of the content of each element of the plating component composition are “mass%”, and are simply “%” unless otherwise specified. Show.
  • the unit of hydrogen concentration is “vol%”, and is simply “%” unless otherwise specified.
  • the high-strength hot-dip galvanized steel sheet having excellent plating adhesion according to the present invention is, in mass%, C: 0.02% to 0.30%, Si: 0.01% to 2.0%, Mn: 0.00. 2% or more and 3.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.001% or more and 0.40% or less, with the balance being Fe and inevitable impurities
  • the steel sheet has a plating layer having a plating adhesion amount of 30 to 90 g / m 2 on one side, and the plating layer contains 0.3 to 1.5 g / m 2 of peeled iron. To do.
  • the present invention is characterized in that the peeled steel is taken into the plating layer, thereby preventing the development of cracks in the plating layer generated during processing and improving the plating adhesion. As a result, a high-strength hot-dip galvanized steel sheet excellent in plating adhesion can be obtained.
  • the C content is 0.02% or more. Preferably it is 0.04% or more.
  • the C content is set to 0.30% or less. Preferably it is 0.20% or less.
  • Si 0.01% or more and 2.0% or less Si is effective as a solid solution strengthening element, and 0.01% or more is necessary to increase the strength of the steel sheet.
  • the Si content is set to 0.01% or more and 2.0% or less.
  • Mn 0.2% or more and 3.0% or less Mn is an element useful for increasing the strength of steel. In order to obtain this effect, it is necessary to contain 0.2% or more of Mn. However, if Mn is contained excessively, the wettability during hot dipping is impaired, and the alloying reactivity is impaired, so that it is difficult to adjust the alloying, and the plating appearance and plating adhesion are reduced. From the above, the Mn content is 0.2% or more and 3.0% or less. Preferably they are 0.3% or more and 2.6% or less.
  • S 0.02% or less S needs to be 0.02% or less in order to reduce toughness when segregation or MnS is produced in large amounts at grain boundaries. Therefore, the S content is 0.02% or less.
  • the lower limit of the content is not particularly limited, and may be about the impurity level.
  • Al 0.001% or more and 0.40% or less Al is added for the purpose of deoxidation of molten steel, but when the content is less than 0.001%, the purpose is not achieved. On the other hand, if the content exceeds 0.40%, a large amount of inclusions are generated, which causes wrinkling of the steel sheet. Therefore, the Al content is set to be 0.001% or more and 0.40% or less.
  • the balance is Fe and inevitable impurities.
  • Ti 0.01% or more and 0.40% or less
  • Nb 0.001% or more and 0.200% or less
  • V 0.001% or more and 0.000% or more by mass%.
  • Mo 0.01% or more and 0.50% or less
  • W 0.001% or more and 0.200% or less
  • B 0.0003% or more and 0.01% or less, 1 type or 2 types or more Can be contained.
  • Ti, Nb, V, Mo, W and B are elements necessary for precipitating precipitates (particularly carbides) in the base steel sheet, and one or two elements selected from the group consisting of these elements It is preferable to add the above. Usually, these elements are often contained in the form of precipitates containing these elements in the base steel sheet. Among these elements, Ti is particularly an element that has a high precipitation strengthening ability and is effective from the viewpoint of cost. However, if the addition amount is less than 0.01%, the amount of precipitates in the base steel sheet necessary for containing precipitates (particularly carbides) in the alloyed hot-dip plated layer may be insufficient. On the other hand, if it exceeds 0.40%, the effect is saturated and the cost is increased.
  • the Ti content is set to 0.01% or more and 0.40% or less.
  • Nb, V, Mo, W, and B for the same reason regarding the upper and lower limits of the Ti content range, when Nb content is contained, the Nb content is 0.001% or more and 0.200% or less, and V content is contained. The amount is 0.001% to 0.500%, the Mo content is 0.01% to 0.50%, the W content is 0.001% to 0.200%, and the B content is 0.0003. % Or more and 0.01% or less.
  • the plating layer has a plating adhesion amount of 30 to 90 g / m 2 on one side. Adhesion amount to secure the corrosion resistance is difficult is less than 30 g / m 2. On the other hand, if it exceeds 90 g / m 2 , the plating peel resistance deteriorates.
  • the peeled steel is included in the plating layer. This is an important requirement in the present invention.
  • exfoliated iron in the plating layer, it is possible to prevent the development of cracks in the plating layer that occur during processing.
  • the interface between the plating layer and the base iron becomes non-smooth because the base iron peels off.
  • an alloyed plating layer for example, in alloyed hot dip galvanizing, cracks generally develop in the interface of ⁇ phase and ⁇ phase or in that phase, but exfoliated iron is included, that is, brittle. It is considered that cracks are suppressed by the presence of soft ground iron in the alloy plating layer.
  • the peeled iron contained in the plating layer is 0.3 to 1.5 g / m 2 .
  • the amount of contained peeled iron is less than 0.3 g / m 2, the effect of improving plating adhesion cannot be expected.
  • it exceeds 1.5 g / m 2 it leads to variations in the amount of plating adhesion and the appearance is deteriorated.
  • the peeled iron is equivalent to the part surrounded by the solid line shown in FIG. 1, and is the part that is peeled off from the steel and completely separated and taken into the plating layer.
  • the content of the peeled steel can be measured by the method of Examples described later.
  • the steel slab having the above composition is hot-rolled, rolled at a reduction rate of 1 to 10%, pickled, and then rolled at a reduction rate of 0.3 to 5%, followed by hot dipping treatment.
  • hot rolling after rough rolling, after rolling at a finish rolling temperature of 820 ° C. or higher, winding is performed at a winding temperature of 450 to 650 ° C.
  • a furnace atmosphere with a hydrogen concentration of 2 to 30 vol% and a dew point of ⁇ 60 to ⁇ 10 ° C. a steel sheet reaching temperature of 600 to 950 ° C. before rolling and hot dipping treatment at a rolling reduction of 0.3 to 5%.
  • an alloying treatment may be further performed after the hot dipping treatment.
  • rolling before and after pickling is an important requirement. Specific conditions for pickling before and after pickling will be described later.
  • Hot rolling hot rolling start temperature (slab heating temperature) (preferred conditions)
  • the heating temperature (slab heating temperature) before hot rolling is preferably 1100 ° C. or higher.
  • the slab heating temperature before hot rolling is preferably 1100 ° C. or higher and 1300 ° C. or lower. More preferably, it is 1100 degreeC or more and 1200 degrees C or less.
  • the finish rolling temperature is preferably set to 820 ° C. or higher.
  • the finish rolling temperature is 820 ° C. or higher, more preferably 820 ° C. or higher and 1000 ° C. or lower. More preferably, it is 850 degreeC or more and 950 degrees C or less.
  • Hot rolling coiling temperature (preferred conditions)
  • the steel plate according to the present invention contains oxidizable elements such as Si, Mn, and Ti. Therefore, in order to suppress excessive oxidation of the steel sheet and ensure good surface properties, the winding temperature is preferably 650 ° C. or lower.
  • the hot rolling coiling temperature is preferably 450 ° C. or higher and 650 ° C. or lower. More preferably, it is 450 degreeC or more and 600 degrees C or less.
  • the hot-rolled steel sheet obtained by rolling hot rolling / rolling with a reduction ratio of 1 to 10% is descaled by pickling and then rolled.
  • Pickling is not particularly limited, and may be performed by a conventional method.
  • rolling is performed at a stage before pickling. It is an important requirement in the present invention that rolling is performed before the pickling.
  • moderate unevenness is imparted to the steel sheet surface by pressing the scale, and the incorporation of peeled iron into the plating layer is promoted.
  • the rolling reduction is 1 to 10%.
  • the rolling reduction is less than 1%, unevenness is not sufficiently imparted to the steel sheet surface, and a sufficient plating adhesion improving effect cannot be obtained.
  • the rolling reduction exceeds 10%, the scale bites into the ground iron, and the descaling property is remarkably lowered.
  • Rolling with a rolling reduction of 0.3 to 5% after pickling In the present invention, rolling with a rolling reduction of 0.3 to 5% is performed after pickling. It is an important requirement in the present invention to perform rolling at a rolling reduction of 0.3 to 5% after pickling.
  • the surface shape is controlled and the residual stress is introduced into the base metal surface.
  • the rolling reduction is 0.3% or more, the residual stress is sufficiently introduced, and the plating reactivity on the steel sheet surface is improved. If the rolling reduction exceeds 5%, the effect of improving the plating reactivity due to the introduction of strain is saturated, the steel plate surface is smoothed, and it becomes difficult to incorporate the peeled iron into the plating layer.
  • Annealing It is preferable to perform annealing after rolling at a rolling reduction of 0.3 to 5% and before hot dipping treatment.
  • the preferred conditions are a furnace atmosphere with a hydrogen concentration of 2 to 30 vol% and a dew point of ⁇ 60 to ⁇ 10 ° C., and a steel sheet temperature of 600 to 950 ° C.
  • the annealing temperature is lower than 600 ° C., the oxide film after pickling is not completely reduced, and desired plating characteristics may not be obtained.
  • the surface of Si, Mn, etc. may be concentrated and the plating property may be deteriorated. More preferably, the steel iron reaching temperature is 650 ° C.
  • the atmosphere in the furnace is preferably a hydrogen concentration of 2 to 30% and a dew point of ⁇ 60 to ⁇ 10 ° C.
  • the atmosphere in the furnace may be any reducing atmosphere, and an atmosphere composed of an inert gas with a dew point of ⁇ 60 to ⁇ 10 ° C., a hydrogen concentration of 2 to 30%, and the balance is preferable.
  • the dew point is higher than ⁇ 10 ° C., the form of the Si oxide generated on the steel sheet surface tends to be a film. More preferably, the dew point is ⁇ 20 ° C. or lower. On the other hand, a dew point lower than ⁇ 60 ° C. is difficult to realize industrially.
  • the hydrogen concentration is lower than 2%, the reducibility is weak. If it is 30% or less, sufficient reducing ability can be obtained. More preferably, the hydrogen concentration is 5% or more and 20% or less.
  • Hot dipping treatment is preferably carried out in a continuous hot dipping line using a hot dipping bath after reduction annealing of the steel sheet.
  • the composition of the hot dip bath is, for example, in the case of hot dip galvanizing treatment, the Al concentration is in the range of 0.01 to 0.25%, and the balance is Zn and inevitable impurities.
  • the Al concentration is less than 0.01%, a Zn—Fe alloying reaction occurs during the plating treatment, a brittle alloy layer develops at the interface between the plating and the steel plate (base material), and the plating adhesion deteriorates.
  • the Al concentration exceeds 0.25%, the growth of the Fe—Al alloy layer becomes remarkable and the plating adhesion is hindered.
  • the plating bath temperature is not particularly limited, and may be 440 ° C. or higher and 480 ° C. or lower which is a normal operation range.
  • the alloying temperature is preferably 550 ° C. or lower. More preferably, it is 530 degrees C or less.
  • the alloying treatment temperature is desirably 480 ° C. or higher.
  • the alloying treatment time is preferably 10 seconds or more and 60 seconds or less from the viewpoint of cost and control.
  • the heating method in the alloying treatment is not particularly limited, and any known method such as radiant heating, current heating, high frequency induction heating, or the like may be used. After alloying, cool to room temperature.
  • the post-treatment after plating is not particularly limited, and post-treatment that is usually performed such as adjustment of the material by temper rolling, adjustment of the flat shape by leveling or the like, and further chromate treatment may be performed as necessary.
  • the galvanizing bath temperature was 460 ° C., and the amount of adhesion was adjusted to 50 g / m 2 by wiping.
  • the alloying treatment was performed at an alloying temperature of 520 ° C.
  • the amount of peeled iron in the galvanized layer was measured by the following procedure using ICP emission spectroscopic analysis. First, only the galvanized layer in the plated steel sheet is peeled and dissolved by dissolving the galvanized layer with dilute hydrochloric acid to which an inhibitor is added.
  • the inhibitor is an additive used for preventing overdissolution of the base iron, and may be a commercially available one.
  • the hydrochloric acid pickling corrosion inhibitor “Ibit No. 700BK” manufactured by Asahi Chemical Industry Co., Ltd. was added to hydrochloric acid diluted 10 to 100 times to a concentration of 0.6 g / L.
  • a solution containing undissolved peeled iron is extracted and divided into two solutions. Of these solutions, one solution is subjected to composition analysis with the undissolved steel strip undissolved, and the other solution is re-dissolved with hydrochloric acid without addition of the inhibitor and then subjected to composition analysis. The difference between the results thus obtained was defined as the amount of peeled iron.
  • TS Tensile strength
  • JIS Z2201 JIS No. 5 tensile specimen taken from a hot-dip galvanized steel sheet (GI) or alloyed hot-dip galvanized steel sheet (GA) in a direction perpendicular to the rolling direction, and a strain rate of 10 -3 / s.
  • a tensile test in accordance with Z 2241 was conducted to obtain TS.
  • Appearance Appearance after hot-dip plating and alloying treatment was visually observed.
  • the case without unplating and non-uniformity of the alloy was evaluated as “ ⁇ ”, and the case with non-plating and non-uniformity of the alloy was evaluated as “X”.
  • the plating adhesion of the hot dip galvanized steel sheet was evaluated by a ball impact test.
  • a ball impact test was performed under the conditions of a ball weight of 2.8 kg and a drop height of 1 m, the processed part was peeled off with tape, and the presence or absence of peeling of the plating layer was visually determined.
  • ⁇ No plating layer peeling ⁇ Plating layer peeling Plating adhesion of the galvannealed steel sheet was evaluated by testing the powdering resistance. A cellophane tape is applied to the galvannealed steel sheet, the tape surface is bent 90 degrees, bent back, and the tape is peeled off.
  • the amount of plating peeled per bent portion 10 mm ⁇ 40 mm was measured as a Zn count number by fluorescent X-rays and evaluated in accordance with the following criteria.
  • the examples of the present invention have good surface appearance and plating adhesion (powdering resistance).
  • any one or more of surface appearance and plating adhesion (powdering resistance) is inferior.
  • the high-strength hot-dip galvanized steel sheet according to the present invention is suitably used as an automobile part whose strength and thickness are rapidly increasing in recent years.

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Abstract

めっき密着性に優れた高強度溶融めっき鋼板およびその製造方法を提供する。　成分組成として、質量％で、Ｃ：０．０２％以上０．３０％以下、Ｓｉ：０．０１％以上２．０％以下、Ｍｎ：０．２％以上３．０％以下、Ｐ：０．０８％以下、Ｓ：０．０２％以下、Ａｌ：０．００１％以上０．４０％以下を含有し、残部がＦｅおよび不可避的不純物からなる鋼板上に、片面あたりのめっき付着量が３０～９０ｇ／ｍ２のめっき層を有し、めっき層中には、剥離地鉄を０．３～１．５ｇ／ｍ２含有する。

Description

めっき密着性に優れた高強度溶融めっき鋼板およびその製造方法
　本発明は、Ｓｉ、Ｍｎを含有する高強度鋼板を母材とする、めっき密着性に優れた高強度溶融めっき鋼板およびその製造方法に関するものである。
　従来、自動車用鋼板の分野を中心に、素材鋼板に防錆性を付与した表面処理鋼板、中でも防錆性に優れた溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板が使用されてきた。しかし近年、燃費の向上や耐衝突特性の向上のため、鋼板の薄肉化、高強度化が推進されている。
　一般的に、溶融めっき鋼板は、スラブを熱間圧延や冷間圧延した鋼板を母材として用い、母材鋼板をＣＧＬの焼鈍炉で再結晶焼鈍し、その後、溶融めっき処理を行い製造される。また、合金化溶融めっき鋼板は、溶融めっき後、さらに合金化処理を行い製造される。
　上記のような用途に使用される溶融めっき鋼板は、表面外観がよいことに加え、穴広げ加工を初めとした厳しい加工に耐えられるめっき密着性が極めて重要である。しかしながら、易酸化性元素であるＳｉやＭｎを含有する溶融めっき鋼板は、再結晶焼鈍中に鋼表面にＳｉ及びＭｎ酸化物が形成されるため、不めっきや合金化未処理などの欠陥が生じやすいだけでなく、めっき密着性が損なわれる。
　上記問題を解決するためにいくつかの提案がなされている。例えば、特許文献１では、鋼板を加熱する前にＮｉめっきを施し、次いで溶融めっき処理をすることで耐パウダリング性を改善する方法が提案されている。
　特許文献２には、母材表面を研削した後に還元性雰囲気中で６００℃以上に加熱し、冷却して溶融めっき処理し、次いで合金化処理するめっき皮膜の密着性改善方法が提案されている。
　特許文献３には、熱延鋼板または焼鈍済みの冷延鋼板に、圧下率が１．０～２０％の軽圧下を施し、５２０～６５０℃で５秒以上保持する低温加熱処理を施し、質量％でＡｌ：０．０１～０．１８％を含有する溶融亜鉛めっき浴に浸漬し、次いで合金化処理する合金化溶融亜鉛めっき鋼板の製造方法が提案されている。
　しかしながら、特許文献１で提案されている方法は、加熱前に母材表面にＮｉめっきを施す工程を必要とする。特許文献２で提案されている方法は研削処理を必要とする。このように、特許文献１、特許文献２のいずれもコストと手間がかかる処理を必要とするため、生産性の低下を招くという問題があった。
　また、特許文献３で提案されている方法では、現在の高強度鋼板において要求される高い強度、加工性に対応できる、十分に高いレベルのめっき密着性は得られておらず、加工部の耐食性に必ずしも寄与するものではなかった。
特開２０１０－１９６１４６号公報 特開平１０－８１９４８号公報 特開２００２－３１７２５７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、めっき密着性に優れた高強度溶融めっき鋼板およびその製造方法を提供することを目的とする。
　本発明者らは、上記の課題を解決すべく、鋭意検討した。その結果、めっき層中に剥離地鉄を含有させることによって、めっき密着性に優れた高強度溶融めっき鋼板が得られることを見出した。
　本発明は上記知見に基づくものであり、特徴は以下の通りである。
［１］成分組成として、質量％で、Ｃ：０．０２％以上０．３０％以下、Ｓｉ：０．０１％以上２．０％以下、Ｍｎ：０．２％以上３．０％以下、Ｐ：０．０８％以下、Ｓ：０．０２％以下、Ａｌ：０．００１％以上０．４０％以下を含有し、残部がＦｅおよび不可避的不純物からなる鋼板上に、片面あたりのめっき付着量が３０～９０ｇ／ｍ２のめっき層を有し、該めっき層中には、剥離地鉄を０．３～１．５ｇ／ｍ２含有することを特徴とするめっき密着性に優れた高強度溶融めっき鋼板。
[２]成分組成として、さらに、質量％で、Ｔｉ：０．０１％以上０．４０％以下、Ｎｂ：０．００１％以上０．２００％以下、Ｖ：０．００１％以上０．５００％以下、Ｍｏ：０．０１％以上０．５０％以下、Ｗ：０．００１％以上０．２００％以下、Ｂ：０．０００３％以上０．０１％以下のうち１種または２種以上を含有することを特徴とする上記［１］に記載のめっき密着性に優れた高強度溶融めっき鋼板。
［３］上記［１］または［２］に記載の成分組成を有する鋼スラブを、熱間圧延し、圧下率１～１０％で圧延し、酸洗を行い、次いで、圧下率０．３～５％で圧延し、溶融めっき処理を行うことを特徴とするめっき密着性に優れた高強度溶融めっき鋼板の製造方法。
［４］前記熱間圧延において、粗圧延後、仕上げ圧延温度８２０℃以上で圧延終了した後、巻取温度４５０～６５０℃で巻き取ることを特徴とする上記［３］に記載のめっき密着性に優れた高強度溶融めっき鋼板の製造方法。
［５］圧下率０．３～５％で前記圧延後前記溶融めっき処理前に、水素濃度２～３０ｖｏｌ％かつ露点－６０～－１０℃の炉内雰囲気、鋼板到達温度６００～９５０℃で連続焼鈍を行うことを特徴とする上記［３］または［４］に記載のめっき密着性に優れた高強度溶融めっき鋼板の製造方法。
［６］前記溶融めっき処理後、さらに、合金化処理を行うことを特徴とする上記［３］～［５］のいずれかに記載のめっき密着性に優れた高強度溶融めっき鋼板の製造方法。
　なお、本発明の高強度溶融めっき鋼板は、引張強度（TS）５９０ＭＰａ以上の鋼板であり、熱延鋼板あるいは冷延鋼板を母材とし、溶融めっき処理を行うもの、溶融めっき処理後さらに合金化処理を行うもの、いずれも含むものである。また、めっきとしては、Ｚｎめっき、Ｚｎ－Ａｌめっき、Ａｌめっきなどのめっきを対象とする。
　本発明によれば、めっき密着性に優れた高強度溶融めっき鋼板が得られる。加工後にも高い耐食性を有することから、複雑な成型を有する部材の製造に効果的であり、本発明により得られる工業上の効果は大きい。
図１は、合金化溶融亜鉛めっき鋼板の剥離地鉄の状態を示す図である。
　以下、本発明について具体的に説明する。
なお、以下の説明において、鋼成分組成の各元素の含有量の単位およびめっきの成分組成の各元素の含有量の単位はいずれも「質量％」であり、特に断らない限り単に「％」で示す。また、水素濃度の単位はいずれも「ｖｏｌ％」であり、特に断らない限り単に「％」で示す。
　本発明のめっき密着性に優れた高強度溶融めっき鋼板は、質量％で、Ｃ：０．０２％以上０．３０％以下、Ｓｉ：０．０１％以上２．０％以下、Ｍｎ：０．２％以上３．０％以下、Ｐ：０．０８％以下、Ｓ：０．０２％以下、Ａｌ：０．００１％以上０．４０％以下を含有し、残部がＦｅおよび不可避的不純物からなる鋼板上に、片面あたりのめっき付着量が３０～９０ｇ／ｍ２のめっき層を有し、めっき層中には、剥離地鉄を０．３～１．５ｇ／ｍ２含有することを特徴とする。すなわち本発明は、めっき層中に剥離地鉄を取り込ませることによって、加工中に生じるめっき層内のクラックの進展を防ぎ、めっき密着性を向上させることを特徴とする。以上の結果、めっき密着性に優れた高強度溶融めっき鋼板が得られることになる。
　まず、本発明の対象とするめっき密着性に優れた高強度溶融めっき鋼板の成分組成の限定理由について説明する。
　Ｃ：０．０２％以上０．３０％以下
Ｃは少ないほど母材の成形性が良好となるが、Ｃを含有させることで鋼板の強度を安価に高めることができる。従って、Ｃ含有量は０．０２％以上とする。好ましくは０．０４％以上である。一方、Ｃを過剰に含有させると鋼板の靱性や溶接性が低下するので、Ｃ含有量は０．３０％以下とする。好ましくは０．２０％以下である。
　Ｓｉ：０．０１％以上２．０％以下
Ｓｉは固溶強化元素として有効であり、鋼板の強度を高めるためにも０．０１％以上が必要である。しかしながら、Ｓｉを過度に含有させると溶融めっき時の濡れ性を損ない、合金化反応性を損なうために合金化の調整が困難となり、めっき外観やめっき密着性の低下を招く。以上より、Ｓｉ含有量は０．０１％以上２．０％以下とする。
　Ｍｎ：０．２％以上３．０％以下
Ｍｎは鋼の強度を高めるのに有用な元素である。この効果を得るには、Ｍｎを０．２％以上含有させる必要がある。しかしながら、Ｍｎを過度に含有させると溶融めっき時の濡れ性を損ない、合金化反応性を損なうために合金化の調整が困難となり、めっき外観やめっき密着性の低下を招く。以上より、Ｍｎ含有量は０．２％以上３．０％以下とする。好ましくは０．３％以上２．６％以下である。
　Ｐ：０．０８％以下
Ｐが０．０８％を超えて含有すると溶接性が劣化すると共に、表面品質が劣化する。また、合金化処理時には合金化処理温度をより高くしないと所望の合金化度とすることができないが、合金化処理温度を上昇させると母材鋼板の延性が劣化すると同時に合金化溶融めっき層の密着性が劣化する。そのため、Ｐ含有量は０．０８％以下とする。
　Ｓ：０．０２％以下
Ｓは粒界に偏析またはＭｎＳが多量に生成した場合、靭性を低下させるため、含有量を０．０２％以下とする必要がある。そのため、Ｓ含有量は０．０２％以下とする。含有量の下限は特に限定されず、不純物程度であってもよい。
　Ａｌ：０．００１％以上０．４０％以下
Ａｌは溶鋼の脱酸を目的に添加されるが、その含有量が０．００１％未満の場合、その目的が達成されない。一方、０．４０％を超えて含有すると、介在物が多量に発生し、鋼板の疵の原因となる。そのため、Ａｌ含有量は０．００１％以上０．４０％以下とする。
　残部はＦe および不可避的不純物である。
　本発明では、下記を目的として、さらに、質量％で、Ｔｉ：０．０１％以上０．４０％以下、Ｎｂ：０．００１％以上０．２００％以下、Ｖ：０．００１％以上０．５００％以下、Ｍｏ：０．０１％以上０．５０％以下、Ｗ：０．００１％以上０．２００％以下、Ｂ：０．０００３％以上０．０１％以下のうち１種または２種以上を含有することができる。
　Ｔｉ、Ｎｂ、Ｖ、Ｍｏ、ＷおよびＢは、母材鋼板中に析出物（特に、炭化物）を析出させるために必要な元素であり、これらの元素からなる群から選ばれる１種または２種以上を添加することが好ましい。通常、これらの元素は、母材鋼板中でこれらの元素を含む析出物の形で含有される場合が多い。
これらの元素のなかで、特にＴｉは析出強化能が高く、コストの観点からも有効な元素である。しかしながら、添加量が０．０１％未満では合金化溶融めっき層中に析出物（特に、炭化物）を含有させるために必要な母材鋼板中の析出物量が不十分な場合がある。一方、０．４０％を超えるとその効果は飽和し、コストアップとなる。そのため、Ｔｉを含有する場合は、Ｔｉ含有量は、０．０１％以上０．４０％以下とする。
なお、Ｎｂ、Ｖ、Ｍｏ、ＷおよびＢについても上記Ｔｉの含有範囲の上限および下限に関する同様の理由から、含有する場合は、Ｎｂ含有量は０．００１％以上０．２００％以下、Ｖ含有量は０．００１％以上０．５００％以下、Ｍｏ含有量は０．０１％以上０．５０％以下、Ｗ含有量は０．００１％以上０．２００％以下、Ｂ含有量は０．０００３％以上０．０１％以下である。
　続いて、めっき層について説明する。
めっき層は、片面あたりのめっき付着量が３０～９０ｇ／ｍ２である。付着量が３０ｇ／ｍ２未満では耐食性の確保が困難になる。一方、９０ｇ／ｍ２を超えると耐めっき剥離性が劣化する。
　また、めっき層中に剥離地鉄を含有させる。本発明において、重要な要件である。めっき層中に剥離地鉄を含有させることで、加工中に生じるめっき層内のクラックの進展を防ぐことができる。溶融めっきの場合、地鉄が剥離することでめっき層と地鉄の界面が非平滑化する。その結果、めっき層と地鉄の界面で亀裂が進展しにくくなり、密着性が向上すると考えられる。また、合金化めっき層の場合、例えば合金化溶融亜鉛めっきでは、一般的にはΓ相とδ相の界面あるいはその相内で亀裂が進展するが、剥離地鉄を含有させる、すなわち、脆性な合金めっき層中に軟質な地鉄が存在することで亀裂が抑制されると考えられる。
　めっき層中に含有させる剥離地鉄は０．３～１．５ｇ／ｍ２である。含有剥離地鉄量が０．３ｇ／ｍ２よりも小さいと、めっき密着性向上効果が期待できない。一方、１．５ｇ／ｍ２を超えると、めっき付着量のばらつきにつながり、外観が劣化する。
　なお、剥離地鉄とは、図１に示す実線で囲まれた部分に相当し、地鉄から剥離し完全に分離し、めっき層内に取り込まれている部分である。
　剥離地鉄は、後述する実施例の方法にて含有量を測定することができる。
　次に、本発明のめっき密着性に優れた高強度溶融めっき鋼板の製造方法について、説明する。
　上記成分組成を有する鋼スラブを、熱間圧延し、圧下率１～１０％で圧延し、酸洗を行い、次いで、圧下率０．３～５％で圧延し、溶融めっき処理を行う。好ましくは、熱間圧延において、粗圧延後、仕上げ圧延温度８２０℃以上で圧延終了した後、巻取温度４５０～６５０℃で巻き取る。また、好ましくは、圧下率０．３～５％での圧延後溶融めっき処理前に、水素濃度２～３０ｖｏｌ％かつ露点－６０～－１０℃の炉内雰囲気、鋼板到達温度６００～９５０℃で連続焼鈍を行う。また、溶融めっき処理後、さらに、合金化処理を行ってもよい。本発明において、酸洗前後に圧延を行うことは重要な要件である。酸洗前後に酸洗を行う際の具体的な条件については後述する。
　熱間圧延
熱間圧延開始温度（スラブ加熱温度）（好適条件）
ＴｉやＮｂ等の微細析出の分散を行うためには、熱間圧延を行う前にＴｉやＮｂ等を一旦鋼板中に溶解させる必要がある。そのため、熱間圧延する前の加熱温度（スラブ加熱温度）は１１００℃以上が好ましい。一方で、１３００℃を超えて加熱した場合には、鋼表層での内部酸化が促進され、表面性状が劣化する恐れがある。よって、熱間圧延前のスラブ加熱温度は１１００℃以上１３００℃以下が好ましい。より好ましくは１１００℃以上１２００℃以下である。
　仕上げ圧延温度（好適条件）
熱間圧延時の変形抵抗を小さくし、操業を容易にするために、仕上げ圧延温度を８２０℃以上とすることが好ましい。一方、１０００℃を超えて仕上げ圧延した場合には、スケール疵が発生しやすくなり、表面性状が劣化することがある。よって、仕上げ圧延温度は８２０℃以上、より好ましくは８２０℃以上１０００℃以下とする。より好ましくは８５０℃以上９５０℃以下である。
　熱延巻取り温度（好適条件）
本発明にかかる鋼板は、ＳｉやＭｎ、Ｔｉを初めとした易酸化性元素を含有する。そのため、鋼板の過度な酸化を抑制し、良好な表面性状を確保するためには、巻取り温度は６５０℃以下であることが好ましい。一方、巻取り温度が４５０℃未満の場合には、冷却ムラに起因したコイル性状不良が生じやすくなるために、生産性を損なう恐れがある。よって、熱延巻取り温度は４５０℃以上６５０℃以下が好ましい。より好ましくは４５０℃以上６００℃以下である。
　熱延巻取り後、酸洗前に圧下率１～１０％の圧延
熱間圧延・熱延巻取り工程によって得られた熱延鋼板は、酸洗によって脱スケールを施し、その後、圧延を実施する。酸洗は特に限定せず、常法でよい。ここで、本発明では、酸洗の前段階で圧延を施す。酸洗の前段階で圧延を施すことは本発明において重要な要件である。酸洗の前段階で圧延を施すことで、スケール押付による鋼板表面への適度な凹凸付与が行われ、めっき層内への剥離地鉄取り込みが促進される。圧下率は１～１０％である。圧下率が１％未満の場合、鋼板表面への凹凸付与が十分に行われず、十分なめっき密着性改善効果が得られない。一方、圧下率が１０％超えの場合、スケールが地鉄に噛み込み、脱スケール性が著しく低下する。
　酸洗後に圧下率０．３～５％の圧延
本発明では、酸洗後に圧下率０．３～５％の圧延を施す。酸洗後に圧下率０．３～５％の圧延を施すことは本発明において重要な要件である。圧下を加えることで表面形状の制御と母材表面に残留応力の導入を行う。圧下率を０．３％以上とすることで残留応力の導入が充分となり、鋼板表面のめっき反応性が改善される。圧下率が５％超えでは、歪導入によるめっき反応性の改善効果が飽和することに加え、鋼板表面が平滑化し、剥離地鉄をめっき層内に取り込ませることが困難となる。
　焼鈍（好適条件）
圧下率０．３～５％の圧延後溶融めっき処理前に焼鈍を行うことが好ましい。好適条件は、水素濃度２～３０ｖｏｌ％かつ露点－６０～－１０℃の炉内雰囲気、鋼板到達温度６００～９５０℃である。焼鈍到達温度が６００℃より低い温度の場合、酸洗後の酸化皮膜が完全には還元されず、所望するめっき特性を得ることができない場合がある。また、９５０℃より高い温度では、Ｓｉ、Ｍｎなどが表面濃化してめっき性が劣化する恐れがある。より好ましくは、鋼鈑到達温度は６５０℃以上８５０℃以下である。
炉内雰囲気は水素濃度２～３０％かつ露点－６０～－１０℃とすることが好ましい。炉内雰囲気は還元性であれば良く、露点－６０～－１０℃、水素濃度２～３０％で残部が不活性ガスからなる雰囲気が好適である。露点が－１０℃より高いと、鋼板表面に生成するＳｉ酸化物の形態が膜状となり易い。より好ましくは、露点－２０℃以下である。一方、－６０℃より低い露点は工業的に実現が困難である。水素濃度が２％より低い場合は、還元性が弱い。３０％以下であれば十分な還元能力が得られる。より好ましくは、水素濃度５％以上２０％以下である。
　溶融めっき処理
溶融めっき処理は連続溶融めっきラインにて、好ましくは鋼板を還元焼鈍したのち、溶融めっき浴を用いて実施する。
溶融めっき浴の組成は、例えば、溶融亜鉛めっき処理の場合は、Ａｌ濃度０．０１～０．２５％の範囲とし、残部をＺｎおよび不可避的不純物とする。Ａｌ濃度が０．０１％未満の場合、めっき処理時にＺｎ－Ｆｅ合金化反応が起こり、めっきと鋼板（母材）の界面に脆い合金層が発達し、めっき密着性が劣化する。Ａｌ濃度が０．２５％を超えるとＦｅ－Ａｌ合金層の成長が顕著となり、めっき密着性を阻害する。めっき浴温度は特に限定する必要はなく、通常の操業範囲である４４０℃以上４８０℃以下でよい。
　合金化処理（好適条件）
合金化処理温度が５５０℃を超えると、合金化処理時に、鋼板（母材）とめっき皮膜の界面に硬質で脆いΓ相の生成が著しく、表面粗さが大きくなると共に耐パウダリング性が劣化するため、合金化処理温度は５５０℃以下が好ましい。さらに好ましくは５３０℃以下である。一方で、合金化処理温度４８０℃未満では十分に合金化がなされず、十分なめっき特性を得ることができない。そのため、合金化処理温度は４８０℃以上が望ましい。
合金化処理時間は、コストや制御上の問題点から、１０秒以上６０秒以下とするのが好ましい。より好ましくは４０秒以下である。
合金化処理における加熱方法は特に限定する必要がなく、輻射加熱、通電加熱、高周波誘導加熱など、公知のいずれの方法でもよい。合金化処理を施した後は常温まで冷却する。めっき後の後処理は特に限定する必要はなく、調質圧延による材質の調整やレベリング等による平坦形状の調整、さらには必要に応じてクロメート処理等通常行われる後処理を施しても構わない。
　以下、本発明を実施例に基づいて具体的に説明するが、本発明は本実施例に限定されるものではない。
表１に示す成分組成を有するスラブを用い、通常の鋳造後、表２に示す条件で熱間圧延、圧延、酸洗、圧延、焼鈍、溶融亜鉛めっき処理、さらに一部については合金化処理を行った。
　溶融亜鉛めっき処理を施すに際し、亜鉛めっき浴温度は４６０℃で行い、ワイピングで付着量を５０ｇ／ｍ2に調整した。合金化処理は、合金化温度５２０℃で実施した。
　亜鉛めっき層中の剥離地鉄量
亜鉛めっき層中の剥離地鉄量については、ＩＣＰ発光分光分析法を使用し、以下の手順で測定した。まず、亜鉛めっき層を、インヒビターを添加した希塩酸で溶解することで、めっき鋼板中の亜鉛めっき層のみを剥離・溶解する。インヒビターとは、地鉄の過溶解防止のために使用する添加剤であり、市販のもので良い。本発明では、１０～１００倍に希釈した塩酸に、０．６ｇ／Ｌの濃度になるよう朝日化学工業株式会社製の塩酸酸洗用腐食抑制剤「イビットＮｏ．７００ＢＫ」を添加した。希塩酸でのめっき鋼板溶解後、未溶解の剥離地鉄を含んだ溶液を抽出し、２つの溶液に分ける。そのうち、片方の溶液については剥離地鉄未溶解のまま組成分析を行い、もう一方の溶液についてはインヒビター未添加の塩酸で再溶解後、組成分析を行う。こうして得られた結果の差分を、剥離地鉄量とした。
　以上により得られた溶融亜鉛めっき鋼板について、下記に示す試験を行い、引張強度を測定し、めっき表面外観及びめっき密着性を評価した。測定方法および評価基準を下記に示す。
　引張強度（ＴＳ）
溶融亜鉛めっき鋼板（GI）もしくは合金化溶融亜鉛めっき鋼板（GA）より圧延方向に対して直角方向にJIS5号引張試験片（JIS Z2201）を採取し、歪速度が10-3/sとするJIS Z 2241の規定に準拠した引張試験を行い、TSを求めた。
　外観性
溶融めっき後及び合金化処理後の外観を目視観察し、不めっき、合金ムラがないものを○、不めっきや合金ムラがあるものは×とした。
　めっき密着性
溶融亜鉛めっき鋼板のめっき密着性は、ボールインパクト試験で評価した。ボール重量2.8ｋｇ、落下高さ1ｍの条件で、ボールインパクト試験を行い、加工部をテープ剥離し、めっき層の剥離有無を目視判定した。
○　めっき層の剥離なし
×　めっき層が剥離
　耐パウダリング性
合金化溶融亜鉛めっき鋼板のめっき密着性は、耐パウダリング性を試験することで評価した。合金化溶融めっき鋼板にセロハンテープを貼り、テープ面に９０度曲げ、曲げ戻しを施し、テープを剥がす。剥がしたテープに付着した鋼板から、曲げ戻し部１０ｍｍ×４０ｍｍ当たりの剥離しためっきの量を、蛍光Ｘ線によるＺｎカウント数として測定し、下記基準に照らして評価した。
蛍光Ｘ線カウント数　　 　　　ランク
３０００未満 　　　　　　：　◎（良）
３０００以上６０００未満 ：　○
６０００以上 　　　　　　：　×（劣）
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
　表２より、本発明例は、表面外観、めっき密着性（耐パウダリング性）のいずれも良好である。一方、比較例では、表面外観、めっき密着性（耐パウダリング性）のいずれか一つ以上が劣る。
　本発明の高強度溶融めっき鋼板は、近年急速に高強度化・薄肉化が進んできている自動車部品として好適に用いられる。

Claims (6)

  1. 　成分組成として、質量％で、Ｃ：０．０２％以上０．３０％以下、
    Ｓｉ：０．０１％以上２．０％以下、
    Ｍｎ：０．２％以上３．０％以下、
    Ｐ：０．０８％以下、
    Ｓ：０．０２％以下、
    Ａｌ：０．００１％以上０．４０％以下を含有し、残部がＦｅおよび不可避的不純物からなる鋼板上に、片面あたりのめっき付着量が３０～９０ｇ／ｍ２のめっき層を有し、該めっき層中には、剥離地鉄を０．３～１．５ｇ／ｍ２含有することを特徴とするめっき密着性に優れた高強度溶融めっき鋼板。
  2. 　成分組成として、さらに、質量％で、Ｔｉ：０．０１％以上０．４０％以下、
    Ｎｂ：０．００１％以上０．２００％以下、
    Ｖ：０．００１％以上０．５００％以下、
    Ｍｏ：０．０１％以上０．５０％以下、
    Ｗ：０．００１％以上０．２００％以下、
    Ｂ：０．０００３％以上０．０１％以下のうち１種または２種以上を含有することを特徴とする請求項１に記載のめっき密着性に優れた高強度溶融めっき鋼板。
  3. 　請求項１または２に記載の成分組成を有する鋼スラブを、熱間圧延し、圧下率１～１０％で圧延し、酸洗を行い、次いで、圧下率０．３～５％で圧延し、溶融めっき処理を行うことを特徴とするめっき密着性に優れた高強度溶融めっき鋼板の製造方法。
  4. 　前記熱間圧延において、粗圧延後、仕上げ圧延温度８２０℃以上で圧延終了した後、巻取温度４５０～６５０℃で巻き取ることを特徴とする請求項３に記載のめっき密着性に優れた高強度溶融めっき鋼板の製造方法。
  5. 　圧下率０．３～５％で前記圧延後前記溶融めっき処理前に、水素濃度２～３０ｖｏｌ％かつ露点－６０～－１０℃の炉内雰囲気、鋼板到達温度６００～９５０℃で連続焼鈍を行うことを特徴とする請求項３または４に記載のめっき密着性に優れた高強度溶融めっき鋼板の製造方法。
  6. 　前記溶融めっき処理後、さらに、合金化処理を行うことを特徴とする請求項３～５のいずれか一項に記載のめっき密着性に優れた高強度溶融めっき鋼板の製造方法。
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