WO2018142849A1 - 高強度溶融亜鉛めっき熱延鋼板およびその製造方法 - Google Patents

高強度溶融亜鉛めっき熱延鋼板およびその製造方法 Download PDF

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Publication number
WO2018142849A1
WO2018142849A1 PCT/JP2018/000177 JP2018000177W WO2018142849A1 WO 2018142849 A1 WO2018142849 A1 WO 2018142849A1 JP 2018000177 W JP2018000177 W JP 2018000177W WO 2018142849 A1 WO2018142849 A1 WO 2018142849A1
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Prior art keywords
hot
steel sheet
less
rolling
rolled steel
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PCT/JP2018/000177
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English (en)
French (fr)
Inventor
正貴 木庭
祐介 伏脇
長滝　康伸
Original Assignee
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 MX2019009043A priority Critical patent/MX2019009043A/es
Priority to CN201880009445.2A priority patent/CN110268088B/zh
Priority to US16/481,646 priority patent/US10927441B2/en
Priority to JP2018515322A priority patent/JP6376310B1/ja
Priority to KR1020197022503A priority patent/KR102259118B1/ko
Priority to EP18747894.6A priority patent/EP3578679B1/en
Publication of WO2018142849A1 publication Critical patent/WO2018142849A1/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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
    • B21B45/04—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for de-scaling, e.g. by brushing
    • B21B45/08—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for de-scaling, e.g. by brushing hydraulically
    • 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
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    • 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
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    • 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
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    • 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
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    • 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
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/60—Aqueous agents
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    • C—CHEMISTRY; METALLURGY
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    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • 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
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    • C—CHEMISTRY; METALLURGY
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    • 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
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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
    • 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
    • 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
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    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0405—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing of ferrous alloys
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
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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
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    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0478—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing involving a particular surface treatment
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    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
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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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    • C23C30/005—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process on hard metal substrates
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Definitions

  • the present invention relates to a high-strength hot-dip galvanized hot-rolled steel sheet excellent in surface appearance, plating adhesion and post-processing corrosion resistance, and a method for producing the same, using a high-strength hot-rolled steel sheet containing Si and Mn as a base material. .
  • hot-dip galvanized steel sheets are manufactured by using hot-rolled or cold-rolled steel sheets as slabs as raw steel sheets, re-annealing the raw steel sheets in a CGL annealing furnace, and then performing hot-dip galvanizing treatment.
  • the alloyed hot-dip galvanized steel sheet is manufactured by further alloying after hot-dip galvanizing.
  • the hot-dip galvanized steel sheet used for the above-mentioned applications has extremely good surface appearance and plating adhesion, and the corrosion resistance of the part subjected to processing such as hole expansion processing is extremely important.
  • hot-dip galvanized steel sheets containing Si often have local scale residue on the surface after pickling or local smud formation due to per-acid washing, and there are defects such as unplating and untreated alloying. Prone to occur.
  • Non-plating is a phenomenon in which plating does not adhere to a part of the surface of the steel sheet in the hot dip galvanizing process, and the surface of the steel sheet is exposed. Its size is usually in the order of mm, and its presence can be visually observed.
  • Patent Document 1 discloses pickling descaling of a hot-rolled steel sheet as a base material as a measure for preventing plating defects such as poor alloying when producing an alloyed hot-dip galvanized steel sheet containing Si, Mn, and P. At the time, methods of performing shot blasting and brush grinding have been proposed.
  • Patent Document 2 proposes a method for improving the adhesion of a plating film in which a base metal surface is ground, heated to 600 ° C. or higher in a reducing atmosphere, cooled, hot dip galvanized, and then alloyed. Yes.
  • Patent Document 3 proposes a method for improving plating adhesion by forming an internal oxide layer on a steel surface layer in a hot rolling process and performing descaling by high-pressure water injection on a rolled steel sheet. Yes.
  • Patent Document 4 a hot-rolled 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 shot blasting or brush grinding on the base material before plating.
  • 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.
  • Patent Document 3 although the plating adhesion is improved, there is a problem in that fine cracks are generated in the steel sheet surface layer portion and the plating layer based on internal oxidation during processing, and the corrosion resistance of the processed portion is deteriorated.
  • JP-A-6-158254 Japanese Patent Laid-Open No. 10-81948 JP2013-108107A 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 hot-rolled steel sheet excellent in surface appearance, plating adhesion and post-processing corrosion resistance, and a method for producing the same.
  • the present inventors diligently studied to solve the above problems. As a result, a high strength hot-dip galvanized hot-rolled steel sheet with excellent surface appearance, plating adhesion and post-processing corrosion resistance can be obtained by controlling the steel sheet surface shape and the amount of oxide on the steel sheet surface after annealing before plating. I found.
  • the present invention is based on the above findings, and the gist is as follows.
  • C 0.02% to 0.30%, Si: 0.01% to 2.5%, Mn: 0.3% to 3.0%, P: 0.08% or less, S: 0.02% or less, Al: 0.001% or more and 0.20% or less, with the balance being plated on one side on a steel plate made of Fe and inevitable impurities
  • a zinc plating layer having an amount of 20 to 120 g / m 2 , a specific surface area ratio r 1 of the steel sheet surface of 2.5 or less, and an amount of Si (g / m 2 ) contained in the zinc plating layer;
  • a roll having hot rolling, pickling, and then surface roughness (Ra) of 0.3 to 1.0 on the steel slab having the composition described in [1] or [2] above A method for producing a high-strength hot-dip galvanized hot-rolled steel sheet, which is rolled at a reduction rate of 1 to 10% using hot rolled galvanized steel and then hot-dip galvanized.
  • descaling is performed by high-pressure water injection at a collision pressure of 0.3 MPa to less than 1.8 MPa, and finish rolling is performed at a finish rolling temperature of 800 ° C. or more.
  • the furnace atmosphere Prior to the hot dip galvanizing treatment, the furnace atmosphere is set to a hydrogen concentration of 2 to 30 vol% and a dew point of ⁇ 60 to ⁇ 10 ° C., and continuous annealing is performed at a steel plate reaching temperature of 600 to 950 ° C. [3] or [4]
  • the high-strength hot-dip galvanized hot-rolled steel sheet is a hot-rolled steel sheet having a tensile strength (TS) of 590 MPa or more.
  • a high-strength hot-dip galvanized hot-rolled steel sheet having excellent surface appearance, plating adhesion and post-processing corrosion resistance can be obtained. Since it has high corrosion resistance even after processing, it is effective as a member having complicated molding, and the industrial effect obtained by the present invention is great.
  • 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. Further, the unit of hydrogen concentration is “vol%”, and is simply indicated by “%” unless otherwise specified.
  • the high-strength hot-dip galvanized hot-rolled steel sheet excellent in surface appearance and plating adhesion of the present invention is in mass%, C: 0.02% to 0.30%, Si: 0.01% to 2.5% Mn: 0.3% or more and 3.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.001% or more and 0.20% or less, with the balance being Fe And on a steel plate made of unavoidable impurities, a galvanized layer having a plating adhesion amount of 20 to 120 g / m 2 on one side, a specific surface area ratio r 1 of the steel plate surface being 2.5 or less, It is important that the amount of Si (g / m 2 ) and the amount of Mn (g / m 2 ) contained in the galvanized layer satisfy Si amount ⁇ r ⁇ 0.06 and Mn amount ⁇ r ⁇ 0.10.
  • the specific surface area ratio r corresponds to the ratio (actual surface area / area) of the actual surface area considering the fine irregularities on the steel sheet surface and the area of the two-dimensional plane not considering the irregularities on the steel sheet surface.
  • the method for measuring the specific surface area ratio is not particularly limited, and can be determined, for example, using a laser microscope.
  • a laser microscope is considered to be most suitable from the viewpoint that fine irregularities on the order of nm can be evaluated over a wide range.
  • the amount of Si and the amount of Mn contained in the galvanized layer refers to the Si formed in the heat treatment step that is taken into the galvanized layer by the reaction between the plating bath and the steel sheet to form the FeAl or FeZn alloy phase. And correspond to Mn oxide.
  • the specific surface area ratio r is multiplied by the Si amount and the Mn amount based on the “Wenzel model” which is one of the models showing the relationship between the surface roughness and the wettability. This is because when the surface area of the solid-liquid interface increases by r times due to the micro unevenness, the influence of the Si and Mn oxide on the steel plate surface on the plating property becomes more obvious.
  • the present invention suppresses reaction unevenness by controlling the steel plate surface shape and the amount of oxide after annealing. Furthermore, local deterioration of the corrosion resistance of the processed part is prevented. As a result, a high-strength hot-dip galvanized hot-rolled steel sheet excellent in surface appearance, plating adhesion and post-processing corrosion resistance can be obtained.
  • the C content is 0.02% or more and 0.30% or less
  • the C content is 0.04% or more. More preferably, the C content is 0.06% or more.
  • the C content is set to 0.30% or less.
  • the C content is 0.20% or less.
  • Si 0.01% or more and 2.5% 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% to 2.5%.
  • the Si content is 0.03% or more and 2.00% or less.
  • Mn 0.3% to 3.0% Mn is an element useful for increasing the strength of steel. In order to obtain this effect, it is necessary to contain 0.3% or more of Mn. However, excessively containing Mn impairs wettability during hot dip galvanizing and impairs alloying reactivity, making it difficult to adjust the alloying, leading to a decrease in plating appearance and plating adhesion. From the above, the Mn content is set to 0.3% or more and 3.0% or less. Preferably, the Mn content is 0.3% or more and 2.6% or less. More preferably, the Mn content is 1.0% or more and 2.2% or less.
  • the P content is 0.08% or less.
  • the P content is 0.03% or less.
  • the lower limit of the P content is not particularly limited, but there is a concern about an increase in cost due to an unnecessary reduction. Therefore, the P content is preferably 0.001% or more.
  • 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.
  • the S content is 0.005% or less.
  • the lower limit of the S content is not particularly limited, and may be an impurity level.
  • Al 0.001% or more and 0.20% or less Al is contained for the purpose of deoxidation of molten steel, but if the content is less than 0.001%, the purpose is not achieved. On the other hand, when Al is contained in excess of 0.20%, 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 0.20% or less. Preferably, the Al content is 0.005% or more and 0.1% 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%.
  • One or two or more of 500% or less, Mo: 0.01% or more and 0.50% or less, W: 0.001% or more and 0.200% or less can be contained.
  • Ti, Nb, V, Mo, and W are elements necessary for depositing precipitates (particularly carbides) in the raw steel sheet, and one or more selected from the group consisting of these elements are added. It is preferable to do. Usually, these elements are often contained in the form of precipitates containing these elements in the raw 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 Ti content is less than 0.01%, the amount of precipitates in the material steel plate necessary for containing precipitates (particularly carbides) in the alloyed hot-dip coating layer may be insufficient. When the Ti content exceeds 0.40%, the effect is saturated and the cost is increased.
  • the Ti content when Ti is contained, the Ti content is 0.01% or more and 0.40% or less. More preferably, the Ti content is 0.02% or more.
  • the Nb content when Nb, V, Mo, and W are contained, the Nb content is 0.001 to 0.200%, and the V content is 0.00. 001 to 0.500%, Mo content is 0.01 to 0.50%, and W content is 0.001 to 0.200%.
  • the galvanized layer has a plating adhesion amount of 20 to 120 g / m 2 per side. If the adhesion amount is less than 20 g / m 2, it is difficult to ensure corrosion resistance. Preferably, the adhesion amount is 30 g / m 2 or more. On the other hand, when the adhesion amount exceeds 120 g / m 2 , the plating peel resistance deteriorates. Preferably, the adhesion amount is 90 g / m 2 or less.
  • the specific surface area ratio r of the material steel plate surface is 2.5 or less.
  • the specific surface area ratio is preferably as low as possible. If the specific surface area ratio is high, even if the amount of Si oxide and Mn oxide formed after annealing is small, the plating wettability is greatly impaired. Furthermore, when the specific surface area ratio is high, the unevenness of the amount of plating tends to increase, and when the specific surface area ratio r exceeds 2.5, the powdering resistance and post-processing corrosion resistance are significantly deteriorated.
  • the specific surface area ratio r is 2.3 or less.
  • the Si oxide and Mn oxide formed in the heat treatment step before plating are taken into the galvanized layer when the plating bath reacts with the material steel plate to form the FeAl or FeZn alloy phase.
  • the amount of Si oxide and the amount of Mn oxide are excessive, the amount of Si oxide and the amount of Mn oxide remain at the interface between the plating and the ground iron, and the plating adhesion is deteriorated. Therefore, there is no lower limit to the amount of Si oxide and the amount of Mn oxide contained in the galvanized layer, and the lower the better.
  • the amounts of Si and Mn in the galvanized layer must satisfy the following: Si amount ⁇ r ⁇ 0.06, Mn amount ⁇ r ⁇ 0.10.
  • Si and Mn amounts of zinc plating layer are respectively 0.06 g / m 2, greater than insufficient formation reaction of FeAl or FeZn alloy phase at 0.10 g / m 2, greater than generation and resistance to plating peeling resistance of the non-coating Cause a decline.
  • the measurement of the amount of Si and Mn in a galvanization layer is performed by the method as described in an Example.
  • the steel slab having the above component composition is hot-rolled, pickled, and then rolled at a rolling reduction of 1 to 10% using a roll having a surface roughness (Ra) of 0.3 to 1.0. Then, a hot dip galvanizing process is performed.
  • descaling is performed by high-pressure water jet at a collision pressure of 0.3 MPa to less than 1.8 MPa, finish rolling is performed at a finish rolling temperature of 800 ° C. or more, and a winding temperature of 400 to It is preferable to wind at 650 degreeC.
  • the furnace atmosphere has a hydrogen concentration of 2 to 30 vol%, a dew point of ⁇ 60 to ⁇ 10 ° C., and continuous annealing at a steel sheet reaching temperature of 600 to 950 ° C. Further, after the hot dip galvanizing treatment, an alloying treatment may be further performed.
  • 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.
  • Descaling by high-pressure water injection It is preferable to perform descaling by high-pressure water injection at a collision pressure of 0.3 MPa to less than 1.8 MPa after rough rolling and before finish rolling.
  • a collision pressure of high pressure water in descaling by high pressure water injection is less than 0.3 MPa, a large amount of scale remains, which may cause a scale defect.
  • the impact pressure of descaling by high-pressure water jet is large from the viewpoint of scale peeling.
  • high-pressure descaling is generally used because scale peelability deteriorates.
  • the impact pressure may differ in the scale peeling because a difference occurs in the steel plate width direction due to the distance from the nozzle and the interference of high-pressure water from the adjacent descaling nozzle.
  • the collision pressure is preferably 0.3 MPa or more and less than 1.8 MPa. More preferably, the collision pressure is 0.5 MPa or more and 1.6 MPa or less.
  • Finish rolling temperature In order to reduce deformation resistance during hot rolling and facilitate operation, it is preferable that the finish rolling temperature be 800 ° C. or higher. On the other hand, when finish rolling is performed at a temperature exceeding 1000 ° C., scale wrinkles are likely to occur, and the surface properties may deteriorate. Therefore, the finish rolling temperature is preferably 800 ° C. or higher. Preferably it shall be 1000 degrees C or less. More preferably, the finish rolling temperature is 850 ° C. or higher and 950 ° C. or lower.
  • 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 coiling temperature is preferably 650 ° C. or lower.
  • the hot rolling coiling temperature is preferably 400 ° C. or higher and 650 ° C. or lower.
  • the hot-rolled steel sheet obtained by pickling hot rolling is descaled by pickling, and then mild rolling is performed.
  • Pickling is not particularly limited, and may be performed by a conventional method. In addition, you may give 5% or less rolling before the pickling, and this mild rolling improves descaling property and leads to improvement of surface properties.
  • Roll with a roll having a rolling surface roughness (Ra) of 0.3 to 1.0 at a rolling reduction of 1 to 10% In order to improve the plateability of the steel sheet surface, rolling is performed after hot rolling and pickling. At this time, by rolling with a roll having surface roughness, it is possible to efficiently correct the hot-rolled plate surface shape having irregularities including the scale portion. By setting the roughness (Ra) of the roll surface to 0.3 or more, the dents in the hot-rolled sheet can be efficiently rolled, leading to an improvement in plating properties. On the other hand, if the roughness (Ra) exceeds 1.0, it may cause uneven plating adhesion and local stress concentration during bending, which may cause poor plating appearance, powdering resistance and post-processing corrosion resistance.
  • the roughness (Ra) is 0.8 or less.
  • the rolling reduction during rolling is 1% or more and 10% or less.
  • the surface shape is controlled and the residual stress is introduced into the base metal surface.
  • the reduction ratio is set to 1% or more, the introduction of residual stress is sufficient, and the plating adhesion on the steel sheet surface is improved.
  • the rolling reduction exceeds 10%, the effect of improving plating adhesion is saturated, and a large amount of dislocations are introduced into the surface layer of the steel sheet, so that the surface layer structure becomes coarse during annealing before plating treatment, resulting in a decrease in strength. Connected.
  • the furnace atmosphere Prior to the hot dip galvanizing treatment, the furnace atmosphere is preferably set to a hydrogen concentration of 2 to 30%, a dew point of ⁇ 60 to ⁇ 10 ° C., and continuous annealing at a steel sheet temperature of 600 to 950 ° C.
  • the steel sheet arrival temperature that is, 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.
  • the annealing temperature is 650 ° C. or higher and 900 ° C. or lower.
  • 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 is not particularly limited, and an atmosphere having a hydrogen concentration of 2 to 30%, a dew point of ⁇ 60 to ⁇ 10 ° C., and the balance of an inert gas 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.
  • 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 dew point is ⁇ 55 ° C. or higher and ⁇ 20 ° C. or lower. More preferably, the hydrogen concentration is 5% or more and 20% or less.
  • Hot dip galvanizing treatment is carried out using a hot dip galvanizing bath after reduction annealing of the steel sheet in a continuous hot dip plating line.
  • the composition of the hot dip galvanizing bath is, for example, in the range of Al concentration of 0.01 to 0.25%, with the balance being Zn and inevitable impurities.
  • Al concentration is less than 0.01%, a Zn—Fe alloying reaction occurs during the plating treatment, and a brittle alloy layer develops at the interface between the plating and the steel sheet, which may deteriorate the plating adhesion.
  • 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 treatment temperature is preferably 550 ° C. or less. More preferably, the alloying temperature is 530 ° C. or lower.
  • the alloying treatment time is preferably 10 seconds or more and 60 seconds or less from the viewpoint of cost and control. More preferably, the alloying treatment time is within 40 seconds.
  • 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 530 ° C.
  • the amount of Si and the amount of Mn in the galvanized layer were measured by dissolving the plated layer with hydrofluoric acid to which an inhibitor was added and using ICP emission spectroscopy.
  • Specific surface area ratio r of steel sheet surface The surface shape of the steel sheet surface after dissolution of the plating layer was scanned using a laser microscope (manufactured by Keyence: VK-X250). Observation and analysis were performed under the conditions that the observation magnification was 3000 times, the resolution of the z-axis was 0.5 nm, and the resolution of the x-axis and the y-axis was 0.1 ⁇ m. In deriving the area ratio, five arbitrary visual fields were selected for each steel plate, and the average value was defined as the specific surface area ratio r.
  • 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 was visually observed after hot-dip galvanization and after alloying, and was marked with symbols ( ⁇ , x) in light of the following criteria. ⁇ No plating, no alloy unevenness ⁇ No plating or alloy unevenness
  • the plating adhesion of the hot dip galvanized steel sheet was evaluated by a ball impact test.
  • a ball impact test is performed under the conditions of a ball weight of 2.8 kg and a drop height of 1 m, the processed part is peeled off with tape, the presence or absence of peeling of the plating layer is visually judged, and symbols ( ⁇ , ⁇ ) are attached according to the following criteria. did.
  • ⁇ 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 according to the following criteria, and symbols ( ⁇ , ⁇ , ⁇ ) It is inferior, so that the Zn count number by a fluorescent X ray is large.
  • SST Salt spray test
  • This test is performed after applying a cutting wrinkle that reaches the plating with a cutter on the surface of the bent part in the alloyed hot-dip galvanized steel sheet and on the ball impact part of the hot-dip galvanized steel sheet.
  • the piece was subjected to a salt spray test for 240 hours in accordance with a neutral salt spray test specified in JIS Z2371: 2000 using a 5 mass% NaCl aqueous solution, and then subjected to a tape peel test on the crosscut collar, The maximum total peel width of the cut buttock left and right was measured.
  • Symbols ( ⁇ , ⁇ ) are attached in light of the following criteria.
  • the examples of the present invention have good surface appearance, plating adhesion (powdering resistance), and post-processing corrosion resistance.
  • any one or more of surface appearance, plating adhesion (powdering resistance), and post-processing corrosion resistance are inferior.
  • the high-strength hot-dip galvanized hot-rolled steel sheet of the present invention is suitably used as an automotive part that has been rapidly strengthened and thinned in recent years.

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Abstract

表面外観、めっき密着性及び加工後耐食性に優れた高強度溶融亜鉛めっき熱延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２．５％以下、Ｍｎ：０．３％以上３．０％以下、Ｐ：０．０８％以下、Ｓ：０．０２％以下、Ａｌ：０．００１％以上０．２０％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２０～１２０ｇ／ｍ２の亜鉛めっき層を有する。鋼板表面の比表面積率ｒ が２．５以下であり、さらに、亜鉛めっき層に含まれるＳｉ量(ｇ／ｍ２)及びＭｎ量(ｇ／ｍ２)が、Ｓｉ量×ｒ ≦ 0.06、Ｍｎ量×ｒ ≦ 0.10を満たす。

Description

高強度溶融亜鉛めっき熱延鋼板およびその製造方法
　本発明は、Ｓｉ、Ｍｎを含有する高強度熱延鋼板を母材とする、表面外観、めっき密着性及び加工後耐食性に優れた高強度溶融亜鉛めっき熱延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（以下、母材と称することもある）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。また、素材鋼板としては、製造コストが安価な熱延鋼板が使用される場合があった。
　一般的に、溶融亜鉛めっき鋼板は、スラブを熱間圧延や冷間圧延した鋼板を素材鋼板として用い、素材鋼板をＣＧＬの焼鈍炉で再結晶焼鈍し、その後、溶融亜鉛めっき処理を行い製造される。また、合金化溶融亜鉛めっき鋼板は、溶融亜鉛めっき後、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亜鉛めっき鋼板は、表面外観やめっき密着性がよいことに加え、穴広げ加工を初めとした加工を施された部分の耐食性が極めて重要である。しかしながら、Ｓｉを含有する溶融亜鉛めっき鋼板は、酸洗後の表面に局所的なスケール残りや過酸洗による局所的なスマッド生成がある場合が多く、不めっきや合金化未処理などの欠陥が生じやすい。不めっきは、溶融亜鉛めっき処理でめっきが鋼板表面の一部に付着せず、鋼板表面が露出する現象であり、そのサイズは通常ｍｍオーダーのため、その存在を目視することができる。
　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には、Ｓi 、Ｍn 、Ｐなどを含有する合金化溶融亜鉛めっき鋼板を製造する際の合金化不良などのめっき欠陥防止対策として、母材の熱延鋼板の酸洗脱スケールに際してショットブラスト処理やブラシ研削を施す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、冷却して溶融亜鉛めっき処理し、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では、熱間圧延工程において鋼表層に内部酸化層を形成させることに加え、圧延後の鋼板に高圧水噴射によるデスケーリングを施すことで、めっき密着性を改善する方法が提案されている。
　特許文献４には、熱間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、めっき前の母材に対してショットブラストやブラシ研削を必要とする。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。 
　特許文献３では、めっき密着性は改善するものの、加工時に内部酸化を基点として鋼板表層部及びめっき層に微細なクラックが生じ、加工部の耐食性が劣化するという問題があった。
　また、特許文献４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。
特開平６－１５８２５４号公報 特開平１０－８１９４８号公報 特開２０１３－１０８１０７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、表面外観、めっき密着性及び加工後耐食性に優れた高強度溶融亜鉛めっき熱延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、鋼板表面形状やめっき前の焼鈍後の素材鋼板表面の酸化物量を制御することにより、表面外観、めっき密着性及び加工後耐食性に優れた高強度溶融亜鉛めっき熱延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、要旨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２．５％以下、Ｍｎ：０．３％以上３．０％以下、Ｐ：０．０８％以下、Ｓ：０．０２％以下、Ａｌ：０．００１％以上０．２０％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２０～１２０ｇ／ｍ２の亜鉛めっき層を有し、前記鋼板表面の比表面積率ｒ が２．５以下であり、さらに、前記亜鉛めっき層に含まれるＳｉ量(ｇ／ｍ２)及びＭｎ量(ｇ／ｍ２)が、Ｓｉ量×ｒ ≦ 0.06、Ｍｎ量×ｒ ≦ 0.10を満たす高強度溶融亜鉛めっき熱延鋼板。
［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％以下のうち１種または２種以上を含有する上記［１］に記載の高強度溶融亜鉛めっき熱延鋼板。
［３］上記［１］または［２］に記載の成分組成を有する鋼スラブに対して、熱間圧延、酸洗し、次いで、表面粗度(Ｒａ)０．３～１．０を有するロールを用いて圧下率１～１０％で圧延し、次いで、溶融亜鉛めっき処理を行う高強度溶融亜鉛めっき熱延鋼板の製造方法。
［４］前記熱間圧延において、粗圧延後、仕上げ圧延前に、衝突圧０．３ＭＰａ以上１．８ＭＰａ未満で高圧水噴射によるデスケーリングを行い、仕上げ圧延温度８００℃以上で仕上げ圧延し、巻取温度４００～６５０℃で巻き取る上記［３］に記載の高強度溶融亜鉛めっき熱延鋼板の製造方法。
［５］前記溶融亜鉛めっき処理前に、炉内雰囲気を水素濃度２～３０ｖｏｌ％かつ露点－６０～－１０℃とし、鋼板到達温度６００～９５０℃で連続焼鈍する上記［３］または［４］に記載の高強度溶融亜鉛めっき熱延鋼板の製造方法。
［６］前記溶融亜鉛めっき処理後、さらに、合金化処理を行う上記［３］～［５］のいずれかに記載の高強度溶融亜鉛めっき熱延鋼板の製造方法。
　なお、本発明において、高強度溶融亜鉛めっき熱延鋼板とは、引張強度（TS）が５９０ＭＰａ以上の溶融亜鉛めっき熱延鋼板である。
　本発明によれば、表面外観、めっき密着性及び加工後耐食性に優れた高強度溶融亜鉛めっき熱延鋼板が得られる。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として効果的であり、本発明により得られる工業上の効果は大きい。
　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「ｖｏｌ％」であり、特に断らない限り単に「％」で示す。
　本発明の表面外観及びめっき密着性に優れた高強度溶融亜鉛めっき熱延鋼板は、質量％で、Ｃ：０．０２％以上０．３０％以下、Ｓｉ：０．０１％以上２．５％以下、Ｍｎ：０．３％以上３．０％以下、Ｐ：０．０８％以下、Ｓ：０．０２％以下、Ａｌ：０．００１％以上０．２０％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２０～１２０ｇ／ｍ２の亜鉛めっき層を有し、前記鋼板表面の比表面積率ｒ が２．５以下であり、さらに、前記亜鉛めっき層に含まれるＳｉ量(ｇ／ｍ２)及びＭｎ量(ｇ／ｍ２)が、Ｓｉ量×ｒ ≦ 0.06、Ｍｎ量×ｒ ≦ 0.10を満たすことが重要である。
　ここで比表面積率ｒとは、鋼板表面の微小凹凸を考慮した実表面積と鋼板表面の凹凸を考慮しない二次元平面の面積の比率（実表面積／面積）に相当する。比表面積率の測定法は特に限定せず、例えばレーザー顕微鏡を用いて求めることができる。他にも３次元ＳＥＭ観察や断面ＴＥＭによる評価も考えられるが、ｎｍオーダーの微細な凹凸を広範囲に評価可能な点から、レーザー顕微鏡が最も適していると考えられる。また、亜鉛めっき層に含まれるＳｉ量及びＭｎ量とは、めっき浴と鋼板が反応し、ＦｅＡｌあるいはＦｅＺｎ合金相が形成されることで、亜鉛めっき層中に取り込まれる熱処理工程で形成されたＳｉ及びＭｎ酸化物に相当する。なお、Ｓｉ量及びＭｎ量に比表面積率ｒを掛けるのは、表面粗さと濡れ性の関係を示すモデルの一つ「Ｗｅｎｚｅｌモデル」に基づく。これは、微小凹凸によって固液界面の表面積がr倍に増えると、鋼鈑表面のＳｉ及びＭｎ酸化物がめっき性に与える影響がより顕在化するためである。
　以上のように、本発明は、鋼板表面形状及び焼鈍後の酸化物量を制御することで、反応ムラを抑制する。さらに、加工部の局所的な耐食性劣化を防ぐ。その結果、表面外観、めっき密着性及び加工後耐食性に優れた高強度溶融亜鉛めっき熱延鋼板が得られることになる。
　まず、本発明の対象とする表面外観、めっき密着性及び加工後耐食性に優れた高強度溶融亜鉛めっき熱延鋼板の鋼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、Ｃ含有量は０．０４％以上である。より好ましくは、Ｃ含有量は０．０６％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。従って、Ｃ含有量は０．３０％以下とする。好ましくは、Ｃ含有量は０．２０％以下である。
　Ｓi：０．０１％以上２．５％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亜鉛めっき時の濡れ性を損ない、合金化反応性を損なうため、合金化の調整が困難となって、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２．５％以下とする。好ましくは、Ｓｉ含有量は０．０３％以上であり、２．００％以下である。
　Ｍn：０．３％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０．３％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亜鉛めっき時の濡れ性を損ない、合金化反応性を損なうため、合金化の調整が困難となって、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０．３％以上３．０％以下とする。好ましくは、Ｍn含有量は０．３％以上２．６％以下である。より好ましくは、Ｍn含有量は１．０％以上であり、２．２％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。しかし、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。好ましくは、Ｐ含有量は０．０３％以下である。なお、Ｐ含有量の下限は特に限定されないが、必要以上の低減によりコストの増大が懸念される。そのため、Ｐ含有量は０．００１％以上とすることが好ましい。
　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。好ましくは、Ｓ含有量は０．００５％以下である。Ｓ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０．２０％以下
Ａｌは溶鋼の脱酸を目的に含有されるが、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、Ａｌは０．２０％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０．２０％以下とする。好ましくは、Ａｌ含有量は０．００５％以上であり、０．１％以下である。
　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およびＷは、素材鋼板中に析出物（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、素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、Ｔｉの含有量が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素材鋼板中の析出物量が不十分な場合がある。Ｔｉの含有量が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％以下である。より好ましくは、Ｔｉ含有量は０．０２％以上である。
なお、Ｎｂ、Ｖ、Ｍｏ、Ｗ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０．００１～０．２００％、Ｖ含有量は０．００１～０．５００％、Ｍｏ含有量は０．０１～０．５０％、Ｗ含有量は０．００１～０．２００％である。
　次に、亜鉛めっき層及び素材鋼板の形状について説明する。
本発明において、亜鉛めっき層は、片面あたりのめっき付着量が２０～１２０ｇ／ｍ２である。付着量が２０ｇ／ｍ２未満では耐食性の確保が困難になる。好ましくは、付着量は３０ｇ／ｍ２以上である。一方、付着量は１２０ｇ／ｍ２を超えると耐めっき剥離性が劣化する。好ましくは、付着量は９０ｇ／ｍ２以下である。
　素材鋼板表面の比表面積率ｒは、２．５以下である。比表面積率は低い程好ましく、比表面積率が高いと焼鈍後に形成されるＳｉ酸化物量及びＭｎ酸化物量が少量であっても、めっき濡れ性を大きく損なう。さらに、比表面積率が高い場合にはめっき付着量ムラも大きくなりやすい傾向にあり、比表面積率ｒは２．５を超えると耐パウダリング性・加工後耐食性が著しく劣化する。好ましくは、比表面積率ｒは２．３以下である。なお、比表面積率ｒの下限は特に限定されないが、粗度を付与したロールで圧延した場合、鋼鈑表面を完全な平滑状態（ｒ＝１）にすることは理論上不可能である。そのため、比表面積率ｒは１．１以上とすることが好ましい。
　めっき前の熱処理工程で形成されたＳｉ酸化物、Ｍｎ酸化物は、めっき浴と素材鋼板が反応しＦｅＡｌあるいはＦｅＺｎ合金相が形成される際に亜鉛めっき層中にとりこまれる。Ｓｉ酸化物量及びＭｎ酸化物量が過剰な場合は、めっきと地鉄界面にＳｉ酸化物量およびＭｎ酸化物量が残留し、めっき密着性を劣化させる。そのため、亜鉛めっき層中に含まれるＳｉ酸化物量及びＭｎ酸化物量に下限はなく、低いほど好ましい。具体的には、亜鉛めっき層中のＳｉ及びＭｎ量がＳｉ量×ｒ ≦ 0.06、Ｍｎ量×ｒ ≦ 0.10を満たす必要がある。亜鉛めっき層中のＳｉ及びＭｎ量が、それぞれ０．０６ｇ／ｍ２超、０．１０ｇ／ｍ２超ではＦｅＡｌあるいはＦｅＺｎ合金相の形成反応が不十分となり、不めっきの発生や耐めっき剥離性の低下を招く。なお、亜鉛めっき層中のＳｉ及びＭｎ量の測定は実施例に記載の方法で行う。
　次に、表面外観、めっき密着性及び加工後耐食性に優れた高強度溶融亜鉛めっき熱延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に対して、熱間圧延、酸洗し、次いで、表面粗度(Ｒａ)が０．３～１．０を有するロールを用いて圧下率１～１０％で圧延し、次いで、溶融亜鉛めっき処理を行う。熱間圧延において、粗圧延後、仕上げ圧延前に、衝突圧０．３ＭＰａ以上１．８ＭＰａ未満で高圧水噴射によるデスケーリングを行い、仕上げ圧延温度８００℃以上で仕上げ圧延し、巻取温度４００～６５０℃で巻き取ることが好ましい。また、溶融亜鉛めっき処理前に、炉内雰囲気を水素濃度２～３０ｖｏｌ％かつ露点－６０～－１０℃とし、鋼板到達温度６００～９５０℃で連続焼鈍することが好ましい。また、溶融亜鉛めっき処理後、さらに、合金化処理を行ってもよい。
　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。
　高圧水噴射によるデスケーリング（好適条件）
粗圧延後、仕上げ圧延前に、衝突圧０．３ＭＰａ以上１．８ＭＰａ未満で高圧水噴射によるデスケーリングを行うことが好ましい。高圧水噴射によるデスケーリングでの高圧水の衝突圧が０．３ＭＰａ未満ではスケールが多量に残存するため、スケール性欠陥の原因となる場合がある。高圧水噴射によるデスケーリングの衝突圧はスケール剥離の観点から一般的には大きい方が好ましい。特にＳｉを含有する鋼板では、スケールの剥離性が劣化するため高圧のデスケーリングが一般的である。しかし、衝突圧はノズルからの距離および隣り合うデスケーリングノズルからの高圧水の干渉により鋼板幅方向で差が生じるためスケール剥離に差が生じる場合がある。スケールの剥離ムラが生じた領域では、付着量および合金化度がムラにならない場合でも、表面性状が異なるため合金化後にスジ状の模様となる場合がある。これらの鋼板幅方向のムラが生じる傾向は、衝突圧が１．８ＭＰａ以上で顕著になる。このため、衝突圧は０．３ＭＰａ以上１．８ＭＰａ未満が好ましい。より好ましくは、衝突圧は０．５ＭＰａ以上であり、１．６ＭＰａ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８００℃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、仕上げ圧延温度は好ましくは８００℃以上とする。好ましくは１０００℃以下とする。より好ましくは、仕上げ圧延温度は８５０℃以上であり、９５０℃以下である。
　熱延巻取温度（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、巻取温度は６５０℃以下であることが好ましい。一方、巻取り温度が４００℃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、熱延巻取温度は４００℃以上６５０℃以下が好ましい。
　酸洗
熱間圧延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、軽度の圧延を実施する。酸洗は特に限定せず、常法でよい。なお、酸洗の前段階で５％以下の圧延を施しても良く、この軽度の圧延によって脱スケール性が向上し、表面性状の改善に繋がる。
　圧延
表面粗度(Ｒａ)０．３～１．０を有するロールで圧下率１～１０％で圧延する。鋼板表面のめっき性を改善するために、熱延、酸洗後に圧延を実施する。この際、表面粗度を有するロールで圧延することで、スケール部をはじめとした凹凸を有する熱延板表面形状を、効率的に矯正することができる。ロール表面の粗度(Ｒａ)は、０．３以上とすることで熱延板での凹み部を効率的に圧延することができ、めっき性の改善につながる。一方、粗度(Ｒａ)が１．０超えになると、めっき付着量ムラや曲げ加工時局部応力集中を誘発し、めっき外観不良や耐パウダリング性・加工後耐食性低下の要因となりうる。より好ましくは、粗度(Ｒａ)は０．８以下である。
圧延の際の圧下率は、１％以上１０％以下とする。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１％以上とすることで残留応力の導入が充分となり、鋼板表面のめっき密着性が改善される。圧下率が１０％超えでは、めっき密着性の改善効果が飽和することに加え、鋼板表層に多量の転位が導入されることでめっき処理前の焼鈍中に表層組織が粗大化し、強度の低下につながる。
　焼鈍（好適条件）
溶融亜鉛めっき処理前に、炉内雰囲気を水素濃度２～３０％かつ露点－６０～－１０℃とし、鋼板到達温度６００～９５０℃で連続焼鈍することが好ましい。鋼板到達温度すなわち焼鈍温度が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、焼鈍温度は６５０℃以上であり、９００℃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、水素濃度２～３０％、露点－６０～－１０℃、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。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、露点は－５５℃以上であり、―２０℃以下である。より好ましくは、水素濃度は５％以上であり、２０％以下である。
　溶融亜鉛めっき処理
溶融めっき処理は連続溶融めっきラインにて、前記鋼板を還元焼鈍したのち、溶融亜鉛めっき浴を用いて実施する。
溶融亜鉛めっき浴の組成は、例えば、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、めっきと鋼板の界面に脆い合金層が発達し、めっき密着性が劣化する場合がある。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
合金化処理温度が５５０℃を超えると、合金化処理時に、（素材）鋼板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場合がある。そのため、合金化処理温度は５５０℃以下が好ましい。さらに好ましくは合金化処理温度は５３０℃以下である。
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。より好ましくは合金化処理時間は４０秒以内である。
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に示す成分を有する鋼スラブを用い、通常の鋳造後、表２に示す条件で熱間圧延、軽圧延、連続焼鈍、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５３０℃で実施した。
　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、めっき表面外観、めっき密着性及び加工後耐食性を評価した。測定方法および評価基準を下記に示す。
　亜鉛めっき層中のＳｉ量及びＭｎ量
　亜鉛めっき層中のＳｉ量及びＭｎ量については、めっき層を、インヒビターを添加したフッ酸で溶解し、ＩＣＰ発光分光分析法を使用して測定した。
　鋼板表面の比表面積率ｒ
　上記、めっき層の溶解を行った後の鋼板表面について、レーザー顕微鏡(Ｋｅｙｅｎｃｅ製:ＶＫ-Ｘ２５０)を用いて表面形状を走査した。観察倍率を３０００倍とし、ｚ軸の分解能０．５ｎｍ、ｘ軸及びｙ軸の分解能０．１μｍの条件下で観察・解析を行った。また面積率の導出に当たり、鋼板毎に任意の視野を５箇所選択し、その平均値を比表面積率ｒとした。
　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
外観性は、溶融亜鉛めっき後及び合金化処理後の外観を目視観察し、次の基準に照らして記号（○、×）を付した。
○　不めっき、合金ムラがないもの
×　不めっきや合金ムラがあるもの
　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、下記の基準に照らして記号（○、×）を付した。
○　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、記号（◎、○、×）を付した。蛍光Ｘ線によるＺｎカウント数が大きいほど劣る。
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）
　加工後耐食性
耐めっき剥離性試験と同様の加工を行い、テープ剥離をしない試験片を用意し、日本パーカライジング社製の脱脂剤：ＦＣ－Ｅ２０１１、表面調整剤：ＰＬ－Ｘおよび化成処理剤：パルボンドＰＢ－Ｌ３０６５を用いて、下記の標準条件で化成処理皮膜付着量が１．７～３．０ｇ／ｍ２となるよう化成処理を施した。
＜標準条件＞
・脱脂工程：処理温度が４０℃、処理時間が１２０秒
・スプレー脱脂、表面調整工程；ｐＨが９．５、処理温度が室温、処理時間が２０秒
・化成処理工程；化成処理液の温度が３５℃、処理時間が１２０秒
　上記化成処理を施した試験片の表面に、日本ペイント社製の電着塗料：Ｖ－５０を用いて、膜厚が２５μｍとなるように電着塗装を施し、下記の腐食試験に供した。
＜塩水噴霧試験（ＳＳＴ）＞
　化成処理、電着塗装を施した上記試験片の合金化溶融亜鉛めっき鋼板では曲げ加工部表面及び溶融亜鉛めっき鋼板ではボールインパクト部分に、カッターでめっきに到達するカット疵を付与した後、この試験片を、５ｍａｓｓ％ＮａＣｌ水溶液を使用して、ＪＩＳ　Ｚ２３７１：２０００に規定される中性塩水噴霧試験に準拠して２４０時間の塩水噴霧試験を行った後、クロスカット疵部についてテープ剥離試験し、カット疵部左右を合わせた最大剥離全幅を測定した。下記の基準に照らして記号（○、×）を付した。この最大剥離全幅が２．０ｍｍ以下であれば、塩水噴霧試験における耐食性は良好と評価することができる。
○：カット疵からの最大膨れ全幅２．０ｍｍ以下
×：カット疵からの最大膨れ全幅２．０ｍｍ超え
　以上の結果を条件と併せて、表２に示す。
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）、加工後耐食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）、加工後耐食性のいずれか一つ以上が劣る。
　本発明の高強度溶融亜鉛めっき熱延鋼板は、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２．５％以下、
    Ｍｎ：０．３％以上３．０％以下、
    Ｐ：０．０８％以下、
    Ｓ：０．０２％以下、
    Ａｌ：０．００１％以上０．２０％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２０～１２０ｇ／ｍ２の亜鉛めっき層を有し、
    前記鋼板表面の比表面積率ｒ が２．５以下であり、
    さらに、前記亜鉛めっき層に含まれるＳｉ量(ｇ／ｍ２)及びＭｎ量(ｇ／ｍ２)が、
    Ｓｉ量×ｒ ≦ 0.06
    Ｍｎ量×ｒ ≦ 0.10
    を満たす高強度溶融亜鉛めっき熱延鋼板。
  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
    のうち１種または２種以上を含有する請求項１に記載の高強度溶融亜鉛めっき熱延鋼板。
  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に対して、熱間圧延、酸洗し、
    次いで、表面粗度(Ｒａ)０．３～１．０を有するロールを用いて圧下率１～１０％で圧延し、
    次いで、溶融亜鉛めっき処理を行う高強度溶融亜鉛めっき熱延鋼板の製造方法。
  4. 　前記熱間圧延において、粗圧延後、仕上げ圧延前に、衝突圧０．３ＭＰａ以上１．８ＭＰａ未満で高圧水噴射によるデスケーリングを行い、
    仕上げ圧延温度８００℃以上で仕上げ圧延し、
    巻取温度４００～６５０℃で巻き取る請求項３に記載の高強度溶融亜鉛めっき熱延鋼板の製造方法。
  5. 　前記溶融亜鉛めっき処理前に、
    炉内雰囲気を水素濃度２～３０ｖｏｌ％かつ露点－６０～－１０℃とし、鋼板到達温度６００～９５０℃で連続焼鈍する請求項３または４に記載の高強度溶融亜鉛めっき熱延鋼板の製造方法。
  6. 　前記溶融亜鉛めっき処理後、さらに、合金化処理を行う請求項３～５のいずれか一項に記載の高強度溶融亜鉛めっき熱延鋼板の製造方法。
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