US20220364214A1 - Methods respectively for manufacturing hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet - Google Patents

Methods respectively for manufacturing hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet Download PDF

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US20220364214A1
US20220364214A1 US17/625,209 US202017625209A US2022364214A1 US 20220364214 A1 US20220364214 A1 US 20220364214A1 US 202017625209 A US202017625209 A US 202017625209A US 2022364214 A1 US2022364214 A1 US 2022364214A1
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steel sheet
hot
dip
mass
annealing
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Takayuki Maeda
Hiroshi Irie
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Kobe Steel Ltd
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Kobe Steel Ltd
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    • 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
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
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    • 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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    • C23C2/29—Cooling or quenching
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    • 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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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
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    • C21D6/00—Heat treatment of ferrous alloys
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    • 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
    • C21D8/0257—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
    • 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
    • C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
    • 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
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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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
    • 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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    • C23C2/0038—Apparatus characterised by the pre-treatment chambers located immediately upstream of the bath or occurring locally before the dipping process
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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
    • 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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    • 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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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
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    • 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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Definitions

  • the present invention relates to methods for manufacturing a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet.
  • high-tensile strength steel sheets have been used for automobile members for the purpose of improving fuel efficiency by weight reduction and collision safety performance.
  • various reinforcing elements are often contained in steel.
  • Mn and Si are known as inexpensive and effective elements.
  • Si is oxidized outward to a surface of a steel sheet during reduction annealing, so that it forms an oxide film to inhibit wettability with molten zinc during plating and cause an appearance failure such as bare spots and uneven alloying. Therefore, in Si-containing steel, it is difficult to produce a hot-dip galvannealed steel sheet having a good appearance.
  • Patent Literature 1 proposes a method that involves controlling a temperature of a steel sheet entering into a plating bath. Specifically, there is proposed a method for manufacturing a hot-dip galvanized steel sheet in which a steel sheet having a Si content of 0.05% by weight or more is plated by being immersed in a hot-dip galvanizing bath having a bath temperature of 440° C. or higher and containing 0.05 to 0.5% of Al at an entering sheet temperature T satisfying 350+30/t ⁇ T(° C.) ⁇ 420+30/t, wherein t is a sheet thickness (mm), and T(° C.) ⁇ 460.
  • this condition can suppress the occurrence of bare spots, but may make the plating thickness uneven and fail to obtain a beautiful appearance.
  • Patent Literature 2 proposes a method for manufacturing a hot-dip galvannealed steel sheet having a beautiful appearance by using a base steel sheet having a Si content of 0.8 to 2.5% by mass and by controlling, in a redox plating process, a Fe-based oxide film thickness, a plating bath temperature, a temperature of a steel sheet entering into a plating bath, and an effective Al concentration in the bath.
  • a redox plating process however, a step of oxidizing a steel sheet is required, and it is difficult to stably and appropriately control an oxide film thickness and to appropriately control the reduction amount according to the oxide film thickness, and it is difficult to stably manufacture a hot-dip galvannealed steel sheet having a beautiful appearance.
  • Patent Literature 3 proposes a method for manufacturing a hot-dip galvanized steel sheet with no bare spots by plating a steel sheet having a Si content of 0.5 to 2.0% under very high entering sheet temperature conditions satisfying T(Zn)+100° C. ⁇ T ⁇ T(Zn)+180° C. and 440° C. ⁇ T(Zn) ⁇ 470° C. wherein T is a temperature (° C.) of a steel sheet entering into a plating bath and T (Zn) is a bath temperature (° C.) of the plating bath.
  • T is a temperature (° C.) of a steel sheet entering into a plating bath
  • T (Zn) is a bath temperature (° C.) of the plating bath.
  • the occurrence of bare spots can be suppressed by this method, but it is difficult to appropriately control a plating deposition weight by wiping after plating due to the progress of alloying in the plating bath, and uneven alloying may occur.
  • a high dew point plating method is known as one of the methods of plating on Si-containing steel.
  • the high dew point plating method is a method in which Si in steel is internally oxidized by increasing an atmosphere dew point in an annealing furnace, and plating is performed while suppressing outward oxidation.
  • plating can be applied even on high Si steel having a Si content of 1.0% by mass or more.
  • the resulting hot-dip galvannealed steel sheet has an appearance in which ash floating in a zinc hot-dip galvanizing bath is attached, and there is a problem that the appearance is significantly deteriorated.
  • Patent Literature 4 proposes a method for manufacturing a hot-dip galvannealed steel sheet having a beautiful appearance by using a steel sheet having a relatively small Si content of 0.1% by mass or less and controlling the temperature of the steel sheet entering into a plating bath.
  • this method since the content of Si in the steel sheet to be used is small and the amount of outward oxidation formed during annealing is small, plating can be performed without using the high dew point plating method, and an appearance failure does not occur in the resulting plated steel sheet.
  • the plated steel sheet obtained using the method described in Patent Literature 4 has a problem that sufficient strength cannot be obtained because the content of Si in the steel is small.
  • the present invention has been made in view of the above-described circumstances, and an object thereof is to provide a method for manufacturing a hot-dip galvanized steel sheet capable of stably manufacturing a hot-dip galvanized steel sheet that can be used as a material of a hot-dip galvannealed steel sheet having a beautiful appearance using high Si steel, and a method for manufacturing a hot-dip galvannealed steel sheet capable of stably manufacturing a hot-dip galvannealed steel sheet having a beautiful appearance.
  • Patent Literature 1 JP-A-2000-303158
  • Patent Literature 2 JP-A-2010-189734
  • Patent Literature 3 JP-A-2010-156029
  • Patent Literature 4 JP-A-H7-216526
  • a method for manufacturing a hot-dip galvanized steel sheet includes an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of ⁇ 20° C. or higher; and a hot-dip galvanizing step of making the annealed steel sheet enter into a galvanizing bath to form a galvanized layer on a surface of the steel sheet, wherein a temperature of the steel sheet entering into the galvanizing bath is set to 390° C. or lower.
  • a method for manufacturing a hot-dip galvannealed steel sheet according to another aspect of the present invention includes an alloying step of alloying the galvanized layer formed in the hot-dip galvanized steel sheet obtained by the aforementioned method for manufacturing a hot-dip galvanized steel sheet.
  • FIGS. 1A and 1B are optical micrographs of a cross section of a steel sheet after annealing, where FIG. 1A shows a case where an annealing temperature is A3 point or higher, and FIG. 1B shows a case where the annealing temperature is lower than A3 point.
  • the present inventors have repeatedly studied a method for manufacturing a hot-dip galvannealed steel sheet having a beautiful appearance using a steel sheet of high Si steel having a Si content of 1.0% by mass or more as follows.
  • a high dew point plating method that involves increasing the atmosphere dew point during annealing is known.
  • the high dew point plating method since the entire steel sheet is exposed to a uniform atmosphere in an annealing furnace, the appearance of plating and the thickness of a plated layer become uniform.
  • the steel sheet of the high Si steel was annealed with the dew point of the annealing atmosphere set to 0° C. and with the annealing temperature set to lower than the A3 point or set to the A3 point or higher.
  • the annealing temperature was the A3 point or higher, the internal oxide layer generated at the surface of the steel sheet was in a state dominated by grain boundary oxidation as shown in FIG. 1A .
  • the present inventors have accomplished the present invention based on these findings.
  • a hot-dip galvanized steel sheet which can be used as a material of a hot-dip galvannealed steel sheet having a beautiful appearance by using high Si steel.
  • a method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment includes an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of ⁇ 20° C. or higher; and a hot-dip galvanizing step of making the annealed steel sheet enter into a galvanizing bath to form a galvanized layer on a surface of the steel sheet, wherein a temperature of the steel sheet entering into the galvanizing bath is set to 390° C. or lower.
  • a hot-dip galvanized steel sheet that can be used as a material of a hot-dip galvannealed steel sheet is stably obtained.
  • a steel sheet of high Si steel having a Si content of 1.0% by mass or more and 3.0% by mass or less is subjected to reduction annealing at a temperature equal to or higher than the A3 point of the steel sheet in an atmosphere having a dew point of ⁇ 20° C. or higher.
  • reduction annealing for example, an indirect heating type annealing furnace such as an all radiant type annealing furnace can be used.
  • the steel sheet of the high Si steel to be used may be a steel sheet manufactured by a conventional method and, and can be obtained, for example, by smelting a cast piece of steel satisfying the Si content described above, and then subjecting the cast piece to hot-rolling, pickling, and cold-rolling.
  • the annealing temperature is set to an A3 point or higher in order to obtain a steel sheet having desired strength characteristics.
  • the upper limit of the annealing temperature is not particularly determined as long as the annealing temperature is a temperature at which no liquid phase is generated.
  • the annealing temperature is lower than 1000° C. in consideration of the durability of the annealing furnace.
  • the holding time at the annealing temperature is preferably 1 s or more and preferably 600 s or less from the viewpoint of obtaining a steel sheet having desired strength characteristics.
  • the A3 point can be calculated by the following formula (i) (“The Physical Metallurgy of Steels” (published by Maruzen Co., Ltd., written by William C. Leslie, p. 273)).
  • the element symbol enclosed in [] in the formula (i) represents the content (% by mass) of the element.
  • A3 (° C.) 910 ⁇ 203 ⁇ [C] 1/2 ⁇ 15.2 ⁇ [Ni]+44.7 ⁇ [Si]+104 ⁇ [V]+31.5 ⁇ [Mo]+13.1 ⁇ [W] ⁇ 30 ⁇ [Mn]+11 ⁇ [Cr]+20 ⁇ [Cu] ⁇ 700 ⁇ [P] ⁇ 400 ⁇ [Al] ⁇ 120 ⁇ [As] ⁇ 400 ⁇ [Ti] ⁇ (i)
  • the dew point of the annealing atmosphere is set to ⁇ 20° C. or higher.
  • a hot-dip galvanized steel sheet as a material of a hot-dip galvannealed steel sheet having a beautiful appearance can be stably manufactured in the subsequent hot-dip galvanizing step. This is because when the dew point of the annealing atmosphere is high, the oxygen potential in the annealing atmosphere increases, and Si and oxygen solid-dissolved in the steel sheet react with each other to form internal oxidation, so that solid-dissolved Si that adversely affects the plating property can be rendered harmless.
  • the entire steel sheet is exposed to a uniform atmosphere in the annealing furnace and falls into a uniform state, so that the appearance of the plating formed in the hot-dip galvanizing step and the thickness of the plated layer are uniform.
  • the upper limit of the dew point of the annealing atmosphere is preferably 10° C.
  • the annealing atmosphere can be, for example, a reducing atmosphere of N 2 -5 vol % H 2 .
  • the steel sheet is made to enter a galvanizing bath, and a galvanized layer is formed on the surface of the steel sheet to complete a hot-dip galvanized steel sheet.
  • a galvanized layer is formed on the surface of the steel sheet to complete a hot-dip galvanized steel sheet.
  • the galvanizing bath one used for manufacturing a normal hot-dip galvanized steel sheet can be used.
  • the temperature of the steel sheet entering into the galvanizing bath (hereinafter, the temperature is also referred to as “entering sheet temperature”) is 390° C. or lower.
  • entering sheet temperature the temperature of the steel sheet entering into the galvanizing bath
  • the reason why the appearance failure of these plated steel sheets is suppressed by setting the entering sheet temperature to 390° C. or lower is considered to be that the reactivity between the surface of the steel sheet and ashes in a snout decreases even when the mode of the internal oxidation of the steel sheet generated during the annealing step is mainly grain boundary oxidation.
  • the entering sheet temperature is preferably 350° C. or higher.
  • the immersion time of the steel sheet in the galvanizing bath can be adjusted according to a desired plating deposition amount.
  • the method for manufacturing a hot-dip galvannealed steel sheet according to the present embodiment includes an alloying step of alloying the galvanized layer formed on the hot-dip galvanized steel sheet obtained by the method described above. That is, the method for manufacturing a hot-dip galvannealed steel sheet according to the present embodiment includes an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of ⁇ 20° C.
  • the hot-dip galvanized steel sheet is heated to a prescribed alloying temperature to diffuse iron atoms constituting the steel sheet into the plated layer, thereby alloying the plated layer.
  • the alloying temperature is preferably 400° C. or higher and preferably 600° C. or lower.
  • the time for holding at the alloying temperature (hereinafter, it is also referred to as “alloying time”) is preferably 1 s or more and preferably 60 s or less from the viewpoint of optimizing the alloying state.
  • the heating atmosphere may be atmospheric air.
  • the hot-dip galvanizing step and the alloying step are preferably performed using a continuous galvanizing line (CGL).
  • CGL continuous galvanizing line
  • the Si content in the steel sheet before the formation of the galvanized layer is 1.0% by mass or more and 3.0% by mass or less.
  • Si is an element that has high solid solution reinforcing performance in a steel sheet and enhances strength without reducing the ductility of the steel sheet.
  • the lower limit of the Si content of the steel sheet is set to 1.0% by mass.
  • the Si content is excessive, the strength of the steel sheet is excessively high, so that the rolling load increases, and Si scales are generated during hot-rolling to deteriorate the surface quality of the steel sheet.
  • the upper limit of the Si content is set to 3.0% by mass.
  • the lower limit of the Si content is preferably 1.1% by mass and more preferably 1.2% by mass.
  • the upper limit of the Si content is preferably 2.7% by mass and more preferably 2.5% by mass.
  • the balance of the steel sheet other than Si contains Fe as a main component and also contains unavoidable impurities.
  • the contents of C and Mn may be set as follows from the viewpoint of securing excellent mechanical properties.
  • the C is an element that enhances the strength of the steel sheet.
  • the C content is preferably 0.05% by mass or more.
  • the C content is preferably 0.5% by mass or less.
  • Mn is an element that enhances the strength of the steel sheet and promotes the generation of retained austenite to enhance the workability.
  • the Mn content is preferably 1.6% by mass or more.
  • the Mn content is preferably 4.0% by mass or less.
  • the steel sheet may further contain, for example, one or more of the following elements.
  • Al is an element that acts as a deoxidizing agent in smelting steel.
  • the Al content is preferably 0,001% by mass or more in order to effectively exhibit the effect.
  • the Al content is preferably 0.5% by mass or less.
  • Cr is an element that suppresses oxidation.
  • the amount of Si oxide generated at grain boundaries is reduced, so that the amount of solid solute Si increases.
  • Both of the solid solute Si and Cr act as an oxidation inhibiting element to prevent rapid progress of oxidation in an oxidation step.
  • the Cr content is preferably 0.01% by mass or more.
  • the Cr content is preferably 0.3% by mass or less.
  • Ti is an element that forms a carbide or a nitride to improve the strength of a steel sheet. It is also an element for forming Ti nitride to reduce the N content in steel, thereby suppressing the formation of B nitride and effectively utilizing the quenching property of the solid solute B. In order to effectively exhibit such an effect, the Ti content is preferably 0.0005% by mass or more. On the other hand, if the Ti content is excessive, the Ti carbide and the Ti nitride will be excessive, and the ductility, the stretch flangeability, and the stretch workability will be deteriorated. Therefore, the Ti content is preferably 0.2% by mass or less.
  • the unavoidable impurities refer to elements brought into steel depending on the situation of raw materials, materials, manufacturing facilities, etc., and examples thereof include S, N, O, Pb, Bi, Sb, and Sn in addition to P described below.
  • P is an clement which a steel sheet contains as an unavoidable impurity. P not only segregates at grain boundaries to promote grain boundary embrittlement, but also deteriorates hole expandability. Therefore, the P content is preferably as low as possible, for example, 0.03% by mass or less.
  • the method for manufacturing a hot-dip galvanized steel sheet includes an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of ⁇ 20° C. or higher; and a hot-dip galvanizing step of making the annealed steel sheet enter into a galvanizing bath to form a galvanized layer on a surface of the steel sheet, wherein a temperature of the steel sheet entering into the galvanizing bath is set to 390° C. or lower.
  • a hot-dip galvanized steel sheet as a material of a hot-dip galvannealed steel sheet having a beautiful appearance can be stably manufactured.
  • a method for manufacturing a hot-dip galvannealed steel sheet according to another aspect of the present invention includes an alloying step of alloying the galvanized layer formed in the hot-dip galvanized steel sheet obtained by the above-described method for manufacturing a hot-dip galvanized steel sheet.
  • the hot-dip galvanizing step and the alloying step may be performed using a continuous galvanizing line.
  • the formation of hot-dip galvanizing on a steel sheet and the alloying of the formed galvanized layer can be continuously performed, and a hot-dip galvannealed steel sheet having a beautiful appearance can be efficiently obtained.
  • Cast pieces manufactured so as to have the compositions of steel types A and B shown in Table 1 were smelted, and the cast pieces smelted were subjected to hot-rolling, pickling, and cold-rolling, to thereby obtain steel sheets of high Si steels.
  • Table 1 also shows the A3 points of the respective steel types calculated by the formula (i).
  • the obtained steel sheet was sequentially subjected to reduction annealing and hot-dip galvanization using a continuous galvanizing line (CGL) equipped with an all radiant tube type annealing furnace, to obtain a hot-dip galvanized steel sheet. Furthermore, alloying treatment was continuously performed on the resulting hot-dip galvanized steel sheet, to obtain a hot-dip galvannealed steel sheet.
  • CGL continuous galvanizing line
  • Table 2 shows the steel types and the A3 points of the steel sheets, the dew points of the annealing atmospheres, the annealing temperatures, and the temperatures of the steel sheets entering into the galvanizing bath used in Test Nos. 1 to 6.
  • Annealing temperature A3 point or higher
  • the present invention has broad industrial applicability in the technical field related to methods for manufacturing a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet.

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