WO2012115291A1 - Method for manufacturing hot dip plated steel - Google Patents

Method for manufacturing hot dip plated steel Download PDF

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Publication number
WO2012115291A1
WO2012115291A1 PCT/KR2011/001247 KR2011001247W WO2012115291A1 WO 2012115291 A1 WO2012115291 A1 WO 2012115291A1 KR 2011001247 W KR2011001247 W KR 2011001247W WO 2012115291 A1 WO2012115291 A1 WO 2012115291A1
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WIPO (PCT)
Prior art keywords
zone
steel sheet
hot dip
bath
balance
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PCT/KR2011/001247
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French (fr)
Inventor
Jung-Bong Kim
Bae-Geun LEE
Hyun-Yong Seo
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Posco Coated & Color Steel Co., Ltd.
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Priority to PCT/KR2011/001247 priority Critical patent/WO2012115291A1/en
Publication of WO2012115291A1 publication Critical patent/WO2012115291A1/en

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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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/54—Furnaces for treating strips or wire
    • C21D9/56—Continuous furnaces for strip or wire
    • C21D9/573—Continuous furnaces for strip or wire with cooling
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/68—Temporary coatings or embedding materials applied before or during heat treatment
    • C21D1/70—Temporary coatings or embedding materials applied before or during heat treatment while heating or quenching
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • 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
    • C23C2/003—Apparatus
    • C23C2/0038—Apparatus characterised by the pre-treatment chambers located immediately upstream of the bath or occurring locally before the dipping process
    • 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
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • 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
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • 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
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching

Definitions

  • the present invention relates to a method for manufacturing a hot dip plated steel sheet, and more particularly, to a method for manufacturing a hot dip plated steel sheet having a hot dip plated layer having excellent uniformity and plating adhesion by using, as a base material, a steel sheet containing special elements that deteriorate plating properties.
  • a base steel sheet contains more than a certain amount of special elements, for example, carbon (C), manganese (Mn), sulfur (S), phosphorous (P), silicon (Si), nitrogen (N), hydrogen (H), oxygen (O), copper (Cu), aluminum (Al), arsenic (As), boron (B), cobalt (Co), chromium (Cr), molybdenum (Mo), nickel (Ni), titanium (Ti), tin (Sn), cerium (Ce), calcium (Ca), niobium (Nb), tellurium (Te), lead (Pb), vanadium (V), tungsten (W), and zirconium (Zr), etc., wettability becomes poor, thereby causing plating properties to be deteriorated. Resultingly, unplated portions may remain
  • plating can be performed after removing the binding force of oxidizing materials by raising the temperature of a heating furnace to extreme levels.
  • one related art method is a method in which an iron (Fe) oxide layer is necessarily formed in a direct-fired furnace positioned at a front end of an annealing furnace, and thereafter, the formation of metal oxides is suppressed by controlling the Fe oxide layer to be reduced in an indirect heating furnace directly before being dipped into the plating bath.
  • another related art method is a manufacturing method for a hot dip plate steel in which a hot-rolled steel sheet with black scale attached is heat-treated within a certain heating range to internally oxidize additive elements, and is then subjected to cleaning, cold rolling, and hot dip plating.
  • an oxide layer formed in the direct-fired furnace may be detached from a steel sheet surface and adhered to the surface of a roll while the steel sheet is passing through the heating furnace, thus deteriorating the surface quality of the steel sheet.
  • An aspect of the present invention provides a method for manufacturing a hot dip plated steel sheet having a hot dip plated layer with uniformity and excellent plating adhesion by using, as a base material, a steel sheet containing special elements that deteriorate plating properties.
  • a method for manufacturing a hot dip plated steel sheet manufactured by sequentially passing a steel sheet through an annealing furnace having a preheating zone, a warming zone, a heating zone, a cooling zone, and a holding zone, and a hot dip plating bath positioned at a rear end of the annealing furnace, the method being characterized in that: the steel sheet includes at least one or more special elements that deteriorate hot dip plating properties; the preheating, warming, heating and holding of the steel sheet are performed in an indirect heating manner; a temperature of the preheating zone is 200-400°C , a temperature of the warming zone is 450-600°C, a temperature of the heating zone is 600-920°C, temperature of the cooling zone is 480-750°C, and temperature of the holding zone is 440-700°C; the preheating zone has an atmosphere of 1 vol.% or less of hydrogen, 300 ppm-0.5 vol.% of oxygen and a balance of nitrogen and una
  • a hot dip plated steel sheet which has a hot dip plated layer with better uniformity and excellent adhesion, can be manufactured by using a steel sheet containing special elements.
  • the hot dip plated steel sheet is applicable to various fields.
  • Fig. 1 is a schematic diagram illustrating an example of a hot dip plating apparatus which is applicable to the present invention.
  • the present invention which utilizes an indirect heating method, is related to a method for manufacturing a hot dip plated steel sheet in which adhesion as well as plating properties are improved using, as a base material, a steel sheet containing special elements.
  • a steel sheet of the present invention is a steel sheet in which at least one of a plurality of special elements that deteriorate hot dip plating properties is included.
  • the special elements may include any element enabling a complex oxide layer to be formed with iron (Fe) in preheating and warming zones
  • the representative element may include titanium (Ti), chromium (Cr), nickel (Ni), niobium (Nb), boron (B), copper (Cu), aluminum (Al), molybdenum (Mo), tungsten (W), tin (Sn), zirconium (Zr), lead (Pb), vanadium (V), cerium (Ce), calcium (Ca), cobalt (Co), arsenic (As), and the like.
  • contents of the special elements are not particularly limited, the contents are limited as being not less than an amount allowing hot-dip plating properties to be degraded.
  • Fig. 1 is a schematic diagram illustrating a preferable example of a hot dip plating apparatus which is applicable to the present invention.
  • the hot dip plating apparatus includes an annealing furnace 10 having a preheating zone 1, a warming zone 2, a heating zone 3, a cooling zone 4, and a holding zone 5 in sequence of a transfer direction of a steel sheet 11, and a hot dip plating bath 6 positioned at the rear end of the annealing furnace 10, and a cooling apparatus 7.
  • the steel sheet 11 is hot-dip plated by sequentially passing the steel sheet 11 through the annealing furnace 10 having the preheating zone 1, the warming zone 2, the heating zone 3, the cooling zone 4, and the holding zone 5, and the hot dip plating bath 6 positioned at the rear end of the annealing furnace 10, and thereafter, the steel sheet 11 is cooled by the cooling apparatus 7.
  • the preheating, warming, heating and holding of the steel sheet are performed in an indirect heating manner.
  • a temperature of the preheating zone is 200-400°C
  • a temperature of the warming zone is 450-600°C
  • a temperature of the heating zone is 600-920°C
  • a temperature of the cooling zone is 480-750°C
  • a temperature of the holding zone is 440-700°C.
  • the preheating zone there is a passage connected to an outside of the heating furnace such that gas is introduced and mixed relatively freely. Since it is impossible to completely prevent the gas from being introduced and mixed, controlling oxide layer thickness, which can be achieved by the present invention, is difficult.
  • the preheating zone only plays a role in raising the temperature of the steel sheet in order to easily control the oxide layer thickness by activating a surface of the steel sheet before the steel sheet enters the warming zone.
  • the temperature of the preheating zone when the temperature of the preheating zone is lower than 200 °C, oxidation may not be sufficiently performed due to a limitation of heat capacity in the warming zone.
  • the temperature when the temperature is higher than 400°C, the oxidation is rapidly performed in the preheating zone such that plating properties are deteriorated due to the thick oxide layer. Therefore, it is preferable that the temperature of the preheating zone be limited to 200-400 °C.
  • the steel sheet surface is oxidized by artificially supplying oxygen thereto.
  • An oxidation reaction progresses relatively slowly to a temperature of 450°C or less. Since reactivity increases to 600°C or more, reduction becomes difficult in the heating zone. Therefore, it is preferable that the temperature of the warming zone is limited to 450-600°C.
  • the steel sheet passes through an atmosphere of hydrogen without impurities and inert nitrogen gas.
  • a proper temperature is limited to a minimum of 600°C and a maximum of 920°C or less.
  • the cooling zone plays a role in cooling the steel sheet to a predetermined temperature before it is transferred to the holding zone. If cooling is not performed, a problem of workability according to material changes may be caused because the homogeneous structure of Fe is not achieved in the holding zone. On the contrary, if supercooled, plating adhesion is deteriorated due to the deviation from the temperature of the plating bath. Therefore, it is preferable that the temperature of the cooling zone is limited to 480-750°C.
  • the holding zone is a section that gives a predetermined time necessary for an Fe structure to uniformly grow under a predetermined temperature after the cooling zone.
  • the temperature of the holding zone be limited to 440-700°C.
  • the preheating zone 1 has an atmosphere of 1 vol.% or less of hydrogen, 300 ppm-0.5 vol.% of oxygen, and a balance of nitrogen and unavoidable impurities
  • the warming zone 2 has an atmosphere of 1 vol.% or less of hydrogen, 10 ppm-0.1 vol.% of oxygen, and a balance of nitrogen and unavoidable impurities.
  • the heating zone 3 has an atmosphere of 20-50 vol.% of hydrogen and a balance of nitrogen and unavoidable impurities
  • the cooling zone 4 and the holding zone 5 have atmospheres of 20 vol.% or less of hydrogen and a balance of nitrogen and unavoidable impurities.
  • the warming zone like the preheating zone, a separate control is not performed, except in the case that hydrogen is inevitably introduced from the heating zone.
  • An oxygen concentration is maintained to 10ppm-0.1 vol.% for forming a thin oxide layer of 0.04-1.00 ⁇ m in order not to induce internal oxidation by penetrating into the interior of the steel sheet.
  • a mixture of hydrogen and nitrogen is supplied.
  • nitrogen as an inert gas does not participate in a surface reaction of the steel sheet, but is only used for maintaining furnace pressure.
  • hydrogen triggers a reduction reaction to ionize Fe by reacting with oxygen which combines with Fe in the complex oxide layer formed in the warming zone.
  • oxygen combined with the special elements forms a stabilized layer such that some oxygen may be reduced in a hydrogen atmosphere at a temperature of 700-900°C, it is difficult to completely separate bonds therebetween. In order to separate the bonds, there is a difference per each special element, but higher temperature, a high concentration hydrogen atmosphere, a strict dew point control or the like is required.
  • Hydrogen remaining after reaction in the heating zone diffuses to be introduced into the cooling and holding zones.
  • the reduction reaction occurring insufficiently in the heating zone occurs additionally in the cooling and holding zones; however, the reduction degree is relatively small as compared to that in the heating zone.
  • the cooling and holding zones maintain a stable activated state of the steel sheet surface reduced in the inert nitrogen gas atmosphere, and transfer the steel sheet into the plating bath.
  • the Fe complex oxide layer containing the special elements is formed to have a thickness of 0.04-1.00 ⁇ m on the steel sheet surface before entering the heating zone 3 after passing through the preheating zone 1 and the warming zone 2.
  • the forming of the complex oxide layer is performed by properly controlling temperatures and atmospheres or the like of the preheating and warming zones.
  • the complex oxide layer formed as above is reduced in the heating zone 3 and the holding zone 5, and as a result, oxygen combined with Fe is removed.
  • the steel sheet with oxygen removed therefrom has a porous form.
  • the thickness of the complex oxide layer which is formed on the steel sheet surface in the preheating zone 1 and the warming zone 2 is less than 0.04 ⁇ m, the special elements is not sufficiently removed; and when the thickness is more than 1.00 ⁇ m, it is too thick to make a contact between Fe and the plating bath. As a result, it is difficult to form a uniform plated layer. Therefore, the thickness of the complex oxide layer is limited to 0.04-1.00 ⁇ m.
  • the steel sheet which has undergone the heat treatment as above enters into the hot dip plating bath 6 and is hot dip plated. Subsequently, the steel sheet is cooled by the cooling apparatus 7, and consequently a hot dip plated steel sheet with excellent adhesion is manufactured.
  • the entry temperature of the steel sheet into the hot dip plating bath should be set to a temperature 10-30°C lower than that of the plating bath.
  • the purpose of a difference between the entry temperature of the steel sheet and the temperature of the plating bath is to improve wettability between the plating bath and Fe during plating in the hot dip plating bath, by weakening the binding force between the non-reduced special elements and the reduced Fe by thermal shock, and also to promote removing the non-reduced special elements having an oxide form into the hot dip plating bath.
  • the difference between the entering temperature of the steel sheet and the temperature of the plating bath is less than 10°C, the thermal shock is too small to weaken the binding force between the non-reduced special elements and the Fe, and plating adhesion is deteriorated when the temperature difference is more than 30°C. Therefore, it is preferable that the difference between the entry temperature of the steel sheet and the temperature of the plating bath be limited to 10-30°C.
  • a hot dip plating bath may include: a molten zinc (Zn) bath consisting of 0.18-0.22 wt.% of Al and a balance of Zn; a molten aluminum bath consisting of 8.7-9.3 wt.% of Si, 2.0-3.0 wt.% of Fe and a balance of Al; and a molten aluminum-zinc alloy bath consisting of 41.4-45.4 wt.% of Zn, 1.2-2.0 wt.% of Si and a balance of Al.
  • Zn molten zinc
  • Al molten aluminum bath
  • a molten aluminum-zinc alloy bath consisting of 41.4-45.4 wt.% of Zn, 1.2-2.0 wt.% of Si and a balance of Al.
  • the porous form is obtained. Also, the steel sheet enters into the hot dip plating bath in a state where the steel sheet received the thermal shock, and thus the wettability between Fe and the plating bath becomes better so that plating properties are improved. Therefore, a uniform plated layer can be obtained without non-plating portions, and adhesion of the hot dip plated layer is also improved.
  • the non-reduced special elements during hot dip plating are removed in the form of oxides into the hot dip plating bath.
  • the preheating zone had an atmosphere of 1 vol.% or less of hydrogen, 300ppm-0.5vol.% of oxygen, and a balance of nitrogen and unavoidable impurities
  • the warming zone had an atmosphere of 1 vol.% or less of hydrogen, 10 ppm-0.1 vol.% of oxygen, and a balance of nitrogen and unavoidable impurities.
  • the heating zone had an atmosphere of 20-50 vol.% of hydrogen and a balance of nitrogen and unavoidable impurities
  • the cooling zone and the holding zone had atmospheres of 20 vol.% or less of hydrogen and a balance of nitrogen and unavoidable impurities.
  • An Fe complex oxide layer containing special elements was formed to a thickness of 0-0.93 ⁇ m on a steel sheet surface before entering the heating zone after passing through the preheating and warming zones.
  • the entry temperature of the steel sheet into a hot dip plating bath was lower than the temperature of the plating bath by 10-30°C.
  • An Al plating bath of Table 2 was composed of 9.0 wt.% of Si, 2.2 wt.% of Fe and a balance of Al, and a Zn plating bath of Table 3 is composed of 0.20 wt.% of Al and a balance of Zn.
  • a temperature of the Al plating bath of the following Table 2 was 660°C, and a temperature of the Zn plating bath of the following Table 3 was 460°C. Coating weight per unit area was controlled to 50g/m 2 by gas wiping.
  • Cooling of the steel sheet after the hot dip plating was maintained at 20°C/sec right after the plating.

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  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
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  • Physics & Mathematics (AREA)
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  • Crystallography & Structural Chemistry (AREA)
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Abstract

Provided is a method for manufacturing a hot dip plated steel sheet by sequentially passing a steel sheet through an annealing furnace having preheating, warming, heating, cooling, and holding zones, and a hot dip plating bath at the rear end of the annealing furnace. In the method, the steel sheet includes at least one or more of the special elements that deteriorate hot dip plating properties, the preheating, warming, heating and holding of the steel sheet are performed in an indirect heating manner, an Fe complex oxide layer containing the special elements is formed to have a thickness of 0.04-1.00㎛ on a surface of the steel sheet passing through the preheating and warming zones, the complex oxide layer is reduced in the heating and holding zones, and entering temperature of the steel sheet into the hot dip plating bath is 10-30℃ lower than that of the plating bath.

Description

METHOD FOR MANUFACTURING HOT DIP PLATED STEEL
The present invention relates to a method for manufacturing a hot dip plated steel sheet, and more particularly, to a method for manufacturing a hot dip plated steel sheet having a hot dip plated layer having excellent uniformity and plating adhesion by using, as a base material, a steel sheet containing special elements that deteriorate plating properties.
Conventionally, in a method for manufacturing hot dip plated steels having a molten zinc base, a molten aluminum base, a molten aluminum-zinc alloy base or the like, if a base steel sheet contains more than a certain amount of special elements, for example, carbon (C), manganese (Mn), sulfur (S), phosphorous (P), silicon (Si), nitrogen (N), hydrogen (H), oxygen (O), copper (Cu), aluminum (Al), arsenic (As), boron (B), cobalt (Co), chromium (Cr), molybdenum (Mo), nickel (Ni), titanium (Ti), tin (Sn), cerium (Ce), calcium (Ca), niobium (Nb), tellurium (Te), lead (Pb), vanadium (V), tungsten (W), and zirconium (Zr), etc., wettability becomes poor, thereby causing plating properties to be deteriorated. Resultingly, unplated portions may remain after plating, or a non-uniform plated layer may be formed.
That is, in the case of carrying out a non-oxidation heat treatment and a reduction process after degreasing and cleaning the steel sheet containing the special elements as described above, then cooling the steel sheet to a required temperature, and subsequently preparing a hot dip plated steel by dipping the steel sheet in a hot dip plating bath, an oxidizing material is formed on a surface of the steel sheet in the air, thereby leading to deterioration of wettability. This causes unplated portions to remain and a non-uniform plated layer to be formed after all.
In order to solve these problems, plating can be performed after removing the binding force of oxidizing materials by raising the temperature of a heating furnace to extreme levels. However, it is physically impossible to maintain such extreme temperatures in the heating furnace.
To solve these problems, several related art methods have been proposed. For example, one related art method is a method in which an iron (Fe) oxide layer is necessarily formed in a direct-fired furnace positioned at a front end of an annealing furnace, and thereafter, the formation of metal oxides is suppressed by controlling the Fe oxide layer to be reduced in an indirect heating furnace directly before being dipped into the plating bath. In addition, another related art method is a manufacturing method for a hot dip plate steel in which a hot-rolled steel sheet with black scale attached is heat-treated within a certain heating range to internally oxidize additive elements, and is then subjected to cleaning, cold rolling, and hot dip plating.
However, these related art methods still have problems, as described below, to be solved.
That is, in the method of reducing the Fe-based oxide layer formed in the direct-fired furnace directly before dipping the steel sheet in the plating bath, contamination in the heating furnace and surface defects due to adsorption on the steel sheet surface, which are caused by carbides produced during the direct combustion of a fuel gas, are generated. This causes plating properties to be deteriorated when a relatively thickly formed oxide layer is insufficiently reduced. On the contrary, when the reduction is performed rapidly, reoxidation of the steel sheet surface occurs in a cooling zone.
Therefore, technology for extremely precise control of the heating furnace is required, making it difficult to perform stable operations in an industrial mass production system.
Also, an oxide layer formed in the direct-fired furnace may be detached from a steel sheet surface and adhered to the surface of a roll while the steel sheet is passing through the heating furnace, thus deteriorating the surface quality of the steel sheet.
For this reason, a heating furnace has recently being changed to an indirect heating type from a direct-fired heating type.
An aspect of the present invention provides a method for manufacturing a hot dip plated steel sheet having a hot dip plated layer with uniformity and excellent plating adhesion by using, as a base material, a steel sheet containing special elements that deteriorate plating properties.
According to an aspect of the present invention, there is provided a method for manufacturing a hot dip plated steel sheet manufactured by sequentially passing a steel sheet through an annealing furnace having a preheating zone, a warming zone, a heating zone, a cooling zone, and a holding zone, and a hot dip plating bath positioned at a rear end of the annealing furnace, the method being characterized in that: the steel sheet includes at least one or more special elements that deteriorate hot dip plating properties; the preheating, warming, heating and holding of the steel sheet are performed in an indirect heating manner; a temperature of the preheating zone is 200-400℃ , a temperature of the warming zone is 450-600℃, a temperature of the heating zone is 600-920℃, temperature of the cooling zone is 480-750℃, and temperature of the holding zone is 440-700℃; the preheating zone has an atmosphere of 1 vol.% or less of hydrogen, 300 ppm-0.5 vol.% of oxygen and a balance of nitrogen and unavoidable impurities, the warming zone has an atmosphere of 1 vol.% or less of hydrogen, 10 ppm-0.1 vol.% of oxygen and a balance of nitrogen and unavoidable impurities, the heating zone has an atmosphere of 20-50 vol.% of hydrogen and a balance of nitrogen and unavoidable impurities, and the cooling and holding zones have atmospheres of 20 vol.% or less of hydrogen and a balance of nitrogen and unavoidable impurities; an iron (Fe) complex oxide layer containing the special elements is formed to have a thickness of 0.04-1.00㎛ on a surface of the steel sheet before entering the heating zone after passing through the preheating and warming zones, and the complex oxide layer is reduced in the heating and holding zones; the special elements are elements allowing the complex oxide layer to be formed in the preheating and warming zones; an entry temperature of the steel sheet into the hot dip plating bath is lower than that of the plating bath by 10-30℃; and the hot dip plating bath is one selected from a molten zinc bath, a molten aluminum bath, and a molten aluminum-zinc alloy bath.
According to the present invention, a hot dip plated steel sheet, which has a hot dip plated layer with better uniformity and excellent adhesion, can be manufactured by using a steel sheet containing special elements. Thus, the hot dip plated steel sheet is applicable to various fields.
The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
Fig. 1 is a schematic diagram illustrating an example of a hot dip plating apparatus which is applicable to the present invention.
Hereinafter, the present invention will be described in detail.
The present invention, which utilizes an indirect heating method, is related to a method for manufacturing a hot dip plated steel sheet in which adhesion as well as plating properties are improved using, as a base material, a steel sheet containing special elements.
A steel sheet of the present invention is a steel sheet in which at least one of a plurality of special elements that deteriorate hot dip plating properties is included.
The special elements may include any element enabling a complex oxide layer to be formed with iron (Fe) in preheating and warming zones, and the representative element may include titanium (Ti), chromium (Cr), nickel (Ni), niobium (Nb), boron (B), copper (Cu), aluminum (Al), molybdenum (Mo), tungsten (W), tin (Sn), zirconium (Zr), lead (Pb), vanadium (V), cerium (Ce), calcium (Ca), cobalt (Co), arsenic (As), and the like.
In the case that contents of carbon (C), manganese (Mn), sulfur (S), phosphorous (P), silicon (Si) and nitrogen (N) are greater than those of general carbon steel, so as to deteriorate plating properties, these are also included in the special elements.
Although contents of the special elements are not particularly limited, the contents are limited as being not less than an amount allowing hot-dip plating properties to be degraded.
Hereinafter, the present invention will be described in detail with reference to Fig. 1.
Fig. 1 is a schematic diagram illustrating a preferable example of a hot dip plating apparatus which is applicable to the present invention.
As shown in Fig. 1, the hot dip plating apparatus includes an annealing furnace 10 having a preheating zone 1, a warming zone 2, a heating zone 3, a cooling zone 4, and a holding zone 5 in sequence of a transfer direction of a steel sheet 11, and a hot dip plating bath 6 positioned at the rear end of the annealing furnace 10, and a cooling apparatus 7.
A method for manufacturing a hot dip plated steel sheet using the hot dip plating apparatus shown in Fig. 1 according to the present invention will be described below.
In order to manufacture the hot dip plated steel sheet according to the present invention, the steel sheet 11 is hot-dip plated by sequentially passing the steel sheet 11 through the annealing furnace 10 having the preheating zone 1, the warming zone 2, the heating zone 3, the cooling zone 4, and the holding zone 5, and the hot dip plating bath 6 positioned at the rear end of the annealing furnace 10, and thereafter, the steel sheet 11 is cooled by the cooling apparatus 7.
The preheating, warming, heating and holding of the steel sheet are performed in an indirect heating manner.
A temperature of the preheating zone is 200-400℃, a temperature of the warming zone is 450-600℃, a temperature of the heating zone is 600-920℃, a temperature of the cooling zone is 480-750℃, and a temperature of the holding zone is 440-700℃.
In the preheating zone, there is a passage connected to an outside of the heating furnace such that gas is introduced and mixed relatively freely. Since it is impossible to completely prevent the gas from being introduced and mixed, controlling oxide layer thickness, which can be achieved by the present invention, is difficult.
Therefore, the preheating zone only plays a role in raising the temperature of the steel sheet in order to easily control the oxide layer thickness by activating a surface of the steel sheet before the steel sheet enters the warming zone.
However, when the temperature of the preheating zone is lower than 200 ℃, oxidation may not be sufficiently performed due to a limitation of heat capacity in the warming zone. When the temperature is higher than 400℃, the oxidation is rapidly performed in the preheating zone such that plating properties are deteriorated due to the thick oxide layer. Therefore, it is preferable that the temperature of the preheating zone be limited to 200-400 ℃.
In the warming zone, the steel sheet surface is oxidized by artificially supplying oxygen thereto. An oxidation reaction progresses relatively slowly to a temperature of 450℃ or less. Since reactivity increases to 600℃ or more, reduction becomes difficult in the heating zone. Therefore, it is preferable that the temperature of the warming zone is limited to 450-600℃.
In the heating zone, the steel sheet passes through an atmosphere of hydrogen without impurities and inert nitrogen gas. At this time, in order to obtain mechanical properties necessary for machining and performing surface activation for ensuring optimum plating properties, a proper temperature is limited to a minimum of 600℃ and a maximum of 920℃ or less.
The cooling zone plays a role in cooling the steel sheet to a predetermined temperature before it is transferred to the holding zone. If cooling is not performed, a problem of workability according to material changes may be caused because the homogeneous structure of Fe is not achieved in the holding zone. On the contrary, if supercooled, plating adhesion is deteriorated due to the deviation from the temperature of the plating bath. Therefore, it is preferable that the temperature of the cooling zone is limited to 480-750℃.
The holding zone is a section that gives a predetermined time necessary for an Fe structure to uniformly grow under a predetermined temperature after the cooling zone. When the temperature is unnecessarily high or low, the deviation of mechanical properties occurs, thereby making it difficult to obtain a required product. Therefore, it is preferable that the temperature of the holding zone be limited to 440-700℃.
The preheating zone 1 has an atmosphere of 1 vol.% or less of hydrogen, 300 ppm-0.5 vol.% of oxygen, and a balance of nitrogen and unavoidable impurities, and the warming zone 2 has an atmosphere of 1 vol.% or less of hydrogen, 10 ppm-0.1 vol.% of oxygen, and a balance of nitrogen and unavoidable impurities. The heating zone 3 has an atmosphere of 20-50 vol.% of hydrogen and a balance of nitrogen and unavoidable impurities, and the cooling zone 4 and the holding zone 5 have atmospheres of 20 vol.% or less of hydrogen and a balance of nitrogen and unavoidable impurities.
Since hydrogen is inevitably introduced into the preheating zone from the heating zone and air in the atmosphere is introduced through the passage connected to the outside, artificially controlling an oxygen concentration is not performed, but hydrogen is limited to 1 vol.% or less by the operating of a gas mixing preventing apparatus between the warming zone and the heating zone. When hydrogen is more than 1 vol.%, hydrogen combines with oxygen in the preheating zone to form H2O vapor. This is a main cause of defects in plated products due to an increase in a dew point.
In the warming zone, like the preheating zone, a separate control is not performed, except in the case that hydrogen is inevitably introduced from the heating zone. An oxygen concentration is maintained to 10ppm-0.1 vol.% for forming a thin oxide layer of 0.04-1.00㎛ in order not to induce internal oxidation by penetrating into the interior of the steel sheet.
When the oxygen concentration is lower than 10ppm or more than 0.1 vol.%, an oxide layer of a required thickness is not formed such that plating properties become poor.
In the heating zone, a mixture of hydrogen and nitrogen is supplied. Herein, nitrogen as an inert gas does not participate in a surface reaction of the steel sheet, but is only used for maintaining furnace pressure. Furthermore, hydrogen triggers a reduction reaction to ionize Fe by reacting with oxygen which combines with Fe in the complex oxide layer formed in the warming zone.
At this time, since reduction opportunities are insufficient in a low hydrogen concentration of 20 vol.% or less, plating properties are deteriorated. When the hydrogen concentration is 50 vol.% or more, a necessary reduction amount is exceeded. Because this causes manufacturing costs to be increased and equipment to be degraded and deteriorates operational stability, an excessive hydrogen supply, more than necessary, is not preferable.
Although oxygen combined with the special elements forms a stabilized layer such that some oxygen may be reduced in a hydrogen atmosphere at a temperature of 700-900℃, it is difficult to completely separate bonds therebetween. In order to separate the bonds, there is a difference per each special element, but higher temperature, a high concentration hydrogen atmosphere, a strict dew point control or the like is required.
Therefore, reactivity with plating molten metal is increased by using Fe as a main reduced material, and thus plating properties can be improved.
Hydrogen remaining after reaction in the heating zone diffuses to be introduced into the cooling and holding zones. As a result, the reduction reaction occurring insufficiently in the heating zone occurs additionally in the cooling and holding zones; however, the reduction degree is relatively small as compared to that in the heating zone. The cooling and holding zones maintain a stable activated state of the steel sheet surface reduced in the inert nitrogen gas atmosphere, and transfer the steel sheet into the plating bath.
The Fe complex oxide layer containing the special elements is formed to have a thickness of 0.04-1.00㎛ on the steel sheet surface before entering the heating zone 3 after passing through the preheating zone 1 and the warming zone 2.
The forming of the complex oxide layer is performed by properly controlling temperatures and atmospheres or the like of the preheating and warming zones.
The complex oxide layer formed as above is reduced in the heating zone 3 and the holding zone 5, and as a result, oxygen combined with Fe is removed.
At this time, most of the special elements exist in a non-reduced state.
The steel sheet with oxygen removed therefrom has a porous form.
As described above, when the thickness of the complex oxide layer, which is formed on the steel sheet surface in the preheating zone 1 and the warming zone 2, is less than 0.04㎛, the special elements is not sufficiently removed; and when the thickness is more than 1.00㎛, it is too thick to make a contact between Fe and the plating bath. As a result, it is difficult to form a uniform plated layer. Therefore, the thickness of the complex oxide layer is limited to 0.04-1.00㎛.
The steel sheet which has undergone the heat treatment as above enters into the hot dip plating bath 6 and is hot dip plated. Subsequently, the steel sheet is cooled by the cooling apparatus 7, and consequently a hot dip plated steel sheet with excellent adhesion is manufactured.
At this time, the entry temperature of the steel sheet into the hot dip plating bath should be set to a temperature 10-30℃ lower than that of the plating bath.
The purpose of a difference between the entry temperature of the steel sheet and the temperature of the plating bath is to improve wettability between the plating bath and Fe during plating in the hot dip plating bath, by weakening the binding force between the non-reduced special elements and the reduced Fe by thermal shock, and also to promote removing the non-reduced special elements having an oxide form into the hot dip plating bath.
When the difference between the entering temperature of the steel sheet and the temperature of the plating bath is less than 10℃, the thermal shock is too small to weaken the binding force between the non-reduced special elements and the Fe, and plating adhesion is deteriorated when the temperature difference is more than 30℃. Therefore, it is preferable that the difference between the entry temperature of the steel sheet and the temperature of the plating bath be limited to 10-30℃.
Any one of the hot dip plating baths can be used if it is conventionally used. Preferable examples of a hot dip plating bath may include: a molten zinc (Zn) bath consisting of 0.18-0.22 wt.% of Al and a balance of Zn; a molten aluminum bath consisting of 8.7-9.3 wt.% of Si, 2.0-3.0 wt.% of Fe and a balance of Al; and a molten aluminum-zinc alloy bath consisting of 41.4-45.4 wt.% of Zn, 1.2-2.0 wt.% of Si and a balance of Al.
As described above, since oxygen combined with Fe is removed from the complex oxide layer that is formed on the steel sheet surface, the porous form is obtained. Also, the steel sheet enters into the hot dip plating bath in a state where the steel sheet received the thermal shock, and thus the wettability between Fe and the plating bath becomes better so that plating properties are improved. Therefore, a uniform plated layer can be obtained without non-plating portions, and adhesion of the hot dip plated layer is also improved.
The non-reduced special elements during hot dip plating are removed in the form of oxides into the hot dip plating bath.
Hereinafter, the present invention will be described in more detail through an exemplary embodiment.
[Exemplary Embodiment]
After heat treating and hot dip plating steel sheets having compositions shown in following Table 1 under the conditions of following Tables 2 and 3, the presence of non-plated portions and pass or fail of a bending test were investigated, and the results are shown in Tables 2 and 3.
At this time, the preheating zone had an atmosphere of 1 vol.% or less of hydrogen, 300ppm-0.5vol.% of oxygen, and a balance of nitrogen and unavoidable impurities, and the warming zone had an atmosphere of 1 vol.% or less of hydrogen, 10 ppm-0.1 vol.% of oxygen, and a balance of nitrogen and unavoidable impurities. The heating zone had an atmosphere of 20-50 vol.% of hydrogen and a balance of nitrogen and unavoidable impurities, and the cooling zone and the holding zone had atmospheres of 20 vol.% or less of hydrogen and a balance of nitrogen and unavoidable impurities.
An Fe complex oxide layer containing special elements was formed to a thickness of 0-0.93㎛ on a steel sheet surface before entering the heating zone after passing through the preheating and warming zones.
The entry temperature of the steel sheet into a hot dip plating bath was lower than the temperature of the plating bath by 10-30℃.
An Al plating bath of Table 2 was composed of 9.0 wt.% of Si, 2.2 wt.% of Fe and a balance of Al, and a Zn plating bath of Table 3 is composed of 0.20 wt.% of Al and a balance of Zn.
A temperature of the Al plating bath of the following Table 2 was 660℃, and a temperature of the Zn plating bath of the following Table 3 was 460℃. Coating weight per unit area was controlled to 50g/m2 by gas wiping.
Cooling of the steel sheet after the hot dip plating was maintained at 20℃/sec right after the plating.
Figure PCTKR2011001247-appb-I000001
Figure PCTKR2011001247-appb-I000002
Figure PCTKR2011001247-appb-I000003
As shown in Tables 2 and 3, when heat treating is performed under the heat treatment conditions of the present invention, it can be understood that a hot dip plated steel sheet having excellent plating properties and adhesion can be manufactured, even if the steel sheet containing the special elements is used as a base steel sheet.
While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (5)

  1. A method for manufacturing a hot dip plated steel sheet manufactured by sequentially passing a steel sheet through an annealing furnace having a preheating zone, a warming zone, a heating zone, a cooling zone, and a holding zone, and a hot dip plating bath positioned at a rear end of the annealing furnace, the method being characterized in that:
    the steel sheet comprises at least one or more special elements that deteriorate hot dip plating properties;
    the preheating, warming, heating and holding of the steel sheet are performed in an indirect heating manner;
    a temperature of the preheating zone is 200-400℃, temperature of the warming zone is 450-600℃, a temperature of the heating zone is 600-920℃, a temperature of the cooling zone is 480-750℃, and a temperature of the holding zone is 440-700℃;
    the preheating zone has an atmosphere of 1 vol.% or less of hydrogen, 300 ppm-0.5 vol.% of oxygen and a balance of nitrogen and unavoidable impurities, the warming zone has an atmosphere of 1 vol.% or less of hydrogen, 10ppm-0.1 vol.% of oxygen and a balance of nitrogen and unavoidable impurities, the heating zone has an atmosphere of 20-50 vol.% of hydrogen and a balance of nitrogen and unavoidable impurities, and the cooling and holding zones have atmospheres of 20 vol.% or less of hydrogen and a balance of nitrogen and unavoidable impurities;
    an iron (Fe) complex oxide layer containing the special elements is formed to a thickness of 0.04-1.00㎛ on a surface of the steel sheet before entering the heating zone after passing through the preheating and warming zones, and the complex oxide layer is reduced in the heating and holding zones;
    the special elements are elements allowing the complex oxide layer to be formed in the preheating and warming zones;
    an entry temperature of the steel sheet into the hot dip plating bath is lower than that of the plating bath by 10-30℃; and
    the hot dip plating bath is one selected from a molten zinc bath, a molten aluminum bath, and a molten aluminum-zinc alloy bath.
  2. The method of claim 1, wherein the special element is at least one selected from the group consisting of titanium (Ti), chromium (Cr), nickel (Ni), niobium (Nb), boron (B), copper (Cu), aluminum (Al), molybdenum (Mo), tungsten (W), tin (Sn), zirconium (Zr), lead (Pb), vanadium (V), cerium (Ce), calcium (Ca), cobalt (Co), and arsenic (As).
  3. The method of claim 1 or 2, wherein contents of carbon (C), manganese (Mn), sulfur (S), phosphorous (P), silicon (Si) and nitrogen (N) in the steel sheet are greater than those in general carbon steel.
  4. The method of claim 1 or 2, wherein the molten zinc (Zn) bath is composed of 0.18-0.22 wt.% of Al and a balance of Zn; the molten aluminum bath is composed of 8.7-9.3 wt.% of Si, 2.0-3.0 wt.% of Fe and a balance of Al; and the molten aluminum-zinc alloy bath is composed of 41.4-45.4 wt.% of Zn, 1.2-2.0 wt.% of Si and a balance of Al.
  5. The method of claim 3, wherein the molten zinc (Zn) bath is composed of 0.18-0.22 wt.% of Al and a balance of Zn; the molten aluminum bath is composed of 8.7-9.3 wt.% of Si, 2.0-3.0 wt.% of Fe and a balance of Al; and the molten aluminum-zinc alloy bath is composed of 41.4-45.4 wt.% of Zn, 1.2-2.0 wt.% of Si and a balance of Al.
PCT/KR2011/001247 2011-02-23 2011-02-23 Method for manufacturing hot dip plated steel WO2012115291A1 (en)

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