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EP4477771A1 - Method for manufacturing high-strength, hot-dip galvanized steel sheet - Google Patents

Method for manufacturing high-strength, hot-dip galvanized steel sheet Download PDF

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Publication number
EP4477771A1
EP4477771A1 EP23775118.5A EP23775118A EP4477771A1 EP 4477771 A1 EP4477771 A1 EP 4477771A1 EP 23775118 A EP23775118 A EP 23775118A EP 4477771 A1 EP4477771 A1 EP 4477771A1
Authority
EP
European Patent Office
Prior art keywords
steel sheet
less
hot
temperature
volume
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP23775118.5A
Other languages
German (de)
English (en)
French (fr)
Inventor
Shunsuke Yamamoto
Tomomi KANAZAWA
Shogo Tamaki
Katsuya Hoshino
Katsutoshi Takashima
Chikaumi SAWANISHI
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
JFE Steel Corp
Original Assignee
JFE Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by JFE Steel Corp filed Critical JFE Steel Corp
Publication of EP4477771A1 publication Critical patent/EP4477771A1/en
Pending legal-status Critical Current

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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
    • C23C2/02—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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • 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
    • C21D1/26—Methods of 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/34—Methods of heating
    • 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
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • 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
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • C21D9/561—Continuous furnaces for strip or wire with a controlled atmosphere or vacuum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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
    • C23C2/0222—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating in a reactive atmosphere, e.g. oxidising or reducing atmosphere
    • 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
    • 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
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips

Definitions

  • the present invention relates to a method for producing a hot-dip galvanized steel sheet having excellent resistance to resistance-welding cracking and excellent delayed fracture resistance.
  • a residual stress produced in a steel sheet is considered to increase with increase in the strength of the steel sheet.
  • LME cracking there is a fear of the occurrence of LME cracking associated with the increase in the strength of a steel sheet.
  • Delayed fracture refers to a phenomenon where when a high-strength steel material is held under a static load stress (load stress lower than tensile strength), brittle fracture suddenly occurs, without any substantial apparent plastic deformation, upon the elapse of a period of time.
  • Such delayed fracture is caused by corrosion that occurs due to the use environment of a steel sheet, and often caused by hydrogen that has entered the steel sheet.
  • Patent Literature 1 discloses a method for improving bare spot defects that may occur in a Si-added steel. The method involves heating a Si-added steel sheet to 700°C or higher in an atmosphere containing O 2 to oxidize the surface of the steel sheet, and reducing an oxide in a surface layer of the steel sheet in an H 2 -containing atmosphere having a dew point of 5°C or more.
  • the amount of oxidation of the steel sheet is large when it is heated to 700°C or more in an atmosphere containing O 2 . This may cause adhesion of an oxide to the steel sheet in a furnace during reduction annealing, resulting in a deterioration of the appearance quality of the steel sheet.
  • Patent Literature 2 discloses a method which involves heating a Si-added steel sheet to a temperature of not less than 600°C and not more than 850°C in an atmosphere containing O 2 to oxidize the surface of the steel sheet, and reducing an oxide in a surface layer of the steel sheet in an atmosphere having a dew point of 5°C or more and containing H 2 O and H 2 in an amount of not less than 500 ppm by volume and not more than 5000 ppm by volume.
  • Patent Literature 3 discloses a method which involves oxidizing the surface of a Si-added steel sheet by increasing the air ratio in a direct-fired furnace (DFF), and reducing an oxide in a surface layer of the steel sheet in an atmosphere where log (P H2O /P H2 ) is not less than -3.4 and not more than -1.1. These methods can adjust the amount of oxidation of a steel sheet, and can therefore ensure good appearance quality of the steel sheet. However, a large amount of hydrogen, which has entered the steel during annealing, remains in the steel sheet, resulting in a failure to achieve sufficient LME cracking resistance and delayed fracture resistance.
  • DFF direct-fired furnace
  • the present inventors found that the appearance quality of a steel sheet can be ensured by optimizing the O 2 concentration and the temperature during oxidation of the steel sheet according to the Si concentration and the Mn concentration of the steel sheet and thus avoiding excessive oxidation.
  • the present inventors also found that by optimizing the H 2 O concentration, the H 2 concentration, and log (P H2O /P H2 ) during reduction annealing, it becomes possible to provide a steel sheet having excellent resistance to resistance-welding cracking while reducing the deterioration of delayed fracture resistance caused by hydrogen embrittlement, leading to completion of the present invention.
  • the present invention it is possible to provide a high-strength steel sheet which has excellent resistance to resistance-welding cracking in a welding portion and good appearance quality, and contains a sufficiently reduced amount of hydrogen which causes deterioration of the delayed fracture resistance of the steel sheet.
  • the unit of the content of each element in the chemical composition of a Si-containing slab and the unit of the content of each element in the chemical composition of a coated layer are "% by mass", and will be expressed simply as “%” unless otherwise specified.
  • a numerical range expressed as "X to Y” includes X and Y as the lower limit and the upper limit.
  • a steel sheet having "high strength” herein refers to a steel sheet whose tensile strength TS, measured in accordance with JIS Z 2241(2011), is 590 MPa or more.
  • Si not less than 0.45% and not more than 2.0%
  • Si has a significant effect of increasing the strength of steel through solid solution (high solid solution strengthening ability) without materially impairing the formability, and therefore is an effective element for achieving an increase in the strength of a steel sheet.
  • Si has an adverse effect on the resistance to resistance-welding cracking in a welding portion.
  • Si is added to achieve an increase in the strength of a steel sheet, it is necessary to add Si in an amount of 0.45% or more. If the Si content is less than 0.45%, Si poses no significant problem in the resistance to resistance-welding cracking in a welding portion; therefore, there is no significant need for the application of the present invention.
  • Si is added in an amount in the range of not less than 0.45% and not more than 3.0%.
  • the amount of Si is preferably 0.7% or more, more preferably 0.9% or more. Further, the amount of Si is preferably 2.5% or less, more preferably 2.0% or less.
  • C improves the formability of a steel sheet through the formation of martensite or the like as a steel microstructure.
  • the amount of C is preferably made 0.8% or less, more preferably 0.30% or less in order to achieve good weldability and LME cracking resistance.
  • the lower limit of the amount of C is not particularly limited; however, in order to achieve good formability, the amount of C is preferably made 0.03% or more, more preferably 0.05% or more.
  • Mn 1.0% or more and 4.0% or less
  • Mn is an element which increases the strength of steel by solid solution strengthening, improves hardenability, and promotes the formation of retained austenite, bainite, and martensite. Such an effect is produced by inclusion of Mn in an amount of 1.0% or more.
  • the amount of Mn is preferably made not less than 1.0%, and is preferably made not more than 4.0%.
  • the amount of Mn is more preferably made 1.8% or more. Further, the amount of Mn is more preferably made 3.3% or less.
  • the use of a low content of P can prevent a reduction in weldability and, in addition, can prevent segregation of P at grain boundaries, thereby preventing deterioration of ductility, bendability, and toughness.
  • the addition of a large amount of P promotes ferrite transformation, leading to an increase in the size of crystal grains. Therefore, the amount of P is preferably made 0.1% or less. While the lower limit of the amount of P is not particularly limited, the amount is more than 0% due to constraints of production technology, and is generally 0.001% or more.
  • the amount of S is preferably made 0.03% or less, more preferably 0.02% or less.
  • the use of a low amount of S can prevent a reduction in weldability, and can prevent a reduction in ductility during hot rolling, thereby preventing hot cracking and significantly improving the surface quality of a steel sheet. Furthermore, the use of a low amount of S can prevent a reduction in the ductility, bendability, and stretch flangeability of the steel sheet due to the formation of a coarse sulfide by S as an impurity element. These problems are noticeable when the amount of S is more than 0.03%.
  • the S content is preferably made as low as possible. While the lower limit of the amount of S is not particularly limited, the amount is more than 0% due to constraints of production technology, and is generally 0.001% or more.
  • Al 0.1% or less (not including 0%)
  • Al is thermodynamically most easily oxidizable; Al is oxidized before Si and Mn are oxidized.
  • Al has the effect of suppressing the oxidation of Si and Mn in the outermost layer of a steel sheet, and promoting the oxidation of Si and Mn inside the steel sheet. This effect is achieved when the amount of Al is 0.01% or more.
  • the use of Al in an amount of more than 0.1% leads to an increase in cost. Therefore, when Al is added, the amount of Al is preferably made 0.1% or less. While the lower limit of the amount of Al is not particularly limited, the amount is more than 0%, and is generally 0.001% or more.
  • N 0.010% or less (not including 0%)
  • the content of N is preferably made 0.010% or less.
  • N can be prevented from forming a coarse nitride with Ti, Nb, or V at a high temperature. This can prevent a deterioration in the effect of increasing the strength of a steel sheet achieved by the addition of Ti, Nb, or V.
  • the N content is preferably 0.005% or less, more preferably 0.003% or less, and still more preferably 0.002% or less. While the lower limit of the amount of N is not particularly limited, the amount is more than 0% due to constraints of production technology, and is generally 0.0005% or more.
  • the chemical composition may further optionally comprise one, two or more selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.200% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM (Rare Earth Metal): 0.005% or less.
  • B 0.005% or less
  • Ti: 0.2% or less Cr: 1.0% or less
  • Cu 1.0% or less
  • Mo 1.0% or less
  • Nb 0.20% or less
  • V 0.5% or less
  • Sb 0.200% or less
  • Ta 0.1% or less
  • W 0.5% or less
  • Zr 0.1% or less
  • Sn 0.20% or less
  • Ca
  • B is an effective element for improving the hardenability of steel.
  • the amount of B is preferably made 0.0003% or more, more preferably 0.0005% or more.
  • the amount of B is preferably made 0.005% or less.
  • Ti is effective for precipitation strengthening of steel. While the lower limit of the amount of Ti is not particularly limited, the amount is preferably made 0.005% or more in order to achieve the effect of adjusting the strength. However, if Ti is added excessively, a hard phase will be too large and the formability will be poor. Therefore, when Ti is added, the amount of Ti is preferably made 0.2% or less, more preferably 0.05% or less.
  • the amount of Cr is preferably made 0.005% or more. By making the amount of Cr 0.005% or more, it is possible to improve the hardenability, thereby improving and the balance between strength and ductility.
  • the amount of Cr is preferably made 1.0% or less from the viewpoint of avoiding an increase in cost.
  • the amount of Cu is preferably made 0.005% or more. By making the amount of Cu 0.005% or more, the formation of a retained ⁇ phase can be promoted. When Cu is added, the amount of Cu is preferably made 1.0% or less from the viewpoint of avoiding an increase in cost.
  • the amount of Ni is preferably made 0.005% or more. By making the amount of Ni 0.005% or more, the formation of a retained ⁇ phase can be promoted. When Ni is added, the amount of Ni is preferably made 1.0% or less from the viewpoint of avoiding an increase in cost.
  • the amount of Mo is preferably made 0.005% or more. By making the amount of Mo 0.005% or more, the effect of adjusting the strength can be achieved.
  • the amount of Mo is more preferably made 0.05% or more.
  • the amount of Mo is preferably made 1.0% or less from the viewpoint of avoiding an increase in cost.
  • Nb in an amount of 0.005% or more can achieve the effect of increasing the strength.
  • the amount of Nb is preferably made 0.20% or less from the viewpoint of avoiding an increase in cost.
  • V in an amount of 0.005% or more can achieve the effect of increasing the strength.
  • the amount of V is preferably made 0.5% or less from the viewpoint of avoiding an increase in cost.
  • Sb can be contained from the viewpoint of inhibiting nitridation, oxidation, or decarburization that occurs in an area ranging from the surface of a steel sheet to a depth of tens of micrometers due to oxidation. Sb inhibits nitridation and oxidation at the surface of the steel sheet, thereby preventing a decrease in the amount of martensite formed in the surface of the steel sheet, and improving the fatigue properties and surface quality of the steel sheet. In order to achieve such an effect, the amount of Sb is preferably made 0.001% or more. On the other hand, in order to achieve good toughness, the amount of Sb is preferably made 0.200% or less.
  • the inclusion of Ta in an amount of 0.001% or more can achieve the effect of increasing the strength.
  • the amount of Ta is preferably made 0.1% or less from the viewpoint of avoiding an increase in cost.
  • W in an amount of 0.005% or more can achieve the effect of increasing the strength.
  • the amount of W is preferably made 0.5% or less from the viewpoint of avoiding an increase in cost.
  • the inclusion of Zr in an amount of 0.0005% or more can achieve the effect of increasing the strength.
  • the amount of Zr is preferably made 0.1% or less from the viewpoint of avoiding an increase in cost.
  • Sn is an effective element for inhibiting denitrification, deboration, or the like, thereby preventing a reduction in the strength of steel.
  • the amount of Sn is preferably made 0.002% or more.
  • the amount of Sn is preferably made 0.20% or less.
  • the inclusion of Ca in an amount of 0.0005% or more can control the shape of a sulfide, thereby improving the ductility and the toughness.
  • the amount of Ca is preferably made 0.005% or less from the viewpoint of achieving good ductility.
  • Mg in an amount of 0.0005% or more can control the shape of a sulfide, thereby improving the ductility and the toughness.
  • the amount of Mg is preferably made 0.005% or less from the viewpoint of avoiding an increase in cost.
  • the inclusion of REM in an amount of 0.0005% or more can control the shape of a sulfide, thereby improving the ductility and the toughness.
  • the amount of REM is preferably made 0.005% or less from the viewpoint of achieving good ductility.
  • a Si-containing steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet.
  • the hot rolling step is a step of hot-rolling the above-described slab, and coiling the hot-rolled steel sheet at a temperature equal to or lower than a temperature T C (°C) calculated from the below-described equation (1), followed by pickling.
  • the hot rolling step After rolling is completed and the steel sheet is coiled, oxygen diffuses into the steel sheet from oxide scale while the steel sheet is cooled. Accordingly, internal oxides of Si and Mn are formed inside the surface of the steel sheet.
  • the internal oxides of Si and Mn, formed after rolling, are non-uniform.
  • the non-uniform internal oxides cause poor appearance such as uneven adhesion of a coating, uneven alloying after an alloying treatment, etc. Therefore, in hot rolling, it is important to suppress the formation of internal oxidation.
  • the internal oxidation of Si and Mn can be made more uniform by controlling the amount of internal oxidation (the total amount of an internal Si oxide and an internal Mn oxide formed in a surface portion of a hot-rolled steel sheet, located immediately beneath scale and ranging from the sheet surface to a depth of 10 um.
  • the amount of internal oxidation is expressed as the amount of oxygen at a position corresponding to the longitudinal and width-direction center of the coil after rolling) at the longitudinal and width-direction center of the coil to 0.10 g/m 2 or less. Therefore, upon later hot-dip galvanization, uneven adhesion of a coating and uneven appearance after an alloying treatment can be further prevented.
  • the heating temperature before hot rolling and the finishing temperature upon hot rolling are not particularly limited; however, from the viewpoint of microstructural control, it is preferred to heat the slab at 1100 to 1300°C, and to complete finish rolling at 800 to 1000°C.
  • pickling is performed to remove scale.
  • a method for pickling is not particularly limited; any conventional method may be used.
  • the cold rolling step is a step of cold-rolling the hot-rolled sheet obtained in the hot rolling step.
  • Conditions for the cold rolling are not particularly limited.
  • the cooled hot-rolled sheet may be cold-rolled at a predetermined rolling reduction ratio in the range of 30 to 80%.
  • the annealing step of the present invention consists of a step of oxidizing the cold-rolled steel sheet, obtained in the cold rolling step, using a direct-fired furnace having two or more separate zones, and a step of reducing the oxidized steel sheet using a radiant tube-type heating and holding furnace.
  • the direct-fired furnace (oxidation annealing step for the steel sheet) will be described first.
  • Si and Mn can be oxidized within the steel sheet during the annealing step by strictly controlling the annealing conditions (oxidation conditions + reduction annealing conditions) before hot-dip galvanization. This can improve coatability and increase the reactivity between a coating and the steel sheet, thereby improving the adhesion of the coating.
  • an oxidation treatment is performed to oxidize Si and Mn within the steel sheet and to thereby prevent oxidation at the surface of the steel sheet.
  • the steel sheet is subjected to reduction annealing and hot-dip galvanization. If necessary, it is effective to perform an alloying treatment of the galvanized steel sheet.
  • the atmosphere is controlled by controlling the air ratio in the direct-fired furnace.
  • the direct-fired furnace is configured to heat a steel sheet by applying a burner flame, produced by burning a mixture of air and a fuel such as coke oven gas (COG) which is a byproduct gas in a steel mill, directly to the surface of the steel sheet.
  • COG coke oven gas
  • the air ratio is increased to increase the proportion of air to the fuel, unreacted oxygen remains in the flame, and the oxygen can promote oxidation of the steel sheet.
  • coke oven gas it is possible to use natural gas, hydrogen gas, ammonia gas, or the like as a fuel in the direct-fired furnace.
  • CO, CO 2 , H 2 O, NOx, etc. are generated as oxidation products upon combustion of such a fuel.
  • N 2 in the combustion air is also present in the atmosphere.
  • the step of oxidizing the steel sheet using the direct-fired furnace needs to be performed in two or more separate zones where the steel sheet is heated in two or more different atmospheres. An early-stage heating zone and a later-stage heating zone will now be described.
  • the air ratio is adjusted to create an atmosphere containing 1000 ppm by volume or more of O 2 and 1000 ppm by volume or more of H 2 O, and the cold-rolled steel sheet is heated.
  • the O 2 concentration is 1000 ppm by volume or less or the H 2 O concentration is 1000 ppm by volume or less, the oxidation of the steel sheet will be insufficient.
  • the O 2 concentration is less than 1000 ppm by volume and the H 2 O concentration is less than 1000 ppm by volume, the O 2 concentration and the H 2 O concentration do not have a significant influence on the oxidation of the steel sheet, while the temperature of the steel sheet has a significant influence thereon.
  • the O 2 concentration is 10000 ppm by volume or less
  • the H 2 O concentration is 10000 ppm by volume or less.
  • the steel sheet is heated to a temperature in the range of not less than 400°C and not more than 670°C. If the temperature of the steel sheet is less than 400°C, the oxidation of the steel sheet will be insufficient, whereas if the temperature of the steel sheet exceeds 670°C, the oxidation of the steel sheet will be excessive, resulting in the above-described pickup on a roll. Therefore, it is essential to the present invention that the steel sheet be heated to a temperature in the range of not less than 400°C and not more than 670°C.
  • the later stage of heating is an important factor in the present invention for preventing the above-described roll pickup and obtaining a beautiful surface appearance free of roll marks or the like.
  • the air ratio is adjusted so that the O 2 concentration of the atmosphere becomes 500 ppm by volume or less, and the steel sheet that has passed through the early-stage heating zone is heated. If the O 2 concentration exceeds 500 ppm by volume, the steel sheet will be oxidized excessively, resulting in the occurrence of the above-described pickup on a roll.
  • the steel sheet is heated to a temperature in the range of not less than 600°C and not more than 700°C. If the temperature of the steel sheet is less than 600°C, the portion (surface layer) of the surface of the steel sheet will not be sufficiently reduced. If the temperature of the steel sheet exceeds 700°C, the portion (surface layer) of the surface of the steel sheet may not be reduced and oxidation may be promoted, resulting in the occurrence of the above-described pickup on a roll. Therefore, it is essential to the present invention that the steel sheet be heated to a temperature in the range of not less than 600°C and not more than 700°C.
  • the radiant tube-type heating and holding furnace (reduction annealing step for the steel sheet) will now be described.
  • internal oxides of Si and Mn exist in a surface layer of the steel sheet. Upon alloying of a coated layer and the steel substrate, such internal oxides of Si and Mn in the surface layer of the steel sheet will diffuse into the coated layer. This promotes removal of hydrogen, which has entered the steel sheet, from the sheet after production, so that good delayed fracture resistance can be achieved.
  • Radiant-tube type heating and holding can be used for the reduction annealing.
  • the H 2 O concentration of the atmosphere By controlling the H 2 O concentration of the atmosphere to be not less than 5000 ppm by volume and not more than 40000 ppm by volume, LME cracking can be prevented and dehydrogenation can be promoted. If the H 2 O concentration is less than 5000 ppm by volume, the LME cracking resistance and the dehydrogenation promoting effect may be insufficient. On the other hand, if the H 2 O concentration exceeds 40000 ppm by volume, there is a fear of equipment damage. Therefore, the H 2 O concentration is preferably 40000 ppm by volume or less.
  • the difference between the H 2 O concentration at the top of the interior space of the furnace and that at the bottom of the interior space of the furnace needs to be 2000 ppm by volume or less. If the difference in H 2 O concentration exceeds 2000 ppm by volume, Si and Mn in the steel will be oxidized externally without being oxidized internally, which may impair the coatability and form bare spot defects. Further, it is possible that an internal oxidation layer may not be formed sufficiently, resulting in insufficient LME cracking resistance and an insufficient dehydrogenation promoting effect.
  • the H 2 concentration during reduction annealing also greatly influences the formation of an internal oxidation layer.
  • the H 2 concentration needs to be not less than 2% by volume and not more than 20% by volume.
  • the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2 (P H2 ) needs to satisfy the below-described relation (2). If the H 2 concentration is less than 2% by volume, reduction of the oxidized steel sheet may sometimes be insufficient, resulting in the formation of bare spot defects and reduced adhesion of a coating upon hot-dip galvanization. On the other hand, if the hydrogen concentration exceeds 20% by volume, a large amount of hydrogen will remain in the steel sheet.
  • log(P H2O /P H2 ) is preferably 0.5 or less.
  • log (P H2O /P H2 ) may be made -0.90 or more, or -0.7 or more so that the bendability can be still further improved.
  • the upper limit of log(P H2O /P H2 ) is preferably 0.5 or less.
  • the reduction annealing atmosphere preferably contains N 2 from the viewpoint of cost.
  • NOx, SOx, CO, CO 2 , etc. can exist in the atmosphere.
  • the reduction annealing temperature needs to be not less than 650°C and not more than 900°C. If the temperature is less than 650°C, the formation of an internal oxidation layer, which is necessary to improve the LME cracking resistance and to promote dehydrogenation, may be insufficient. If the temperature exceeds 900°C, there is a fear of damage to the furnace body of the annealing furnace; therefore, the temperature should preferably be 900°C or lower.
  • the above-described reducing atmosphere conditions may be satisfied by part or the whole of the atmosphere in the furnace.
  • the reduction annealing atmosphere need not be wholly controlled in the above-described manner throughout the furnace.
  • the cooling and heating step is a step of cooling the steel sheet, which has undergone the reduction annealing, from the final holding temperature in the reduction annealing to a cooling end temperature of 150 to 350°C at an average cooling rate of at least 10°C/sec, and then heating the steel sheet to a reheating temperature of 350 to 600°C and holding it at that temperature for 10 to 600 seconds.
  • the cooling and heating step can further improve the mechanical properties.
  • the cooling and heating step is not an essential step of the present invention, and may be performed as necessary.
  • the cooling rate during cooling from the final holding temperature in the reduction annealing is less than 10°C/sec, pearlite will be formed, resulting in a reduction in "TS ⁇ EL" and in flangeability. Therefore, the cooling rate during cooling from the final holding temperature in the reduction annealing is preferably at least 10°C/sec.
  • the final holding temperature in the reduction annealing herein refers to the temperature when at least one of the annealing temperature, the hydrogen concentration, the dew point, and the holding time in the reduction annealing has come to fall outside the range described above.
  • the cooling end temperature is more than 600°C, the temperature of a galvanizing bath increases in the subsequent hot-dip galvanizing step, which may promote the formation of dross that impairs the surface appearance quality. Therefore, the cooling end temperature is preferably 600°C or less.
  • the mechanical properties can be improved by making the cooling end temperature 350°C or less. If the cooling end temperature is lower than 150°C, most of austenite is transformed into martensite during cooling, and the amount of non-transformed austenite decreases. Therefore, the cooling end temperature is preferably in the range of 150 to 350°C. Any cooling method, such as gas jet cooling, mist cooling, water cooling, or metal quenching, may be used as long as the intended cooling rate and the intended cooling stop temperature (cooling end temperature) can be achieved.
  • the steel sheet after cooling the steel sheet to the cooling end temperature, the steel sheet may be heated to the reheating temperature and held at that temperature for at least 10 seconds.
  • martensite that has been formed during cooling is tempered and becomes tempered martensite, resulting in improved flangeability.
  • non-transformed austenite that has not been transformed into martensite during cooling may be stabilized, and a sufficient amount of retained austenite may be finally obtained, leading to improved ductility.
  • the reheating temperature is made in the range of 350 to 600°C, and the holding time in that temperature range is made 10 to 600 seconds.
  • the hot-dip galvanization step is a step of hot-dip galvanizing the annealed steel sheet after the annealing step in a hot-dip galvanizing bath containing 0.12 to 0.22% by mass of Al.
  • the Al concentration in the galvanizing bath is made 0.12 to 0.22% by mass. If the Al concentration is less than 0.12% by mass, an Fe-Zn alloy phase will be formed during galvanization, which may lead to poor adhesion of a coating and uneven appearance. If the Al concentration exceeds 0.22% by mass, a thick Fe-Al alloy phase will be formed at the coating and steel substrate interface during galvanization, resulting in poor weldability. Further, because of the large amount of Al in the bath, a large amount of Al oxide film will be formed on the surface of the galvanized steel sheet, which may impair not only the weldability but the appearance as well.
  • the Al concentration in the galvanizing bath is preferably 0.12 to 0.17% by mass. If the Al concentration is less than 0.12% by mass, an Fe-Zn alloy phase will be formed during galvanization, which may lead to poor adhesion of a coating and uneven appearance. If the Al concentration exceeds 0.17% by mass, a thick Fe-Al alloy phase may be formed at the coating and steel substrate interface during galvanization. The Fe-Al alloy phase will be an obstacle to an Fe-Zn alloying reaction, resulting in a high alloying temperature and poor mechanical properties.
  • galvanization is performed by immersing the steel sheet at a sheet temperature of 440 to 550°C in the hot-dip galvanizing bath generally at a temperature in the range of 440 to 500°C.
  • the amount of coating can be adjusted, e.g., by gas wiping.
  • the alloying step is a step of alloying the steel sheet after the hot-dip galvanization step at a temperature in the range of 450 to 550°C for 10 to 60 seconds.
  • the alloying degree (i.e., Fe concentration of the coated layer) after the alloying treatment is not particularly limited, it is preferably 7 to 15% by mass. If the alloying degree is less than 7% by mass, an ⁇ phase will remain, leading to poor press formability. If the alloying degree exceeds 15% by mass, the adhesion of the coating will be poor.
  • Molten steels having the chemical compositions shown in Table 1, were each continuously cast into a slab.
  • Example B 0.11 0.45 2.52 0.02 0.001 0.003 0.032 0.001 0.01 0.59 - - - 0.04 -
  • Example C 0.09 0.62 2.72 0.01 0.002 0.005 0.035 0.001 0.01 - - - - 0.02 -
  • Example D 0.15 0.93 2.13 0.03 0.002 0.004 0.034 - - - - - - -
  • Example E 0.18 1.03 3.09 0.01 0.002 0.006 0.037 0.001 0.01 - - - - 0.01 0.007
  • Example F 0.12 1.18 1.86 0.01 0.001 0.004 0.031 0.001 0.01 - - - - 0.01 0.012
  • Example G 0.24 1.42 1.29 0.01 0.001 0.003 0.033 0.001 0.01 - - - - - -
  • Example H 0.13 1.38 1.97 0.02 0.001 0.007 0.034 0.001 0.01 -
  • the amount of internal oxidation was measured by an "impulse furnace melting-infrared absorption method".
  • the concentration of oxygen in the steel was measured before and after polishing a 10 mm ⁇ 70 mm area in a surface layer portion (at the center (width-direction and longitudinal center) of the coil) by 10 um on both sides of the hot-rolled sheet. Further, from the difference between the measured values, the amount of oxygen, existing in a 10-pm region from the steel sheet surface, per unit area of one surface was determined as the amount of internal oxidation of Si and/or Mn (g/m 2 ).
  • the internal oxide, formed in the surface layer portion of the hot-rolled sheet is an oxide of Si and/or Mn was confirmed by SEM observation and by elemental analysis using an EDS (energy dispersive X-ray spectroscope) after embedding the hot-rolled sheet into a resin and polishing a cross-section.
  • EDS energy dispersive X-ray spectroscope
  • the steel sheet was cold-rolled to obtain a cold-rolled sheet having a thickness of 1.2 mm.
  • the cold-rolled sheet was then subjected to annealing and hot-dip galvanization in a CGL.
  • Early-stage heating was performed under the conditions shown in Table 2 in a direct-fired furnace having a nozzle mix burner. Later-stage heating was then performed under the conditions shown in Table 2 in a direct-fired furnace having a premix burner.
  • the oxidation start temperature was 300°C.
  • the oxidation start temperature does not significantly affect the coating appearance; therefore, it is possible to create an oxidizing atmosphere at a temperature of less than 400°C.
  • Reduction annealing was performed in a radiant tube-type heating and holding furnace under the conditions shown in Table 2, followed by cooling.
  • hot-dip galvanization was performed using a zinc bath at 460°C, containing 0.135% of Al, as shown in Table 2, followed by gas wiping to adjust the coating weight to about 50 g/m 2 .
  • An alloying treatment was performed in some cases.
  • each steel sheet was visually observed.
  • a steel sheet was evaluated as " ⁇ ” when it was free of appearance defects such as bare spot portions, roll marks due to the pickup phenomenon, or uneven alloying, evaluated as "o” when it had slight appearance defects, but was acceptable as a product, and evaluated as " ⁇ ”when it had clear uneven alloying, bare spot portions, or roll marks.
  • the appearance of a steel sheet was judged to be good when the above evaluation was " ⁇ " or "o".
  • a tensile test was conducted in accordance with JIS Z 2241 using a JIS No. 5 test specimen with the rolling direction as the tensile direction. A test specimen was judged to be good when TS (MPa) ⁇ EL (%) was 8000 (MPa ⁇ %) or more.
  • test specimen was cut from each hot-dip galvanized steel sheet to a size of 150 mm in the longer direction and 50 mm in the shorter direction, with the longer direction being a direction (TD) perpendicular to the rolling direction, and the shorter direction coinciding with the rolling direction.
  • the test specimen was superimposed on a hot-dip galvanized steel sheet for testing (thickness 1.2 mm, TS: 980 MPa grade) with a coating weight of 50 g/m 2 per one surface, which had the same size as the test specimen, to form a sheet assembly.
  • the sheet assembly was assembled such that the hot-dip galvanized layer of the test specimen met the surface of the hot-dip galvanized layer of the commercially available hot-dip galvanized steel sheet. As shown in FIG.
  • the sheet assembly was fixed to a fixing base, via 2.0 mm-thick spacers, in an inclined position at an angle of 5°, which is the maximum inclination expected for the shape of some part.
  • the spacers were a pair of steel sheets, each having a size of 50 mm in the longer direction ⁇ 45 mm in the shorter direction ⁇ 2.0 mm in thickness, and were disposed such that the long-direction end surfaces of each of the pair of steel sheets were aligned with the short-direction end surfaces of the sheet assembly. Therefore, the distance between the pair of steel sheets, constituting the spacers, is 60 mm.
  • the fixing base is a single plate with a hole in the center.
  • the sheet assembly was subjected to resistance welding with a pressure of 3.5 kN and a holding time of 0.10 seconds or 0.16 seconds while pressing and bending the sheet assembly by means of a pair of electrodes (tip diameter: 6 mm).
  • the welding was performed under such welding current and welding time conditions that would make the weld nugget diameter 5.9 mm (i.e., welding current and welding time were adjusted for each sheet assembly so that the nugget diameter would be 5.9 mm), thereby obtaining a sheet assembly with the welding portion.
  • the pair of electrodes pressed the sheet assembly from above and below in the vertical direction, and the lower electrode pressed the test specimen through the hole of the fixing base.
  • the lower electrode of the pair of electrodes and the fixing base were fixed such that the lower electrode contacts a plane extending from the contact plane between each spacer and the fixing base, while the upper electrode was allowed to move. Further, the upper electrode was brought into contact with the center of the hot-dip galvanized steel sheet for testing.
  • the holding time refers to the time from the end of the application of a welding current to the start of opening of the electrodes.
  • the nugget diameter refers to the distance between the ends 10 of the nugget in the longer direction of the sheet assembly, as shown in FIG. 2 .
  • the sheet assembly with the welding portion was cut along a plane including the welding portion (nugget).
  • the cross-section of the welding portion was observed by an optical microscope (200x), and the resistance to resistance-welding cracking in the welding portion was evaluated according to the following criteria.
  • the upper diagram of FIG. 2 is a plan view of the sheet assembly with the welding portion, showing the cutting position.
  • the lower diagram of FIG. 2 is a diagram showing the thickness-direction cross-section of the sheet assembly after cutting, schematically illustrating a crack formed in the test specimen. If a crack is formed in the hot-dip galvanized steel sheet for testing, a stress in the test specimen is dispersed, and therefore an appropriate evaluation is not possible. Therefore, data obtained without the formation of a crack in the hot-dip galvanized steel sheet for testing was used as data for examples.
  • a rectangular test specimen having a long-axis length of 30 mm and a short-axis length of 5 mm was taken from the center of the width of each hot-dip galvanized steel sheet.
  • the coated layer of the test specimen was removed by a Leutor, and immediately thereafter the test specimen was subjected to a hydrogen analysis using a thermal desorption analyzer under the conditions of an analysis start temperature of 25°C, an analysis end temperature of 300°C, and a heating rate of 200°C/hr to measure the amount of released hydrogen (mass ppm/min), which is the amount of hydrogen released from the surface of the test specimen, at each temperature.
  • the total amount of released hydrogen from the analysis start temperature to 300°C was calculated as the amount of diffusible hydrogen in steel.
  • a test specimen was evaluated as " ⁇ " when the amount of diffusible hydrogen in steel was 0.10 ppm by mass or less, and evaluated as "o" when the amount of diffusible hydrogen in steel was 0.30 ppm by mass or less. Further, based on the empirical fact that the delayed fracture resistance of a steel sheet is often poor when the amount of diffusible hydrogen in steel exceeds 0.30 ppm by mass, a test specimen having such an amount of diffusible hydrogen was evaluated as " ⁇ ". The dehydrogenation behavior was judged to be excellent when the above evaluation was " ⁇ " or "o".
  • Damage to the furnace body was evaluated by visual inspection to check whether discoloration occurred in the steel shell (SUS310S) inside the annealing furnace.
  • a steel sheet which caused no discoloration of the steel shell was evaluated as " ⁇ ”, and judged to be non-damaging to the furnace body.
  • a steel sheet which caused appreciable discoloration of the steel shell was evaluated as " ⁇ ”, and judged to be damaging to the furnace body.
  • a 25 mm ⁇ 100 mm rectangular test specimen was cut from each galvanized steel sheet such that the short sides of the specimen were parallel to the rolling direction.
  • the test specimen was then subjected to a 90° V-bend test in which the test specimen was bent such that the ridge formed extended in the rolling direction.
  • the streak speed was 50 mm/min, and the specimen was pressed against a die for 5 seconds under a load of 10 tons.
  • the test was performed while varying the radius of curvature R at the tip of a V-shaped punch in 0.5 steps. An area around the ridge of the specimen was observed through a 20x lens to check for the presence or absence of a crack(s).
  • R/t was calculated from the minimum R at which no crack was formed and the thickness (t mm, rounded to the nearest hundredth) of the test specimen, and used as an index of bendability. A smaller R/t value indicates better bendability.
  • a test specimen having an R/t value of less than 1.0 was evaluated as " ⁇ +”
  • a test specimen having an R/t value of less than 1.5 was evaluated as " ⁇ ”
  • a test specimen having an R/t value of less than 2.0 was evaluated as "o”
  • a test specimen having an R/t value of less than 4.0 was evaluated as " ⁇ ”
  • the data in Table 3 indicates that the steel sheets of Inventive Examples, despite being high-strength hot-dip galvanized steel sheets containing C, Si, and Mn, are excellent in the LME cracking resistance, have good coating appearance, and have a small amount of diffusible hydrogen in steel sheet, and therefore are expected to achieve good delayed fracture resistance. Further, the steel sheets caused little damage to the furnace body, and are excellent also in ductility and bendability. On the other hand, the steel sheets of Comparative Examples, each produced by a method outside the scope of the present invention, are inferior in at least one of LME cracking resistance, coating appearance, the amount of diffusible hydrogen in steel sheet, and damage to the furnace body.
  • a high-strength hot-dip galvanized steel sheet obtained by the production method of the present invention, has excellent appearance quality and excellent resistance to resistance-welding cracking, and can reduce deterioration of the delayed fracture resistance caused by hydrogen embrittlement.
  • Such a steel sheet can be used as a surface-treated steel sheet to reduce the weight and increase the strength of an automotive body itself.

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  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Thermal Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Coating With Molten Metal (AREA)
EP23775118.5A 2022-03-25 2023-03-24 Method for manufacturing high-strength, hot-dip galvanized steel sheet Pending EP4477771A1 (en)

Applications Claiming Priority (2)

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JP2022049566 2022-03-25
PCT/JP2023/012038 WO2023182525A1 (ja) 2022-03-25 2023-03-24 高強度溶融亜鉛めっき鋼板の製造方法

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EP4477771A1 true EP4477771A1 (en) 2024-12-18

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EP (1) EP4477771A1 (ko)
JP (1) JP7468819B2 (ko)
KR (1) KR20240152885A (ko)
CN (1) CN118843703A (ko)
MX (1) MX2024011568A (ko)
WO (1) WO2023182525A1 (ko)

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JPS5652219U (ko) 1979-09-29 1981-05-08
JPS6052270U (ja) 1983-09-19 1985-04-12 岐阜プラスチック工業株式会社 食品容器
JPH029751Y2 (ko) 1984-10-15 1990-03-12
JP5652219B2 (ja) 2011-01-20 2015-01-14 Ｊｆｅスチール株式会社 めっき密着性および摺動特性に優れた合金化溶融亜鉛めっき鋼板の製造方法
JP5811841B2 (ja) * 2011-12-28 2015-11-11 新日鐵住金株式会社 Ｓｉ含有高強度合金化溶融亜鉛めっき鋼板の製造方法
JP5924332B2 (ja) * 2013-12-12 2016-05-25 Ｊｆｅスチール株式会社 加工性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法
JP6052270B2 (ja) 2013-12-13 2016-12-27 Ｊｆｅスチール株式会社 高強度溶融亜鉛めっき鋼板およびその製造方法
MX2016007518A (es) * 2013-12-13 2016-09-13 Jfe Steel Corp Metodo para la produccion de laminas de acero recocidas despues de galvanizadas de alta resistencia.
MX2017002974A (es) * 2014-09-08 2017-06-19 Jfe Steel Corp Metodo e instalacion para la produccion de laminas de acero galvanizadas de alta resistencia.

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JP7468819B2 (ja) 2024-04-16
CN118843703A (zh) 2024-10-25
MX2024011568A (es) 2024-09-26
KR20240152885A (ko) 2024-10-22
WO2023182525A1 (ja) 2023-09-28
JPWO2023182525A1 (ko) 2023-09-28

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