US20100006184A1 - Alloyed hot-dip galvanized steel sheet and production method thereof - Google Patents

Alloyed hot-dip galvanized steel sheet and production method thereof Download PDF

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US20100006184A1
US20100006184A1 US12/484,553 US48455309A US2010006184A1 US 20100006184 A1 US20100006184 A1 US 20100006184A1 US 48455309 A US48455309 A US 48455309A US 2010006184 A1 US2010006184 A1 US 2010006184A1
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steel sheet
coat layer
alloyed hot
galvanized
dip galvanized
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Mikako Takeda
Fumio Yuse
Shigenobu Namba
Kazutaka Kunii
Yoshihiro Miyake
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Kobe Steel Ltd
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Kobe Steel Ltd
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Assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) reassignment KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KUNII, KAZUTAKA, MIYAKE, YOSHIHIRO, NAMBA, SHIGENOBU, TAKEDA, MIKAKO, YUSE, FUMIO
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0478—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing involving a particular surface treatment
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets

Definitions

  • the present invention relates to an alloyed hot-dip galvanized steel sheet which excels in surface appearance and is used typically in automobiles, home appliances, and building materials; and to a method for producing the alloyed hot-dip galvanized steel sheet.
  • Galvanized steel sheets are widely used typically in automobiles, home appliances, and building materials. Among them, alloyed hot-dip galvanized steel sheets excel in corrosion resistance and spot weldability and are thereby widely used as automobile steel sheets. The alloyed hot-dip galvanized steel sheets are more and more demanded to have higher strength and smaller thickness, with recent requirements for automobiles to have higher collision safety and lighter body weight so as to have higher fuel efficiency.
  • alloyed hot-dip galvanized steel sheets are those further containing easily oxidizable elements such as silicon (Si) and manganese (Mn). These easily oxidizable elements, however, are known to be selectively oxidized upon annealing carried out before galvanization of a steel sheet to thereby significantly impair wettability in galvanization and alloying performance, and it is very difficult to control these elements. Accordingly, stable production of alloyed hot-dip galvanized steel sheets is difficult according to known techniques.
  • JP-A No. 200711/2005 discloses an alloyed hot-dip galvanized steel sheet and a production method thereof, in which a reaction product between an added element in a steel sheet and a component in an annealing atmosphere is formed during an annealing process.
  • JP-A No. 279410/2001 discloses a galvanized steel sheet and a production method thereof, in which a sulfur-containing ammonium salt is attached to the surface of a high tensile strength steel sheet containing manganese, and the steel sheet is then subjected to a heat treatment and subsequently to a galvanization process.
  • JP-A No. No. 88193/1994 discloses a method for producing an alloyed hot-dip galvanized steel sheet, in which the surface layer of a steel sheet is dry-etched before immersing the steel sheet in a plating bath.
  • JP-A No. 328036/2003 discloses a method for improving the plating quality of an alloyed hot-dip galvanized steel sheet, in which a steel sheet after annealing is cooled in a controlled manner in order to reduce grain boundary segregation.
  • JP-A No. 263271/2004 discloses a method for producing a high tensile strength galvanized steel sheet, in which 70% or more of a surface enriched layer containing silicon (Si), manganese (Mn), and aluminum (Al) after annealing is removed by acid pickling, and the treated steel sheet is galvanized.
  • alloyed hot-dip galvanized steel sheets are all complicated in their steps and are difficult to produce alloyed hot-dip galvanized steel sheets in a simple and easy manner. In addition, they are not intended to produce an alloyed hot-dip galvanized steel sheet that is derived from a steel sheet having a high manganese content and excels in surface appearance.
  • an object of the present invention is to provide an alloyed hot-dip galvanized steel sheet which is resistant to uneven alloying and excels in surface appearance even when the base steel has a high manganese content, by controlling the amounts of MnO and an Fe—Al—O alloy layer that cause uneven alloying and whereby accelerating the alloying of the galvanized steel sheet.
  • Another object of the present invention is to provide a method for producing the alloyed hot-dip galvanized steel sheet with superior surface appearance.
  • an alloyed hot-dip galvanized steel sheet which includes a base steel sheet and a galvanized zinc-coat layer on the steel sheet, the galvanized zinc-coat layer and the steel sheet are alloyed, and a steel constituting the base steel sheet contains, on the mass basis, 0.02% to 0.2% of carbon (C), 2.0% to 3.5% of manganese (Mn), 0.03% to 0.5% of chromium (Cr), 0.01% to 0.15% of aluminum (Al), 0.04% or less (including 0%) of silicon (Si), 0.03% or less (including 0%) of phosphorus (P), and 0.03% or less (including 0%) of sulfur (S), with the remainder including iron (Fe) and inevitable impurities, in which MnO particles are present on an arbitrary straight line lying in an interface between the galvanized zinc-coat layer and the steel sheet, which MnO particles are present in an average number of 10 or less per micrometer of the straight line, an Fe—A
  • the steel sheet may further contain, on the mass basis, a total of 0.003% to 1.0% of at least one member selected from the group consisting of 0.003% to 0.5% of copper (Cu), 0.003% to 1.0% of nickel (Ni), and 0.003% to 1.0% of titanium (Ti).
  • the steel sheet may further contain, on the mass basis, at least one member selected from the group consisting of 0.003% to 1.0% of vanadium (V), 0.0039% to 1.0% of niobium (Nb), 0.0002% to 0.1% of boron (B), and 0.003% to 1.0% of molybdenum (Mo).
  • V vanadium
  • Nb niobium
  • B boron
  • Mo molybdenum
  • the steel sheet may further contain, on the mass basis, at least one member selected from the group consisting of 0.0005% to 0.005% of calcium (Ca) and 0.0005% to 0.001% of magnesium (Mg).
  • a method for producing any of the above-mentioned alloyed hot-dip galvanized steel sheets includes the steps of carrying out annealing of the steel sheet under such conditions that an oxygen partial pressure PO 2 (in units of atmospheric pressure (atm)) satisfies the following condition: ⁇ log(PO 2 ) ⁇ 20; galvanizing the annealed steel sheet to form the galvanized zinc-coat layer on the surface of the steel sheet; and alloying the steel sheet bearing the galvanized zinc-coat layer.
  • an oxygen partial pressure PO 2 in units of atmospheric pressure (atm)
  • the alloyed hot-dip galvanized steel sheets according to the present invention are alloyed hot-dip galvanized steel sheets that are resistant to uneven alloying and excel in surface appearance, even through they have a high manganese content of 2.0 to 3.5 percent by mass, because the amounts of MnO and an Fe—Al—O alloy layer that cause uneven alloying are controlled.
  • the method according to the present invention can produce alloyed hot-dip galvanized steel sheets which are resistant to uneven alloying and excel in surface appearance even when their base steels have a high manganese content of 2.0 to 3.5 percent by mass, by controlling the amounts of MnO and an Fe—Al—O alloy layer that cause uneven alloying and thereby accelerating the alloying of the galvanized steel sheet.
  • FIGS. 1A , 1 B, and 1 C are vertical sectional views of a steel sheet illustrating how uneven alloying occurs in the production of an alloyed hot-dip galvanized steel sheet, in which FIG. 1A illustrates the generation of large amounts of MnO particles on the surface of the steel sheet, FIG. 1B illustrates the generation of an Fe—Al—O alloy layer at the interface between the steel sheet and the galvanized zinc-coat layer, and FIG. 1C illustrates that the Fe—Al—O alloy layer acts as a barrier against the diffusion of iron and causes uneven alloying; and
  • FIG. 2 is a vertical sectional view showing an alloyed hot-dip galvanized steel sheet according to an embodiment of the present invention.
  • iron as a main component of the base steel sheet is not oxidized, but easily oxidizable elements such as silicon and manganese, if added, are selectively oxidized and diffuse to the surface layer of the steel sheet.
  • the surface of the steel sheet thereby bears oxides containing each of these easily oxidizable elements alone (single-component oxide) or in combination (multiple-component oxide).
  • silicon if enriched on the surface, forms a thin oxide layer in the outermost surface of the steel sheet and/or causes internal oxidation and thereby significantly impairs the plating quality and suitability for alloying. Therefore, of the easily oxidizable elements, manganese is positively added, but silicon is not positively added to the steel in the present invention, although contamination of silicon as an inevitable impurity is accepted.
  • manganese is also enriched in the surface layer of the steel sheet but less acts as a barrier than silicon, because manganese grows not as an oxide layer and an internal oxidation layer as with silicon but grows as a granular oxide (MnO) (oxide particles) and less impedes the outward diffusion of iron during alloying. Contrarily, a small amount of manganese rather increases the alloying rate. However, manganese should be added in a large amount because of its low reinforcing efficiency, but such a large amount of manganese may often cause MnO particles in the surface layer of the steel sheet. This complicates the alloying behavior and makes it difficult to control the alloying.
  • MnO granular oxide
  • the present inventors made investigations while focusing attention on the relationship between the formation mechanism of MnO and alloying and, as a result, succeeded in revealing a detailed formation mechanism of uneven alloying.
  • the present inventors conceived that uneven alloying can be controlled to thereby give a good appearance of the galvanized zinc-coat surface, by allowing MnO to be dispersed in the steel sheet through internal oxidation and/or by suppressing the oxidation of Mn in the surface layer.
  • the present inventors focused attention on the state of MnO in the surface layer of the steel sheet after annealing; prepared a series of steel sheets having different manganese contents by carrying out annealing under different oxygen partial pressures; observed cross sections of steel sheets suffering from uneven alloying and those of steel sheets without uneven alloying with a scanning electron microscope (SEM) and a transmission electron microscope (TEM); observed cross-sectional structures of these steel sheets after galvanization; and analyzed the iron contents in the galvanized zinc-coat layers. As a result, they succeeded in revealing how the state of uneven alloying varies depending on the state of MnO in the surface layer of the steel sheet after annealing.
  • SEM scanning electron microscope
  • TEM transmission electron microscope
  • Carbon (C) content 0.02to 0.2%
  • the carbon (C) element significantly affects the strength of the steel and affects the amounts and shapes of low-temperature transformation products to thereby affect the extensibility (elongation properties) and stretch flange formability of the steel.
  • the carbon content is therefore 0.02% and preferably 0.04% in its lower limit and is 0.2% and preferably 0.15% in its upper limit.
  • Manganese (Mn) content 2.0% to 3.5%
  • Manganese (Mn) element acts as a reinforcing element and should be contained in a content of at least 2.0% or more for high strength and for properties as a high-strength steel sheet with very superior workability.
  • the manganese content should be 3.5% or less, because manganese, if present in an excessively large content, may impair extensibility (elongation properties) or increase carbon equivalent to thereby adversely affect the weldability. Accordingly, the manganese content should be from 2.0% to 3.5%.
  • Chromium (Cr) content 0.03% to 0.5%
  • Chromium (Cr) element is effective for increasing the hardenability to reinforce the texture. Chromium helps carbon to be enriched and thereby stabilized in austenite and accelerates the formation of martensite.
  • chromium affects the plating quality by allowing an oxide to form on the surface of the steel sheet.
  • the lower limit of the chromium content should be 0.03%, because chromium, if contained in a content of less than 0.03%, may not effectively improve the hardenability.
  • the upper limit of the chromium content should be 0.5%, because if chromium is contained in a content of more than 0.5%, the effect of improving the hardenability may be saturated and the cost may be disadvantageously increase.
  • the upper limit of the chromium content is preferably 0.3%, because chromium, if contained in a content of more than 0.3%, may impair the plating quality.
  • Aluminum (Al) content 0.01% to 0.15%
  • Aluminum (Al) element is effective as a deoxidizer during steel making and should be added in a content of 0.01% or more. However, aluminum, if contained in a content of more than 0.15%, may not only impair the surface appearance but also cause increased production cost. Accordingly, the aluminum content should be 0.01% to 0.15%.
  • Silicon (Si) content 0.04% or less (including 0%)
  • Silicon (Si) element acts to reduce dissolved carbon content in alpha phase to thereby improve the workability such as extensibility (elongation properties); but it forms an oxide film on the surface of steel sheet, significantly impairs the wettability of the galvanized zinc-coat layer, and is not positively added.
  • this element may be contained as an inevitable impurity in the steel, and the Si content should be controlled to 0.04% or less so as to avoid adverse effects thereof, and is preferably controlled to 0.03% or less.
  • Phosphorus (P) content 0.03% or less (including 0%)
  • Phosphorus (P) element is effective for obtaining a high-strength steel sheet.
  • phosphorus if contained in a content of more than 0.03%, may often cause uneven plating, may impede alloying, and is not positively added.
  • this element may be contained as an inevitable impurity in the steel, and the phosphorus content should be controlled to 0.03% or less.
  • S Sulfur (S) content: 0.03% or less (including 0%)
  • S Sulfur element causes hot crack during hot rolling and significantly impairs spot crack resistance. Sulfur is fixed as a precipitate in the steel, but, if contained in a large content, may impair the extensibility (elongation properties) and stretch flange formability, and is not positively added. However, this element may be contained as an inevitable impurity in the steel, and the sulfur content should be controlled to 0.03% or less.
  • the material steel for use herein contains the above elements with the remainder being iron (Fe) and inevitable impurities. Where necessary, the steel may further contain elements mentioned below.
  • Copper (Cu): 0.003% to 0.5%, Nickel (Ni): 0.003% to 1.0% Copper (Cu) and nickel (Ni) elements are effective to increase the strength of the material steel itself and to improve the plating quality. Copper and nickel, if enriched in the surface layer of the steel sheet, serve to change the shapes (states) of oxides of silicon (Si) and manganese (Mn) to thereby prevent deterioration in plating quality, because copper and nickel are less oxidizable than iron mainly constituting the material steel. For sufficiently exhibiting these advantages, they are preferably contained in contents of 0.003% or more. However, these elements, if contained in excessively large contents, may impair the workability and cause the cost to increase, and the upper limits of the copper and nickel contents are preferably 0.5% and 1.0%, respectively.
  • Titanium (Ti) element forms a carbide and is thereby effective for strengthening the steel. This element also fixes carbon and nitrogen to thereby increase the gamma value of the steel sheet.
  • the titanium content is preferably 0.003% or more.
  • this element if present in an excessively large content, may impair the workability and cause the production cost to increase, and the upper limit of its content is preferably 1.0%.
  • Copper, nickel, and titanium if contained in combination, improve the surface cleanliness of the steel sheet and form multiple oxides with iron upon melting of iron so as to improve the plating quality.
  • the total content of these elements, if two or more of them are contained in combination, is preferably 0.003% to 1.0% in consideration of the upper and lower limits of their contents in the case of single use.
  • V Vanadium
  • Nb Niobium
  • Vanadium (V) and niobium (Nb) elements form carbides and are thereby effective for increasing the strength of the steel.
  • their contents are preferably 0.003% or more, respectively.
  • these elements if contained in excessively large contents, may impair the workability and cause the production cost to increase, and the upper limits of their contents are preferably 1.0%, respectively.
  • Boron (B) element serves to improve the weldability and to increase the hardenability.
  • the boron content is preferably 0.0002% or more.
  • the upper limit of its content is preferably 0.1%.
  • Molybdenum (Mo) element is effective for solid-solution strengthening without impairing the plating quality.
  • the molybdenum content is preferably 0.003% or more.
  • this element if present in an excessively large content, may cause the production cost to increase, and the upper limit of its content is preferably 1.0%.
  • Magnesium (Mg): 0.0005% to Calcium (Ca) element acts to control the shapes of inclusions to thereby increase the ductility and to increase the workability.
  • the calcium content is preferably 0.0005% or more.
  • this element if present in an excessively large content, may increase the amounts of inclusions in the steel to thereby reduce the ductility and to impair the workability.
  • the upper limit of its content is therefore preferably 0.005%.
  • Magnesium (Mg) acts in the steel in the same manner as with calcium. The magnesium content is therefore preferably from 0.0005% to 0.001% for the same reasons as with calcium.
  • the term “alloyed hot-dip galvanized steel sheet having good surface appearance with less uneven alloying” refers to an alloyed hot dip galvanized steel sheet, in which MnO particles are present on an arbitrary straight line lying in an interface between the galvanized zinc-coat layer 2 and the steel sheet 1 in an average number of 10 or less per micrometer of the straight line, an Fe—Al—O alloy layer is present at an interface between the MnO particles and the steel sheet 1 , and the length of the Fe—Al—O alloy layer is less than 10% of the overall length of the interface, in which each of the length and the overall length are measured on the arbitrary straight line, as illustrated typically in FIG. 2 .
  • the galvanized zinc-coat layer In this connection, it requires much efforts and time and is practically difficult to observe the entire interface between the galvanized zinc-coat layer and the steel sheet typically with a scanning electron microscope (SEM). Accordingly, it is enough to observe only part of the interface. However, if the area to be observed is excessively narrow, the obtained data may significantly differ from actual data, and the interface should be observed in a length of at least 500 ⁇ m. As used herein the “overall length of an interface” refers to the length of the interface in an area to be observed.
  • the length of the Fe—Al—O alloy layer in FIG. 2 is not illustrated as being less than 10% of the overall length of the interface on an arbitrary straight line.
  • FIG. 2 is, however, an exemplary diagram in order to illustrate the present invention, and, in actual, the length of the Fe—Al—O alloy layer on the arbitrary straight line is less than 10% of the overall length of the interface.
  • the area shown in FIG. 2 is the area to be observed; the overall longitudinal width of FIG.
  • the total length of three parts of the Fe—Al—O alloy layer is the length of the Fe—Al—O alloy layer; and the percentage of the length of the Fe—Al—O alloy layer is calculated according to the formula: (Length of the Fe—Al—O alloy layer)/(Overall length of the interface) ⁇ 100.
  • the average number of MnO particles on an arbitrary straight line lying in the interface between the galvanized zinc-coat layer 2 and the steel sheet 1 is preferably 10 or less per micrometer of the straight line.
  • MnO particles, if present in an average number of more than 10 per micrometer may accelerate uneven alloying of the alloyed hot-dip galvanized steel sheet, and MnO particles, if present in an average number of more than 20 per micrometer, may further impair the wettability in galvanization, cause bare spots when the steel sheet is dipped in a galvanization bath, and this impedes the production of a galvanized steel sheet.
  • the average number of MnO particles is more preferably 5 or less per micrometer.
  • the length of the Fe—Al—O alloy layer generated at an interface between the MnO particle and the steel sheet 1 is preferably less than 107 , of the overall length of the interface.
  • An Fe—Al—O alloy layer if present in a length of 10% or more of the overall length of the interface, may accelerate uneven alloying of the alloyed hot-dip galvanized steel sheet and significantly impair the surface appearance thereof.
  • the resulting alloyed hot dip galvanized steel sheet is not suitable for use typically as an automotive outside sheathing.
  • the length of the Fe—Al—O alloy layer is more preferably less than 5% of the overall length of the interface.
  • Such an alloyed hot dip galvanized steel sheet according to the present invention may be prepared in the following manner.
  • MnO particles are present on an arbitrary straight line lying in the interface between the galvanized zinc-coat layer 2 and the steel sheet 1 in an average number of 10 or less per micrometer, and the length of an Fe—Al—O alloy layer present at an interface between the MnO particle and the steel sheet 1 on the arbitrary straight line is less than 10% of the overall length of the interface.
  • an annealing process is preferably carried out under such conditions that an oxygen partial pressure PO 2 (in units of atmospheric pressure (atm)) satisfies the following condition: ⁇ log(PO 2 ) ⁇ 20.
  • This annealing process is carried out prior to a galvanizing process to form a galvanized zinc-coat layer 2 on the surface of the annealed steel sheet 1 .
  • the annealing process is more preferably carried out under such conditions that the oxygen partial pressure PO 2 (in units of atmospheric pressure (atm)) satisfies the following condition: ⁇ log(PO 2 ) ⁇ 23.
  • a slab of steel containing the components is hot-rolled, wound as a coil, subjected to acid pickling of the surface according to necessity, cold-rolled, and thereby yields a base steel sheet (steel sheet).
  • annealing of the base steel sheet is conducted in a continuous hot-dip galvanizing line.
  • the annealing in the annealing process may be conducted at a temperature of from 750° C. to 900° C. for a duration of 200 seconds or less.
  • the annealing process is conducted under such conditions that an oxygen partial pressure PO 2 (in units of atmospheric pressure (atm)) in the atmosphere satisfies the following condition: ⁇ log(PO 2 ) ⁇ 20.
  • galvanization hot-dip zinc plating
  • a plating bath for use herein may be a galvanization bath containing 0.05 to 0.20 percent by mass of aluminum.
  • the temperature of the steel sheet upon dipping in the galvanization bath herein is 440° C. or higher and lower than 480° C., as being substantially equal to the temperature of the hot-dip galvanization bath.
  • the dipping duration of the steel sheet in the galvanization bath is, for example, 5 seconds or less.
  • the steel sheet after dipping is retrieved from the galvanization bath, and the amount of the zinc coat deposited on the surface of the steel sheet is adjusted.
  • the amount of the zinc coat is adjusted to a suitable amount of 60 ⁇ 5 g/m 2 typically with a gas wiper.
  • alloying of the galvanized steel sheet is conducted as an alloying process to give an alloyed hot-dip galvanized steel sheet.
  • the alloying process is conducted at a temperature of from 450° C. to 600° C. for a duration of 60 seconds or less.
  • test pieces of alloyed hot-dip galvanized steel sheets were prepared by working cold-rolled steel sheets having component compositions given in Table 1 to a size of 100 mm wide and 250 mm long, and sequentially subjecting the same to annealing, galvanizing, and alloying with a hot-dip galvanization simulator.
  • the surfaces of the cold-rolled steel sheets having component compositions given in Table 1 were cleaned by acid pickling, and annealing was conducted in an atmosphere of N-3% H 2 .
  • the annealing conditions are shown in Table 2.
  • the annealing was conducted at temperatures of from 750° C. to 900° C. for a duration of 120 seconds.
  • the parameter ⁇ log(PO 2 ) was controlled by setting the annealing temperature within a range of from 750° C. to 900° C. and varying the dew point within a range of from ⁇ 75° C. to 0° C., as shown in Table 2.
  • Annealing temperature Dew H 2 concentration log ⁇ log Type (° C.) point (° C.) (% by volume) (H 2 O/H 2 ) (PO 2 ) a 750 ⁇ 75 3 ⁇ 4.399 28.5 b 750 ⁇ 50 3 ⁇ 2.887 25.5 c 750 ⁇ 25 3 ⁇ 1.68 23.1 d 750 0 3 ⁇ 0.697 21.1 e 800 ⁇ 75 3 ⁇ 4.399 27.3 f 800 ⁇ 50 3 ⁇ 2.887 24.3 g 800 ⁇ 25 3 ⁇ 1.68 21.9 h 800 0 3 ⁇ 0.697 19.9 i 850 ⁇ 50 3 ⁇ 2.887 24.3 j 850 ⁇ 25 3 ⁇ 1.68 20.8 k 850 0 3 ⁇ 0.697 18.8 l 900 ⁇ 75 3 ⁇ 4.399 25.2 m 900 ⁇ 50 3 ⁇ 2.887 22.2 n 900 ⁇ 25 3 ⁇ 1.68 19.8 o 900 0 3 ⁇ 0.697 17.8
  • the annealed steel sheets were dipped in a hot-dip galvanization bath containing 0.13 percent by mass of aluminum to thereby form a galvanized zinc-coat layer on the surface of the steel sheets.
  • the dipping was conducted at a temperature of the steel sheet of 460° C. for a duration of 2 seconds.
  • the amount of zinc coat was adjusted to 60 g/m 2 with a gas wiper, and thereby yielded galvanized steel sheets.
  • the temperature of the galvanization bath was set to the same temperature as that of the steel sheet.
  • the alloying was conducted with an infrared heating furnace in the galvanizing simulator immediately after the galvanization. The alloying was conducted at a temperature of 550° C. for a duration of 10 seconds. In the tests, the test pieces of the alloyed hot-dip galvanized steel sheets were examined on properties and alloying performance of the steel sheets after alloying. The results are shown in Table 3.
  • a 5- ⁇ m square cross section sample was sampled at random from an area of 500 ⁇ m length including the interface between the galvanized zinc-coat layer and the steel sheet of each alloyed hot-dip galvanized steel sheet, according to a focused ion beam (FIB) microsampling process.
  • the sample was machined to a thickness of about 0.1 ⁇ m through focused ion beam micromachining and used as a test specimen to be observed in the tests.
  • test specimen was observed with a TEM (JEM-2010F) equipped with a high angle annular dark field detector (HAADF) (EM-24015BU) at an accelerating voltage of 200 kV, to determine the number of the MnO particles, and the percentage of the length of the Fe—Al—O alloy layer to the overall length of the interface.
  • HAADF high angle annular dark field detector
  • Samples Nos. 1, 2, 4, 5, 9-12, 14, 15, 17, 20-25, 27, and 30-40 each had an average number of MnO particles present on an arbitrary straight line lying in the interface between the galvanized zinc-coat layer and the steel sheet of 10 or less per micrometer and each had a length of the Fe—Al—O alloy layer present at the interface between the series of the MnO particles and the steel sheet of less than 10% of the overall length of the interface.
  • These samples are therefore examples according to the present invention. These examples, in which uneven alloying occurs in a quantity of less than 10%, are resistant to uneven alloying and excel in surface appearance.
  • Samples Nos. 3, 6-8, 13, 16, 18, 19, and 26 each had an average number of MnO particles present on an arbitrary straight line lying in the interface between the galvanized zinc-coat layer and the steel sheet of more than 10 and equal to or less than 20 per micrometer and each had a length of an Fe—Al—O alloy layer present at the interface between the series of the MnO particles and the steel sheet of 10% or more of the overall length of the interface (Comparative Examples). These comparative examples are poor in surface appearance.
  • Samples Nos. 28 and 29 each had an average number of MnO particles present on an arbitrary straight line lying in the interface between the galvanized zinc-coat layer and the steel sheet of more than 20 per micrometer (Comparative Examples). These comparative examples had further deteriorated wettability in galvanization and suffered from bare spots when they were dipped in the galvanization bath. Thus, galvanized steel sheets were not produced from these samples.

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US20150184273A1 (en) * 2012-06-25 2015-07-02 Jfe Steel Corporation Galvannealed steel sheet with excellent anti-powdering property (as amended)
US20150337683A1 (en) * 2012-12-29 2015-11-26 United Technologies Corporation Angled cut to direct radiative heat load
CN107858599A (zh) * 2017-09-29 2018-03-30 宁波市胜源技术转移有限公司 一种耐磨增强型风机叶片及其制备方法
US11655531B2 (en) 2017-09-19 2023-05-23 Thyssenkrupp Steel Europe Ag Hot dip coated steel strip having an improved surface appearance and method for production thereof

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JP6246621B2 (ja) * 2013-05-08 2017-12-13 株式会社神戸製鋼所 引張強度が１１８０ＭＰａ以上の強度−曲げ性バランスに優れた溶融亜鉛めっき鋼板もしくは合金化溶融亜鉛めっき鋼板
US20160194744A1 (en) * 2013-08-12 2016-07-07 UFE Steel Corporation Method of producing high-strength hot-dip galvanized steel sheet and method of producing high-strength galvannealed steel sheet
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US20150337683A1 (en) * 2012-12-29 2015-11-26 United Technologies Corporation Angled cut to direct radiative heat load
US11655531B2 (en) 2017-09-19 2023-05-23 Thyssenkrupp Steel Europe Ag Hot dip coated steel strip having an improved surface appearance and method for production thereof
CN107858599A (zh) * 2017-09-29 2018-03-30 宁波市胜源技术转移有限公司 一种耐磨增强型风机叶片及其制备方法

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