US20090025831A1 - Hot-dip galvanized steel sheet and galvannealed steel sheet - Google Patents

Hot-dip galvanized steel sheet and galvannealed steel sheet Download PDF

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US20090025831A1
US20090025831A1 US11/908,431 US90843106A US2009025831A1 US 20090025831 A1 US20090025831 A1 US 20090025831A1 US 90843106 A US90843106 A US 90843106A US 2009025831 A1 US2009025831 A1 US 2009025831A1
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steel sheet
hot
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Katsuhiro Yamamoto
Masaaki Miura
Yukihiro Utsumi
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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: MIURA, MASAAKI, UTSUMI, YUKIHIRO, YAMAMOTO, KATSUHIRO
Publication of US20090025831A1 publication Critical patent/US20090025831A1/en
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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
    • C21D1/34—Methods of heating
    • C21D1/42—Induction 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
    • 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
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/25—Process efficiency

Definitions

  • the present invention relates to a hot-dip galvanized steel sheet and a galvannealed steel sheet (alloyed hot-dip galvanized steel sheet).
  • steel sheets for structural members such as front longitudinal members which play a role as automotive skeletal members to absorb energy upon collision.
  • These steel sheets should have higher strength from the viewpoint of higher safety or from the viewpoint of lightness in weight of automotive bodies so as to enhance fuel economy to thereby remedy environmental issues.
  • the steel sheets for structural members should also have improved rust preventive property.
  • high-strength steel sheets each including a high-strength steel sheet with a hot-dip galvanized surface are used.
  • hot-dip galvanized high-strength steel sheets desirably have a uniform galvanized layer
  • high-strength steel sheets for use as base metal materials desirably have good galvanizing ability and do not cause bare spots upon hot-dip galvanization.
  • Patent Document 1 proposes a technique of reducing the sliding resistance during pressing-molding by configuring a galvannealed steel sheet to have such a surface shape as to easily hold a liquid lubricant such as a rust preventive oil.
  • a liquid lubricant such as a rust preventive oil.
  • the lubricant may be coated unevenly or the lubricant may be degraded due to heat generated when pressing is carried out continuously. This is likely to cause decrease in sliding performance and deterioration in workability.
  • Patent Document 2 discloses a technique of controlling the degree of concentration (degree of enrichment) of alloy elements in a very surface area of a base metal material directly below a galvanized layer. After investigations, however, the present inventors found that even the control of the degree of concentration of alloy elements may not sufficiently improve the adhesion of galvanized layer, and this technique is still susceptible to improvement.
  • Patent Document 3 discloses a technique of improving the peeling resistance by finely dividing crystals of a steel sheet so as to cause outbursts uniformly, as a technique for providing a galvanized layer having good surface properties without bare spots and uneven alloying and for improving the peeling resistance and sliding performance of the galvanized layer.
  • This technique may cause deterioration in sliding performance and/or powdering resistance, because large amounts of alloy elements must be added so as to finely divide the crystals of steel sheet, and this makes it difficult to control the iron concentration in the galvanized layer appropriately.
  • Patent Document 1 Japanese Unexamined Patent Application Publication (JP-A) No. 247949/2001
  • Patent Document 2 JP-A No. 115039/2002
  • Patent Document 3 JP-A No. 323492/1999
  • the present invention has been made under these circumstances, and an object of the present invention is to provide a hot-dip galvanized steel sheet using a steel sheet that is resistant to the occurrence of bare spots upon hot-dip galvanization.
  • Another object of the present invention is to provide a galvannealed steel sheet (alloyed hot-dip galvanized steel sheet) which is prepared through hot-dip galvanization and subsequent alloying (galvannealing) and is excellent in sliding performance and powdering resistance.
  • Yet another object of the present invention is to provide an automotive member using the hot-dip galvanized steel sheet or the galvannealed steel sheet which is obtained by further alloying the hot-dip galvanized steel sheet.
  • the present inventors made investigations to improve the sliding performance and powdering resistance of a galvannealed steel sheet which is prepared by alloying a hot-dip galvanized steel sheet. As a result, they found that the sliding performance and powdering resistance of a galvannealed steel sheet can be improved by appropriately controlling the balance in content among Mn, P, Cr, and Mo, of the component composition of a steel sheet. Such a galvannealed steel sheet is obtained through hot-dip galvanization and subsequent alloying of the steel sheet. The present invention has been made based on these findings.
  • a steel sheet for use in a hot-dip galvanized steel sheet according to the present invention contains, on the percent by mass basis, 0.06% to 0.15% carbon (C); 1% to 3% manganese (Mn); 0.01% to 0.05% phosphorus (P); 0.03% to 1% chromium (Cr); 0.03% to 1% molybdenum (Mo); and 0.02% to 0.15% aluminum (Al).
  • the steel sheet has a silicon (Si) content of 0.2% or less (inclusive of 0%) and a sulfur (S) content of 0.03% or less (inclusive of 0%) and has a K value of ⁇ 2.0 or more, in which the K value is calculated according to following Equation (1):
  • the steel sheet preferably further contains at least one element selected from the group consisting of 0.15% or less (exclusive of 0%) titanium (Ti); 0.15% or less (exclusive of 0%) niobium (Nb); and 0.15% or less (exclusive of 0%) vanadium (V).
  • the steel sheet preferably further contains, on the percent by mass basis, 0.01% (exclusive of 0%) boron (B) and/or 0.01% or less (exclusive of 0%) calcium (Ca) as another element.
  • a steel sheet for use in a galvannealed steel sheet according to the present invention contains, on the percent by mass basis, 0.06% to 0.15% carbon (C); 1% to 3% manganese (Mn); 0.01% to 0.05% phosphorus (P); 0.03% to 1% chromium (Cr); 0.03% to 1% molybdenum (Mo); and 0.02% to 0.15% aluminum (Al).
  • the steel sheet has a silicon (Si) content of 0.2% or less (inclusive of 0%) and a sulfur (S) content of 0.03% or less (inclusive of 0%).
  • the steel sheet has a K value of ⁇ 2.0 or more, in which the K value is calculated according to following Equation (1):
  • the steel sheet has an F value of 0.7 to 3.0, in which the F value is calculated according to following Equation (2):
  • a hot-dip galvanized steel sheet and a galvannealed steel sheet according to the present invention can be advantageously used as materials for automotive members.
  • a steel sheet for hot-dip galvanization which is resistant to the occurrence of bare spots upon hot-dip galvanization, because the steel sheet has an appropriately controlled balance in content between Mo and Cr, among constitutional elements of the steel sheet.
  • a steel sheet for alloyed hot-dip galvanization which yields a galvannealed steel sheet excellent in sliding performance and powdering resistance through hot-dip galvanization and subsequent alloying, because the steel sheet has an appropriately controlled balance in content of Mn, P, Cr, and Mo, among the constitutional elements of the steel sheet.
  • a hot-dip galvanized steel sheet without bare spots can be provided by subjecting the steel sheet for hot-dip galvanization to hot-dip galvanization, and a galvannealed steel sheet excellent in sliding performance and powdering resistance can be provided by subjecting the hot-dip galvanized steel sheet to alloying (galvannealing).
  • These hot-dip galvanized steel sheet and galvannealed steel sheet can be advantageously used as materials for automotive members.
  • FIG. 1 is an electron micrograph (photograph in place of drawing) of Sample No. 3 in Table 2.
  • FIG. 2 is a graph showing the relationship among the K value, the iron (Fe) concentration and the occurrence of bare spots.
  • FIG. 3 is a graph showing the relationship among the F value, the Fe concentration, and the sliding performance or powdering resistance.
  • the K value is less than ⁇ 2.0, there occur a multiplicity of pinhole-like bare spots (ungalvanized portions) in steel sheet surface upon hot-dip galvanization, and this significantly impairs the appearance and quality of steel sheet.
  • the K value is preferably controlled to ⁇ 1.5 or more, and more preferably to ⁇ 1 or more.
  • Equation (1) “[Element] ” represents the content (percent by mass) of the element.
  • Chromium (Cr) content 0.03% to 1%
  • Molybdenum (Mo) content 0.03% to 1%
  • a steel sheet containing Cr alone is resistant to the occurrence of bare spots at a Cr content up to 0.4% but causes the occurrence of bare spots at a Cr content of 0.4% or more.
  • Cr and Mo when added in combination, they form a multiple oxide in steel sheet surface to thereby prevent the occurrence of bare spots.
  • the combination addition of Cr and Mo also yields a steel sheet having good balance between strength and elongation and thereby having both satisfactory mechanical properties and good galvanizing ability, as is described in examples mentioned later.
  • the elements Cr and Mo act to improve hardenability and to concentrate carbon (C) in austenite so as to improve the stability of austenite. They also act to form martensite so as to ensure satisfactory strength of the steel sheet. If the Cr content or Mo content is less than 0.03%, the hardenability is not sufficiently improved, and the amount of oxide in the steel sheet surface is small.
  • the Cr content is preferably 0.05% or more, and more preferably 0.1% or more.
  • the Mo content is preferably 0.05% or more, and more preferably 0.1% or more. The upper limits of these contents are each 1%, because the effects of Cr and Mo become saturated and the cost increases when the Cr content or Mo content is more than 1%.
  • the Cr content is preferably 0.9% or less, and more preferably 0.8% or less.
  • the Mo content is preferably 0.9% or less, and more preferably 0.8% or less.
  • a steel sheet for use in the present invention contains, as other components, 0.06% to 0.15% carbon (C); 1% to 3% manganese (Mn); 0.01% to 0.05% phosphorus (P); and 0.02% to 0.15% aluminum (Al). Reasons for specifying these ranges will be described below.
  • Carbon (C) content 0.06% to 0.15%
  • the element carbon (C) is important to improve the strength of steel sheet and affects the amounts and shapes of products of low-temperature transformation, such as bainite and martensite, so as to improve workability including extensibility (elongation property and stretch flange formability).
  • the C content is set at 0.06% or more, because, if the C content is less than 0.06%, it is difficult to ensure a strength of 590 MPa or more.
  • the C content is preferably 0.07% or more, and more preferably 0.08% or more. If the C content is more than 0.15%, the weldability of the resulting hot-dip galvanized high-strength steel sheet and galvannealed high-strength steel sheet deteriorates.
  • the C content is therefore set at 0.15% or less.
  • the C content is preferably 0.14% or less, and more preferably 0.13% or less.
  • Manganese (Mn) content 1% to 3%
  • the element manganese (Mn) is important to stabilize austenite and to ensure the strength of steel sheet itself.
  • the element Mn acts to improve the hardenability and effectively acts to yield a desired metal structure.
  • Mn must be contained in a content of 1% or more.
  • the Mn content is preferably 1.5% or more, and more preferably 1.8% or more.
  • the Mn content is preferably as large as possible, but if the Mn content is more than 3%, ingot making becomes difficult, and the weldability of the resulting hot-dip galvanized high-strength steel sheet and galvannealed high-strength steel sheet is adversely affected.
  • the Mn content is therefore set at 3% or less.
  • the Mn content is preferably 2.9% or less, and more preferably 2.8% or less.
  • Phosphorus (P) content 0.01% to 0.05%
  • the element phosphorus (P) is fixed as a deposit in steel sheet and is effective to ensure the balance between strength and elongation. To exhibit these activities, phosphorus must be contained in a content of 0.01% or more.
  • the P content is preferably 0.015% or more. However, if the P content is more than 0.05%, for example, galvanizing failure occurs, and the upper limit of the P content is therefore set at 0.05%.
  • the P content is preferably 0.04% or less, and more preferably 0.03% or less.
  • Aluminum (Al) content 0.02% to 0.15%
  • the element aluminum (Al) is contained for deoxidization and should be added in a content of 0.02% or more.
  • the Al content is preferably 0.03% or more, and more preferably 0.04% or more. It should be noted that the excessive addition of Al causes increased oxide inclusions and thereby impairs the surface properties and toughness. Accordingly, the upper limit of the Al content is set at 0.15%.
  • the Al content is preferably 0.13% or less, and more preferably 0.1% or less.
  • the elements silicon (Si) and sulfur (S) are basically preferably not contained in a high-strength steel sheet for use in the present invention and are contained, if any, as inevitable impurities.
  • a high-strength steel sheet for use in the present invention should be controlled so as to have a Si content of 0.2% or less and a S content of 0.03% or less.
  • Silicon (Si) content 0.2% or less (inclusive of 0%)
  • the element silicon (Si) forms oxide films such as a SiO 2 film on steel sheet surface, impairs the wettability of galvanized layer, and is harmful. Accordingly, the Si content is preferably as small as possible and is reduced to 0.2% or less, and preferably 0.1% or less.
  • S Sulfur (S) content: 0.03% or less (inclusive of 0%)
  • S Sulfur
  • S is fixed as MnS deposits in steel sheet, and increased amounts of MnS deposits cause deterioration in extensibility and stretch flange formability. Accordingly, the content of sulfur should be reduced to 0.03% or less even if sulfur is contained as inevitable impurities. The S content is preferably reduced to 0.015% or less.
  • a high-strength steel sheet according to the present invention satisfies the contents of the compositional elements and may preferably further contain, as other elements, additional components such as (a) at least one selected from the group consisting of 0.15% or less (exclusive of 0%) titanium (Ti); 0.15% or less (exclusive of 0%) niobium (Nb); and 0.15% or less (exclusive of 0%) vanadium (V), (b) 0.01% (exclusive of 0%) boron (B), and/or (c) 0.01% or less (exclusive of 0%) calcium (Ca).
  • additional components such as (a) at least one selected from the group consisting of 0.15% or less (exclusive of 0%) titanium (Ti); 0.15% or less (exclusive of 0%) niobium (Nb); and 0.15% or less (exclusive of 0%) vanadium (V), (b) 0.01% (exclusive of 0%) boron (B), and/or (c) 0.01% or less (exclusive of
  • the elements titanium (Ti), niobium (Nb), and vanadium (V) act to enhance deposition and finely divide the structure and are useful for providing higher strength. To exhibit these activities effectively, it is recommended to incorporate these elements in a content of each 0.01% or more, and particularly preferably 0.02% or more. The excessive addition of these elements may cause excessively high strength of a hot-rolled steel sheet and thereby invite defects of shape in cold rolling. Accordingly, the contents of these elements are each set at 0.15% or less, and more preferably 0.13% or less.
  • the element boron (B) increases the hardenability and improves the strength (tensile strength (TS) and yield strength (YS)) of steel sheet.
  • the addition of boron (B) in combination with Mo controls the hardenability during accelerated cooling after rolling and optimizes the balance between the strength (TS) and toughness of base metal material.
  • the B content is preferably 0.0005% or more, and more preferably 0.001% or more.
  • an excessively high B content of more than 0.01% may impair the toughness of base metal material, and the B content is preferably controlled to 0.01% or less, and more preferably 0.05% or less.
  • Calcium (Ca) content 0.01% or less (exclusive of 0%)
  • the element calcium (Ca) allows sulfides in steel to have spherical shapes and thereby contributes to improvement in workability. To exhibit these activities effectively, it is recommended to add Ca in a content of 0.0003% or more, and more preferably 0.0005% or more. However, if Ca is contained in a content exceeding 0.01%, the effects may be saturated and be economically useless.
  • the Ca content is more preferably 0.05% or less.
  • the remainder (balance) of a high-strength steel sheet for use in the present invention contains iron (Fe) and inevitable impurities and may further contain other elements within ranges not adversely affecting the advantages of the present invention.
  • the occurrence of bare spots upon hot-dip galvanization of a high-strength steel sheet can be prevented by appropriately controlling the balance in content between Cr and Mo in the high-strength steel sheet.
  • the resulting high-strength steel sheet for hot-dip galvanization is excellent in galvanizing ability.
  • the obtained galvannealed high-strength steel sheets may not be sufficiently improved in sliding performance and powdering resistance in some cases.
  • the present inventors therefore focused attention on the composition of an alloyed galvanized layer after the alloying process and made various investigations.
  • the present inventors found that the alloying rate is particularly affected by, of the component composition of a high-strength steel sheet, the contents of Mn, P, Cr, and Mo. Specifically, the elements Mn, P, Cr, and Mo are necessary for improving the hardenability of steel sheet and ensuring the strength thereof; however, a slight variation in their contents significantly affects the alloying rate of galvanized layer.
  • the alloying of the galvanized layer may be unlikely to proceed and a soft ⁇ phase (FeZn 13 ) may be formed on the surface of the alloyed galvanized layer.
  • the component composition is desirably controlled so as to have an F value of 0.9 or more, and more preferably 1.1 or more.
  • the galvanized layer may be excessively alloyed, and a hard and brittle ⁇ phase (Fe 3 Zn 10 ) is formed at the interface between the alloyed galvanized layer and the base metal material.
  • the component composition is desirably controlled so as to have an F value of 2.8 or less, and more preferably 2.6 or less.
  • the iron (Fe) concentration of the galvanized layer is preferably 7 to 15 percent by mass. If the Fe concentration is less than 7 percent by mass, the diffusion of Fe as a result of the alloying process may be insufficient, a soft ⁇ phase (FeZn 13 ) may be formed on the surface of alloyed galvanized layer, and the sliding performance upon pressing may deteriorate.
  • the Fe concentration is more preferably 8 percent by mass or more, and further preferably 9 percent by mass or more. In contrast, if the Fe concentration is more than 15 percent by mass, Fe may excessively diffuse, a hard and brittle ⁇ phase (Fe 3 Zn 10 ) may be formed at the interface between the alloyed galvanized layer and the base metal material, and powdering may be likely to occur.
  • the Fe concentration is more preferably 14 percent by mass or less, and further preferably 13 percent by mass or less.
  • the aluminum (Al) content of a molten zinc bath may be controlled. This point will be described later.
  • a galvannealed high-strength steel sheet excellent in sliding performance and powdering resistance can be obtained by subjecting the high-strength steel sheet for hot-dip galvanization which satisfies the condition on the F value to hot-dip galvanization and alloying the galvanized steel sheet.
  • the metal structure of the base steel sheet of the hot-dip galvanized steel sheet or galvannealed steel sheet is preferably a composite structure (dual phase) mainly containing ferrite and martensite.
  • a composite structure mainly containing ferrite and martensite By having a composite structure mainly containing ferrite and martensite, the entire galvanized steel sheet can have higher strength.
  • the phrase “mainly containing” as used herein means that the ratio of a total area of the composite structure to that of the entire metal structure of the base steel sheet is 70% or more. The a real ratio is more preferably 80% or more. In this connection, as long as not impairing the strength of the galvanized steel sheet, intrusion of other structural components such as bainite or pearlite may not be eliminated.
  • the ratio by area of ferrite to martensite is preferably about 90:10 to about 25:75.
  • the metal structure of the base steel sheet may be observed with a scanning electron microscope at a magnification of 3000 times.
  • the hot-dip galvanized high-strength steel sheet and galvannealed high-strength steel sheet each have a tensile strength (TS) of about 590 to about 1180 MPa and show good balance between the strength and elongation (El), in which the product of TS and El is 12000 or more.
  • TS tensile strength
  • El elongation
  • a method for manufacturing a high-strength steel sheet for hot-dip galvanization for use in the present invention is not particularly limited.
  • the high-strength steel sheet can be manufactured, for example, by hot-rolling slabs satisfying the above-specified component composition, coiling the hot-rolled steel at a temperature of 700° C. or lower, carrying out pickling according to necessity, and then carrying out cold rolling.
  • the hot rolling can be carried out according to a common procedure.
  • a heating temperature of slabs is preferably in a range from 1000° C. to 1300° C. from the viewpoints of ensuring a satisfactory finishing temperature and preventing coarse austenite particles.
  • a finishing temperature of the hot rolling is preferably 800° C. to 950° C. so as to prevent the formation of a texture adversely affecting the workability.
  • a cooling rate after finish rolling is preferably 30° C. to 120° C. per second, so as to prevent the formation of pearlite.
  • a coiling temperature is preferably set at 700° C. or lower. If the coiling temperature is higher than 700° C., scale on the steel sheet surface may be excessively thick, to thereby impair pickling processability.
  • the lower limit of the coiling temperature is not particularly limited; however, an excessively low coiling temperature may cause the steel sheet to have excessively high hardness and thereby have decreased cold rolling processability; and the lower limit may be set at about 250° C.
  • the coiling temperature is preferably 400° C. or higher.
  • a reduction ratio in cold rolling is preferably set at 30% or more. If the reduction ratio in cold rolling is set at less than 30%, the thickness of the hot-rolled steel sheet must be reduced, it takes a long time to conduct pickling, and the productivity may deteriorate.
  • Methods for manufacturing a hot-dip galvanized high-strength steel sheet and a galvannealed high-strength steel sheet according to the present invention are not particularly limited.
  • the hot-dip galvanized high-strength steel sheet can be obtained, for example, by carrying out cold rolling according to the above procedure, soaking the cold rolled steel sheet at a temperature equal to or higher than the Ac1 point in a continuous hot-dip galvanization line, and cooling the soaked steel sheet at an average cooling rate of 1° C. per second or more to the temperature of a molten zinc bath so as to cover the surface of the high-strength steel sheet with a galvanized layer.
  • the galvannealed high-strength steel sheet can be obtained by subjecting the hot-dip galvanized high-strength steel sheet prepared by the above procedure to an alloying process according to a common procedure, and cooling the alloyed steel sheet at an average cooling rate of 5° C. per second or more.
  • a soaking temperature in the continuous hot-dip galvanization line may be set to be equal to or higher than the A c1 point.
  • the soaking is preferably carried out at a temperature higher than the A c1 point by about 50° C., in order to allow the high-strength steel sheet to have a composite structure including ferrite and austenite to thereby have increased workability. More specifically, the soaking temperature is preferably about 780° C. or higher.
  • the upper limit of the soaking temperature is not particularly limited, but is generally set at 900° C. or lower.
  • a holding time of the soaking is also not particularly limited, and may be set, for example, at about 10 seconds or more.
  • the temperature of molten zinc bath is 400° C. to 500° C., and preferably 440° C. to 470° C. If the cooling rate is less than 1° C. per second, pearlite may be formed and remain as a final structure to thereby impair the workability.
  • the cooling rate is preferably set at 5° C. per second or more.
  • the upper limit of the cooling rate is not particularly limited but is preferably set at 50° C. per second in consideration of easy control of the sheet temperature and the costs of facilities.
  • the composition of the molten zinc bath is not particularly limited, and a known molten zinc bath can be used. It should be noted that the Al content of molten zinc bath is preferably 0.05% to 0.2%.
  • the element aluminum (Al) acts to control the alloying rate of a galvanized layer. When a steel sheet is dipped in a molten zinc bath containing aluminum (Al), an Fe—Al metal layer is formed on a surface of the steel sheet, i.e., at the interface between the steel sheet and the galvanized layer.
  • the Al content is less than 0.05%, a formed Fe—Al alloy layer may be excessively thin, and, when the steel sheet is dipped in the molten zinc bath, the alloying between the steel sheet and zinc may immediately proceed. Accordingly, the F phase may grow excessively before the completion of alloying up to the galvanized surface during the alloying process, and this may impair the powdering resistance (peeling resistance).
  • the Al content is more preferably 0.07% or more. If the Al content, however, is more than 0.2%, the Fe—Al alloy layer may be excessively thick, and the alloying between Fe and Zn may be inhibited so as to retard the alloying of the galvanized layer in the alloying process. In this case, the alloying line should be elongated or another alloying process at high temperatures should be carried out in order to proceed alloying.
  • the Al content is therefore more preferably 0.18% or less.
  • the galvanized steel sheet is cooled at a cooling rate of 5° C. per second or more to ordinary temperature so as to transform austenite into martensite.
  • a composite (dual-phase) structure mainly containing ferrite and martensite is obtained. If the cooling rate is less than 5° C. per second, martensite may not be formed, and, contrarily, pearlite and/or bainite may be formed.
  • the cooling rate is preferably set at 10° C. per second or more.
  • the hot-dip galvanized steel sheet may be heated to about 500° C. to about 750° C., and preferably about 500° C. to about 600° C. according to a common procedure.
  • a heating procedure for the alloying process is not particularly limited and can be selected from among various known procedures such as gas heating or induction heating.
  • a composite structure mainly containing ferrite and martensite can be obtained by cooling the steel sheet after the alloying process at a cooling rate of 5° C. per second or more to ordinary temperature.
  • the hot-dip galvanization was conducted by dipping the steel sheets in the molten zinc bath at a temperature of 450° C. to 470° C. for 3 seconds. Next, the steel sheets were heated to a temperature within a range from 540° C. to 560° C., held at this temperature for 15 seconds for carrying out an alloying process, cooled at a cooling rate of 30° C. per second or more to room temperature, subjected to temper rolling at a rolling reduction of 1.0%, and thereby yielded galvannealed steel sheets.
  • the metal structures at a center position in a thickness direction of the base steel sheets of the galvannealed steel sheets were observed with a scanning electron microscope (SEM) at a magnification of 3000 times. These steel sheets were found to have composite structures mainly containing ferrite and martensite in an a real ratio of 80% or more, with the ratios of the area of ferrite to that of martensite of 74:26 to 32:68.
  • SEM scanning electron microscope
  • the Fe concentrations of the galvannealed steel sheets were measured by dissolving their alloyed galvanized layers in hydrochloric acid and carrying out atomic absorption spectrometry. The results are shown in Table 2 below.
  • TS tensile strengths
  • El elongations
  • TS tensile strength
  • El elongation
  • the galvanizing ability, sliding performance, and powdering resistance of the galvannealed steel sheets were evaluated in the following manner.
  • the galvanizing ability of a sample was evaluated by visually observing the sample on whether or not bare spots occurred. A sample with no bare spot observed was evaluated as good (“Good”) in galvanizing ability, and a sample with bare spots observed was evaluated as poor (“Poor”) in galvanizing ability. The evaluation results are shown in Table 2 below.
  • the sliding performance was evaluated in the following manner.
  • a rust preventive oil supplied from Parker Industries, Inc., “NOX-RUST 550HN (trade name)” was applied in a coating amount of about 1.5 g/m 2 to the both sides of a sample galvannealed steel sheet (test piece), and 20-mm square flat tools were pressed to the both sides of the test piece at a contact pressure of about 30 N/mm 2 .
  • the powdering resistance was evaluated by caring out a V-shaped bending test using a V-shaped punch having a bending angle of 60 degrees and a bending radius of 1 mm, and measuring the amount of peeled deposit (galvanized layer) inside of the bent portion. The evaluation was conducted based on such criteria that a sample having a peeling amount of 10 mg or less was evaluated as good in powdering resistance, and a sample having a peeling amount more than 10 mg was evaluated as poor in powdering resistance. The results are shown in Table 2 below.
  • Tables 1 and 2 demonstrate as follows. Samples Nos. 1 to 10 satisfy conditions as specified in the present invention, are free from the occurrence of bare spots, and are excellent in sliding performance and powdering resistance. In contrast, Samples Nos. 11 to 20 do not satisfy the conditions as specified in the present invention and are poor in mechanical properties, particularly in the balance between strength and elongation. In addition, they suffer from the occurrence of bare spots or are poor in at least one of sliding performance and powdering resistance.
  • FIG. 2 is a graph showing the relationship among the K value, the Fe concentration, and the occurrence of bare spots.
  • data indicated by the open circle, closed triangle, and open triangle represent data of samples without the occurrence of bare spots; and data indicated by the closed rhombus and “X” represent data of samples with the occurrence of bare spots.
  • data indicated by the closed triangle are the data of a sample evaluated as poor in powdering resistance; data indicated by the open triangle are the data of a sample evaluated as poor in sliding performance; and data indicated by the closed rhombus are the data of a sample evaluated as poor both in powdering resistance and sliding performance.
  • FIG. 2 demonstrates that a sample having a K value within the range as specified in the present invention is free from bare spots, but a sample having a K value out of the range suffers from bare spots.
  • FIG. 3 is a graph showing the relationship among the F value, the Fe concentration, and the sliding performance or powdering resistance.
  • data indicated by the open circle represent data of a sample having good sliding performance and good powdering resistance
  • data indicated by “X” represent data of a sample having poor sliding performance or powdering resistance
  • data indicated by the closed triangle represent the data of a sample which is good in sliding performance and powdering resistance but suffers from the occurrence of bare spots.
  • FIG. 3 demonstrates that a sample having an F value within the range as specified in the present invention is excellent in sliding performance and powdering resistance; but, in contrast, a sample having an F value out of this range is poor in sliding performance or powdering resistance.

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US20100132848A1 (en) * 2008-11-28 2010-06-03 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel Ltd) Ultrahigh-strength steel sheet excellent in hydrogen embrittlement resistance and workability, and manufacturing method therefor
US9181613B2 (en) 2011-04-20 2015-11-10 Kobe Steel, Ltd. High tensile strength hot-dip galvannealed steel sheet having excellent coated-layer adhesiveness and method for producing same
US10190187B2 (en) 2008-05-21 2019-01-29 Arcelormittal Manufacturing method for very high-strength, cold-rolled, dual-phase steel sheets

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JP4788291B2 (ja) * 2005-10-27 2011-10-05 Ｊｆｅスチール株式会社 伸びフランジ成形性に優れた高強度溶融亜鉛めっき鋼板の製造方法
JP5257981B2 (ja) * 2007-07-11 2013-08-07 Ｊｆｅスチール株式会社 プレス成形性に優れた高強度溶融亜鉛めっき鋼板の製造方法
JP5272412B2 (ja) * 2008-01-17 2013-08-28 Ｊｆｅスチール株式会社 高強度鋼板およびその製造方法
KR20100034118A (ko) * 2008-09-23 2010-04-01 포항공과대학교 산학협력단 마르텐사이트 조직을 가진 초고강도 용융아연도금 강판 및 그 제조 방법
JP6228741B2 (ja) * 2012-03-27 2017-11-08 株式会社神戸製鋼所 板幅方向における中央部と端部の強度差が少なく、曲げ加工性に優れた高強度溶融亜鉛めっき鋼板、高強度合金化溶融亜鉛めっき鋼板、およびこれらの製造方法
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
CN109154044B (zh) * 2016-07-15 2020-09-04 日本制铁株式会社 热浸镀锌钢板

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