WO2008082146A9 - High strength zn-coated steel sheet having excellent mechanical properites and surface quality and the method for manufacturing the same - Google Patents

High strength zn-coated steel sheet having excellent mechanical properites and surface quality and the method for manufacturing the same

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Publication number
WO2008082146A9
WO2008082146A9 PCT/KR2007/006867 KR2007006867W WO2008082146A9 WO 2008082146 A9 WO2008082146 A9 WO 2008082146A9 KR 2007006867 W KR2007006867 W KR 2007006867W WO 2008082146 A9 WO2008082146 A9 WO 2008082146A9
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WO
WIPO (PCT)
Prior art keywords
steel sheet
steel
added
high strength
surface quality
Prior art date
Application number
PCT/KR2007/006867
Other languages
French (fr)
Other versions
WO2008082146A1 (en
Inventor
Il Ryoung Sohn
Hee Jae Kang
Kwang Geun Chin
Original Assignee
Posco
Il Ryoung Sohn
Hee Jae Kang
Kwang Geun Chin
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 Posco, Il Ryoung Sohn, Hee Jae Kang, Kwang Geun Chin filed Critical Posco
Priority to JP2009528186A priority Critical patent/JP5354600B2/en
Priority to EP07851792A priority patent/EP2097547A1/en
Publication of WO2008082146A1 publication Critical patent/WO2008082146A1/en
Publication of WO2008082146A9 publication Critical patent/WO2008082146A9/en

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    • 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
    • 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
    • C21D8/0447—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 characterised by the heat treatment
    • C21D8/0473—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
    • 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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—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
    • 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
    • C22C38/60—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
    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
    • C23C28/04—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings of inorganic non-metallic 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
    • 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
    • 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
    • C21D8/0421—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 characterised by the working steps
    • C21D8/0426—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
    • C21D8/04—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
    • C21D8/0421—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 characterised by the working steps
    • C21D8/0436—Cold rolling

Definitions

  • the present invention relates to a high strength dual phase (DP) steel having excellent mechanical properties and surface quality, which is mainly used for inner and outer planes and structural parts of a vehicle, and more particularly, to a high strength Zn-coated steel sheet which can be more easily manufactured with superior mechanical properties and surface quality than a conventional high strength DP steel, and a method for manufacturing the same.
  • DP dual phase
  • material-reinforcing elements such as Si, Mn, Ti, Nb, and Al are added to manufacture the steel sheet.
  • material-reinforcing elements such as Si, Mn, Ti, Nb, and Al are added to manufacture the steel sheet.
  • most of these elements have higher affinity for oxygen than Fe, and thus cause surface concentration of oxide during cold-rolled annealing.
  • This surface concentration of oxide may lead to poorer coating quality such as non- coating. Also, the surface concentration, if coarse, are adsorbed onto a hearth roll of a continuous annealing furnace, thereby triggering defects such as dent defects on a surface of the coated steel sheet.
  • Japanese Patent Laid-open Application Nos. 2002-146477, 2001-64750, 2002-294397, and 2002-155317, and Korean Patent No. 2005-0128666 disclose a method of enhancing coating performance by adding specific elements such as Cr,Sb, and Sn.
  • Japanese Patent Laid-open Application No. 2001-288550 teaches a method of inhibiting surface concentration from being formed during cold-rolled annealing by pre-oxidizing a hot-rolled coil before cold rolling.
  • effects of the specific elements added are not evident and metallurgical behavior of the added elements has not been clearly examined. Thus, the methods are considered insufficient and hamper workability.
  • the present invention has been made to solve the foregoing problems of the prior art and therefore an aspect of the present invention is to provide a high strength Zn-coated steel sheet having superior mechanical properties and surface quality whose alloy elements and hot-rolling coiling temperature are adequately regulated to ensure the steel sheet to be manufactured more easily than a conventional high-strength dual phase (DP) steel.
  • DP dual phase
  • the invention provides a high strength Zn- coated steel sheet having excellent mechanical properties and surface quality including, by weight: 0.01 to 0.2% C, 0.01 to 1.5% Si, 0.2 to 4.0% Mn, 0.001 to 0.1% P, ⁇ 0.03% S, 0.01 to 1.5% Al, 0.001 to 0.03% N, 0.0002 to 0.005% B, 0.01 to 2.0% Cr, 0.005 to 0.5% Mo, 0.005 to 0.1% Sb, and the balance of Fe and unavoidable impurities, the steel sheet satisfying a relation of
  • the invention provides a method for manufacturing a high strength Zn-coated steel sheet having excellent mechanical properties and surface quality, the method including: reheating a steel slab to 1100 to 125O 0 C, the steel slab comprising, by weight: 0.01 to 0.2% C, 0.01 to 1.5% Si, 0.2 to 4.0% Mn, 0.001 to 0.1% P, ⁇ 0.03% S, 0.01 to 1.5% Al, 0.001 to 0.03% N, 0.0002 to 0.005% B, 0.01 to 2.0% Cr, 0.005 to 0.5% Mo, 0.005 to 0.1% Sb, and the balance of Fe and unavoidable impurities, the steel slab satisfying a relation of 5 ⁇ (7Mn+4Si+2Al+18B)/(4C+3P+20Sb) ⁇ 18; hot rolling the steel slab; coiling the steel slab at a coiling temperature satisfying a relation of
  • CT (700-(30Mn+18Si+54P+15Cr+7Mo)+150B)+20°C; pickling and cold-rolling the steel slab; and annealing the steel slab at a temperature of 700 0 C to 86O 0 C.
  • FIG. 1 is a picture illustrating the shape of oxides of an Inventive Steel and a
  • FIG. 2 is a graph illustrating the size distribution of oxides formed on a surface of an
  • the inventors have conducted a research on a method for manufacturing a high- strength dual phase (DP) steel which can be more easily manufactured with superior mechanical properties and surface quality than a conventional high-strength DP phase.
  • DP high- strength dual phase
  • alloy elements and hot-rolling coiling temperature of a steel can be adequately regulated to suppress increase in diameter of oxide grains on a surface of a steel sheet, thereby ensuring excellent surface quality and improving mechanical properties.
  • Carbon (C) is preferably in a range of 0.01 to 0.2%.
  • C increases strength of the steel sheet, and is very important in attaining a composite structure consisting of ferrite and martensite. C added at less than 0.01% does not ensure desired strength of the present invention. Meanwhile, C added at more than 0.2% is highly likely to degrade tensile strength and weldability. Thus, C may be added in an amount of 0.01 to 0.2%.
  • Si is preferably in a range of 0.01 to 1.5%.
  • Si is beneficial for ensuring strength while not impairing ductility of the steel sheet.
  • Si facilitates ferrite formation, and martensite formation by increasing C concentration of non-transformed austenite.
  • Si added at less than 0.01% does not bring about the above effects.
  • Si added at more than 1.5% is highly likely to deteriorate surface properties and weldability.
  • Si may be added in an amount of 0.01 to 1.5%.
  • Manganese (Mn) is preferably in a range of 0.2 to 4.0%.
  • Mn is greatly effective for strengthening solid solution and facilitates formation of a composite structure consisting of ferrite and martensite. Mn added at less than 0.2% does not yield effects enough to realize high strength. Meanwhile, Mn added at more than 4.0% is highly likely to deteriorate weldability and hot-rolling properties. Thus, Mn may be added in an amount of 0.2 to 4.0%.
  • Phosphor (P) is preferably in a range of 0.001 to 0.1 % .
  • P is a major solid solution-strengthening element added to increase strength, together with Mn. P added at less than 0.001% does not bring about desired effects. Meanwhile, P added at more than 0.1% impairs weldability and leads to great difference in properties of the steel due to central segregation occurring during continuous casting. Thus, P may be added in an amount of 0.001 to 0.1%.
  • Sulfer (S) is preferably added in an amount up to 0.03%.
  • Aluminum (Al) is preferably in a range of 0.01 to 1.5%.
  • Al is added to deoxidize a steel typically but increase ductility according to the present invention. Also, Al inhibits carbide from being formed during austempering and increases strength. Al added at less than 0.01% does not yield sufficient effects. On the other hand, Al added at more than 1.5% accelerates internal oxidation during annealing of the cold-rolled steel to prevent alloying of GA-plated layer, thereby requiring a high alloying temperature. Thus, Al may be added in an amount of 0.01% to 1.5%.
  • N Nitrogen (N) is preferably in a range of 0.001 to 0.03%.
  • N is an effective element for stabilizing austenite. N added at less than 0.001% does not yield such an effect. Meanwhile, N, when added at more than 0.03%, binds with Al to produce a coarse AlN, thereby undermining mechanical properties. Thus, the N amount may not exceed 0.03%.
  • Boron (B) is preferably in a range of 0.0002 to 0.005%.
  • B is a grain boundary- strengthening element which enhances fatigue properties of a spot welding portion and prevents brittleness of P grain boundary. Also, in manufacturing the steel with high Al and Si contents, B improves hot ductility. In addition, B delays austenite from being transformed into perlite in a cooling process during annealing. B added at more than 0.005% causes an excessive amount of B to be concentrated on a surface of the steel, thus degrading coating adhesion. Therefore, to achieve desired effects, B should be added in an amount of at least 0.0002% but the B amount exceeding 0.005% drastically undermines workability and deteriorates surface properties of the coated steel sheet. Thus, B may be added in an amount of 0.0002 to 0.005%.
  • Chrome (Cr) is preferably in a range of 0.01 to 2.0%.
  • Cr is added to enhance hardenability and ensure high strength.
  • the Cr amount less than 0.01% does not assure such effects.
  • the Cr amount exceeding 2.0% yields no further effect and is highly likely to degrade ductility.
  • Cr may be added in an amount of 0.01 to 2.0%.
  • Molybdenum (Mo) is preferably in a range of 0.005 to 0.5%.
  • Mo amount less than 0.005% does not ensure desired effects. Meanwhile, the Mo amount exceeding 0.5% does not lead to much improved effect and is economically disadvantageous. Thus, Mo may be added in an amount of 0.005 to 0.5%.
  • Antimony (Sb) is preferably in a range of 0.005 to 0.1%.
  • Sb itself does not form an oxidation layer at a high temperature but is concentrated on a surface of the steel and a crystal grain boundary thereof to prevent the elements from spreading onto the surface. This as a result suppresses formation of an oxide. Also, Sb is remarkably effective in inhibiting selective oxidation from being developed along a grain boundary of the steel sheet. Sb is added to inhibit penetration of various oxides formed along the grain boundary of the steel sheet surface in a hot-rolling process due to a considerable amount of Si, Mn, and Al. When a grain boundary oxide of the hot-rolled steel sheet has a depth exceeding l ⁇ m, the oxide remains inside the metal even after prickling, thereby causing various scale defects in a following cold- rolling process.
  • Sb added to effectively inhibit the selective grain boundary oxidation of the hot-rolled steel sheet is also noticeably effective in suppressing scale defects.
  • Sb is added to suppress formation of the oxide in an annealing due to a great amount of Si, Mn, and Al to thereby improve coating properties.
  • Sb when added in conjunction with Mn and B effectively inhibits coarsening of a surface oxide layer.
  • elements such as Sn, Se, and Y could be added to achieve similar effects but Sn is less capable of inhibiting the grain boundary oxidation of the hot- rolled steel sheet.
  • Se and Y are more likely to be concentrated on the surface of the steel than other elements and also may coarsen the oxide since the oxide is formed under SiO and Al O formed on the surface of the steel.
  • Sb should be added in an amount of at least 0.005% but, when added beyond a specific limit, does not ensure any further effects. Therefore, the Sb amount may not exceed 0.1%.
  • an alloy of the steel sheet having the element content range as described above may satisfy a relation of 5 ⁇ (7Mn+4Si+2Al+18B)/(4C+3P+20Sb) ⁇ 18.
  • Mn, Si, Al, and B characteristically form concentration on the surface of the steel in annealing. A greater amount of concentration of these elements degrades coating properties. Meanwhile, C, P, and Sb are prone to segregation at the crystal grain boundary. With segregation of these elements such as C, P, and Sb, the concentrated elements are blocked off the diffusion of Mn, Si, Al and B on the grain boundary, thus contributing to surface quality.
  • the above relation indicates an empirical numerical value capable of preventing deterioration of the steel characteristics and ensuring superior surface quality depending on surface concentration behavior and segregation behavior of the elements. For example, in a case where a value calculated by the relation is smaller than 5, the steel is degraded in mechanical properties. On the other hand, in a case where a value calculated by the relation is greater than 18, the steel cannot attain desired surface quality.
  • the steel sheet containing the elements described above may further include at least one of 0.01 to 1.0% Go, 0.001 to 0.1% Zr, 0.001 to 0.1% Ti, 0.001 to 0.1% Nb, 0.0005 to 0.040% La, 0.0005 to 0.040% Ce and 0.0005 to 0.030% Ca.
  • Cobalt Cb is preferably in a range of 0.01 to 1.0% .
  • Cb is added to boost strength of the steel.
  • Cb inhibits formation of the oxide in high- temperature annealing, thereby improving wettability against a molten zinc steel sheet during molten coating.
  • Cb should be added in an amount of at least 0.01%.
  • Cb added beyond a specific limit significantly reduces elongation of the steel.
  • the Cb amount may not exceed 1.0%.
  • Zirconium (Zr) is preferably in a range of 0.001 to 0.1%
  • Zr is solved in a columnar grain boundary to increase a melting temperature of a Al- concentrated and low-melting point compound, thereby blocking formation of a liquid film at a temperature of 1300 0 C or less and strengthening the columnar grain boundary.
  • Zr added at less than 0.001% does not ensure such effects.
  • Zr added at more than 0.1% does not yield any further effects. Therefore, Zr may be added in an amount of 0.001 to 0.1%.
  • Titanium (Ti) and Niobium (Nb) are preferably in a range of 0.001 to 0.1%, respectively.
  • Ti and Nb are effective in increasing strength of the steel sheet and attaining fine grain size. Ti and Nb added at less than 0.001% do not ensure such effects. Meanwhile, Ti and Nb added at more than 0.1% may raise manufacturing costs and deteriorate ferrite ductility due to excessive precipitate. Therefore, Ti and Nb may be added in an amount of 0.001 to 0.1%, respectively.
  • Lanthanum (La) and cerium (Ce) are preferably in a range of 0.0005 to 0.04%, respectively.
  • La and Ce decrease the size and amount of columnar grains which cause em- brittlement to the grain boundary and increase the amount of equiaxed grains with superior high-temperature ductility, thereby enhancing hot-rolling workability of a cast structure.
  • La and Ce are segregated at the grain boundary to form a compound with P and S which degrade rupture strength of the grain boundary, thereby ensuring P and S to have less negative effects.
  • La and Ce added at less than 0.0005% do not bring about any effects.
  • La and Ce added at more than 0.04% yield no further effects. Therefore, La and Ce may be added in an amount of 0.0005 to 0.04%, respectively.
  • Ca is preferably in a range of 0.0005 to 0.03%.
  • Calcium Ca forms a compound together with a non-metallic inclusion such as MnO, and MnS in a molten steel to spherodize the non-metallic inclusion. This accordingly increases rupture strength of the columnar grains, allows the steel sheet to be less prone to the flange crack and increases bore expandability. However, the Ca amount exceeding 0.03% yields no further effect. Thus Ca may be added in an amount of 0.0005 to 0.030%.
  • the steel sheet contains the balance of Fe and unavoidable impurities.
  • an oxide layer on the surface of the steel sheet has a thickness of l ⁇ m or less.
  • the oxide layer formed on a metal surface in an annealing process after cold-rolling serves as an impediment between a metal matrix and a coating layer in the case of coating, thereby degrading coating adhesion.
  • the oxide layer grown to a thickness exceeding 1/M causes dent defects and coating defects due to detachment of the oxide. Therefore, the oxide layer, when formed with a uniform but not great thickness in the annealing, advantageously ensures better quality in the coating layer.
  • Sb added in an amount of 0.005 to 0.1% is not oxidized but concentrated on a metal surface layer to inhibit oxidization reaction. This ensures the oxide layer to be formed with a uniform thickness of 1/M or less.
  • the steel slab At a reheating temperature of less than 1100 0 C, the steel slab is not uniform in structure and has Ti and Nb re-solved insufficiently therein. Meanwhile, at a reheating temperature of more than 125O 0 C, an oxidized scale is formed and a great amount of oxides such as SiO , MnO, and Al O are generated at a boundary with the metal and
  • the reheating temperature may range from 1100 to 125O 0 C.
  • the steel slab is hot finish rolled at a temperature ranging from an Ar transformation point to 95O 0 C.
  • the steel slab hot finish rolled at a temperature of an Ar transformation point is very likely to become more highly resistant to hot deformation and may have manufacturing problems.
  • the steel slab hot finish rolled at more than 95O 0 C experiences too thick an oxidized scale and is very likely to be coarsened. Therefore, the hot finish rolled temperature may range from an Ar transform to 95O 0 C.
  • CT (700-(30Mn+18Si+54P+15Cr+7Mo)+150B)+20°C.
  • elements such as Mn, Si, P, Cr, and Mo are added as an alloy element associated with formation of martensite out of microstructures.
  • these elements are more likely to experience segregation at a higher coiling temperature.
  • the added Mn, Si, P, Cr, and Mo achieve less effect, thereby not assuring desired strength and workability.
  • the above relation has been devised empirically as a means to attain desired strength and workability depending on the coiling temperature and the amount of the alloy elements. The coiling process described above ensures high-quality steel to be obtained.
  • a hot-rolled sheet manufactured by the above process is prickled, and cold-rolled to a desired thickness, and then recrystalized and annealed at a temperature of 700 to 86O 0 C to remove microstructural defects.
  • annealing temperature less than 700 0 C annealed oxides are formed minutely so that the added Sb does not yield noticeable effects.
  • oxides are grown excessively so that the surface concentration can not be sufficiently inhibited.
  • a steel slab containing elements as noted in Table 1 below was heated at 1200 0 C to be produced. Then the steel slab was hot rolled and coiled at a temperature(+20°C) satisfying relation 2 and cold-rolled to manufacture a cold-rolled steel sheet. Subsequently, the cold rolled steel strip was annealed and heat treated in an N -10%H O atmosphere at a temperature of 780 to 83O 0 C and at a heating rate of 3°C/sec per 90 seconds. Thereafter, the steel strip was coated in a Zn bath having a temperature of 46O 0 C and an Al range of 0.12 to 0.19%, and the coated steel was alloyed and heat- treated at a temperature of 540 to 56O 0 C for 24 seconds. Then, the steel sheet was observed to identify surface quality.
  • the category of coating appearance includes non-coating, O for having no other coating defects and a defect name for any defect that occurs. Also, to evaluate the category of coating adhesion, the coating sheet was cut into a size of 2Qnmx5Onm and subjected to a 60° bending test. Then, the coating sheet was straightened out to tape a bent portion. Here, a coating layer detached had a width measured as below.
  • Inventive Steels 1 to 11 satisfying the element content range and manufacturing method of the present invention can be manufactured as a high strength DP steel superior in coating surface properties and strength elongation balance (TSxEl).
  • Comparative steels 12 to 16 not satisfying the element content range and manufacturing method of the present invention were found inferior in terms of coating surface properties and strength elongation balance.
  • FIG. 1 shows the shape of oxides of Inventive Steel 11 and Comparative Steel 16, respectively depending on addition or non-addition of Sb.
  • FIG. IA denotes Comparative Steel 16 and
  • FIG. IB denotes Inventive Steel 11.
  • Inventive Steel 11 shows an oxide whose grains are noticeably smaller. That is, the oxide growth is inhibited due to addition of Sb.
  • FIG. 2 shows the size distribution of oxides formed on a surface of Inventive Steel 11.
  • the oxides formed on a metal surface in an annealing process have sizes stably distributed around ⁇ m or less. That is, the minute oxides uniformly distributed serve to form a more uniform coating layer.

Abstract

There is provided a high strength Zn-coated steel sheet having excellent mechanical properties and surface quality, which is mainly used for inner and outer panels and structural parts of a vehicle. The steel sheet having excellent mechanical properties and surface quality includes, by weight: 0.01 to 0.2% C, 0.01 to 1.5% Si, 0.2 to 4.0% Mn, 0.001 to 0.1% P, ≤0.03% S, 0.01 to 1.5% Al, 0.001 to 0.03% N, 0.0002 to 0.005% B, 0.01 to 2.0% Cr, 0.005 to 0.5% Mo, 0.005 to 0.1% Sb, and the balance of Fe and unavoidable impurities, the steel sheet satisfying a relation of 5≤(7Mn+4Si+2Al+18B)/(4C+3P+20Sb)≤18 and having an oxide layer with a thickness of 1μm or less on a surface thereof. According to the present invention, a high-strength dual phase steel having excellent mechanical properties and surface quality can be manufactured.

Description

Description HIGH STRENGTH ZN-COATED STEEL SHEET HAVING
EXCELLENT MECHANICAL PROPERITES AND SURFACE QUALITY AND THE METHOD FOR MANUFACTURING THE SAME Technical Field
[1] The present invention relates to a high strength dual phase (DP) steel having excellent mechanical properties and surface quality, which is mainly used for inner and outer planes and structural parts of a vehicle, and more particularly, to a high strength Zn-coated steel sheet which can be more easily manufactured with superior mechanical properties and surface quality than a conventional high strength DP steel, and a method for manufacturing the same.
[2]
Background Art
[3] Recently, a steel sheet for vehicles needs to possess higher formability since forming products thereof tend to be complicated and integrated. In the meantime, as generally known, the steel sheet for vehicles should be superior in work embrittlement and fatigue characteristics of welding parts to enhance usability of the vehicles. Moreover, the steel sheet for vehicles should have an aesthetically satisfying coating surface.
[4] In general, to enhance formability and strength of the steel sheet, material-reinforcing elements such as Si, Mn, Ti, Nb, and Al are added to manufacture the steel sheet. However, most of these elements have higher affinity for oxygen than Fe, and thus cause surface concentration of oxide during cold-rolled annealing.
[5] This surface concentration of oxide may lead to poorer coating quality such as non- coating. Also, the surface concentration, if coarse, are adsorbed onto a hearth roll of a continuous annealing furnace, thereby triggering defects such as dent defects on a surface of the coated steel sheet.
[6] To improve coating defects as described above, a known technology of manufacturing a thin steel sheet for deep processing has been developed by Japanese blast furnace companies. In a brief description, Japanese Patent Laid-open Application Nos. 2002-146477, 2001-64750, 2002-294397, and 2002-155317, and Korean Patent No. 2005-0128666 disclose a method of enhancing coating performance by adding specific elements such as Cr,Sb, and Sn. Also, Japanese Patent Laid-open Application No. 2001-288550 teaches a method of inhibiting surface concentration from being formed during cold-rolled annealing by pre-oxidizing a hot-rolled coil before cold rolling. However, in these methods, effects of the specific elements added are not evident and metallurgical behavior of the added elements has not been clearly examined. Thus, the methods are considered insufficient and hamper workability.
[7] In addition, some of the conventional technologies cannot be realized in the current general hot-rolling and cold-rolling-continuous annealing facility, not ensuring any commercially viable production. Disclosure of Invention Technical Problem
[8] The present invention has been made to solve the foregoing problems of the prior art and therefore an aspect of the present invention is to provide a high strength Zn-coated steel sheet having superior mechanical properties and surface quality whose alloy elements and hot-rolling coiling temperature are adequately regulated to ensure the steel sheet to be manufactured more easily than a conventional high-strength dual phase (DP) steel.
[9]
Technical Solution
[10] According to an aspect of the invention, the invention provides a high strength Zn- coated steel sheet having excellent mechanical properties and surface quality including, by weight: 0.01 to 0.2% C, 0.01 to 1.5% Si, 0.2 to 4.0% Mn, 0.001 to 0.1% P, <0.03% S, 0.01 to 1.5% Al, 0.001 to 0.03% N, 0.0002 to 0.005% B, 0.01 to 2.0% Cr, 0.005 to 0.5% Mo, 0.005 to 0.1% Sb, and the balance of Fe and unavoidable impurities, the steel sheet satisfying a relation of
5<(7Mn+4Si+2Al+18B)/(4C+3P+20Sb)<18 and having an oxide layer with a thickness of 1/M or less on a surface thereof.
[11] According to another aspect of the invention, the invention provides a method for manufacturing a high strength Zn-coated steel sheet having excellent mechanical properties and surface quality, the method including: reheating a steel slab to 1100 to 125O0C, the steel slab comprising, by weight: 0.01 to 0.2% C, 0.01 to 1.5% Si, 0.2 to 4.0% Mn, 0.001 to 0.1% P, <0.03% S, 0.01 to 1.5% Al, 0.001 to 0.03% N, 0.0002 to 0.005% B, 0.01 to 2.0% Cr, 0.005 to 0.5% Mo, 0.005 to 0.1% Sb, and the balance of Fe and unavoidable impurities, the steel slab satisfying a relation of 5<(7Mn+4Si+2Al+18B)/(4C+3P+20Sb)<18; hot rolling the steel slab; coiling the steel slab at a coiling temperature satisfying a relation of
CT=(700-(30Mn+18Si+54P+15Cr+7Mo)+150B)+20°C; pickling and cold-rolling the steel slab; and annealing the steel slab at a temperature of 7000C to 86O0C. [12]
Advantageous Effects
[13] According to the present invention, a high-strength DP steel having excellent surface properties and mechanical properties can be manufactured. [14]
Brief Description of the Drawings
[15] FIG. 1 is a picture illustrating the shape of oxides of an Inventive Steel and a
Comparative Steel, respectively depending on addition or non-addition of Sb; and
[16] FIG. 2 is a graph illustrating the size distribution of oxides formed on a surface of an
Inventive Steel. Best Mode for Carrying Out the Invention
[17] Exemplary embodiments of the present invention will now be described in detail.
[18] The inventors have conducted a research on a method for manufacturing a high- strength dual phase (DP) steel which can be more easily manufactured with superior mechanical properties and surface quality than a conventional high-strength DP phase. In the course of the research, the inventors have discovered that alloy elements and hot-rolling coiling temperature of a steel can be adequately regulated to suppress increase in diameter of oxide grains on a surface of a steel sheet, thereby ensuring excellent surface quality and improving mechanical properties.
[19]
[20] Hereinafter, an element content range of a steel according to the present invention will be described.
[21]
[22] Carbon (C) is preferably in a range of 0.01 to 0.2%.
[23] C increases strength of the steel sheet, and is very important in attaining a composite structure consisting of ferrite and martensite. C added at less than 0.01% does not ensure desired strength of the present invention. Meanwhile, C added at more than 0.2% is highly likely to degrade tensile strength and weldability. Thus, C may be added in an amount of 0.01 to 0.2%.
[24]
[25] Silicon (Si) is preferably in a range of 0.01 to 1.5%. [26] Si is beneficial for ensuring strength while not impairing ductility of the steel sheet.
Also, Si facilitates ferrite formation, and martensite formation by increasing C concentration of non-transformed austenite. Si added at less than 0.01% does not bring about the above effects. On the other hand, Si added at more than 1.5% is highly likely to deteriorate surface properties and weldability. Thus, Si may be added in an amount of 0.01 to 1.5%.
[27]
[28] Manganese (Mn) is preferably in a range of 0.2 to 4.0%.
[29] Mn is greatly effective for strengthening solid solution and facilitates formation of a composite structure consisting of ferrite and martensite. Mn added at less than 0.2% does not yield effects enough to realize high strength. Meanwhile, Mn added at more than 4.0% is highly likely to deteriorate weldability and hot-rolling properties. Thus, Mn may be added in an amount of 0.2 to 4.0%.
[30]
[31] Phosphor (P) is preferably in a range of 0.001 to 0.1 % .
[32] P is a major solid solution-strengthening element added to increase strength, together with Mn. P added at less than 0.001% does not bring about desired effects. Meanwhile, P added at more than 0.1% impairs weldability and leads to great difference in properties of the steel due to central segregation occurring during continuous casting. Thus, P may be added in an amount of 0.001 to 0.1%.
[33]
[34] Sulfer (S) is preferably added in an amount up to 0.03%.
[35] S is inevitably added in manufacturing the steel and thus the S amount may not exceed 0.03%.
[36]
[37] Aluminum (Al) is preferably in a range of 0.01 to 1.5%.
[38] Al is added to deoxidize a steel typically but increase ductility according to the present invention. Also, Al inhibits carbide from being formed during austempering and increases strength. Al added at less than 0.01% does not yield sufficient effects. On the other hand, Al added at more than 1.5% accelerates internal oxidation during annealing of the cold-rolled steel to prevent alloying of GA-plated layer, thereby requiring a high alloying temperature. Thus, Al may be added in an amount of 0.01% to 1.5%.
[39]
[40] Nitrogen (N) is preferably in a range of 0.001 to 0.03%. [41] N is an effective element for stabilizing austenite. N added at less than 0.001% does not yield such an effect. Meanwhile, N, when added at more than 0.03%, binds with Al to produce a coarse AlN, thereby undermining mechanical properties. Thus, the N amount may not exceed 0.03%.
[42]
[43] Boron (B) is preferably in a range of 0.0002 to 0.005%.
[44] B is a grain boundary- strengthening element which enhances fatigue properties of a spot welding portion and prevents brittleness of P grain boundary. Also, in manufacturing the steel with high Al and Si contents, B improves hot ductility. In addition, B delays austenite from being transformed into perlite in a cooling process during annealing. B added at more than 0.005% causes an excessive amount of B to be concentrated on a surface of the steel, thus degrading coating adhesion. Therefore, to achieve desired effects, B should be added in an amount of at least 0.0002% but the B amount exceeding 0.005% drastically undermines workability and deteriorates surface properties of the coated steel sheet. Thus, B may be added in an amount of 0.0002 to 0.005%.
[45]
[46] Chrome (Cr) is preferably in a range of 0.01 to 2.0%.
[47] Cr is added to enhance hardenability and ensure high strength. The Cr amount less than 0.01% does not assure such effects. On the other hand, the Cr amount exceeding 2.0% yields no further effect and is highly likely to degrade ductility. Thus, Cr may be added in an amount of 0.01 to 2.0%.
[48]
[49] Molybdenum (Mo) is preferably in a range of 0.005 to 0.5%.
[50] Mo is added to improve work embrittlement and coating properties. However, the
Mo amount less than 0.005% does not ensure desired effects. Meanwhile, the Mo amount exceeding 0.5% does not lead to much improved effect and is economically disadvantageous. Thus, Mo may be added in an amount of 0.005 to 0.5%.
[51]
[52] Antimony (Sb) is preferably in a range of 0.005 to 0.1%.
[53] Sb itself does not form an oxidation layer at a high temperature but is concentrated on a surface of the steel and a crystal grain boundary thereof to prevent the elements from spreading onto the surface. This as a result suppresses formation of an oxide. Also, Sb is remarkably effective in inhibiting selective oxidation from being developed along a grain boundary of the steel sheet. Sb is added to inhibit penetration of various oxides formed along the grain boundary of the steel sheet surface in a hot-rolling process due to a considerable amount of Si, Mn, and Al. When a grain boundary oxide of the hot-rolled steel sheet has a depth exceeding lμm, the oxide remains inside the metal even after prickling, thereby causing various scale defects in a following cold- rolling process. Therefore, it is important to regulate the depth of the grain boundary oxide of the hot-rolled steel sheet to \μm or less. Sb added to effectively inhibit the selective grain boundary oxidation of the hot-rolled steel sheet is also noticeably effective in suppressing scale defects. Sb is added to suppress formation of the oxide in an annealing due to a great amount of Si, Mn, and Al to thereby improve coating properties. Particularly, Sb when added in conjunction with Mn and B effectively inhibits coarsening of a surface oxide layer.
[34]
[55] In a case where an annealed oxide is grown coarsely, the oxide is deposited repeatedly on the roll, thereby causing dents on the surface of the cold-rolling steel and coating steel. Here, Sb added to inhibit the surface oxide is remarkably effective in suppressing such dent defects. Sb added in an appropriate amount enhances strength and ductility of the steel sheet, thereby improving mechanical properties thereof.
[56]
[57] In addition to Sb, elements such as Sn, Se, and Y could be added to achieve similar effects but Sn is less capable of inhibiting the grain boundary oxidation of the hot- rolled steel sheet. Moreover, Se and Y are more likely to be concentrated on the surface of the steel than other elements and also may coarsen the oxide since the oxide is formed under SiO and Al O formed on the surface of the steel.
2 2 3
[58]
[59] Therefore, Sb added is superbly effective in inhibiting surface concentration of MnO,
SiO , and Al O in annealing the cold-rolled steel sheet and serves to improve
2 2 3 mechanical properties. Sb should be added in an amount of at least 0.005% but, when added beyond a specific limit, does not ensure any further effects. Therefore, the Sb amount may not exceed 0.1%.
[60]
[61] In designing an alloy of the steel sheet having the element content range as described above may satisfy a relation of 5<(7Mn+4Si+2Al+18B)/(4C+3P+20Sb)<18.
[62] Mn, Si, Al, and B characteristically form concentration on the surface of the steel in annealing. A greater amount of concentration of these elements degrades coating properties. Meanwhile, C, P, and Sb are prone to segregation at the crystal grain boundary. With segregation of these elements such as C, P, and Sb, the concentrated elements are blocked off the diffusion of Mn, Si, Al and B on the grain boundary, thus contributing to surface quality.
[63]
[64] However, too much elements segregated at the grain boundary deteriorate properties such as elongation. On the other hand, too much elements prone to surface concentration undermine surface properties, thus required to be adequately regulated in the amount. The above relation indicates an empirical numerical value capable of preventing deterioration of the steel characteristics and ensuring superior surface quality depending on surface concentration behavior and segregation behavior of the elements. For example, in a case where a value calculated by the relation is smaller than 5, the steel is degraded in mechanical properties. On the other hand, in a case where a value calculated by the relation is greater than 18, the steel cannot attain desired surface quality.
[65]
[66] According to the present invention, the steel sheet containing the elements described above may further include at least one of 0.01 to 1.0% Go, 0.001 to 0.1% Zr, 0.001 to 0.1% Ti, 0.001 to 0.1% Nb, 0.0005 to 0.040% La, 0.0005 to 0.040% Ce and 0.0005 to 0.030% Ca.
[67]
[68] Cobalt Cb is preferably in a range of 0.01 to 1.0% .
[69] Cb is added to boost strength of the steel. Cb inhibits formation of the oxide in high- temperature annealing, thereby improving wettability against a molten zinc steel sheet during molten coating. To achieve these effects, Cb should be added in an amount of at least 0.01%. Meanwhile, Cb added beyond a specific limit significantly reduces elongation of the steel. Thus, the Cb amount may not exceed 1.0%.
[70]
[71] Zirconium (Zr) is preferably in a range of 0.001 to 0.1%
[72] Zr is solved in a columnar grain boundary to increase a melting temperature of a Al- concentrated and low-melting point compound, thereby blocking formation of a liquid film at a temperature of 13000C or less and strengthening the columnar grain boundary. Zr added at less than 0.001% does not ensure such effects. On the other hand, Zr added at more than 0.1% does not yield any further effects. Therefore, Zr may be added in an amount of 0.001 to 0.1%. [74] Titanium (Ti) and Niobium (Nb) are preferably in a range of 0.001 to 0.1%, respectively.
[75] Ti and Nb are effective in increasing strength of the steel sheet and attaining fine grain size. Ti and Nb added at less than 0.001% do not ensure such effects. Meanwhile, Ti and Nb added at more than 0.1% may raise manufacturing costs and deteriorate ferrite ductility due to excessive precipitate. Therefore, Ti and Nb may be added in an amount of 0.001 to 0.1%, respectively.
[76]
[77] Lanthanum (La) and cerium (Ce) are preferably in a range of 0.0005 to 0.04%, respectively.
[78] La and Ce decrease the size and amount of columnar grains which cause em- brittlement to the grain boundary and increase the amount of equiaxed grains with superior high-temperature ductility, thereby enhancing hot-rolling workability of a cast structure. Also La and Ce are segregated at the grain boundary to form a compound with P and S which degrade rupture strength of the grain boundary, thereby ensuring P and S to have less negative effects. However, La and Ce added at less than 0.0005% do not bring about any effects. On the other hand, La and Ce added at more than 0.04% yield no further effects. Therefore, La and Ce may be added in an amount of 0.0005 to 0.04%, respectively.
[79]
[80] Ca is preferably in a range of 0.0005 to 0.03%.
[81] Calcium Ca forms a compound together with a non-metallic inclusion such as MnO, and MnS in a molten steel to spherodize the non-metallic inclusion. This accordingly increases rupture strength of the columnar grains, allows the steel sheet to be less prone to the flange crack and increases bore expandability. However, the Ca amount exceeding 0.03% yields no further effect. Thus Ca may be added in an amount of 0.0005 to 0.030%.
[82]
[83] According to the present invention, the steel sheet contains the balance of Fe and unavoidable impurities.
[84]
[85] According to the present invention, an oxide layer on the surface of the steel sheet has a thickness of lμm or less.
[86] The oxide layer formed on a metal surface in an annealing process after cold-rolling serves as an impediment between a metal matrix and a coating layer in the case of coating, thereby degrading coating adhesion. The oxide layer grown to a thickness exceeding 1/M causes dent defects and coating defects due to detachment of the oxide. Therefore, the oxide layer, when formed with a uniform but not great thickness in the annealing, advantageously ensures better quality in the coating layer. According to the present invention, Sb added in an amount of 0.005 to 0.1% is not oxidized but concentrated on a metal surface layer to inhibit oxidization reaction. This ensures the oxide layer to be formed with a uniform thickness of 1/M or less.
[87]
[88] Hereinafter, a method for manufacturing a steel sheet containing the element content range as above will be described in detail.
[89] First, a steel slab having a composition as described above is reheated at 1100 to
125O0C. At a reheating temperature of less than 11000C, the steel slab is not uniform in structure and has Ti and Nb re-solved insufficiently therein. Meanwhile, at a reheating temperature of more than 125O0C, an oxidized scale is formed and a great amount of oxides such as SiO , MnO, and Al O are generated at a boundary with the metal and
2 2 3 inside the metal, thereby impairing surface quality. Therefore, the reheating temperature may range from 1100 to 125O0C.
[90]
[91] Thereafter, the steel slab is hot finish rolled at a temperature ranging from an Ar transformation point to 95O0C. The steel slab hot finish rolled at a temperature of an Ar transformation point is very likely to become more highly resistant to hot deformation and may have manufacturing problems. On the other hand, the steel slab hot finish rolled at more than 95O0C experiences too thick an oxidized scale and is very likely to be coarsened. Therefore, the hot finish rolled temperature may range from an Ar transform to 95O0C.
[92]
[93] The hot finish rolling is followed by hot-rolling coiling. Here, a coiling temperature
(CT) should satisfy a relation of
CT=(700-(30Mn+18Si+54P+15Cr+7Mo)+150B)+20°C. According to the present invention, elements such as Mn, Si, P, Cr, and Mo are added as an alloy element associated with formation of martensite out of microstructures. However, these elements are more likely to experience segregation at a higher coiling temperature. In this case, the added Mn, Si, P, Cr, and Mo achieve less effect, thereby not assuring desired strength and workability. [95] The above relation has been devised empirically as a means to attain desired strength and workability depending on the coiling temperature and the amount of the alloy elements. The coiling process described above ensures high-quality steel to be obtained.
[96]
[97] A hot-rolled sheet manufactured by the above process is prickled, and cold-rolled to a desired thickness, and then recrystalized and annealed at a temperature of 700 to 86O0C to remove microstructural defects. At an annealing temperature less than 7000C, annealed oxides are formed minutely so that the added Sb does not yield noticeable effects. Meanwhile, at an annealing temperature exceeding 86O0C, oxides are grown excessively so that the surface concentration can not be sufficiently inhibited. Mode for the Invention
[98] Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
[99] [Example]
[100] A steel slab containing elements as noted in Table 1 below was heated at 12000C to be produced. Then the steel slab was hot rolled and coiled at a temperature(+20°C) satisfying relation 2 and cold-rolled to manufacture a cold-rolled steel sheet. Subsequently, the cold rolled steel strip was annealed and heat treated in an N -10%H O atmosphere at a temperature of 780 to 83O0C and at a heating rate of 3°C/sec per 90 seconds. Thereafter, the steel strip was coated in a Zn bath having a temperature of 46O0C and an Al range of 0.12 to 0.19%, and the coated steel was alloyed and heat- treated at a temperature of 540 to 56O0C for 24 seconds. Then, the steel sheet was observed to identify surface quality.
[101]
[102] In Table 2 below, the category of coating appearance includes non-coating, O for having no other coating defects and a defect name for any defect that occurs. Also, to evaluate the category of coating adhesion, the coating sheet was cut into a size of 2Qnmx5Onm and subjected to a 60° bending test. Then, the coating sheet was straightened out to tape a bent portion. Here, a coating layer detached had a width measured as below.
[103] © : when no coating was detached or a coating had a width less than lmm
[104] O : when the coating detached had a width ranging from 1 to less than 3mm
[105] Δ : when the coating detached had a width ranging from 3 to less than 5mm
[106] x : when the coating detached had a width of 5mm or more [107] Table 1
[Table 1] [Table ]
Figure imgf000014_0001
Figure imgf000015_0001
[108] [109] [110] Table 2
[Table 2] [Table ]
Figure imgf000016_0001
[111]
[112] As seen in Table 2, Inventive Steels 1 to 11 satisfying the element content range and manufacturing method of the present invention can be manufactured as a high strength DP steel superior in coating surface properties and strength elongation balance (TSxEl).
[113]
[114] However, Comparative steels 12 to 16 not satisfying the element content range and manufacturing method of the present invention were found inferior in terms of coating surface properties and strength elongation balance.
[115]
[116] Moreover, FIG. 1 shows the shape of oxides of Inventive Steel 11 and Comparative Steel 16, respectively depending on addition or non-addition of Sb. FIG. IA denotes Comparative Steel 16 and FIG. IB denotes Inventive Steel 11. As shown in FIG. 1, Inventive Steel 11 shows an oxide whose grains are noticeably smaller. That is, the oxide growth is inhibited due to addition of Sb.
[117] In addition, FIG. 2 shows the size distribution of oxides formed on a surface of Inventive Steel 11. In Inventive Steels, the oxides formed on a metal surface in an annealing process have sizes stably distributed around \μm or less. That is, the minute oxides uniformly distributed serve to form a more uniform coating layer.
[118]
[119]
[120]
[121]

Claims

Claims
[1] A high strength Zn-coated steel sheet having excellent mechanical properties and surface quality comprising, by weight:
0.01 to 0.2% C, 0.01 to 1.5% Si, 0.2 to 4.0% Mn, 0.001 to 0.1% P, <0.03% S, 0.01 to 1.5% Al, 0.001 to 0.03% N, 0.0002 to 0.005% B, 0.01 to 2.0% Cr, 0.005 to 0.5% Mo, 0.005 to 0.1% Sb, and the balance of Fe and unavoidable impurities, the steel sheet satisfying a relation of
5<(7Mn+4Si+2Al+18B)/(4C+3P+20Sb)<18 and having an oxide layer with a thickness of 1/M or less on a surface thereof.
[2] The high strength Zn-coated steel sheet of claim 1, wherein a critical oxide thickness of the steel sheet is 1/M or less in a hot-rolling process.
[3] The high strength Zn-coated steel sheet of claim 1, wherein the oxide layer is formed in an annealing process after cold-rolling.
[4] The high strength Zn-coated steel sheet of claim 1, further comprising, by weight, at least one of 0.01 to 1.0% Cb, 0.001 to 0.1% Zr, 0.001 to 0.1% Ti, 0.001 to 0.1% Nb, 0.0005 to 0.040% La, 0.0005 to 0.040% Ce and 0.0005 to 0.030% Ca.
[5] A method for manufacturing a high strength Zn-coated steel sheet having excellent mechanical properties and surface quality, the method comprising: reheating a steel slab to 1100 to 125O0C, the steel slab comprising, by weight:
0.01 to 0.2% C, 0.01 to 1.5% Si, 0.2 to 4.0% Mn, 0.001 to 0.1% P, <0.03% S,
0.01 to 1.5% Al, 0.001 to 0.03% N, 0.0002 to 0.005% B, 0.01 to 2.0% Cr, 0.005 to 0.5% Mo, 0.005 to 0.1% Sb, and the balance of Fe and unavoidable impurities, the steel slab satisfying a relation of
5<(7Mn+4Si+2Al+ 18B)/(4C+3P+20Sb)< 18 ; hot rolling the steel slab; coiling the steel slab at a coiling temperature satisfying a relation of
CT=(70& (30Mn+ 18Si+54P+ 15Cr+7Mo)+ 150B )+20°C; pickling and cold-rolling the steel slab; and annealing the steel slab at a temperature of 7000C to 86O0C.
[6] The method of claim 5, wherein the steel slab further comprises, by weight, at least one of 0.01 to 1.0% Cb, 0.001 to 0.1% Zr, 0.001 to 0.1% Ti, 0.001 to 0.1% Nb, 0.0005 to 0.040% La, 0.0005 to 0.040% Ce and 0.0005 to 0.030% Ca.
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