EP2097546A1 - Zn-coated steel sheet having excellent surface quality and the method for manufacturing the same - Google Patents

Zn-coated steel sheet having excellent surface quality and the method for manufacturing the same

Info

Publication number
EP2097546A1
EP2097546A1 EP07851682A EP07851682A EP2097546A1 EP 2097546 A1 EP2097546 A1 EP 2097546A1 EP 07851682 A EP07851682 A EP 07851682A EP 07851682 A EP07851682 A EP 07851682A EP 2097546 A1 EP2097546 A1 EP 2097546A1
Authority
EP
European Patent Office
Prior art keywords
steel sheet
steel
coated steel
temperature
amount
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP07851682A
Other languages
German (de)
French (fr)
Inventor
Il Ryoung Sohn
Seung Bok Lee
Kwang Geun Jin
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Posco Holdings Inc
Original Assignee
Posco Co Ltd
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 Co Ltd filed Critical Posco Co Ltd
Publication of EP2097546A1 publication Critical patent/EP2097546A1/en
Withdrawn legal-status Critical Current

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Classifications

    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to a high strength thin steel sheet generally used for an inner panel or outer panel of an automobile, and more particularly, to a Zn-coated steel sheet with more excellent formability than that of a conventional high strength steel and an excellent surface quality in a process of manufacturing a high strength steel sheet and a method of manufacturing the same.
  • a steel sheet is manufactured by adding Si, Mn, Ti, Nb, and Al, which are elements strengthening material quality. Most of elements are more easily oxidized than Fe, which cause a surface concentration of oxide in a cold annealing process.
  • 2002-294397, and 2002-155317 disclose methods of improving coating properties by adding certain elements such as Cr, Sb, and Sn.
  • Japanese Patent Laid-Open Publication No. 2001-288550 provides a method of suppressing agglomerates formed on a surface in a cold annealing process by pre-oxidized hot coils before cold rolling.
  • Such methods have problems in which an effect of adding certain elements is unclear and consideration for metallurgical behavior is unclear.
  • An aspect of the present invention provides a Zn-coated steel sheet having fine surface qualities by appropriately controlling alloy elements and a hot coiling temperature of steel in a high strength thin steel sheet with excellent workability and a method of manufacturing the Zn-coated steel sheet.
  • a Zn-coated steel sheet with excellent surface quality including, by weight percent, C: 0.04 to 0.25%, Mn: 0.2 to 2.5%, Si: 0.01 to 1.7%, Al: 0.01 to 1.5%, P: 0.01 to 0.1%, S: 0.02% or less, and Sb: 0.001 to 0.1%, residual Fe, and unavoidable impurities, wherein the Si and Al satisfy 0.5 ⁇ Si+Al ⁇ 2.0 and a thickness of an oxide layer on a surface thereof is 1 D or less.
  • a method of manufacturing a Zn-coated steel sheet with excellent surface quality including: reheating and hot rolling a steel slab at a temperature from 1100 to 125O 0 C, the steel slab including, by weight percent, C: 0.04 to 0.25%, Mn: 0.2 to 2.5%, Si: 0.01 to 1.7%, Al: 0.01 to 1.5%, P: 0.01 to 0.1%, S: 0.02% or less, and Sb: 0.001 to 0.1%, residual Fe, and unavoidable impurities, in which the Si and Al satisfy 0.5 ⁇ Si+Al ⁇ 2.0; coiling the steel slab at a temperature from 450 to 75O 0 C; pickling and cold rolling the steel slab; and annealing the steel slab at a temperature within a range from 700 to 86O 0 C.
  • FIG. 1 is a graph illustrating distribution of some metal elements of oxides on a surface for each type of steel according to whether Sb is added or not;
  • FIG. 2 illustrates photographs illustrating a size of surface oxides for each type of steels according to whether Sb is added or not;
  • FIG. 3 illustrates photographs illustrating a size of surface oxides for each type of steels according to a hot rolling and coiling temperature
  • FIG. 4 illustrates photographs illustrating an external shape of Zn-coating for each type of steels according to whether Sb is added or not.
  • the present invention is provided based on a result of research that coated surface may be improved by properly controlling alloy elements of steel and a hot coiling temperature while researching a method of improving the coated surface quality in a high strength steel sheet with excellent workability.
  • An amount of C may be 0.04 to 0.25%
  • C in steel is concentrated on austenite while two phase regions annealing and rapid cooling and austempering bainite reverse, thereby lowering a transformation temperature of martensite in austenite.
  • the amount is less than 0.04%, since grains are grown and a solid solution strengthening effect and precipitation strengthening effect due to C are reduced, it may be difficult to provide enough tensile strength.
  • the amount of C may be limited to be from 0.04 to 0.25%.
  • An amount of Mn may be from 0.2 to 2.5%.
  • Mn has an effect on solid solution strengthening.
  • the amount of Mn is less than 0.2%, the effect is insignificant.
  • the amount of Mn may be limited to be from 0.2 to 2.5%.
  • An amount of Si may be from 0.01 to 1.7%.
  • Si in steel is capable of increasing strength of the steel without decrease of malleability and may be added to 0.01% or more to provide strength.
  • Si oxide is concentrated on a surface of a steel sheet in annealing at a high temperature in a continuous annealing process and this oxide reduces the wettability of Zn on the surface of the steel sheet during a continuous hot-dip galvanizing process. And also welding characteristics of the steel may be greatly deteriorated by the high amount of Si in the steel.
  • An amount of Al may be from 0.01 to 1.5%.
  • Al is added to improve malleability. Al is added to suppress a formation of carbides formed in an austempering process and to improve strength. When the amount is less than 0.01%, it is difficult to obtain the effect. When more than 1.5%, internal oxidation is increased in annealing a cold rolled steel sheet in such a way that an alloying process of GA coating is prevented and a high alloying temperature is required. Accordingly, the amount of Al may be limited to be from 0.01 to 1.5%.
  • An amount of P may be from 0.01 to 0.1 % .
  • P in steel is a representative solid solution element added to improve strength, together with Mn.
  • the amount is less than 0.01%, it is difficult to obtain an effect.
  • welding characteristics may be deteriorated and a material deviation for each portion of steel may be increased due to central segregation occurring in continuous casting.
  • An amount of S may be from 0.02% or less.
  • S in steel is an element unavoidably added to manufacture the steel.
  • An upper limit of the amount may be limited to be 0.02% or less.
  • An amount of Sb may be from 0.001 to 0.1 % .
  • Sb is most important element in the present invention. Though Sb itself does not form an oxide layer at a high temperature, a diffusion of elements in steel is suppressed, thereby suppressing a formation of oxides. Also, Sb has a notable effect on suppressing selective oxidation growing along a grain boundary of a steel sheet. Adding Sb suppresses penetration of various oxides formed along the grain boundary of the surface of the steel sheet in a hot rolling process, which is caused by a large amount of Si, Mn, and Al. Generally, when a depth of the oxides on the grain boundary of the hot rolled steel sheet is more than 1 D, residual oxides in a metal after pickling causes various scale defects in a next cold rolling process.
  • the suppressing the selective oxidation of the boundary of the hot rolled steel sheet by adding Sb has a great effect on suppressing the scale defects.
  • the adding Sb improves coating properties by suppressing the formation of the oxides in the annealing process due to the large amount of Si, Mn, and Al, and more particularly, effectively suppresses coarsening of the oxide layer on the surface when Mn and B are compositely added.
  • annealed oxides coarsely grow, the oxides are repeatedly stacked on a roll and cause dents on a surface of a cold rolled material or a coating material.
  • the suppressing the oxides on the surface by adding Sb has a great effect on suppressing the dents.
  • Si and Al may satisfy 0.5 ⁇ Si+Al ⁇ 2.0.
  • the sum of Si and Al may be limited to be from 0.5 to 2.0wt%.
  • the steel sheet including the elements further includes one or more of Co: 0.01 to 1.0%, B: 0.0002 to 0.002%, Zr: 0.0005 to 0.1%, Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, La: 0.0005 to 0.040%, Mo: 0.005 to 0.5%, Ce: 0.0005 to 0.040%, and Ca: 0.0005 to 0.030%.
  • An amount of Co may be from 0.01 to 1.0%.
  • Co in steel is added to improve strength of the steel. Since suppressing a formation of oxides in annealing at a high temperature, wettability of a steel sheet in a continuous hot-dip coating process may be improved. To obtain the effect, the amount should be 0.01% or more. However, when Co is added more than a certain limit, an elongation rate of the steel is greatly reduced. Accordingly, an upper limit may be 1.0%.
  • An amount of B may be from 0.0002 to 0.002%.
  • [62] B in steel improves fatigue properties of a spot welding part and prevents brittleness of a P grain boundary. Also, in a process of manufacturing steel containing a large amount of Al and Si, there is an effect of improving high temperature malleability. To obtain the effect, the amount should be 0.0002% or more. However, when more than 0.002%, workability is rapidly decreased and surface properties are deteriorated. Accordingly, the amount may be limited to be from 0.0002 to 0.002%.
  • An amount of Zr may be from 0.0005 to 0.1 %.
  • Zr in steel is in a solid solution with a grain boundary of columnar dendrites and increases a melting temperature of a low melting point compound in which Al is concentrated, thereby preventing a liquid film at a temperature of 1300 0 C or less and strengthening the grain boundary of the columnar dendrites.
  • the amount is less than 0.0005%, it is difficult to provide the effect.
  • the amount is more than 0.1%, it is difficult to provide more improved effect. Accordingly, the amount may be limited to be from 0.0005 to 0.1%.
  • Amounts of Ti and Nb may be from 0.001 to 0.1%, respectively.
  • Ti and Nb in steel have an effect on increasing strength of a steel sheet and grain size refinement.
  • the amounts of the Ti and Nb are less than 0.001%, it is difficult to provide the effect.
  • the amounts are more than 0.1%, due to an increase of manufacturing costs and an excessive amount of precipitates, malleability of ferrites may be decreased. Accordingly, the respective amounts may be limited to be from 0.001 to 0.1%.
  • Amounts of La and Ce may be from 0.0005 to 0.04%, respectively.
  • La and Ce in steel reduce a size and amount of columnar dendrites causing brittleness of a grain boundary and increases an amount of equiaxed dendrites having excellent high temperature malleability in such a way that hot workability of a cast structure.
  • La and Ce reduce a bad effect of P and S, which are segregated from the grain boundary and decrease fracture strength of the grain boundary, by forming a compound of P and S.
  • the amounts of La and Ce are less than 0.0005%, re- spectively, there is no effect. When the amounts are more than 0.04%, there is no more than the effect. Accordingly, the respective amounts of La and Ce may be limited from 0.0005 to 0.04%.
  • An amount of Mo may be from 0.005 to 0.5%.
  • Mo in steel is added to improve work brittleness and coating properties.
  • amount is less than 0.005%, a predetermined effect is not shown.
  • amount is more than 0.05%, not only the effect is greatly decreased but also it is economically disadvantageous.
  • the amount may be limited to be from 0.005 to 0.05%.
  • An amount of Ca may be from 0.0005 to 0.03%.
  • Calcium Ca forms a compound together with a non-metallic inclusion such as
  • Ca may be added in an amount of 0.0005 to 0.030%.
  • a thickness of an oxide layer on a surface of a steel sheet is limited to be 1 D or less.
  • the oxide layer formed on a surface of a metal by an annealing process after cold rolling acts as an obstacle between a steel and a coating layer, which obstructs coating adhesion.
  • a coating layer which obstructs coating adhesion.
  • the oxide layer grows to have a thickness more than 1 D, dents and coating defects occur due to separation of oxides. Accordingly, it is advantageous for providing quality of the coating layer that the oxide layer is uniformly formed.
  • 0.005 to 0.1% of Sb is added and concentrated on a surface of a metal instead of being oxidized in such a way that oxidation is suppressed, thereby forming the oxide layer uniformly and suppressing the thickness thereof to be 1 D or less.
  • a steel slab including the composition described above is reheated at a temperature of 1100 to 125O 0 C.
  • the temperature is less than 1100 0 C, structure equalization and re-solid solution of Ti and Nb are not enough.
  • the temperature is more than 125O 0 C, ' a larg °e amount of oxides such as SiO 2 , MnO, and Al 2 O 3 are formed at an interface of oxide scales and a metal and in the metal, which deteriorates surface quality.
  • the reheating temperature may be limited to be from 1100 to 125O 0 C.
  • finish hot rolling is performed at a temperature from Ar transformation point to 95O 0 C.
  • a temperature of the finish hot rolling is less than the Ar transformation point, hot transformation resistance may be rapidly increased and manufacturing defects may occur.
  • the temperature is more than 95O 0 C, not only excessively thick oxide scales may occur but also the steel sheet may be coarsened. Accordingly, the finish hot rolling temperature may be limited to be from the Ar transformation point to 95O 0 C.
  • the steel slab is coiled at a temperature from 450 to
  • a limitation of the coiling temperature is very important to embody the effect of adding Sb in the present invention.
  • Si, Mn, and Al in steel could react to oxide scales of FeO after the coiling and form oxides at an interface between scales and a metal. It has a great effect on concentration of elements in a top surface metal layer whether the oxides of Si, Mn, and Al.
  • the hot coiling temperature may be limited to be from 450 to 75O 0 C.
  • a hot rolled steel sheet formed by the described process is pickled, cold rolled at a target thickness, and annealed at a temperature from 700 to 86O 0 C to remove recys- tallization and microstructure defects.
  • the annealing temperature is less than 700 0 C, since development of annealed oxides is small, the effect of adding Sb is not distinguishable.
  • the temperature is more than 86O 0 C, since oxides are excessively grown, it is impossible to suppress surface oxides enough by adding Sb.
  • a steel slab having a composition as shown in following Table 1 was extracted from heating furnace at a temperature of 1200 0 C, coiled at a temperature shown in Table 2, and cold rolled, thereby manufacturing a steel sheet.
  • a width of separated fragment was from 1 to 3 mm
  • FIG. 1 illustrates distribution of surface oxides of inventive steels 7, 8, and 15 according to whether Sb is added or not. That is, it may be checked that relative amounts of Mn and Al concentrated on a surface by adding Sb are reduced.
  • FIG. 2 illustrates shapes of oxides formed on a surface of cold rolling steel sheet of inventive steel 1 and comparative steel 7 according to whether Sb is added or not.
  • FIG. 2 (a) illustrates the inventive steel 1 and (b) illustrates the comparative steel 7.
  • FIG. 2 in the case of Sb-added steel, it may be known that a grain size of surface oxides is notably reduced.
  • FIG. 4 photographs illustrating ungalvanized bare spots occurring in Sb- added steel (inventive steel 5) and non-Sb steel (comparative steel 6).
  • the inventive steel 5 is shown in (a) of FIG. 4
  • the comparative steel 6 is shown in (b) of FIG. 4.
  • FIG. 4 in the case of the non-Sb steel, a probability of occurrence of ungalvanized bare spots is high.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Coating With Molten Metal (AREA)

Abstract

There is provided a Zn-coated steel sheet with excellent surface quality, which is generally used for inner and outer panels for automobiles. The Zn-coated steel sheet includes, by weight percent, C: 0.04 to 0.25%, Mn: 0.2 to 2.5%, Si: 0.01 to 1.7%, Al: 0.01 to 1.5%, P: 0.01 to 0.1%, S: 0.02% or less, and Sb: 0.001 to 0.1%, residual Fe, and unavoidable impurities, wherein the Si and Al satisfy 0.5<Si+Al<2.0 and a thickness of an oxide layer on a surface thereof is 1 D or less. The Zn-coated steel sheet may have excellent coating quality in a high strength thin steel sheet having excellent workability.

Description

Description
ZN-COATED STEEL SHEET HAVING EXCELLENT SURFACE QUALITY AND THE METHOD FOR MANUFACTURING THE
SAME
Technical Field
[1] The present invention relates to a high strength thin steel sheet generally used for an inner panel or outer panel of an automobile, and more particularly, to a Zn-coated steel sheet with more excellent formability than that of a conventional high strength steel and an excellent surface quality in a process of manufacturing a high strength steel sheet and a method of manufacturing the same.
[2]
Background Art
[3] Recently, due to complication and integration of components for forming an automobile, there are required steel sheets for automobiles, which have not only higher degree of formability but also excellent work brittleness, fatigue properties of welding portion, and excellent coated surface quality.
[4] Generally, to improve formability and strength, a steel sheet is manufactured by adding Si, Mn, Ti, Nb, and Al, which are elements strengthening material quality. Most of elements are more easily oxidized than Fe, which cause a surface concentration of oxide in a cold annealing process.
[5] When such surface oxide concentration occurs, it is easy to deteriorate coating qualities such as ungalvanized bare spots. When surface oxides are coarse, surface oxides may adsorb a hearth roll and cause defects such as micro dents on a surface of a coated steel sheet.
[6]
[7] To solve the described coating defects, Japanese blast furnace companies provide technologies for manufacturing thin steel sheets for deep drawing, which are well- known.
[8] Japanese Patent Laid-Open Publication Nos. 2002-146477, 2001-64750,
2002-294397, and 2002-155317 disclose methods of improving coating properties by adding certain elements such as Cr, Sb, and Sn. Also, Japanese Patent Laid-Open Publication No. 2001-288550 provides a method of suppressing agglomerates formed on a surface in a cold annealing process by pre-oxidized hot coils before cold rolling. However, such methods have problems in which an effect of adding certain elements is unclear and consideration for metallurgical behavior is unclear.
[9] [10] Also, since it is impossible to embody some of conventional art described above in present, general hot rolling-cold rolling-continuous annealing facilities, commercial production is not actually performed.
[H]
Disclosure of Invention Technical Problem
[12] An aspect of the present invention provides a Zn-coated steel sheet having fine surface qualities by appropriately controlling alloy elements and a hot coiling temperature of steel in a high strength thin steel sheet with excellent workability and a method of manufacturing the Zn-coated steel sheet.
[13]
Technical Solution
[14] According to an aspect of the present invention, there is provided a Zn-coated steel sheet with excellent surface quality, the Zn-coated steel sheet including, by weight percent, C: 0.04 to 0.25%, Mn: 0.2 to 2.5%, Si: 0.01 to 1.7%, Al: 0.01 to 1.5%, P: 0.01 to 0.1%, S: 0.02% or less, and Sb: 0.001 to 0.1%, residual Fe, and unavoidable impurities, wherein the Si and Al satisfy 0.5<Si+Al<2.0 and a thickness of an oxide layer on a surface thereof is 1 D or less.
[15]
[16] According to another aspect of the present invention, there is provided a method of manufacturing a Zn-coated steel sheet with excellent surface quality, the method including: reheating and hot rolling a steel slab at a temperature from 1100 to 125O0C, the steel slab including, by weight percent, C: 0.04 to 0.25%, Mn: 0.2 to 2.5%, Si: 0.01 to 1.7%, Al: 0.01 to 1.5%, P: 0.01 to 0.1%, S: 0.02% or less, and Sb: 0.001 to 0.1%, residual Fe, and unavoidable impurities, in which the Si and Al satisfy 0.5<Si+Al<2.0; coiling the steel slab at a temperature from 450 to 75O0C; pickling and cold rolling the steel slab; and annealing the steel slab at a temperature within a range from 700 to 86O0C.
[17]
Advantageous Effects
[18] According to the present invention, there is provided a Zn-coated steel sheet with excellent workability and high strength, which has excellent coating properties.
[19]
Brief Description of the Drawings
[20] FIG. 1 is a graph illustrating distribution of some metal elements of oxides on a surface for each type of steel according to whether Sb is added or not; [21] FIG. 2 illustrates photographs illustrating a size of surface oxides for each type of steels according to whether Sb is added or not;
[22] FIG. 3 illustrates photographs illustrating a size of surface oxides for each type of steels according to a hot rolling and coiling temperature; and
[23] FIG. 4 illustrates photographs illustrating an external shape of Zn-coating for each type of steels according to whether Sb is added or not.
[24]
Best Mode for Carrying Out the Invention
[25] Hereinafter, embodiments of the present invention will be described in detail.
[26] The present invention is provided based on a result of research that coated surface may be improved by properly controlling alloy elements of steel and a hot coiling temperature while researching a method of improving the coated surface quality in a high strength steel sheet with excellent workability.
[27] Hereinafter, a composition range of elements of steel according to the present invention will be described.
[28]
[29] An amount of C may be 0.04 to 0.25%
[30] C in steel is concentrated on austenite while two phase regions annealing and rapid cooling and austempering bainite reverse, thereby lowering a transformation temperature of martensite in austenite. When the amount is less than 0.04%, since grains are grown and a solid solution strengthening effect and precipitation strengthening effect due to C are reduced, it may be difficult to provide enough tensile strength. On the other hand, when more than 0.25%, since the solid solution effect and an amount of residual austenite are increased, the tensile strength may be increased and delayed fracture resistance may occur due to formation of a large amount of the residual austenite. Also, welding characteristics are greatly deteriorated. Accordingly, the amount of C may be limited to be from 0.04 to 0.25%.
[31]
[32] An amount of Mn may be from 0.2 to 2.5%.
[33] Mn has an effect on solid solution strengthening. When the amount of Mn is less than 0.2%, the effect is insignificant. When more than 2.5%, strength of steel is greatly increased due to excessively high hardenability, workability may be decreased and welding characteristics of the steel may be deteriorated. Accordingly, the amount of Mn may be limited to be from 0.2 to 2.5%.
[34]
[35] An amount of Si may be from 0.01 to 1.7%.
[36] Si in steel is capable of increasing strength of the steel without decrease of malleability and may be added to 0.01% or more to provide strength. On the other hand, when adding more than 1.7%, since Si oxide is concentrated on a surface of a steel sheet in annealing at a high temperature in a continuous annealing process and this oxide reduces the wettability of Zn on the surface of the steel sheet during a continuous hot-dip galvanizing process. And also welding characteristics of the steel may be greatly deteriorated by the high amount of Si in the steel.
[37]
[38] An amount of Al may be from 0.01 to 1.5%.
[39] Al in steel is generally added to deoxidize steel. However, in the present invention,
Al is added to improve malleability. Al is added to suppress a formation of carbides formed in an austempering process and to improve strength. When the amount is less than 0.01%, it is difficult to obtain the effect. When more than 1.5%, internal oxidation is increased in annealing a cold rolled steel sheet in such a way that an alloying process of GA coating is prevented and a high alloying temperature is required. Accordingly, the amount of Al may be limited to be from 0.01 to 1.5%.
[40]
[41] An amount of P may be from 0.01 to 0.1 % .
[42] P in steel is a representative solid solution element added to improve strength, together with Mn. When the amount is less than 0.01%, it is difficult to obtain an effect. When more than 0.10%, welding characteristics may be deteriorated and a material deviation for each portion of steel may be increased due to central segregation occurring in continuous casting.
[43]
[44] An amount of S may be from 0.02% or less.
[45] S in steel is an element unavoidably added to manufacture the steel. An upper limit of the amount may be limited to be 0.02% or less.
[46] An amount of Sb may be from 0.001 to 0.1 % .
[47] Sb is most important element in the present invention. Though Sb itself does not form an oxide layer at a high temperature, a diffusion of elements in steel is suppressed, thereby suppressing a formation of oxides. Also, Sb has a notable effect on suppressing selective oxidation growing along a grain boundary of a steel sheet. Adding Sb suppresses penetration of various oxides formed along the grain boundary of the surface of the steel sheet in a hot rolling process, which is caused by a large amount of Si, Mn, and Al. Generally, when a depth of the oxides on the grain boundary of the hot rolled steel sheet is more than 1 D, residual oxides in a metal after pickling causes various scale defects in a next cold rolling process. Accordingly, it is important to control the depth of the oxides on the grain boundary of the hot rolled steel sheet. The suppressing the selective oxidation of the boundary of the hot rolled steel sheet by adding Sb has a great effect on suppressing the scale defects. The adding Sb improves coating properties by suppressing the formation of the oxides in the annealing process due to the large amount of Si, Mn, and Al, and more particularly, effectively suppresses coarsening of the oxide layer on the surface when Mn and B are compositely added. When annealed oxides coarsely grow, the oxides are repeatedly stacked on a roll and cause dents on a surface of a cold rolled material or a coating material. The suppressing the oxides on the surface by adding Sb has a great effect on suppressing the dents.
[48]
[49] Since adding an appropriate amount of Sb has an effect of simultaneously increasing strength and malleability of a steel material, the adding an appropriate amount of Sb is very effective to improve mechanical characteristics. In addition to Sb, a similar effect is shown in Sn, Se, and Y. However, Sn is inferior to Sb in the case of an ability of suppressing the oxidation of the boundary of the hot rolled steel sheet. Also, Se and Y have surface concentrations of themselves greater than that of another element and form oxides beneath SiO 2 and Al 2 O 3 formed on the surface in such a way that the oxides may be coarsened.
[50]
[51] It is possible to have an excellent effect of suppressing occurrence of the surface concentrations of MnO, SiO , and Al 0 while annealing the cold rolled steel sheet and to improve the mechanical characteristics by addition of Sb. To obtain the effect, 0.001% of Sb is required. However, since there is no more improved effect when adding Sb more than a certain limit, an upper limit may be 0.1%.
[52] In an alloy design of the steel sheet having the element range, Si and Al may satisfy 0.5<Si+Al<2.0.
[53]
[54] When a sum of Si and Al is less than 0.5%, it is difficult to expect certain mechanical characteristics. When the sum is more than 2.0%, since there is formed an oxide layer that is an obstacle to form a sound coating layer, it may be difficult to provide coating quality. Accordingly, the sum of Si and Al may be limited to be from 0.5 to 2.0wt%.
[55]
[56] According to an exemplary embodiment of the present invention, the steel sheet including the elements further includes one or more of Co: 0.01 to 1.0%, B: 0.0002 to 0.002%, Zr: 0.0005 to 0.1%, Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, La: 0.0005 to 0.040%, Mo: 0.005 to 0.5%, Ce: 0.0005 to 0.040%, and Ca: 0.0005 to 0.030%.
[57]
[58] An amount of Co may be from 0.01 to 1.0%.
[59] Co in steel is added to improve strength of the steel. Since suppressing a formation of oxides in annealing at a high temperature, wettability of a steel sheet in a continuous hot-dip coating process may be improved. To obtain the effect, the amount should be 0.01% or more. However, when Co is added more than a certain limit, an elongation rate of the steel is greatly reduced. Accordingly, an upper limit may be 1.0%.
[60]
[61] An amount of B may be from 0.0002 to 0.002%.
[62] B in steel, as a grain boundary strengthening element, improves fatigue properties of a spot welding part and prevents brittleness of a P grain boundary. Also, in a process of manufacturing steel containing a large amount of Al and Si, there is an effect of improving high temperature malleability. To obtain the effect, the amount should be 0.0002% or more. However, when more than 0.002%, workability is rapidly decreased and surface properties are deteriorated. Accordingly, the amount may be limited to be from 0.0002 to 0.002%.
[63]
[64] An amount of Zr may be from 0.0005 to 0.1 %.
[65] Zr in steel is in a solid solution with a grain boundary of columnar dendrites and increases a melting temperature of a low melting point compound in which Al is concentrated, thereby preventing a liquid film at a temperature of 13000C or less and strengthening the grain boundary of the columnar dendrites. When the amount is less than 0.0005%, it is difficult to provide the effect. When the amount is more than 0.1%, it is difficult to provide more improved effect. Accordingly, the amount may be limited to be from 0.0005 to 0.1%.
[66]
[67] Amounts of Ti and Nb may be from 0.001 to 0.1%, respectively.
[68] Ti and Nb in steel have an effect on increasing strength of a steel sheet and grain size refinement. When the amounts of the Ti and Nb are less than 0.001%, it is difficult to provide the effect. When the amounts are more than 0.1%, due to an increase of manufacturing costs and an excessive amount of precipitates, malleability of ferrites may be decreased. Accordingly, the respective amounts may be limited to be from 0.001 to 0.1%.
[69]
[70] Amounts of La and Ce may be from 0.0005 to 0.04%, respectively.
[71] La and Ce in steel reduce a size and amount of columnar dendrites causing brittleness of a grain boundary and increases an amount of equiaxed dendrites having excellent high temperature malleability in such a way that hot workability of a cast structure. Also, La and Ce reduce a bad effect of P and S, which are segregated from the grain boundary and decrease fracture strength of the grain boundary, by forming a compound of P and S. However, the amounts of La and Ce are less than 0.0005%, re- spectively, there is no effect. When the amounts are more than 0.04%, there is no more than the effect. Accordingly, the respective amounts of La and Ce may be limited from 0.0005 to 0.04%.
[72]
[73] An amount of Mo may be from 0.005 to 0.5%.
[74] Mo in steel is added to improve work brittleness and coating properties. When the amount is less than 0.005%, a predetermined effect is not shown. When the amount is more than 0.05%, not only the effect is greatly decreased but also it is economically disadvantageous. The amount may be limited to be from 0.005 to 0.05%.
[75]
[76] An amount of Ca may be from 0.0005 to 0.03%.
[77] 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%.
[78] Others in addition to the described elements are Fe and unavoidable impurities.
[79] According to an exemplary embodiment of the present invention, a thickness of an oxide layer on a surface of a steel sheet is limited to be 1 D or less.
[80]
[81] The oxide layer formed on a surface of a metal by an annealing process after cold rolling acts as an obstacle between a steel and a coating layer, which obstructs coating adhesion. In this case, when the oxide layer grows to have a thickness more than 1 D, dents and coating defects occur due to separation of oxides. Accordingly, it is advantageous for providing quality of the coating layer that the oxide layer is uniformly formed. In the present invention, 0.005 to 0.1% of Sb is added and concentrated on a surface of a metal instead of being oxidized in such a way that oxidation is suppressed, thereby forming the oxide layer uniformly and suppressing the thickness thereof to be 1 D or less.
[82] Hereinafter, a method of manufacturing the steel sheet having the composition described above will be described.
[83]
[84] A steel slab including the composition described above is reheated at a temperature of 1100 to 125O0C. When the temperature is less than 11000C, structure equalization and re-solid solution of Ti and Nb are not enough. When the temperature is more than 125O0C, ' a larg °e amount of oxides such as SiO 2 , MnO, and Al 2 O 3 are formed at an interface of oxide scales and a metal and in the metal, which deteriorates surface quality. Accordingly, the reheating temperature may be limited to be from 1100 to 125O0C.
[85] After that, finish hot rolling is performed at a temperature from Ar transformation point to 95O0C. When a temperature of the finish hot rolling is less than the Ar transformation point, hot transformation resistance may be rapidly increased and manufacturing defects may occur. When the temperature is more than 95O0C, not only excessively thick oxide scales may occur but also the steel sheet may be coarsened. Accordingly, the finish hot rolling temperature may be limited to be from the Ar transformation point to 95O0C.
[86]
[87] After the finish hot rolling, the steel slab is coiled at a temperature from 450 to
75O0C. A limitation of the coiling temperature is very important to embody the effect of adding Sb in the present invention. Si, Mn, and Al in steel could react to oxide scales of FeO after the coiling and form oxides at an interface between scales and a metal. It has a great effect on concentration of elements in a top surface metal layer whether the oxides of Si, Mn, and Al.
[88]
[89] As a result of repeated experiments of adding Sb, when the coiling is performed at a temperature lower than 45O0C, the concentration of Si, Mn, and Al in the top surface metal layer is excessive, it is impossible to provide the effect of suppressing oxides due to Sb. When the temperature is more than 75O0C, a depth of internal oxidation of Si, Mn, and Al is excessive, surface roughness and pickling properties are deteriorated. Accordingly, to obtain the effect of adding Sb in an element range of Si, Mn, and Al defined in the present invention, the hot coiling temperature may be limited to be from 450 to 75O0C.
[90]
[91] A hot rolled steel sheet formed by the described process is pickled, cold rolled at a target thickness, and annealed at a temperature from 700 to 86O0C to remove recys- tallization and microstructure defects. When the annealing temperature is less than 7000C, since development of annealed oxides is small, the effect of adding Sb is not distinguishable. When the temperature is more than 86O0C, since oxides are excessively grown, it is impossible to suppress surface oxides enough by adding Sb.
[92]
Mode for the Invention
[93] Hereinafter, embodiments of the present invention will be described in detail.
[94] [Embodiment]
[95] A steel slab having a composition as shown in following Table 1 was extracted from heating furnace at a temperature of 12000C, coiled at a temperature shown in Table 2, and cold rolled, thereby manufacturing a steel sheet.
[96] Subsequently, the steel slab 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. An amount of Mn and Al concentrated on a surface of the steel sheet was checked, which is shown in Table 1.
[97] Also, the surface of the steel sheet was coated with Zn at a temperature of 46O0C and an external shape and coating adhesion thereof were checked. A result of checking coating conditions and coating properties were shown in following Table 2.
[98] In Table 2, an external shape of coating was shown as O in the case where a spot had not been coated and when there had been no coating defect and the name of a defect was written when the defect had occur. Also, to estimate the coating adhesion, a coated sheet was cut into 20 mmxO mm, bent by 60°straightened, an adhesive tape was attached to a bent portion of the coated sheet, and a width of a fragment separated from the bent portion was checked based on references as follows.
[99]
[100] ©: There is no separated fragment or a width of a separated fragment was 1 mm or less,
[101] O: A width of separated fragment was from 1 to 3 mm, and
[102] Δ : A width of separated fragment was from 3 to 5 mm.
[103]
[104] Table 1
[105] Table 2
[106] [107] As shown in Table 2, Inventive examples 1 to 9 manufactured using inventive steels 1 to 8 satisfying the element range of the present invention by the method according to the present invention showed excellent coating surface properties.
[108] [109] However, Comparative examples 5 to 11 manufacturing using comparative steels 9 to 15 out of the element range of the present invention according to the method according to the present invention and Comparative examples 1 to 4 manufactured using inventive steels satisfying the element range of the present invention according to a different method from the method according to the present invention showed deteriorated coating surface properties.
[HO] [111] FIG. 1 illustrates distribution of surface oxides of inventive steels 7, 8, and 15 according to whether Sb is added or not. That is, it may be checked that relative amounts of Mn and Al concentrated on a surface by adding Sb are reduced.
[112] FIG. 2 illustrates shapes of oxides formed on a surface of cold rolling steel sheet of inventive steel 1 and comparative steel 7 according to whether Sb is added or not. In FIG. 2, (a) illustrates the inventive steel 1 and (b) illustrates the comparative steel 7. As shown in FIG. 2, in the case of Sb-added steel, it may be known that a grain size of surface oxides is notably reduced.
[113] Also, in FIG. 3, to check an effect of a hot coiling temperature, with respect to Sb- added steel (inventive steel 8, shapes of surface oxides formed under the same annealing condition when the hot coiling temperature is 4000C (a) and when the hot coiling temperature is 62O0C (b). As shown in FIG. 3, in the case of low temperature coiling, a grain size of the surface oxides is greatly increased, thereby largely damaging the effect of adding Sb.
[114] Also, in FIG. 4, photographs illustrating ungalvanized bare spots occurring in Sb- added steel (inventive steel 5) and non-Sb steel (comparative steel 6). The inventive steel 5 is shown in (a) of FIG. 4, and the comparative steel 6 is shown in (b) of FIG. 4. As shown in FIG. 4, in the case of the non-Sb steel, a probability of occurrence of ungalvanized bare spots is high.

Claims

Claims
[1] A Zn-coated steel sheet with excellent surface quality, the Zn-coated steel sheet comprising, by weight percent, C: 0.04 to 0.25%, Mn: 0.2 to 2.5%, Si: 0.01 to 1.7%, Al: 0.01 to 1.5%, P: 0.01 to 0.1%, S: 0.02% or less, and Sb: 0.001 to 0.1%, residual Fe, and unavoidable impurities, wherein the Si and Al satisfy 0.5<Si+Al<2.0 and a thickness of an oxide layer on a surface thereof is 1 D or less.
[2] The Zn-coated steel sheet of claim 1, wherein a grain boundary oxide of the Zn- coated steel sheet in hot rolling is 1 D or less.
[3] The Zn-coated steel sheet of claim 1, wherein the oxide layer is formed during annealing after cold rolling.
[4] The Zn-coated steel sheet of claim 1, further comprising one or more of, by weight percent, Co: 0.01 to 1.0%, B: 0.0002 to 0.0020%, Zr: 0.0005 to 0.1%, Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, La: 0.0005 to 0.040%, Mo: 0.005 to 0.5%, Ce: 0.0005 to 0.040%, and Ca: 0.0005 to 0.030%.
[5] A method of manufacturing a Zn-coated steel sheet with excellent surface qual ity, the method comprising: reheating and hot rolling a steel slab at a temperature from 1100 to 125O0C, the steel slab comprising, by weight percent, C: 0.04 to 0.25%, Mn: 0.2 to 2.5%, Si: 0.01 to 1.7%, Al: 0.01 to 1.5%, P: 0.01 to 0.1%, S: 0.02% or less, and Sb: 0.001 to 0.1%, residual Fe, and unavoidable impurities, in which the Si and Al satisfy 0.5<Si+Al<2.0; coiling the steel slab at a temperature from 450 to 75O0C; pickling and cold rolling the steel slab; and annealing the steel slab at a temperature within a range from 700 to 86O0C.
[6] The method of claim 5, wherein the Zn-coated steel sheet further comprises one or more of, by weight percent, Co: 0.01 to 1.0%, B: 0.0002 to 0.0020%, Zr: 0.0005 to 0.1%, Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, La: 0.0005 to 0.040%, Mo: 0.005 to 0.5%, Ce: 0.0005 to 0.040%, and Ca: 0.0005 to 0.030%.
EP07851682A 2006-12-27 2007-12-21 Zn-coated steel sheet having excellent surface quality and the method for manufacturing the same Withdrawn EP2097546A1 (en)

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