Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—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
  • C21D8/0405—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 of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—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
• • 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
  • 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
  • C22C38/02—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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0224—Two or more thermal pretreatments
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
  • C23C2/29—Cooling or quenching
• • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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

Definitions

• the present disclosure relates to a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, and a manufacturing method thereof, and more particularly, to a steel sheet hot dip plated with zinc based layer, having excellent bake hardenability and aging resistance, preferably capable of being used as a material for external automobile panels, and a manufacturing method thereof.
• a phenomenon of bake hardenability is a phenomenon in which yield strength is increased due to fixing solid solution carbon and nitrogen, which are activated during the press, onto dislocations at the time of the baking of paint.
• Steel having excellent bake hardenability is easy to form before the baking of paint, and final products thereof have enhanced dent resistance. Therefore, such steel is very ideal as a material for external automobile panels.
• Japanese Patent Publication No. 2005-264176 discloses a steel sheet having a complex phase mainly composed of martensite as a conventional technique for improving workability in a high-strength steel sheet.
• a method of manufacturing a high-strength steel sheet in which a fine Cu precipitate has a grain size of 1 to 100 nm is disclosed.
• Japanese Patent Publication No. 2004-292891 discloses a steel sheet having a complex phase including ferrite as a main phase and residual austenite and bainite and martensite which are low temperature transformation phases as secondary phases, and a method for improving ductility and stretch flangeability of the steel sheet.
• this technique has problems in that it may be difficult to secure plating quality, and to secure surface quality in a process for making steel and a continuous casting process, since large amounts of Si and Al are added to secure the residual austenite phase.
• yield ratio may be high because an initial YS value is high due to transformation induced plasticity.
• Korean Patent Publication No. 10-2012-0073564 discloses a technique for providing a high tensile hot-dip galvanized steel sheet having good workability.
• a steel sheet comprising soft ferrite and hard martensite as a microstructure, and a manufacturing method for improving an elongation and an r value (a Lankford value) of the steel sheet are disclosed.
• this technology has a problem that it is difficult to secure good plating quality, since large amounts of Si are added, and a problem that manufacturing costs increase due to the addition of large amounts of Ti and Mo.
• One of the objects of the present disclosure is to provide a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance, and a manufacturing method thereof.
• a steel sheet hot-dip plated with zinc based layer having excellent bake hardenability and aging resistance, comprises a cold-rolled steel sheet and a zinc based plating layer formed on a surface of the cold-rolled steel sheet, wherein the cold-rolled steel sheet comprises, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al), comprises one or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B), and comprises a remainder of iron (
• a method of manufacturing a steel sheet hot-dip plated with zinc based layer, having excellent bake hardenability and aging resistance comprises reheating a steel slab comprising, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al), comprising one or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B), and comprising a remainder of iron (Fe) and unavoidable impurities; hot-rolling the reheated steel
• the galvanized steel sheet according to an embodiment of the present disclosure may be suitably applied to a material for external automobile panels, because of its excellent bake hardenability and aging resistance.
• the inventors of the present disclosure have conducted intensive research into providing a steel sheet hot-dip plated with zinc based layer securing excellent strength and ductility simultaneously to have excellent formability, as well as excellent bake hardenability and aging resistance, so as to be suitable as a material for external automobile panels.
• a steel sheet hot-dip plated with zinc based layer which satisfies the intended properties by optimally controlling a composition range of a cold-rolled steel sheet, a substrate, and optimizing production conditions thereof.
• the present disclosure has been accomplished based on this finding.
• the steel sheet hot-dip plated with zinc based layer of the present disclosure may include a cold-rolled steel sheet and a zinc based hot-dip plating layer formed on one or both surfaces of the cold-rolled steel sheet.
• a composition of the zinc based hot-dip plating layer is not particularly limited, and may be a pure Zinc plating layer, or a Zinc based alloy plating layer containing Si, Al, Mg, or the like.
• the zinc based hot-dip plating layer may be agalva-annealed layer.
• the alloying element and the preferable content range thereof of the cold-rolled steel sheet as a substrate will be described in detail. It is to be noted in advance that the content of each component described below is on a weight basis unless otherwise specified.
• Carbon may be an indispensable element to be added to secure the desired complex phase in the present disclosure.
• carbon is advantageous for producing a complex phase since martensite may be easily formed as the content of carbon increases.
• yield strength/tensile strength it is necessary to control the content in a proper amount.
• the content of carbon is less than 0.02%, it may be difficult to achieve the desired strength in the present disclosure, and formation of an appropriate level of martensite may be difficult.
• the content thereof exceeds 0.08%, the formation of bainite at the grain boundary may be promoted during cooling after annealing to increase the yield ratio of the steel, and bending and surface defects may be easily caused in machining into automobile parts. Therefore, in the present disclosure, the content of carbon may be controlled to be 0.02 to 0.08%, and more preferably 0.03 to 0.06%.
• Manganese may be an element which improves the hardenability in the complex phase steel, and, in particular, plays an important role in forming martensite.
• the content of manganese is less than 1.3%, the formation of martensite may be impossible, and complex phase steel may be difficult to be produced.
• the content of manganese exceeds 2.1%, martensite may be excessively formed to make a material property unstable, and there may be a problem that the risk of processing crack and strip breakage is significantly increased due to the formation of a band of manganese in the structure.
• the manganese oxide is precipitated on the surface upon annealing, which significantly deteriorates plating characteristics. Therefore, in the present disclosure, the content of manganese may be controlled to be 1.3 to 2.1%, and more preferably to 1.4 to 1.8%.
• Silicon may contribute to an increase in the strength of the steel sheet by solid solution strengthening, but may be not intentionally added in the present disclosure. Further, there may be no problem in securing the properties without adding silicon. However, 0% may be excluded in consideration of an amount that is inevitably added in the manufacturing process. On the other hand, when the content of silicon exceeds 0.3%, there may be a problem that the surface properties of the plating may be poor. Therefore, the content of silicon may be controlled to be 0.3% or less in the present disclosure.
• Chromium may be a component having characteristics similar to manganese, and may be an element added to improve hardenability of steel, and to improve strength of steel.
• chromium may assist in forming martensite.
• chromium since an occurrence of yield stretch YP-El is suppressed by precipitating solid solute carbon to be under a certain level which is proper amount of solute carbon in the steel through forming coarse Cr-based carbides such as Cr23C6 during hot-rolling, chromium may be an element favorable for the production of complex phase steel having a relatively low yield ratio.
• chromium is an element advantageous for manufacturing high strength complex phase steel having a relatively high ductility by relatively reducing ductility drop compared with the increase in strength.
• the content thereof exceeds 1.0%, the martensite structure fraction may be excessively increased to cause a decrease in strength and elongation.
• the content of chromium may be controlled to be 1.0% or less.
• Phosphorus is the most advantageous element in securing strength without significantly impairing formability.
• the possibility of the occurrence of brittle fracture significantly increases when the element is excessively added, the possibility of strip breakage of a slab significantly increases during hot-rolling, and the surface properties of a plated layer may be deteriorated. Therefore, in the present disclosure, the content of phosphorus may be controlled to be 0.1%.
• Sulfur may be an impurity to be inevitably contained in the steel. It may be desirable to control the content of sulfur to be as low as possible. In particular, sulfur in the steel may increase the possibility of generating hot shortness, and the content thereof may be controlled to be 0.01% or less.
• Nitrogen may be an impurity to be inevitably contained in the steel. It may be desirable to control the content of nitrogen as low as possible. However, since the steel refining cost rises sharply to reduce the content of nitrogen, the content thereof may be controlled to be 0.01% or less, a possible range of operation conditions.
• Acid soluble aluminum is an element to be added for grain refinement and deoxidation.
• the content thereof is less than 0.01%, aluminum-killed (Al-killed) steel may not be produced in a normal stable state. Meanwhile, when the content thereof exceeds 0.06%, it is advantageous to increase the strength due to the grain refinement effect.
• the inclusions may be excessively formed. In this case, the possibility of surface defects of a plated steel sheet may increase, and a sharp rise in manufacturing costs may occur. Therefore, in the present disclosure, the content of acid soluble aluminum may be controlled to be 0.01 to 0.06%.
• Molybdenum may be an element added to delay transformation of austenite into pearlite, and to improve ferrite refinement and steel strength. Molybdenum may also assist in improving hardenability of steel. However, when the content of molybdenum exceeds 0.1%, there may be a problem in that manufacturing costs are rapidly increased to lower economical efficiency and to lower ductility of steel. In the present disclosure, the content of molybdenum may be controlled to be 0.1% or less.
• boron may be an element added to prevent secondary work embrittlement caused by phosphorous in the steel. There may be no problem in securing the properties without adding boron. Meanwhile, when the content of boron exceeds 0.003%, there may be a problem that ductility of the steel is lowered. In the present disclosure, the content of boron may be controlled to be 0.003% or less.
• iron (Fe) and unavoidable impurities may be further included as a remainder.
• impurities that are not intended from raw materials or surrounding environments may be inevitably incorporated, such that it may not be excluded.
• impurities are not specifically mentioned in this specification, as they are known to one of ordinary skill in the art.
• the addition of an effective component other than the above-mentioned composition may be not excluded.
• the cold-rolled steel sheet of the present disclosure may include, by area, 90 to 99% of ferrite and 1 to 10% of martensite as a microstructure.
• an area ratio of the martensite is preferably 1 to 10%, more preferably 2 to 5%, by area.
• a ratio (a/b) of an average carbon concentration a in the martensite and an average carbon concentration b in the ferrite located in a virtual circle having a diameter corresponding to a long axis of the martensite at the point of 1/4t of a sheet thickness thereof may be a value of 1.4 or less.
• fine martensite in a ferrite matrix may be appropriately distributed.
• a ratio of the carbon concentration in an interior of martensite and in an interior of ferrite in a periphery of the martensite may be appropriately controlled.
• it may be designed such that the carbon intensively present in martensite can easily diffuse into surrounding ferrite by the conventional baking treatment (about 170°C, about 20 minute).
• the ratio (a/b) of the average carbon concentration exceeds 1.4, the content of the solid solution carbon present in ferrite is too low to secure the desired bake hardenability.
• the ratio (a/b) of the average carbon concentration lowers, the securing of bake hardenability may be relatively high. Therefore, the lower limit is not particularly limited in the present disclosure.
• a ratio (d/c) of an average manganese concentration c in the martensite and an average manganese concentration d in the ferrite located in a virtual circle having a diameter corresponding to a long axis of the martensite at the point of 1/4t of a sheet thickness thereof may be a value of 0.9 or less, more preferably a value of 0.8 or less.
• the ratio (d/c) of the average manganese concentration exceeds 0.9, the content of manganese present in ferrite is too high to facilitate the formation of a manganese band in the structure.
• the possibility of processing cracks in forming may increase due to the decrease in ductility of steel.
• the ratio (d/c) of the average manganese concentration lowers, the securing of ductility may be relatively high. Therefore the lower limit is not particularly limited in the present disclosure.
• the fine martensite having an average circle equivalent diameter of 5 ⁇ m or less (excluding 0 ⁇ m) is mainly present at ferrite grain boundaries rather than inside ferrite crystal grains, it may be advantageous in improving ductility with maintaining a relatively low yield ratio.
• the occupancy ratio (M) of martensite is less than 90%, martensite formed in the crystal grains may increase yield strength during tensile deformation to increase yield ratio. In this case, it may be difficult to control the yield ratio through temper rolling.
• martensite existing in the crystal grains may significantly inhibit a moving of dislocation during processing and weaken ductility of ferrite, such that a reduction of elongation may be caused.
• the cold-rolled steel sheet of the present disclosure may partially contain bainite in addition to the above-mentioned ferrite and martensite. Since solid solute carbon and solid solute nitrogen existing inside the grains of bainite may easily adhere to a dislocation, interfere with the displacement of the dislocation, and exhibit a discontinuous yield behavior to remarkably increase a yield ratio of steel. Therefore, in the present disclosure, the formation of bainite is preferred to be inhibited as much as possible.
• an area ratio (B) of the bainite defined by the following Relationship 2 may be 3 or less.
• the area ratio (B) of the bainite exceeds 3, the carbon concentration around the bainite may increase to deteriorate ductility of steel, and a yield ratio may rise sharply:
• B A B / A F + A M + A B x 100
• a F refers to an area ratio of ferrite
• a m refers to an area ratio of martensite
• a B refers to an area ratio of bainite.
• a plated layer may be formed on a surface of the cold-rolled steel sheet of the present disclosure.
• a plated layer may be any one of a hot-dip galvanized layer or a galva-annealed layer. As described above, when a cold-rolled steel sheet is formed with the plated layer on its surface, corrosion resistance may be remarkably improved.
• the steel sheet hot-dip plated with zinc based layer of the present disclosure described above may be produced by various methods, and the production method thereof is not particularly limited. As a preferable example, it may be produced by the following methods.
• a steel slab having the above-mentioned component system may be reheated. This operation may be carried out to smoothly perform the subsequent hot-rolling operation, and to sufficiently obtain the targeted properties of the steel sheet.
• process conditions of the reheating operation are not particularly limited, and may be normal conditions.
• a reheating operation may be performed in a temperature range of 1100 to 1300°C.
• the reheated steel slab may be hot-rolled in a single phase temperature region of austenite to obtain a hot-rolled steel sheet.
• the reason why a hot-rolling operation is carried out in the single phase temperature region of austenite may be to increase the uniformity of the structure.
• a finish rolling temperature may be within a range of (Ar3 + 50) to 950°C.
• the finish rolling temperature is lower than (Ar3 + 50)°C, ferrite and austenite two-phase region rolling is highly likely to cause non-uniformity of material.
• the temperature exceeds 950°C, non-uniformity of material due to coarse grain caused by high-temperature rolling may occur, and a coil twisting phenomenon may occur during cooling of the hot-rolled steel sheet.
• the hot-rolled steel sheet may be coiled.
• the coiling temperature may be within a range of 450 to 700°C.
• the coiling temperature is lower than 450°C, excess formation of martensite or bainite may lead to an excessive increase in strength of the hot-rolled steel sheet, which may cause problems such as poor shape, and the like, due to the subsequent load during cold-rolling.
• the coiling temperature exceeds 700°C, surface enrichment of elements which lower wettability of hot-dip galvanized steel such as Si, Mn, B, and the like in the steel may be significantly increased.
• the rolled hot-rolled steel sheet may be cold-rolled to obtain a cold-rolled steel sheet.
• a cold-rolling reduction ratio in the cold-rolling operation may be 40 to 80%.
• the cold-rolling reduction ratio is less than 40%, it may be difficult to secure the target thickness, and it may be also difficult to correct a shape of the steel sheet.
• the cold-rolling reduction ratio exceeds 80%, cracks may occur at an edge portion of the steel sheet, and a cold-rolling load may be caused.
• the cold-rolled steel sheet may be continuously annealed. This operation may be performed to form ferrite and austenite simultaneously with recrystallization, and to distribute carbon therein.
• an annealing temperature may preferably be within a range of 760 to 850°C.
• the annealing temperature is lower than 760°C, sufficient recrystallization may be not achieved, and sufficient formation of austenite may be difficult, which make it difficult to secure the desired strength in the present disclosure.
• the productivity may be lowered, austenite may be excessively formed, bainite may be formed in the subsequent cooling operation, and ductility of steel may be deteriorated.
• the above annealing temperature range may correspond to a two-phase region (ferrite + austenite) temperature range, but annealing is preferably carried out at a temperature range containing as much ferrite as possible. This is why as initial ferrite at the annealing temperature of the two-phase region is relatively more, a growth of crystal grain after annealing may be promoted to enhance ductility. Further, a degree of carbon enrichment in austenite may be increased to lower a martensitic transformation starting temperature (Ms). In this case, it is possible to form martensite upon cooling after plating process, the subsequent operation.
• Ms martensitic transformation starting temperature
• the annealing temperature may more preferably be within a range of 770 to 810°C.
• the cold-rolled steel sheet subjected to the continuously annealing operation may be firstly cooled in a temperature range of 630 to 670°C at an average cooling rate of 2 to 14°C/sec.
• the firstly cooling end temperature is controlled to be relatively high, or the firstly cooling rate is controlled to be relatively slow, tendency of uniformity and coarsening of ferrite may be enhanced, advantageous for ensuring ductility of steel.
• a sufficient time may be provided to allow carbon to diffuse into austenite during the firstly cooling operation, which is significant in the present disclosure. More specifically, in the two-phase temperature region, carbon may diffuse into austenite having a high degree of carbon enrichment.
• the firstly cooling end temperature is lower than 630°C, such an excessively low temperature may result in a relatively low carbon diffusion activity. In this case, carbon concentration in ferrite may increase to result in an increase in yield ratio and an increase in a tendency toward cracking during processing.
• the firstly cooling end temperature exceeds 670°C, it may be advantageous in terms of diffusion of carbon, but require an excessively high cooling rate in a secondly cooling operation of the subsequent process.
• the firstly cooling rate is lower than 2°C/sec, it may be disadvantageous in terms of productivity.
• the firstly cooling rate exceeds 14°C/sec, diffusion of carbon may not sufficiently occur, thereby being not preferred.
• the firstly cooled cold-rolled steel sheet may be secondly cooled to a temperature in a range of (Ms + 20) to (Ms + 50)°C at an average cooling rate of 3 to 12°C/sec.
• the temperature range of a conventional hot-dip galvanizing bath coarse martensite may be formed on the cold-rolled steel sheet to be finally obtained, thereby a low yield ratio may be not achieved.
• martensite may be generated during the secondly cooling operation.
• a cooling rate before introducing into the plating bath after the secondly cooling should be controlled to be relatively high.
• the secondary cooling rate is lower than 3°C/sec, martensite may be not formed, but it is disadvantageous in terms of productivity.
• the rate exceeds 12°C/sec the overall speed of passing a sheet may be increased to generate problems such as shape warping of a sheet.
• Ms ° C 539 ⁇ 423 C ⁇ 30.4 Mn ⁇ 12.1 Cr ⁇ 17.7 Ni ⁇ 7.5 Mo where each of [C], [Mn], [Cr].
• Ni] and [Mo] refers to weight% of the respective elements.
• the secondly cooled cold-rolled steel sheet may be thirdly cooled to a temperature range of 440 to 480°C at a rate of 4 to 8°C/sec.
• the above temperature range may be a temperature range of a conventional galvanizing bath, and this operation may be carried out to prevent formation of a martensite structure before the cold-rolled steel sheet is immersed in the galvanizing bath.
• the thirdly cooling rate is lower than 4°C/sec, martensite may be not formed, but it is disadvantageous in terms of productivity.
• the rate exceeds 8°C/sec martensite may be partially formed and bainite may be partially formed in the grains. In this case, ductility may be deteriorated, as well as an increase in yield strength.
• the thirdly cooled cold-rolled steel sheet may be immersed in a zinc based hot bath to obtain a steel sheet hot-dip plated with zinc based layer.
• a composition of the zinc based hot bath is not particularly limited, and may be a pure galvanizing bath or an alloyed galvanizing bath containing Si, Al, Mg, or the like.
• the hot-dip galvanized steel sheet may be finally cooled to a temperature in a range of (Ms-100)°C or lower at an average cooling rate of 3°C/sec or higher.
• the final cooling end temperature is lower than (Ms-100)°C, not only fine martensite may not be obtained, but also a defective problem regarding a plate shape may be caused.
• the average cooling rate is lower than 3°C/sec, martensite may be irregularly formed in the grain boundaries or in the crystal grains, due to the excessively slow cooling rate.
• steel having a relatively low yield ratio may be not manufactured.
• the steel sheet hot-dip plated with zinc based layer may be subjected to an alloying heat treatment before the final cooling to obtain a galva-annealed steel sheet.
• conditions of the alloying heat treatment process are not particularly limited, and may be conventional conditions.
• an alloying heat treatment process may be performed in a temperature range of 480 to 600°C.
• the final cooled steel sheet plated with zinc based layer or the galva-annealed steel sheet is subjected to temper rolling to form large amounts of dislocations in ferrite disposed around martensite, thereby further improving bake hardenability.
• a reduction ratio is preferably 0.3 to 1.6%, more preferably 0.5 to 1.4%.
• the reduction ratio is less than 0.3%, sufficient dislocations may be not formed and it is disadvantageous from the viewpoint of a plate form. In particular, defects of the plated surface may occur.
• the reduction ratio exceeds 1.6%, it is advantageous in terms of formation of dislocation, but it may cause side effects such as occurrence of strip breakage due to facility capability limit.
• a hot-dip galvanized steel sheet (GI steel sheet) or a galva-annealed steel sheet (GA steel sheet) was prepared using a manufacturing process described in Table 2 below.
• inventive steels 1, 2, 4 and 5 and comparative examples 1 and 2 correspond to galva-annealed steel sheets in Table 1
• invention steels 3 and 6 correspond to hot-dip galvanized steel sheets.
• a firstly cooling end temperature was constantly set to be 650°C
• a secondly cooling end temperature was constantly set to be 560°C
• a thirdly cooling end temperature was constantly set to be 460°C
• a plating bath temperature was constantly set to be 480°C.
• fractions of microstructures and concentration ratios of C and Mn were results from analysis of structures at the point of 1/4t of a sheet thickness of the steel sheet.
• the fractions of microstructures were measured by observing martensite and bainite through Lepera etching using an optical microscope, observing them with SEM (3,000 times), and measuring size and distribution of martensite at three times averages through Count Point operation.
• the concentration ratios of C and Mn were performed by preferentially measuring concentrations of C and Mn existing on the respective phases by a CPS (Count Per Sec) method, in a line and point manner using a TEM and an EDS (Energy Dispersive Spectroscopy) analysis method, thereby quantitatively measuring the ratios.
• concentrations of C and Mn measured in a position in contact with a virtual circle having a diameter corresponding to a short axis of martensite were taken as an average carbon concentration in martensite
• concentrations of C and Mn measured in a ferrite in contact with a virtual circle having a diameter corresponding to a short axis of martensite were taken as an average carbon concentration in ferrite.
• Comparative Example 1 since the annealing temperature thereof was lower than the range proposed in the present disclosure, austenite was not sufficiently formed during the annealing operation, and martensite was not sufficiently formed in a final structure. Thus, the desired ductility and bake hardenability could not be obtained.
• the annealing temperature exceeded the range proposed in the present disclosure. In this case, bake hardenability was secured by a formation of a martensite structure, but an aging problem was caused.
• Comparative Examples 3 and 4 the secondly or thirdly cooling rate exceeded the range proposed in the present disclosure. In these cases, the intended curing properties were not secured, or aging problems were caused.
• Comparative Example 5 the firstly cooling rate exceeded the range suggested in the present disclosure.

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