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
  • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
  • B21C47/02—Winding-up or coiling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • 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
• • 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
  • 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
  • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
  • C23C2/36—Elongated material
  • C23C2/40—Plates; Strips
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/22—Electroplating: Baths therefor from solutions of zinc
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D7/00—Electroplating characterised by the article coated
  • C25D7/06—Wires; Strips; Foils
  • C25D7/0614—Strips or foils
• • 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
  • C21D2211/001—Austenite
• • 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
  • C21D2211/002—Bainite
• • 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

Definitions

• the present disclosure relates to a cold-rolled steel sheet and a manufacturing method therefor and, more specifically, to a cold-rolled steel sheet that is preferably applicable to collision energy absorption members such as a body-in-white (BIW) structural element, and a manufacturing method for the cold-rolled steel sheet.
• a cold-rolled steel sheet that is preferably applicable to collision energy absorption members such as a body-in-white (BIW) structural element, and a manufacturing method for the cold-rolled steel sheet.
• BIW body-in-white
• Patent Document 1 relates to manufacturing a steel having a martensite volume ratio of 80 to 97% and a balance of ferrite by continuously annealing a steel including C in the content of 0.18 to 0.3% and cooling the steel to room temperature with water, and then performing an overaging treatment at a temperature of 120 to 300°C for 1 to 15 minutes.
• Ultra-high strength steel may be manufactured by tempering a cold-rolled steel sheet after rapid cooling to room temperature after annealing in a dual phase zone or a single phase zone. In this case, a yield strength and hole expandability are excellent, but the shape quality of the coil deteriorates due to temperature deviation in width and length directions, and problems such as poor material quality and reduced workability may occur depending on the area during processing roll-forming components.
• the elongation thereof decreases, which causes problems such as reduced formability, and thus, an application thereof as a material for cold stamping is limited.
• the elongation should be high, and a representative method for increasing the elongation is a method of introducing retained austenite and utilizing a TRIP phenomenon, as in Patent Document 2.
• a representative method for increasing the elongation is a method of introducing retained austenite and utilizing a TRIP phenomenon, as in Patent Document 2.
• the yield strength and hole expandability may be inferior.
• An aspect of the present disclosure is to provide a cold-rolled steel sheet and a manufacturing method therefor.
• a preferred aspect of the present disclosure is to provide a cold-rolled steel sheet having excellent strength, excellent elongation and excellent hole expandability and a manufacturing method therefor.
• a cold-rolled steel sheet including: by wt%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.1%, molybdenum (Mo): 0.01% or less (excluding 0%), boron (B): 0.001% or less (excluding 0%), and a balance of Fe and other inevitable impurities, wherein a microstructure includes, in area%, ferrite: more than 10% and 45% or less, retained austenite: 7 to 15%, fresh martensite: 10% or less (including 0%), and a sum of tempered martensite and bainite: 40 to 80%, and the cold-rolled steel sheet includes a soft layer having a predetermined depth (t) of the steel sheet in a thickness direction from
• a fraction of the tempered martensite may be 40% or more.
• the predetermined depth (t) may be 50 to 100 ⁇ m.
• the cold-rolled steel sheet may have one of a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), and an electrolytic galvanized layer (EG) formed on at least one surface thereof.
• a method for manufacturing a cold-rolled steel sheet including: heating a slab at 1100 to 1300°C, the slab including, by wt%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.1%, molybdenum (Mo): 0.01% or less (excluding 0%), boron (B): 0.001% or less (excluding 0%), and a balance of Fe and other inevitable impurities; finishing hot-rolling the heated slab at Ar3 or higher to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 700°C or less; pickling and then cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; primarily heating the cold-rolled steel sheet at 7
• the cold rolling may be performed at a cold reduction ratio of 30 to 80%.
• the method for manufacturing a cold-rolled steel sheet may further include: immersing the cold-rolled steel sheet in a hot-dip galvanized bath at 440 to 480°C after the secondary heating and holding, to form a hot-dip galvanized layer.
• the method for manufacturing a cold-rolled steel sheet may further include: performing an alloying heat treatment on the cold-rolled steel sheet at 450 to 520°C after the formation of the hot-dip galvanized layer.
• the method for manufacturing a cold-rolled steel sheet may further include: forming an electro-galvanized layer after the secondary heating and holding.
• a cold-rolled steel sheet and a manufacturing method therefor could be provided.
• a cold-rolled steel sheet having excellent strength, excellent elongation and excellent hole expandability and a manufacturing method therefor could be provided.
• FIG. 1 is a microstructure image of Inventive Example 1 according to an embodiment of the present disclosure observed with an SEM microscope.
• C is an interstitial solid-solution element and is the most effective and important element for improving the strength of steel.
• the content of C is less than 0.15%, it may be difficult to obtain the yield ratio and tensile strength targeted in the present disclosure.
• the content of C exceeds 0.25%, the strength thereof may increase rapidly due to excessive formation of martensite during cooling due to an increase in hardenability, which may result in poor elongation and reduced weldability. Accordingly, it is preferable that the content of C is in the range of 0.15 to 0.25%.
• a lower limit of the content of C is more preferably 0.18%, and a lower limit of the content of C is more preferably 0.2%.
• the upper limit of the content of C is more preferably 0.24%.
• Mn is an element added to secure strength.
• the content of Mn is less than 1.5%, it may be difficult to secure the level of strength desired in the present disclosure.
• Mn exceeds 2.5%, an Ms temperature decreases during cooling after annealing, which may make it difficult to smoothly secure an initial martensite phase. It may be difficult to simultaneously secure the strength, elongation, and hole expandability targeted in the present disclosure due to a decrease in a tempered martensite fraction in a Quenching & Partitioning (Q&P) process.
• Q&P Quenching & Partitioning
• Mn may segregate in a thickness direction, making it easy to form an Mn band in a slab, which may increase the possibility of defects occurring during a rolling process along with continuous casting cracks.
• the content of Mn is preferably in the range of 1.5 to 2.5%.
• a lower limit of the content of Mn is more preferably 1.8%, and a lower limit of the content of Mn is more preferably 2.0%.
• the upper limit of the content of Mn is more preferably 2.4%.
• Si is a key element of Transformation Induced Plasticity (TRIP) steel securing an appropriate level of retained austenite fraction and increasing an elongation by suppressing precipitation of cementite.
• TRIP Transformation Induced Plasticity
• the content of Si is less than 1.0%, the control of cementite precipitation during the reheating and overaging operations may be not smoothly performed, and thus, a fraction of a finally obtained retained austenite may be reduced or the stability thereof may be low, resulting in a poor elongation.
• the content of Si exceeds 2.0%, the properties of a welded portion may deteriorate due to the occurrence of Liquid Metal Embrittlement (LME) cracks, and the surface characteristics and plating properties of a steel may also deteriorate.
• the content of Si is preferably in the range of 1.0 to 2.0%.
• a lower limit of the content of Si is more preferably 1.2%.
• An upper limit of the content of Si is more preferably 1.8%.
• P is an impurity element included in steel, and when a content thereof exceeds 0.1%, weldability thereof deteriorates and there is a risk of steel brittleness.
• S similarly to P, is an impurity element included in steel, and when a content thereof exceeds 0.03%, ductility and weldability thereof may deteriorate.
• Al is an element added to remove oxygen in molten steel, and is effective in stabilizing retained austenite by suppressing precipitation of cementite during reheating and overaging operations, similarly to Si.
• the content of Al is less than 0.01%, the deoxidation effect may not be sufficiently obtained, which may impair the cleanliness of a steel.
• the content of Al exceeds 0.1%, not only may the castability of the slab deteriorate, but the temperature required for single-phase region heating during annealing may also increase, which may cause production and facility problems. Accordingly, the content of Al is preferably in the range of 0.01 to 0.1%. An upper limit of the content of Al is more preferably 0.05%.
• Mo is a representative element that may improve hardenability, but in the present disclosure, since the balance of strength, an elongation and hole expandability is important, and a steel has a tensile strength of 980 MPa, there is no need to add Mo for the purpose of improving hardenability and facilitating the formation of martensite.
• Mo is added, there is a problem of increasing the manufacturing costs, so that it is preferable not to intentionally add Mo in the present disclosure. Accordingly, in the present disclosure, the content of Mo may be limited to 0.01% or less. Meanwhile, considering a case in which Mo is inevitably included during the manufacturing process, a lower limit thereof may be 0.001%.
• B is a representative element that may improve hardenability, but in the present disclosure, since the balance of strength, an elongation and hole expandability is important, and a steel has a tensile strength of 980 MPa, there is no need to add B for the purpose of improving hardenability and facilitating the formation of martensite. Accordingly, it is preferable not to add B in the present disclosure. Accordingly, in the present disclosure, the content of B may be limited to 0.001% or less. Meanwhile, considering a case in which B is unavoidably included during the manufacturing process, a lower limit thereof may be 0.0001%.
• the remaining component is iron (Fe).
• Fe iron
• unintended impurities may inevitably be mixed during a normal manufacturing process from raw materials or the surrounding environment, this may not be excluded. Since these impurities may be known to anyone who is skilled in the normal manufacturing process, not all of their contents are specifically mentioned in this specification.
• a microstructure of the cold-rolled steel sheet of the present disclosure may include, in area%, ferrite: more than 10% and 45% or less, retained austenite: 7 to 15%, fresh martensite: 10% or less (including 0%), and a sum of tempered martensite and bainite: 40 to 80%.
• the ferrite is a structure advantageous for securing an elongation. When a fraction of the ferrite is 10% or less, it may be difficult to secure the elongation targeted in the present disclosure, and when the fraction of the ferrite exceeds 45%, it may be difficult to secure the strength and hole expandability targeted in the present disclosure.
• the retained austenite is a structure absolutely necessary for securing the elongation together with the ferrite formed during the annealing process.
• the fraction of the retained austenite is less than 7%, it may be difficult to secure the elongation targeted in the present disclosure.
• the fraction of the retained austenite exceeds 15%, it may be difficult to secure the targeted elongation due to insufficient stability of the retained austenite.
• the fraction of the fresh martensite exceeds 10%, it may be difficult to obtain a steel having excellent strength, an excellent elongation and excellent hole expandability characteristics.
• the tempered martensite and bainite are structures necessary for securing strength and hole expandability, and when a phase transformation is advanced to include the fraction in the above-described range, the retained austenite that is stable at room temperature may ultimately include 7 to 15%.
• the fraction of the tempered martensite and bainite is less than 40%, it may be difficult to secure the retained austenite fraction targeted by the present disclosure due to insufficient total transformation amount, and since 10% or more of the fresh martensite is secured, it may be difficult to secure the strength, elongation, and hole expandability targeted by the present disclosure.
• the fraction of the tempered martensite and bainite exceeds 80%, it may be possible to secure strength and hole expandability, but it may be difficult to secure the high elongation targeted by the present disclosure due to the lack of the fraction of ferrite and retained austenite. Meanwhile, the fraction of the tempered martensite is more preferably 40% or more.
• the cold-rolled steel sheet of the present disclosure includes a soft layer having a predetermined depth (t) of the steel sheet in a thickness direction from a surface thereof, and it is preferable that the soft layer satisfies the following relational expressions 1 and 2.
• the predetermined depth (t) may be 50 to 100 ⁇ m.More specifically, the predetermined depth (t) may be 50 to 80 ⁇ m.
• the surface refers to a surface of a base steel sheet, and a plating layer that may be formed on the surface of the base steel sheet is excluded.
• the soft layer satisfies the following relational expressions 1 and 2.
• [C t/5 ] represents an average C content in a region from the surface to 1/5 of the predetermined depth (t) of the steel sheet in a thickness direction
• [C 3t/5 ] represents an average C content in a region corresponding to 3/5 of the predetermined depth (t) of the steel sheet in the thickness direction from 1/5 of the predetermined depth (t)
• [C M ] represents an average C content of the steel sheet.
• the [C 1/5t ] may include a microstructure including, area%, 80% or more of ferrite, and a balance of bainite and tempered martensite.
• the cold-rolled steel sheet of the present disclosure may have one of a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), and an electrogalvanized layer (EG) formed on at least one surface thereof.
• GI hot-dip galvanized layer
• GA alloyed hot-dip galvanized layer
• EG electrogalvanized layer
• the present disclosure does not specifically limit the specific conditions of the hot-dip galvanized layer (GI), the alloyed hot-dip galvanized layer (GA), or the electrogalvanized layer (EG), and all types commonly used in the relevant technical field may be used.
• the cold-rolled steel sheet of the present disclosure may have a yield strength (YS): 600 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T-El): 21% or more, and a hole expansion ratio (HER): 20 to 40%. Since the yield strength, the tensile strength, the total elongation, and the uniform elongation are advantageous as they are higher, the present disclosure does not specifically limit upper limits of the yield strength, the tensile strength, and the total elongation.
• the control of the value X is to secure the yield strength of 600 MPa or more, targeted by the present disclosure, while simultaneously securing excellent elongation and excellent hole expandability.
• the value of X is less than 30000 MPa% or exceeds 70000 MPa%, one or more of the properties of the strength, the elongation, and the hole expandability, desired by the present disclosure, may be inferior, making it difficult to use the cold-rolled steel sheet as a member for absorbing impact energy.
• a lower limit of the value X is more preferably 35000 MPa%.
• An upper limit of the value X is more preferably 65000 MPa%, and 60000 MPa% is even more preferably.
• the slab is heated to a temperature of 1100 to 1300°C.
• the slab heating is performed to smoothly perform the subsequent hot rolling process and obtain the target properties of the steel sheet.
• the slab heating temperature is lower than 1100°C, a problem of a rapid increase in the hot rolling load may occur.
• the slab heating temperature exceeds 1300°C, the amount of surface scale may increase, which may reduce productivity.
• the heated slab is subjected to a finishing hot rolling at Ar3 or higher to obtain a hot rolled steel sheet.
• a finishing hot rolling temperature is lower than Ar3
• a two-phase zone of ferrite + austenite or a ferrite zone rolling is performed, resulting in generating a mixed grain structure, and equipment malfunction may occur due to a change in the hot rolling load.
• the hot-rolled steel sheet is coiled at 700°C or lower.
• a coiling temperature exceeds 700°C, an oxide film may be excessively formed on the surface of the steel sheet, which may cause defects.
• the coiling temperature is more preferably 650°C or lower.
• the strength of the hot-rolled steel sheet increases, which has the disadvantage of increasing the rolling load as the subsequent cold rolling process, but since this is not a factor making actual production impossible, the present disclosure does not specifically limit a lower limit thereof.
• the lower limit of the coiling temperature may be 300°C.
• the coiled hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet.
• the pickling is a process for removing the oxide layer formed on the surface of the coiled hot-rolled steel sheet.
• the cold-rolling may be performed at a cold reduction ratio of 30 to 80%.
• the cold reduction ratio is less than 30%, it is difficult to secure the target thickness, and there is a concern that the formation of austenite and securing of physical properties may be affected during annealing heat treatment due to retained crystal grains formed during hot rolling.
• the cold reduction ratio exceeds 80%, a material deviation may occur due to the uneven rolling amount in length and width directions due to the work hardening occurring during cold rolling, and it may be difficult to secure a target thickness due to the rolling load.
• the cold-rolled steel sheet is primarily heated to a temperature of 780°C or higher and less than Ac3-10°C for 30 seconds or longer under atmospheric conditions having a dew point temperature of 0 to 30°C.
• the primary heating is to form some annealed ferrite in addition to the retained austenite in order to secure an elongation of 21% or more.
• the dew point temperature is less than 0°C, the soft layer targeted by the present disclosure is not sufficiently formed on the surface of the steel sheet.
• the dew point temperature exceeds 30°C, there are problems of reduced equipment life and productivity.
• a lower limit of the dew point temperature is more preferably 2°C.
• a upper limit of the dew point temperature is more preferably 25°C.
• the primary heating temperature is less than 780°C, annealed ferrite may be excessively formed, making it difficult to secure strength and hole expandability.
• the primary heating temperature is Ac3-10°C or higher, the fraction of annealed ferrite may be insufficient due to heating at a single-phase region level, resulting in poor elongation.
• a lower limit of the primary heating temperature is more preferably 790°C.
• An upper limit of the primary heating temperature is more preferably Ac3-15°C.
• the primary heating time is less than 30 seconds, there is a disadvantage in that a sufficient annealing effect is not obtained. Meanwhile, the longer the primary heating time is, the more advantageous it is, and thus the present disclosure is not particularly limited to the lower limit thereof.
• the upper limit of the primary heating time may be 500 seconds.
• the above-described Ac3 may be obtained through the following relational expression 2.
• Ac3(°C) 910 - 203 ⁇ [C] - 15.2[Ni] + 44.7[Si] + 104[V] + 31.5[Mo] + 13.1[W]
• the heated cold-rolled steel sheet is primarily cooled to 600 to 750°C at an average cooling rate of 1 to 10°C /s.
• a primary cooling end temperature is less than 600°C, there is a concern that phases such as ferrite or bainite may be formed, resulting in a decrease in strength.
• the primary cooling end temperature exceeds 750°C, problems may occur in an actual production line.
• a lower limit of the primary cooling end temperature is more preferably 610°C, and more preferably 630°C.
• An upper limit of the primary cooling end temperature is more preferably 740°C, and more preferably 730°C.
• the primary average cooling rate exceeds 10°C/s, the average cooling rate decreases during the second cooling, making it difficult to secure sufficient martensite, which in turn leads to a decrease in the fraction of tempered martensite, making it difficult to secure strength and hole expandability at the same time.
• An upper limit of the primary average cooling rate is more preferably 6°C/s.
• the primarily-cooled cold-rolled steel sheet is secondarily cooled to 150°C ⁇ Ms at an average cooling rate of 10 to 45°C/s.
• the secondary cooling end temperature preferably has a range of 150°C-Ms.
• a lower limit of the secondary cooling end temperature is more preferably 180.
• the secondary average cooling rate is less than 10°C/s, some bainite structure may be formed from the primary cooling section to the secondary cooling.
• the secondary average cooling rate exceeds 45°C/s, a surface shape of the steel sheet may become inferior due to the rapid martensite transformation rate at the time of the secondary cooling, and a material deviation problem in a width direction may occur.
• a lower limit of the secondary average cooling rate is more preferably 12°C/s.
• An upper limit of the secondary average cooling rate is more preferably 42°C/s.
• Ms (°C) 539 - 423 [C] - 30.4 [Mn] - 7.5 [Si] + 30 [Al] - 12.1 [Cr] - 17.7 [Ni] - 7.5 [Mo]
• the secondarily-cooled cold-rolled steel sheet is secondarily heated to a temperature of Ms ⁇ 480°C, and then subjecting to overaging for 1 to 30 minutes.
• the secondary heating and overaging are intended to improve the toughness by changing the high dislocation density and hard martensite formed during the second cooling into tempered martensite.
• C is enriched in the austenite remaining from the annealing (Partitioning).
• the martensite transformation start temperature (Ms) of the austenite in which C is enriched is lowered to a temperature equal to or lower than room temperature, and a large amount of retained austenite is ultimately formed, thereby securing the properties targeted by the present disclosure.
• Ms martensite transformation start temperature
• the secondary heating temperature is less than Ms or exceeds 480°C, it may be difficult to secure the fraction of the microstructure targeted by the present disclosure.
• a lower limit of the secondary heating temperature is more preferably 360°C.
• An upper limit of the secondary heating temperature is more preferably 460°C.
• the overaging treatment time is less than 1 minute, it is difficult to obtain a partitioning effect because sufficient transformation is not advanced.
• the overaging treatment time exceeds 30 minutes the secondary heating and overaging treatment section should be significantly long, and productivity decreases, so that it may be difficult to apply to an actual production line.
• the cold-rolled steel sheet may be immersed in a hot-dip galvanized bath of 440 to 480°C to form a hot-dip galvanized layer.
• a hot-dip galvanized bath temperature is less than 440°C, it may be difficult to manage the molten zinc plating bath, and when the hot-dip galvanized bath temperature exceeds 480°C, a final elongation may decrease.
• the cold-rolled steel sheet on which the hot-dip galvanized layer is formed may be subjected to an alloying heat treatment at 450 to 520°C.
• the alloying heat treatment temperature is less than 450°C, it may be difficult to form a sufficient Fe-Zn alloy plating layer, and when the alloying heat treatment temperature exceeds 520°C, the final elongation may be inferior due to the decomposition of the retained austenite formed in a previous operation.
• an electro-galvanized layer may be formed.
• a slab having an alloy composition as described in Table 1 below was heated to a temperature of 1100 to 1300°C, and then finishing hot-rolled at 900 to 1000°C to manufacture a hot-rolled steel sheet.
• the hot-rolled steel sheet was coiled at 350 to 650°C, pickled, and cold-rolled at a cold reduction ratio of 45 to 65%, and then, a cold-rolled steel sheet was manufactured by applying the conditions described in Table 2 below. Meanwhile, the conditions described in Table 2 below are based on a surface temperature of the steel sheet. Accordingly, the manufactured cold-rolled steel sheet was subjected to hot-dip galvanizing or hot-dip galvanizing and alloying heat treatment under the conditions described in Table 2 below.
• a phase fraction of the microstructure was measured using XRD and EBSD for t/4 (t: thickness of steel) of the cold-rolled steel sheet.
• the formation of a soft layer having a predetermined depth (t) was measured using GDS.
• An average C content [C t/5 ] of a region from a surface thereof to 1/5 of a predetermined depth (t) of the steel sheet in a thickness direction and an average C content [C 3t/5 ] of a region corresponding to 3/5 of the predetermined depth (t) of the steel sheet in the thickness direction from 1/5 of the predetermined depth (t) were calculated by the average value of the content of C measured through GDS, and [C M ] was measured using the results of OES and ICP C component analysis of a parent material.
• Yield strength (YS), tensile strength (TS), total elongation (T-El), and uniform elongation (U-El) were measured by processing the cold-rolled steel sheet into specimens of the JIS standard (gauge length width ⁇ length: 25 ⁇ 50 mm, total length of specimen: 200 to 260 mm), and then performing a tensile test at a test speed of 28 mm/min.
• Hole expansion ratio was measured according to the ISO 16330 standard, and holes were sheared at a clearance of 12% using a 10 mm diameter punch.
• FIG. 1 is a microstructure image of Inventive Example 1 observed with an SEM microscope. As can be seen from FIG. 1 , in the case of Inventive Example 1, it can be seen that the microstructure of an appropriate fraction to be obtained by the present disclosure is secured.

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• 2022
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