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 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
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties 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
  • C21D8/0263—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 following hot 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
  • C22C38/02—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
  • C22C38/18—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
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • 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
• • 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
  • C23C2/34—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

Definitions

• the present disclosure relates to a material applied to vehicle members and, more specifically, to a cold-rolled steel sheet (and a plated steel sheet) having highly excellent bending properties, and a method for manufacturing the same.
• a composite structure steel sheet in which a ferrite phase and a low-temperature transformation phase such as martensite or bainite are both present may be used as a high-strength steel sheet with excellent processability.
• a composite steel sheet is formed by dispersing a hard, low-temperature transformation phase in soft ferrite to simultaneously improve strength and processability.
• the steel material may be manufactured using a cold forming method, and there may be demand for the development of ultra-high strength steel with higher processing characteristics, in particular, excellent bending properties, such that research is actively being conducted on a method for manufacturing ultra-high strength steel having a tensile strength of 1500 MPa or more using a single martensite phase.
• a Hot Press Forming (HPF) method of forming a material at high temperature which may be easy to form, and then securing the required strength through water cooling between a die and the material, is being developed.
• HPF Hot Press Forming
• the HPF method is widely used in manufacturing parts because it can secure high strength as compared to the same thickness, but due to a problem in application caused by excessive facility investment and increased process costs, the development of a material for cold stamping is required. Therefore, the development of a cold-rolled steel sheet, which is suitable for use as the material for cold stamping, has a high strength and high yield ratio to secure collision performance, and has excellent bending properties, is required.
• Patent Document 1 discloses steel having a steel composition including: 0.25 to 0.4% of C, 1.0% or less of Si, 1.5 to 2.5% of Mn, 0.02% or less of P, 0.003% or less of S, 0.01 to 0.1% of Al, 0.005% or less of N, 0.0005 to 0.005% of B, and also including 0.005 to 0.1% of Ti, 0.005 to 0.1% of Nb, a total of 0.005 to 0.1%, wherein a metal structure is a martensite single-phase structure.
• Patent Document 2 discloses a technology for manufacturing a plate steel sheet having a steel structure including, 0.05% or more and 0.35% or less of C, 0.01% or more and 2.0% or less of Si, 0.8% or more and 3.0% or less of Mn, 0.05% or less of P, 0.005% or less of S, 0.005% or more and 0.10% or less of Al, and 0.0060% or less of N, wherein a ferrite area ratio is 0% or more and 90% or less, a bainite area ratio is 5% or less (including 0%), a martensite and tempered martensite area ratio is 10% or more
• An aspect of the present disclosure is to overcome the limitations of the prior art described above, and to provide a steel sheet having ultra-high strength having a tensile strength of 1500 MPa or more, while having excellent shape and bending properties by optimizing a steel composition and manufacturing process.
• An object of the present disclosure is not limited to the above description.
• the object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.
• a steel sheet According to an aspect of the present disclosure, provided is a steel sheet,
• the steel sheet may include in area %, 10% or less (excluding 0%) of one or more phases selected from the group consisting of ferrite and bainite, as a microstructure, in the region within 20 ⁇ m from the surface.
• the steel sheet may have a remainder of martensite, as a microstructure, in the region within 20 ⁇ m from the surface.
• the steel sheet may include in area%, 90 to 99 % of martensite, as a microstructure, in the region within 20 ⁇ m from the surface.
• the total thickness, t may be 0.6 to 2.5 mm.
• the steel sheet may include a zinc-based plating layer on the surface of the steel sheet.
• the steel sheet may have a tensile strength (TS) of 1500 MPa or more and bendability (R/t) of 3.7 or less.
• TS tensile strength
• R/t bendability
• the method may further include an over-aging heat treatment which is reheating the secondarily-cooled steel sheet to a temperature within a range of 150 to 240°C.
• the over-aging heat treatment may be performed for 400 to 1000 seconds.
• FIG. 1 is a photograph of a cross-section of a steel sheet in a thickness direction according to Inventive Example 1 of the present disclosure, captured using a scanning electron microscope (SEM).
• SEM scanning electron microscope
• Carbon (C) is an interstitial solid-solution element, the most effective and important element for improving strength of steel, and is an element which should be added to secure strength of martensite steel.
• carbon is added in an amount of 0.1% or more, and more preferably, carbon may be added in an amount of 0.15% or more.
• a content of C exceeds 0.3%, martensite may be excessively formed during cooling due to increased hardenability, which may rapidly increase the strength and deteriorate elongation.
• an upper limit of the content of C it is preferable to limit an upper limit of the content of C to 0.3%, and more preferably, the upper limit of the content of C may be 0.28% or less. Meanwhile, a lower limit of the content of C may be 0.2%, or the upper limit of the content of C may be 0.25%.
• Silicon is known as a key element of Transformation Induced Plasticity (TRIP) steel, which acts to increase a fraction and elongation of retained austenite.
• TRIP Transformation Induced Plasticity
• the addition of Si is a factor of suppressing the occurrence of cracks during bending processing by suppressing the precipitation of cementite. Therefore, in order to obtain the above-described effect, an Si content of 0 wt% is excluded. However, if the content of Si exceeds 0.5%, not only will the weldability deteriorated, but the surface properties and plating properties of the steel sheet will also deteriorate, so the content of Si is included in an amount of 0.5 wt%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Si may be 0.01%, or an upper limit of the content of Si may be 0.3%.
• Manganese (Mn) is an element added to secure strength. If the content of Mn is less than 1.3%, hardenability is low, so when a cooling speed is not fast enough during cooling after annealing, martensite is not formed, making it difficult to secure a level of strength required in the present disclosure. On the other hand, when the content of Mn exceeds 2.5%, a Ms temperature is lowered during cooling after annealing, which lowers a final cooling temperature and thus the shape of the steel sheet becomes poor, and it also becomes difficult to secure an initial martensite structure. In addition, during steelmaking/continuous casting operations, a segregation zone occurs in a length direction of a Mn-based slab, which acts as a factor deteriorating bendability, and thus sets the upper limit thereof.
• the content of manganese (Mn) be 1.3 to 2.5%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Mn may be 1.5%, or an upper limit of the content of Mn may be 2.1%.
• Chromium (Cr) is an alloying element facilitating securing a low-temperature transformation structure by suppressing ferrite transformation, and when using a continuous annealing process with slow cooling as in the present disclosure, there is an advantage of suppressing ferrite formation.
• a Cr content of 0 wt% is excluded.
• carbides such as CrC, or the like may be formed, to impair hole expandability and bending processability, and costs may increase due to excessive alloy input. Therefore, the Cr content is preferably in a range of 0.2% or less.
• B Boron
• B is an element that suppresses formation of ferrite, and accordingly, in the present disclosure, B has an advantage of suppressing the formation of ferrite during cooling after annealing.
• the content of B exceeds 0.003%, ductility may be significantly decreased.
• the content of B is less than 0.0005%, there is no hardening effect at all, so not only is the target strength not secured, but ferrite is formed on a surface layer, which tends to result in poor bending properties, thus limiting a lower limit of the B content.
• the lower limit of the content of B may be 0.0008%, or an upper limit of the content of B may be 0.0022%.
• Phosphorus (P) is an impurity element contained in steel, and the lower an amount of phosphorous (P) added in steel, the better, but considering the cases in which it is unavoidably included during the manufacturing process, a Cr content of 0 wt% is excluded. However, if a P content exceeds 0.1%, weldability deteriorates and there is a risk of steel brittleness, so an upper limit of the P content is limited to 0.1%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the P content may be 0.0001%, or the upper limit of the P content may be 0.03%.
• S Sulfur
• S is an impurity inevitably contained in steel, which impairs ductility and weldability of a steel sheet.
• S content it is preferable to manage an S content to be as low as possible, and therefore, it is preferable to limit the S content to 0.01 wt% or less in the present disclosure.
• 0% is excluded considering cases in which it is inevitably included during the manufacturing process.
• a lower limit of the S content may be.
• an upper limit of the S content may be 0.008% or 0.005%.
• N Nitrogen
• a lower limit of the N content may be 0.0001%, or, more preferably, the upper limit of the N content is 0.008%, and even more preferably, 0.006%.
• Niobium is an element that segregates at austenite grain boundaries and contributes to increased strength through precipitation strengthening by suppressing growth of austenite grains during annealing heat treatment.
• Nb content exceeds 0.05%, precipitation of carbides and nitrides increases, which reduces processability of a base material, and the cost increases as an alloy input amount becomes excessive.
• the Nb content is less than 0.01%, it does not contribute to increasing the strength thereof at all, so a lower limit of the Nb content is limited to 0.01%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Nb content may be 0.02%, or an upper limit of the Nb content may be 0.04%.
• Titanium (Ti) is a nitride forming element, and is an element scavenging by precipitating N in steel into TiN.
• Ti When Ti is not added, there is a possibility that cracks may occur during continuous casting due to AlN formation.
• the Ti content exceeds 0.05%, the strength of martensite may be reduced by additional carbide precipitation in addition to removal of dissolved N, and hole expandability and bending processability may be impaired by the formation of carbides and nitrides such as TiC and TiN.
• the Ti content is less than 0.01%, it does not contribute to increasing strength at all, similar to the Nb element, so a lower limit of the Ti content is set. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Ti content may be 0.02%, or an upper limit of the Ti content may be 0.04%.
• the remaining component of the present disclosure is iron (Fe).
• Fe iron
• the component since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.
• a ratio (a/b ⁇ 100) of a total content (a) of C and Mn in a region within 20 ⁇ m from a surface of the steel sheet, and a total content (b) of C and Mn at a point (1/4) ⁇ t, where t is a total thickness of the steel sheet, from the surface is 75% or more (excluding 100%).
• the inventors have repeatedly conducted extensive research, and as a result thereof, have discovered the tendency for the bending properties to be improved when the total content of C and Mn in a region within 20 ⁇ m from the surface of the steel sheet satisfies a specific ratio relative to a point (1/4) ⁇ t, where t is a total thickness of the steel sheet, from the surface of the steel sheet (in a thickness direction of the steel sheet), thereby completing the present disclosure.
• ferrite or bainite may be excessively formed in the form of clusters on the surface layer portion, and cracks may occur at a boundary between the phases of ferrite and martensite, resulting in reduced bendability.
• a lower limit of the ratio (a/b) may be 78%, or an upper limit of the ratio (a/b) may be 87%.
• the steel sheet may include in area %, 10% or less (excluding 0%) of one or more phases selected from the group consisting of ferrite and bainite, as a microstructure.
• a ratio (A) of one or more phases selected from the group consisting of ferrite and bainite exceeds 10%, ferrite or bainite, which are soft phases, may be excessively formed around martensite, a hard phase, causing cracks to occur when bending.
• a lower limit of the ratio (A) is not particularly limited, but it is advantageous if it can be managed as low as possible within a practically manufacturable range.
• the steel sheet may have a balance of martensite, other than ferrite and bainite, described above, as a microstructure, in the region within 20 ⁇ m from the surface.
• the steel sheet may include, in area %, 90 to 99% of martensite, as a microstructure, in a region within 20 ⁇ m from the surface.
• a lower limit of a fraction of martensite may be, in area %, 95%, or in a region within 20 ⁇ m from the surface, an upper limit of the fraction of martensite in area %, may be 98%.
• the total thickness, t may be 0.6 to 2.5 mm.
• the reheated slab is hot rolled.
• hot rolling may be performed at a temperature within a range of Ar3 to 1000°C.
• a final hot rolling temperature of the reheated slab is limited to Ar3 (a temperature at which ferrite begins to appear when austenite is cooled) or higher, limited because, below Ar3, rolling in a dual phase region of ferrite and austenite or ferrite region is performed to create a mixed structure, and there may be concerns about malfunctions due to fluctuations in the hot rolling load.
• the hot-rolled steel sheet is coiled at a temperature within a range of 400 to 600°C.
• a coiling temperature exceeds 600°C, an oxide film may be excessively generated on a surface of the steel sheet, which may cause defects, and surface properties of a plating material may deteriorate, so an upper limit thereof is limited.
• a lower limit of the coiling temperature is limited to 400°C or higher.
• the lower limit of the coiling temperature may be 420°C
• an upper limit of the coiling temperature may be 520°C
• water cooling treatment cooling may be performed after coiling.
• a lower limit of the annealing temperature may be 823°C, or an upper limit of the annealing temperature may be 916°C.
• Ac 3 910 ⁇ 203 ⁇ C ⁇ 15.2 Ni + 44.7 Si + 104 V + 31.5 Mo + 13.1 W
• a ratio of other phase structures one or more phases selected from the group consisting of ferrite and bainite
• the ratio (a/b) is less than 75%, which may result in a bendability (R/t) evaluation value exceeding 3.7, which may deteriorate the formability.
• a method for manufacturing a steel sheet according to an embodiment of the present disclosure may satisfy the following Relational Expression 1.
• the present inventors have repeatedly conducted extensive research, and as a result thereof, the inventors have additionally confirmed that a steel sheet having excellent properties such as strength and bendability may be secured by satisfying a specific relationship, such as the following Relational Expression 1, between cooling end temperatures during primary and secondary cooling. 4.5 ⁇ T 1 / T 2 ⁇ 7
• T2 is a cooling end temperature of secondary cooling (°C)
• T3 is a temperature of over-aging heat treatment (°C).
• the over-aging heat treatment time When the over-aging heat treatment time is less than 400 seconds, tempering may not be sufficiently performed, which may result in lower yield strength.
• an upper limit of the over-aging heat treatment time there is no specific limitation on an upper limit of the over-aging heat treatment time, but it is difficult to exceed 1000 seconds due to the characteristics of continuous annealing facility. Accordingly, the over-aging heat treatment time may be performed for 400 to 1000 seconds, and in terms of further improving the above-described effect, a lower limit of the over-aging heat treatment time may be 428 seconds, or the upper limit of the over-aging heat treatment time may be 600 seconds.
• the method for manufacturing a steel sheet may further include an operation of forming a plating layer on a surface of the steel sheet.
• the plating may be performed by a hot-dip plating method in which a plating bath is installed and the steel sheet is dipped in a molten plating solution, or by an electroplating method in an electrolyte after annealing is completed.
• the plating conditions are not particularly limited as long as the conditions are generally known in the technical field to which the present disclosure belongs.
• Molten steel having the alloy composition shown in Table 1 below was cast into an ingot and then sized and rolled to prepare a steel slab.
• the steel slab was heated to a temperature of 1200°C, maintained for 1 hour, and then was subjected to finish hot rolling at a temperature of 900°C, charged into a heated furnace, which set with various conditions, maintained for 1 hour, and then furnace-cooled to simulate hot rolling.
• cold rolling was performed at a cold rolling reduction rate of 50%, and then was subjected to annealing heat treatment, primary cooling (slow cooling), secondary cooling (rapid cooling), reheating, and overaging heat treatment under the conditions shown in Table 2 below to form a cold-rolled steel sheet and then electrogalvanized under normal conditions.
• a tensile strength (TS) and yield strength (YS) were measured by collecting a tensile test sample of a size of JIS No. 5 in a direction perpendicular to a rolling direction and then performing a tensile test at a strain rate of 0.01/s.
• R/t bending properties
• a cold-rolled steel sheet was processed into a sample having a width of 100mm * a length of 30 mm, then was subjected to a 90° bending test at a test speed of 100 mm/min, and then checking for cracks in a bent portion using a microscope to improve the reliability of the results.
• Comparative examples 1 and 2 when the first and second cooling end temperatures and cooling rates are outside, the flatness or bendability is outside the range required in the present disclosure, and it can be seen that the bendability is inferior because a ratio of one or more mixed grain structures selected from the group consisting of ferrite and bainite other than a martensite structure within a surface layer of 20 ⁇ m exceeds 10%, or the total content ratio of C and Mn is outside the required range of the present disclosure.
• a 1500 MPa grade annealed and electrogalvanized steel sheet having the target shape (flatness) and excellent bendability may be prepared as in the case of Inventive Example.

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