Steel sheet and method for manufacturing same

Abstract

The present invention relates to a steel sheet and a method for manufacturing same, and, more specifically, to a high-strength steel sheet having excellent formability and a high yield ratio, and a method for manufacturing same

Description

Technical Field

[0001] The present disclosure relates to a steel sheet and a method for manufacturing the same, and, more specifically, to a high-strength steel sheet having excellent formability and a high yield ratio, and a method for manufacturing the same.Background Art

[0002] Recently, the automobile industry has paid a lot of attention to ensuring passenger safety and improving the fuel efficiency of automobiles. For this reason, the application of high-strength steel sheets to automobile body materials to meet safety and weight reduction requirements has increased.

[0003] In order to improve the collision performance of automobile bodies, when the yield strength of steel is increased, collision energy may be absorbed efficiently even at low deformation amounts. There are two methods for increasing yield strength: solid solution strengthening and precipitation strengthening.

[0004] Solid solution strengthening steel is a steel that increases yield strength by performing solid-solution strengthening on solid solution strengthening elements (Mn, Si, Cr, and the like) in a ferrite phase having excellent formability. However, Si or Cr are elements that easily form oxides on a surface of a steel sheet in continuous annealing lines or continuous hot-dip galvanized lines. Additionally, Mn is an element that promotes a low-temperature transformation phase (bainite or martensite), and the low-temperature transformation phase may have the characteristic of lowering the yield strength. Accordingly, the solid solution strengthening steel to which a large amount of Mn, Si, and Cr is added is not an appropriate method for increasing a yield ratio of high-strength steel having a tensile strength of 610 MPa or more.

[0005] On the other hand, precipitation strengthening steel using Nb, Ti, V, and the like, improves yield strength by precipitating fine carbides in ferrite. Since precipitation strengthening steel increases a yield ratio without deteriorating the workability, it is a strengthening mechanism suitable for a high-strength steel sheet having a tensile strength of 610 MPa or more that have excellent crash performance and workability. As a technology for improving the formability and yield ratio of a steel sheet, a method of introducing unrecrystallized ferrite and utilizing the addition of Ti or Nb is disclosed in Patent Documents 1 and 2. Precipitation strengthening and unrecrystallized ferrite using Ti or Nb are effective in increasing yield strength without significantly increasing the tensile strength by directly strengthening the ferrite.

[0006] However, the techniques described in Patent Documents 1 and 2 did not have an appropriate fraction of unrecrystallized ferrite, and thus did not have both excellent formability and a high yield ratio.

[0007] The technique described in Patent Document 3 included an appropriate area fraction of unrecrystallized ferrite, but the balance of tensile strength, elongation, hole expandability, and yield ratio were not appropriate.[Prior Art Documents][Patent Documents]

[0008] (Patent Document 1) Japanese Patent Laid-open Publication No. 2009-114523 (Patent Document 2) Japanese Patent Laid-open Publication No. 2017-002333 (Patent Document 3) Japanese Patent Laid-open Publication No. 2017-002332 Summary of InventionTechnical Problem

[0009] According to an embodiment of the present disclosure, provided is a steel sheet and a method for manufacturing the same.

[0010] According to an embodiment of the present disclosure, provided is a high-strength steel sheet having excellent formability and a high yield ratio, and a method for manufacturing the same.

[0011] The aspects of the present disclosure are not limited to the above-described contents. Those skilled in the art will have no difficulty in understanding additional aspects of the present disclosure from the overall contents of this specification.Solution to Problem

[0012] According to an embodiment of the present disclosure, provided is a steel sheet comprising: by wt.%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and comprising at least one selected from Ti, Nb, and V in an amount of 0.22% or less, wherein a microstructure comprises, in area%, 80 to 99% of recrystallized ferrite and 1 to 20% of cementite, and a product of a square of a tensile strength (TS) and a square root of an elongation (El) (TS 2< ×√EL) is 1.8 to 2.3×10 6< MPa 2< % 0.5< .

[0013] The steel sheet further may comprise at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.

[0014] The steel sheet may comprise at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.

[0015] The steel sheet may have a tensile strength (TS) of 610 MPa or more and a yield ratio (YR) of 0.8 to 0.95.

[0016] According to an embodiment of the present disclosure, provided is a steel sheet, comprising, by wt.%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and including at least one selected from Ti, Nb, and V in an amount of 0.22% or less, wherein a microstructure comprises, in area%, 80 to 99% of recrystallized ferrite and 1 to 20% of cementite, and a product of a square of a tensile strength (TS) and a square root of a pore expansion ratio (HER) (TS 2< ×√HER) is 2.5 to 3.8×10 6< MPa 2< % 0.5< .

[0017] The steel sheet may further comprise at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.

[0018] The steel sheet may include at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.

[0019] The steel sheet may have a tensile strength (TS) of 610 MPa or more and a yield ratio (YR) of 0.8 to 0.95.

[0020] According to an embodiment of the present disclosure, provided is a steel sheet, comprising, by wt%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and including at least one selected from Ti, Nb, and V in an amount of 0.22% or less, wherein a microstructure includes, in area%, 80 to 99% of recrystallized ferrite and 1 to 20% of cementite, and a product of a square of a tensile strength (TS) and a square root of a pore expansion ratio (HER) (TS 2< ×√HER) is 1.8 to 2.3×10 6< MPa 2< % 0.5< .

[0021] The steel sheet may further include at least one selected from, by wt %, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.

[0022] The steel sheet may include at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.

[0023] The steel sheet may have a tensile strength (TS) of 610 MPa or more and a yield ratio (YR) of 0.8 to 0.95, and a product of a square of a tensile strength (TS) and a square root of a pore expansion ratio (HER) (TS 2< ×√HER) is 2.5 to 3.8×10 6< MPa 2< % 0.5<

[0024] According to another embodiment of the present disclosure, provided is a method for manufacturing a steel sheet comprising: a reheating a steel slab comprising, by wt.%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and including at least one selected from Ti, Nb, and V in an amount of 0.22% or less; hot-rolling the reheated slab; coiling the hot-rolled steel sheet at a temperature range of 25 to 300°C; a heat treatment operation of heating the coiled steel sheet to a temperature within a range of 650 to 800°C and holding the coiled steel sheet for 600 to 1,700 seconds; cold-rolling the heat-treated steel sheet; primarily heating the cold-rolled steel sheet to a temperature within a range of 720 to 880°C and holding for 50 seconds or longer, and primarily cooling to a temperature within a range of 600 to 760°C at an average cooling rate of 1°C / s or higher; secondarily cooling the primarily cooled steel sheet to a temperature within a range of 520 to 620°C at an average cooling rate of 2°C / s or higher and holding for 20 seconds or longer; and thirdly cooling the secondarily cooled and held steel sheet to a temperature within a range of 420 to 520°C at an average cooling rate of 2°C / s or higher and then holding for 20 seconds or longer.

[0025] The steel slab may further comprise at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.

[0026] The steel sheet may comprise at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.

[0027] The reheating may be performed at a temperature range of 1000 to 1350°C, the hot-rolling may be performed at a finishing rolling temperature of 800 to 1000°C, and the cold-rolling may be performed at a reduction ratio of 30% or more.

[0028] The steel sheet may be cooled to a coiling temperature at an average cooling rate of 10°C / s or more after the hot-rolling.

[0029] The method for manufacturing a steel sheet may further include: pickling the steel sheet after the heat treatment operation.

[0030] The method for manufacturing a steel sheet may further include: plating the steel sheet after the third cooling and holding operation.Advantageous Effects of Invention

[0031] According to an embodiment of the present disclosure, a steel sheet and a method for manufacturing the steel sheet may be provided.

[0032] According to an embodiment of the present disclosure, a high-strength steel sheet having excellent formability and a high yield ratio and a method for manufacturing the same may be provided.

[0033] According to an embodiment of the present disclosure, a high-strength steel sheet that may be used for various purposes including automobile components and has excellent formability such as ductility and hole expandability, and a method for manufacturing the same may be provided.Best Mode for Invention

[0034] Hereinafter, preferred embodiments of the present disclosure will be described. The embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present disclosure in more detail to those skilled in the art to which the present disclosure pertains.

[0035] The inventors of the present disclosure has confirmed that an alloy composition and microstructure of steel may be optimized to manufacture a high-strength steel sheet having excellent formability and a high yield ratio, and have completed the present disclosure.

[0036] Hereinafter, the present disclosure will be described in detail.

[0037] First, a steel composition of the present disclosure will be described in detail.

[0038] Unless otherwise specifically stated in the present disclosure, % indicating the content of each element is based on weight.

[0039] A steel sheet according to an embodiment of the present disclosure may include, in weight %, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and may include at least one selected from Ti, Nb, and V at 0.22% or less.Carbon (C): 0.04 to 0.25%

[0040] Carbon (C) is an essential element for forming precipitates with Ti, Nb, or V in a ferrite phase to provide strength to a steel sheet. When the content of carbon (C) is less than 0.04%, it may be difficult to secure a desired level of strength. According to an embodiment of the present disclosure, carbon (C) may be comprised at the content of 0.05% or more. On the other hand, when the content thereof exceeds 0.25%, it may be difficult to secure the welding strength of a welded portion. According to an embodiment, an upper limit of the content of carbon (C) may be limited to 0.24%.Silicon (Si): 0.7% or less

[0041] Silicon (Si) is an element that has an effect of improving strength through solid solution strengthening, and is an element that strengthens ferrite, uniformizes a structure, and improves workability. Additionally, silicon (Si) is an element necessary for deoxidation during steelmaking. When the content of silicon (Si) exceeds 0.7%, there is a concern that plating defects such as underplating may occur in a plating process and that the weldability of the steel sheet may be reduced. According to an embodiment of the present disclosure, silicon (Si) may be included in the content of 0.68% or less to further improve weldability if necessary. On the other hand, a lower limit of the content may be limited to 0.01% to strengthen ferrite and uniformize the structure.Manganese (Mn): 0.46-1.8%

[0042] Manganese (Mn) is a useful element for increasing both strength and ductility. When the content of manganese (Mn) is less than 0.46%, it is difficult to secure the above-described effect. According to an embodiment of the present disclosure, manganese (Mn) may be included in the content of 0.47% or more to further improve strength and ductility if necessary. On the other hand, when the content thereof exceeds 1.8%, the formation of a low-temperature transformation phase from austenite to martensite or bainite may be promoted, which may lower a yield ratio of the steel sheet. According to an embodiment, an upper limit of the content thereof may be limited to 1.78% as needed.Aluminum (Al): 0.7% or less

[0043] Aluminum (Al) is an element combined with oxygen in steel to perform a deoxidizing effect. Additionally, like Si, aluminum (Al) is an element that strengthens ferrite, uniformizes the structure, and improves workability. When the content of aluminum (Al) exceeds 0.7%, aluminum (Al) may cause plating defects such as underplating in a plating process and may reduce the weldability of the steel sheet. According to an embodiment of the present disclosure, an upper limit thereof may be limited to 0.68% as needed to more effectively secure plating and weldability. Meanwhile, a lower limit of the content thereof may be limited to 0.01% to strengthen ferrite and uniformize the structure.Phosphorus (P): 0.05% or less

[0044] Phosphorus (P) is an element contained as an impurity in steel to deteriorate impact toughness. Accordingly, the content of phosphorus (P) may be controlled to be 0.05% or less. However, considering a portion thereof that is inevitably added during a manufacturing process, 0% is excluded.Sulfur (S): 0.03% or less

[0045] Sulfur (S) is an element contained as an impurity in steel to form MnS in a steel sheet and deteriorate ductility. Accordingly, the content of the sulfur (S) may be preferably controlled to be 0.03% or less. However, considering a portion thereof that is inevitably added during the manufacturing process, 0% is excluded.Nitrogen (N): 0.03% or less

[0046] Nitrogen (N) is an element contained as an impurity in steel to generate nitrides during continuous casting, causing cracks in a slab. Accordingly, the content of the nitrogen (N) may be preferably controlled to be 0.03% or less. However, considering a portion thereof that is inevitably added during the manufacturing process, 0% is excluded.At least one selected from titanium (Ti), niobium (Nb), and vanadium (V) in an amount of 0.22% or less

[0047] Titanium (Ti), niobium (Nb) and vanadium (V) are important elements forming precipitates of a steel sheet. Titanium (Ti), niobium (Nb) and vanadium (V) may be contained to improve the strength and impact toughness of the steel sheet. In an embodiment of the present disclosure, a sum of these contents may be 0.01% or more. When the content of at least one of titanium (Ti), niobium (Nb) and vanadium (V) exceeds 0.22%, unrecrystallized ferrite may be excessively formed due to the formation of excessive precipitates, which may cause excessive characteristic effects as well as an increase in manufacturing costs. According to an embodiment of the present disclosure, the content thereof may be limited to 0.20% or less. According to an embodiment of the present disclosure, titanium (Ti) may be 0.01 to 0.15%, niobium (Nb) may be 0.01 to 0.12%, and vanadium (V) may be 0.01 to 0.12%.

[0048] The steel of the present disclosure may comprise a balance of iron (Fe) and unavoidable impurities in addition to the composition described above. Since unavoidable impurities may be unintentionally mixed in during a normal manufacturing process, they may not be excluded. Since these impurities are known to anyone skilled in the art of normal steel manufacturing, all of their contents are not specifically mentioned in this specification.

[0049] According to an embodiment of the present disclosure, the steel sheet may further include at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, Hf: 0.2% or less.Chromium (Cr): 0.8% or less, and Molybdenum (Mo) : 0.8% or less

[0050] Chromium (Cr) and molybdenum (Mo) are elements that suppress austenite decomposition during alloying treatment and, stabilize austenite, similarly to Mn. When the content of chromium (Cr) or molybdenum (Mo) exceeds 0.8%, a low-temperature transformation phase of martensite or bainite may be promoted, which may lower a yield ratio of the steel sheet.Copper (Cu): 0.8% or less, and Nickel (Ni): 0.8% or less

[0051] Copper (Cu) and nickel (Ni) are elements that stabilize austenite and suppress corrosion. Additionally, the copper (Cu) and nickel (Ni) are concentrated on a surface of the steel sheet to prevent hydrogen from penetrating into the steel sheet, thereby suppressing hydrogen-delayed destruction. When the content of copper (Cu) or nickel (Ni) exceeds 0.8%, copper (Cu) or nickel (Ni) may cause excessive characteristic effects as well as increased manufacturing costs.Boron (B): 0.005% or less

[0052] Boron (B) is an element that improves hardenability, increases strength, and suppresses nucleation at grain boundaries. When the content of boron (B) exceeds 0.005%, not only excessive characteristic effects but also manufacturing costs may increase.Calcium (Ca): 0.05% or less, Magnesium (Mg): 0.05% or less, and rare earth elements (REM) excluding yttrium (Y): 0.05% or less

[0053] Rare earth elements (REM) refer to a total of 17 elements, including scandium (Sc), yttrium (Y), and lanthanides. Rare earth elements (REM) excluding calcium (Ca), magnesium (Mg) and yttrium (Y) are elements that improve the ductility of steel sheets by spheroidizing sulfides. When the content of rare earth elements (REM) excluding calcium (Ca), magnesium (Mg) and yttrium (Y) exceeds 0.05%, it may cause excessive characteristic effects as well as increased manufacturing costs.Tungsten (W): 0.5% or less, and zirconium (Zr): 0.5% or less

[0054] Tungsten (W) and zirconium (Zr) are elements that improve hardenability and increase the strength of the steel sheet. When the content of tungsten (W) or zirconium (Zr) exceeds 0.5%, tungsten (W) or zirconium (Zr) may cause excessive characteristic effects as well as increased manufacturing costs.Antimony (Sb): 0.5% or less, and tin (Sn): 0.5% or less

[0055] Antimony (Sb) and tin (Sn) are elements that improve the plating wettability and plating adhesion of the steel sheet. When the content of antimony (Sb) or tin (Sn) exceeds 0.5%, the brittleness of the steel sheet increases, which may cause cracks to occur during hot working or cold working.Yttrium (Y): 0.2% or less, and hafnium (Hf): 0.2% or less

[0056] Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of the steel sheet. When the content of yttrium (Y) or hafnium (Hf) exceeds 0.2%, there is a concern that the ductility of the steel sheet may deteriorate.

[0057] Hereinafter, a microstructure of the steel of the present disclosure will be described in detail.

[0058] In the present disclosure, unless otherwise specifically stated, % indicating the fraction of the microstructure is based on an area.

[0059] A microstructure of the steel sheet according to an embodiment of the present disclosure may include, in area %, 80 to 99% of recrystallized ferrite and 1 to 20% of cementite.

[0060] In the present disclosure, the microstructure may be observed through a scanning electron microscope (SEM) after etching the steel with nital. After nital etching, a structure without irregularities on a surface of the specimen may be determined as ferrite, and a structure having a spherical or lamellar structure may be determined as cementite.

[0061] On the other hand, unrecrystallized ferrite containing a large amount of dislocations has a difference in crystal orientation within grains. Accordingly, after measuring the crystal orientation of ferrite using FESEM-EBSD, unrecrystallized ferrite among ferrites may be distinguished using a Kernel Average Misorientation (KAM) method. That is, the ferrite proposed in the present disclosure may denote recrystallized ferrite, and in an embodiment of the present disclosure, recrystallized ferrite may be included in an amount of 80 to 99%.

[0062] In the present disclosure, 80% or more of ferrite may be included to secure appropriate strength and ductility. On the other hand, when an area fraction of ferrite exceeds 99%, there may be a problem of not securing a desired strength of the steel sheet.

[0063] Cementite may be included in an amount of 1% or more to secure the strength of the steel sheet. On the other hand, when an area fraction thereof exceeds 20%, there may be a problem in securing the ductility and hole expandability of the steel sheet.

[0064] Additionally, inevitable structures may be comprised as the balance, and for example, low-temperature transformation structures such as bainite and martensite may be included.

[0065] Hereinafter, a method for manufacturing the steel sheet of the present disclosure will be described in detail.

[0066] A steel sheet according to an embodiment of the present disclosure may be manufactured by reheating, hot rolling, coiling, heat treating, cold rolling, continuous annealing, and cooling a steel slab satisfying the alloy composition described above.Reheating

[0067] A steel slab satisfying the alloy composition of the present disclosure may be reheated in a temperature range of 1000 to 1350°C.

[0068] When a reheating temperature is less than 1000°C, there is a concern that hot rolling may be performed in a temperature range below a finishing rolling temperature proposed by the present disclosure. On the other hand, when the reheating temperature exceeds 1350°C, the steel may melt by reaching a melting point of the steel.Hot rolling

[0069] The reheated steel slab may be hot rolled at a finishing rolling temperature of 800 to 1000°C.

[0070] When the finishing rolling temperature is less than 800°C, the high strength of the steel slab may place a great burden on the hot rolling mill. On the other hand, when the finishing rolling temperature exceeds 1000°C, there is a concern that the grains of the steel sheet after hot rolling may be coarse, which may deteriorate the properties of the high-strength steel sheet.Coiling

[0071] The hot-rolled steel sheet may be coiled in a temperature range of 25~300°C.

[0072] In the present disclosure, the cooling rate to a coiling temperature after the hot rolling is not particularly limited, but cooling may be performed at an average cooling rate of 10°C / s or more in order to further refine the grains of the steel sheet.

[0073] Meanwhile, when a main phase of the coiled hot-rolled steel sheet is a martensite low-temperature transformation phase, a high-strength steel sheet having excellent balance of tensile strength, elongation, hole expandability and yield ratio may be manufactured after annealing heat treatment of the cold-rolled steel sheet. Accordingly, in the present disclosure, a coiling temperature of the hot-rolled steel sheet may be limited to 25 to 300°C. When the coiling temperature exceeds 300°C, it is difficult to manufacture a high-strength steel sheet having excellent balance of tensile strength, elongation, hole expandability, and yield ratio that does not include unrecrystallized ferrite by annealing and heating the cold-rolled steel sheet in a continuous annealing line or a continuous hot-dip galvanizing line. On the other hand, when the temperature is less than 25°C, there is a concern that the workability may be poor and the cold-rollability may deteriorate.Heat treatment

[0074] The coiled steel sheet may be heated in a temperature range of 650 to 800°C and may be held for 600 to 1,700 seconds.

[0075] When a heat treatment is performed at a high temperature for a short time, precipitates may be easily formed, which may improve a yield ratio. When coiling, non-precipitated Ti, Nb, and V may be re-precipitated in the steel sheet during the heat treatment at an appropriate temperature and time, which may contribute to optimizing the precipitates of the steel sheet.

[0076] When the heat treatment temperature is less than 650°C or less than 600 seconds, it may not be easy to optimize the precipitates of the heat-treated steel sheet. On the other hand, when a heat treatment conditions exceed 800°C or exceed 1700 seconds, it may not be easy to form precipitates of the heat-treated steel sheet.Cold rolling

[0077] The heat-treated steel sheet may be cold-rolled at a reduction ratio of 30% or more.

[0078] In the present disclosure, the conditions of the cold rolling process that determine a thickness of a final steel sheet are not particularly limited, but during the cold rolling, a cumulative reduction ratio may be preferably 30 to 90%. During the cold rolling, when the cumulative reduction ratio exceeds 90%, it may be difficult to perform the cold rolling in a short period of time due to the high strength of the steel sheet.

[0079] In an embodiment of the present disclosure, an operation of pickling the steel sheet before the cold rolling may be further comprised. A pickling condition is not particularly limited, and a typical condition may be applied.Primary heating and primary cooling

[0080] The cold-rolled steel sheet may be primarily heated in a temperature range of 720 to 880°C and may be held for 50 seconds or more, and primarily cooled to a temperature within a range of 600 to 760°C at an average cooling rate of 1°C / s or more.

[0081] When the heating temperature is less than 720°C, there is a concern in which non-recrystallized ferrite may be generated. On the other hand, when the heating temperature exceeds 880°C, a yield ratio of the steel sheet may be reduced.

[0082] When the holding time after the primary heating is less than 50 seconds, the heat treatment time may be insufficient to reduce a yield ratio and desired properties of the steel sheet. In the present disclosure, an upper limit of the holding time may be limited to 200 seconds in consideration of the durability and limitations of the production equipment and the production speed.

[0083] When a cooling end temperature is less than 600°C during the primary cooling, a cementite fraction may exceed 20%, which may reduce the desired properties. On the other hand, when the cooling end temperature exceeds 760°C, the strength and ductility may not be secured at a desired level.

[0084] When an average cooling rate is less than 1°C / s during the primary cooling, there is a risk of reducing the desired properties.Secondary cooling

[0085] The primarily cooled steel sheet may be secondarily cooled to a temperature within a range of 520 to 620°C at an average cooling rate of 2°C / s or more and may be held for 20 seconds or more.

[0086] When the cooling end temperature is less than 520°C during the secondary cooling, there is a problem that the yield ratio, tensile strength and hole expandability of the steel sheet may not be secured at a desired level due to the low heat treatment temperature. On the other hand, when the cooling end temperature exceeds 620°C, the cementite fraction may exceed 20%, which may reduce the strength and ductility of the steel sheet.

[0087] When the holding time after the secondary cooling is less than 20 seconds, the heat treatment time may be insufficient, making it impossible to secure the desired level of physical properties. In the present disclosure, the upper limit of the holding time may be limited to 300 seconds in consideration of the durability and restrictions of the production facility and the production speed.

[0088] When an average cooling rate is less than 2°C / s during the secondary cooling, there is a concern that the cementite fraction may exceed 20%, which may reduce the physical properties.Third cooling

[0089] The secondarily cooled and held steel sheet may be thirdly cooled to a temperature within a range of 420 to 520°C at an average cooling rate of 2°C / s or more and may then be held for 20 seconds or more.

[0090] When the cooling end temperature is lower than 420°C during the third cooling, the desired steel sheet properties may not be secured due to the low heat treatment temperature. On the other hand, when the cooling end temperature exceeds 520°C, the yield ratio, tensile strength, and hole expandability of the steel sheet may be reduced.

[0091] When the holding time after the third cooling is less than 20 seconds, the heat treatment time may be insufficient and a desired level of strength and ductility may not be properly secured. In the present disclosure, an upper limit of the holding time may be limited to 200 seconds in consideration of the durability and restrictions of the production facility and the production speed.

[0092] In the present disclosure, the thirdly cooled and held steel sheet may be cooled to room temperature. There are no particular limitations on the cooling condition after the third cooling, but air cooling may be performed as an example.Plating

[0093] According to an embodiment of the present disclosure, the thirdly cooled and held steel sheet may be plated and then cooled.

[0094] The steel sheet manufactured in the present disclosure may be plated to manufacture a plated steel sheet. In order to impart corrosion resistance in the present disclosure, hot-dip galvanizing, electrogalvanizing, and hot-dip galvanizing may be performed, and a plating condition is not particularly limited, but plating may be performed under normal conditions that may be applied in the same technical field.

[0095] The steel sheet of the present disclosure manufactured in this manner has a tensile strength (TS) of 610 MPa or more, a yield ratio (YR) of 0.8 to 0.95, a product of a square of a tensile strength and a square root of the elongation (TS 2< ×√EL) of 1.8 to 2.3×10 6< MPa 2< % 0.5< , and a product of a square of the tensile strength and a square root of a hole expansion ratio (TS 2< ×√HER) of 2.5 to 3.8×10 6< MPa 2< % 0.5< , thereby securing characteristics of an excellent balance of strength, elongation, hole expansion, and a yield ratio.Mode for Invention

[0096] Hereinafter, the present disclosure will be described in more detail through examples. However, it should be noted that the following examples are only intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the rights of the present disclosure.[Example]

[0097] A steel slab having a thickness of 100 mm, which satisfies a composition system described in Table 1, was manufactured, and the steel slab was heated at 1200°C, and then hot-rolled at a finishing rolling temperature of 900°C. The hot-rolled steel sheet was cooled at an average cooling rate of 30°C / s, and was coiled at a coiling temperature of Table 2 to manufacture a steel sheet having a thickness of 3 mm. Then, the steel sheet was heated and held according to the heat treatment conditions of Table 2. The heat-treated steel sheet was pickled to remove a surface scale, and then cold-rolled at a thickness of 1.5 mm. Additionally, the steel sheet was heated and held at the heating temperature described in Table 2, and cooled under the conditions of primary cooling, secondary cooling, and third cooling. In this case, a heating rate was 10°C / s during heating, and after the third cooling, the steel sheet was cooled to room temperature at 10°C / s. Table 1:Steel TypeAlloy Composition (wt.%)CSiMnPSAlNTiNbVTi+Nb+ VOthersA0.120.451. 410.0100.000 80.360.003 20.130.020.010.16-B0.100.381.350.0100.001 20.310.002 90.010.150.010.17-C0.130.401.380.0090.001 00.010.002 70.010.010.120.14-D0.120.391.280.0120.001 10.370.002 80.060.080.050.19-E0.150.441.390.0130.001 00.280.003 00.090.080.030.2Cr: 0.42F0.050.681.770.0090.001 20.680.002 70.080.040.050.17Mo: 0.38G0.240.011.150.0110.000 80.640.003 10.020.070.080.17Ni: 0.33H0.140.451.130.0080.000 90.310.003 00.100.020.12Cu: 0.41I0.130.391.280.0090.000 90.250.003 40.010.040.10.15B: 0.0021J0.120.321.300.0090.000 70.290.002 80.090.030.030.15Ca: 0.005K0.200.640.470.0110.000 80.550.003 00.120.020.010.15REM: 0.001L0.120.371.310.0120.001 10.380.003 20.010.080.020.11Mg: 0.002M0.140.411.420.0100.001 20.410.003 000.030.030.06W: 0.18N0.120.431.380.0090.001 00.320.002 80.010.110.020.14Zr: 0.15O0.210.500.580.0080.000 90.600.003 10.040.080.010.13Sb: 0.13P0.110.361.200.0090.001 20.230.002 90.030.020.090.14Sn: 0.08Q0.130.421.340.0100.000 80.310.003 20.010.010.110.13Y: 0.04R0.140.451.330.0120.000 90.280.002 70.020.0300.05Hf: 0.05XA0.030.431.170.0110.001 00.370.002 80.110.010.080.2-XB0.260.391. 240.0090.001 10.380.003 10.120.010.020.15-XC0.110.721.280.0070.001 00.270.003 10.130.020.010.16-XD0.130.480.440.0120.000 90.320.003 40.140.020.010.17-XE0.120.391.820.0110.001 30.290.003 00.010.120.020.15-XF0.100.421.420.0090.001 00.730.003 20.010.140.010.16-XG0.130.381.460.0110.000 80.350.003 30.240.010.020.27-XH0.110.431.370.0090.000 90.300.002 90.010.230.010.25-XI0.120.461.320.0120.001 00.270.002 80.020.020.230.27-XJ0.140.411.300.0080.000 90.330.002 70.080.090.070.24- Table 2: Speci men Numbe rSteel TypeCoil ing Temperat ure (°C)Heat treatmentPrimary Heating and Primary CoolingSecondary Cooling and MaintenanceThird Cooling and MaintenanceTem per atu re (°C)Time (seco nd)Hea tin g Tem per atu re (°C)Hold ing Time (sec ond)End Temp erat ure (°C)Avera ge Cooli ng Rate (°C / s)End Temp erat ure (°C)Avera ge Cooli ng Rate (°C / s)Hold ing Time (sec ond)End Temp erat ure (°C)Average Cooli ng Rate (°C / s)Holding Time (sec ond)1A3075011007801007001057020100470101002A55075011007801007001057020100470101003A35075011007801007001057020100470101004A3082012008001007001057020100470101005A3063012008001007001054020100470101006A307501750800120700105402080470101007A307505508001206501054020100470101008A3070011009001206501060020100440101009A30700110070010065010600201004401010010A307001100780406501060020804401010011A307001200780100770106002080500108012A307501200780120580105702080500108013A307501000780120700105700.51005001010014A30750100080012070010640201005001010015A30700120080010070010500201004701010016A3070011008601007001057020104701010017A30670110084010065010620201005401010018A30780120074010065010520201004001010019A27070012007601206501057020100470101020B150700130080012070010570201004701010021C3070012008001207001057020804701010022D3075012007801007401061020805101010023E3075011007808062010530201004301010024F307501100780606501057020100440108025G3070011008201006501057020100440108026H3070016008001006501054020804401010027I3070070080012070010540201004401010028J3070014007601207001057020805001010029K307508007801007001057020805001010030L30750100082012070010540201005001010031M3075010007809065010570201005001010032N30750100078010065010570201004701010033O3070011008001206501054020804701010034P3070011008001007001057020804701010035Q3075012008201006501054020100470108036R3075012008001006501057020804701010037XA3075012007801207001057020100470108038XB30700120078012070010540201005001010039XC30700130078010070010570201005001010040XD30700130078010065010570201005001010041XE3070013008001006501054020804701010042XF30750110080012070010540201004701010043XG30750110080012070010570201004701010044XH30700120078010065010540201004701010045XI30700120078010065010570201004701010046XJ307001200780100650105402010047010100

[0098] Table 3 below illustrates a microstructure and physical properties of the manufactured steel sheet observed and measured. A microstructure of the steel sheet was observed through a scanning electron microscope (SEM) after performing nital etching on a polished specimen cross-section with. After the nital etching, a structure without irregularities on a specimen surface was determined to be recrystallized ferrite, and a structure having a spherical or lamellar structure was determined to be cementite. Unrecrystallized ferrite including a large amount of dislocations has crystal orientation differences within the grains. Accordingly, after measuring the crystal orientation of ferrite using FESEM-EBSD, the unrecrystallized ferrite among the ferrites was distinguished using the Kernel Average Misorientation (KAM) method.

[0099] The physical properties of the steel sheet were evaluated by a tensile test and a hole expansion test. By means of the tensile test was conducted for evaluation with a test piece collected in accordance with the JIS No. 5 standard based on the 0° direction with respect to a rolling direction of a rolled plate, thus calculating a yield ratio (YR) and a product (TS 2< ×√EL) of a square of the tensile strength and a square root of the elongation. The yield ratio (YR) refers to a value obtained by dividing the yield strength (YS) by the tensile strength (TS). The hole expansion test was conducted by pressing and expanding a cone punch at a top angle of 60° to a punching hole having a diameter of 10mm (die internal diameter of 10.3mm, clearance of 12.5%) in a direction in which a burr of the punching hole is an outer side. Accordingly, the hole expansion ratio (HER) was calculated using the following formula. Accordingly, the product of the square of the tensile strength and the square root of the hole expansion ratio (TS 2< ×√HER) was calculated.

[0100] HER % = D - D 0 / D 0 × 100

[0101] (In the formula, D is a hole diameter (mm) when cracks penetrate through a plate thickness, and D 0 is an initial hole diameter (mm).) Table 3:Speci men Numbe rSteel TypeMicrostructure (area%)PropertiesDivisionRecrysta llized FerriteUnrecrys tallized ferriteCementiteYield ratioTS 2< ×√EL (MPa 2< % 0.5< )TS 2< ×√HER (MPa 2< % 0.5< )1A860140.872,054,6973,228,146Inventive Example 12A789130.761,965,5722,937,628Comparative Example 13A778150.741,827,9152,849,204Comparative Example 24A840160.821,662,2852,305,812Comparative Example 35A850150.831,584,3182,278,615Comparative Example 46A880120.861,765,0042,306,259Comparative Example 57A870130.881,691,3472,275,381Comparative Example 68A788140.751,927,3062,730,537Comparative Example 79A797140.962,483,2803,921,342Comparative Example 810A789130.972,357,5494,055,316Comparative Example 911A900100.811,741,2642,240,370Comparative Example 1012A780220.851,634,5032,349,862Comparative Example 1113A730270.821,531,1972,129,367Comparative Example 1214A790210.832,560,7633,945,612Comparative Example 1315A870130.981,534,0962,290,303Comparative Example 1416A92080.881, 621, 7452,161,538Comparative Example 1517A799120.972,455,2984,037,142Comparative Example 1618A870130.831,723,6342,334,908Comparative Example 1719A860140.851,522,5922,430,652Comparative Example 1820B880120.842,056,3473,168,559Inventive Example 221C880120.922,181,5493,756,845Inventive Example 322D97030.862,295,6343,608,472Inventive Example 423E91090.851,821,0572,620,335Inventive Example 524F820180.831,865,3482,539,856Inventive Example 625G810190.932, 152, 6052,862,307Inventive Example 726H91090.842,267,3503,342,178Inventive Example 827I840160.832,196,7423,079,613Inventive Example 928J900100.851, 953, 9652,899,584Inventive Example 1029K880120.822,076,5392,931,608Inventive Example 1130L850150.932,161,5323,165,205Inventive Example 1231M880120.902,272,6103,210,487Inventive Example 1332N860140.872,195,1243,352,296Inventive Example 1433O900100.912,087,9873,085,631Inventive Example 1534P92080.882, 194, 9523,173,724Inventive Example 1635Q870130.851,985,2932,837,918Inventive Example 1736R890110.862,0928,06 82,797,257Inventive Example 1837XA93070.761,605,7212,367,862Comparative Example 1938XB7710130.912,568,5903,935,167Comparative Example 2039XC850150.891,529,2572,382,069Comparative Example 2140XD831070.851,643,7242,287,525Comparative Example 2241XE779140.902,546,4104,067,283Comparative Example 2342XF870130.891,525, 6982,404,376Comparative Example 2443XG788140.982,492,8143,934,620Comparative Example 2544XH7711120.972,670,1364,165,305Comparative Example 2645XI787150.982,569,5313,937,543Comparative Example 2746XJ7610140.962,685,3924,037,934Comparative Example 28

[0102] As shown in Table 3, in the case of inventive examples satisfying an alloy composition and manufacturing conditions of the present disclosure, the microstructure characteristics proposed by the present disclosure were satisfied, and the properties targeted by the present disclosure may also be secured.

[0103] On the other hand, in Comparative Examples 1 and 2, the coiling temperature exceeded the temperature range proposed by the present disclosure. Since the coiling temperature was high, unrecrystallized ferrite existed after the heat treatment of the cold-rolled steel sheet, and the desired yield ratio may not be secured.

[0104] Comparative Examples 3 and 4 are examples in which the heat treatment temperature was outside the range of the present disclosure. In Comparative Example 3, since the heat treatment temperature was excessively high, precipitation was not easily performed. As a result, it was difficult to secure the desired properties. In Comparative Example 4, since the heat treatment temperature was below the range of the present disclosure, precipitation optimization was not easy, and the desired strength and ductility were not be secured.

[0105] Comparative Examples 5 and 6 are cases in which the heat treatment time was outside the range of the present disclosure. Comparative Examples 5 and 6 are cases in which the heat treatment time is excessively long or short, and since precipitation was not easily performed in Comparative Examples 5 and 6, the properties targeted by the present disclosure were not secured.

[0106] Comparative Examples 7 and 8 are cases in which the first heating and holding temperature are outside the temperature range proposed by the present disclosure. In Comparative Example 7, since the heating temperature was excessively high, an area fraction of recrystallized ferrite was below a value proposed by the present disclosure, and as a result, the yield ratio was inferior. In Comparative Example 8, since the heating temperature was low, unrecrystallized ferrite was present, and the desired strength and ductility were not secured.

[0107] Comparative Example 9 is a case in which the holding time after the first heating was insufficient, and unrecrystallized ferrite was formed. As a result, the desired strength and elongation were not secured.

[0108] Comparative Examples 10 and 11 are cases in which the cooling end temperature proposed by the present disclosure was not satisfied during the primary cooling. In Comparative Example 10, since the cooling end temperature was exceeded during the primary cooling, the properties were inferior. In Comparative Example 11, since the cooling end temperature was significantly low during the primary cooling, so that cementite was excessively formed, and as a result, the desired strength and elongation were not secured.

[0109] In Comparative Example 12, since the average cooling rate was low during the secondary cooling, the cementite fraction was excessive. As a result, the physical properties were inferior.

[0110] In Comparative Examples 13 and 14, the cooling end temperature during the secondary cooling was outside the range of the present disclosure. In Comparative Example 13, since the cooling end temperature was exceeded during the secondary cooling, the cementite fraction was excessive, and the desired physical properties were not secured. In Comparative Example 14, the yield ratio exceeded the desired range because the secondary cooling end temperature was low, and the strength and elongation were also inferior.

[0111] In Comparative Example 15, since the holding time after the secondary cooling was below the range proposed by the present disclosure, the desired physical properties were not secured.

[0112] In Comparative Examples 16 and 17, the cooling end temperature during the third cooling was outside the range of the present disclosure. In Comparative Example 16, the cooling end temperature was excessively high during the third cooling, and unrecrystallized ferrite was formed. As a result, the recrystallized ferrite fraction was insufficient, the yield ratio exceeded the proposed range, and the desired strength and ductility were not secured. Comparative Example 17 is a case in which the cooling end temperature was insufficient during the third cooling, which made it difficult to secure strength and ductility.

[0113] In Comparative Example 18, since the holding time was short after the third cooling, the desired physical properties and elongation were not secured.

[0114] In Comparative Examples 19 and 20, the carbon content was outside the range of the present disclosure. Comparative Example 19 is a case in which the content of carbon was insufficient, where the yield ratio was insufficient, and the strength and elongation were reduced. Comparative Example 20 is a case in which the carbon content was excessive, where non-recrystallized ferrite was formed, and as a result, the recrystallized ferrite fraction was insufficient, and the desired physical properties were not secured.

[0115] Comparative Example 21 did not secure the desired strength and elongation because the silicon content exceeded the range proposed by the present disclosure.

[0116] Comparative Examples 22 and 23 are examples in which the content of manganese was outside the range of the present disclosure. Comparative Example 22 did not secure the desired strength and ductility because the content of manganese was insufficient, and in Comparative Example 23, the content of manganese was excessive, and unrecrystallized ferrite was formed, which exceeded a proposed property level.

[0117] Comparative Example 24 did not satisfy the proposed property level of the present disclosure because the aluminum content was excessive.

[0118] Comparative examples 25 to 28 are a case in which a total content of titanium, niobium, and vanadium exceeded the range proposed by the present invention, where unrecrystallized ferrite was formed and recrystallized ferrite was insufficient. As a result, the yield ratio exceeded a desired level, and the strength and elongation were also excessive.

[0119] Although the present disclosure has been described in detail through examples, other forms of examples are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the embodiments.

Claims

1. A steel sheet, comprising: by wt.%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and comprising at least one selected from Ti, Nb, and V in an amount of 0.22% or less, wherein a microstructure comprises, in area%, 80 to 99% of recrystallized ferrite and 1 to 20% of cementite, and a product of a square of a tensile strength (TS) and a square root of an elongation (El) (TS2×√EL) is 1.8 to 2.3×106 MPa2%0.5.

2. The steel sheet of claim 1, wherein the steel sheet further comprises at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.

3. The steel sheet of claim 1, wherein the steel sheet comprises at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.

4. The steel sheet of claim 1, wherein the steel sheet has a tensile strength (TS) of 610 MPa or more and a yield ratio (YR) of 0.8 to 0.95.

5. A steel sheet, comprising, by wt.%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and comprising at least one selected from Ti, Nb, and V in an amount of 0.22% or less, wherein a microstructure comprises, in area%, 80 to 99% of recrystallized ferrite and 1 to 20% of cementite, and a product of a square of a tensile strength (TS) and a square root of a pore expansion ratio (HER) (TS2×√HER) is 2.5 to 3.8×106 MPa2%0.5.

6. The steel sheet of claim 5, wherein the steel sheet further comprises at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.

7. The steel sheet of claim 5, wherein the steel sheet comprises at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.

8. The steel sheet of claim 5, wherein the steel sheet has a tensile strength (TS) of 610 MPa or more and a yield ratio (YR) of 0.8 to 0.95.

9. A method for manufacturing a steel sheet, comprising: reheating a steel slab comprising, by wt.%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and comprising at least one selected from Ti, Nb, and V in an amount of 0.22% or less; hot-rolling the reheated slab; coiling the hot-rolled steel sheet at a temperature range of 25 to 300°C; a heat treatment operation of heating the coiled steel sheet to a temperature within a range of 650 to 800°C and holding for 600 to 1,700 seconds; cold-rolling the heat-treated steel sheet; primarily heating the cold-rolled steel sheet to a temperature within a range of 720 to 880°C and holding for 50 seconds or longer, and primarily cooling to a temperature within a range of 600 to 760°C at an average cooling rate of 1°C / s or higher; secondarily cooling the primarily cooled steel sheet to a temperature within a range of 520 to 620°C at an average cooling rate of 2°C / s or higher and holding for 20 seconds or longer; and thirdly cooling the secondarily cooled and held steel sheet to a temperature within a range of 420 to 520°C at an average cooling rate of 2°C / s or higher and then holding the steel sheet for 20 seconds or longer.

10. The method for manufacturing a steel sheet of claim 9, wherein the steel slab further comprises at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.

11. The method for manufacturing a steel sheet of claim 9, wherein the steel sheet comprises at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.

12. The method for manufacturing a steel sheet of claim 9, wherein the reheating is performed at a temperature range of 1000 to 1350°C, the hot-rolling is performed at a finishing rolling temperature of 800 to 1000°C, and the cold-rolling is performed at a reduction ratio of 30% or more.

13. The method for manufacturing a steel sheet of claim 9, wherein the hot-rolled steel sheet is cooled to a coiling temperature at an average cooling rate of 10°C / s or more after the hot-rolling.

14. The method for manufacturing a steel sheet of claim 9, further comprising: pickling the steel sheet after the heat treatment operation.

15. The method for manufacturing a steel sheet of claim 9, further comprising: plating the steel sheet after the third cooling and holding operation.

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