Steel plate and its manufacturing method

Abstract

The present invention relates to a steel plate and a manufacturing method thereof, and more particularly to a high-strength steel plate having excellent formability and a high yield ratio, and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a steel plate and a manufacturing method thereof, and more particularly to a high-strength steel plate having excellent formability and a high yield ratio, and a manufacturing method thereof. [Background technology]

[0002] Recently, the automotive industry has been focusing on ensuring passenger safety and improving fuel efficiency. For these reasons, the use of high-strength steel sheets to meet the requirements for safety and weight reduction has been increasing in the automotive body materials.

[0003] In order to improve the crashworthiness of automobile bodies, increasing the yield strength of steel allows it to efficiently absorb collision energy even with a small amount of deformation. Methods for increasing yield strength include solid solution strengthening and precipitation strengthening.

[0004] Solid-solution strengthened steel is a steel that has increased yield strength by dissolving solid-solution strengthening elements (Mn, Si, Cr, etc.) in the ferrite phase, which has excellent formability. However, Si and Cr are elements that tend to form oxides on the steel sheet surface in continuous annealing lines or continuous hot-dip galvanizing lines. Furthermore, Mn is an element that promotes the formation of low-temperature transformation phases (bainite or martensite), which have the characteristic of reducing yield strength. Therefore, solid-solution strengthened steel with large amounts of Mn, Si, and Cr added is not suitable as a method for increasing the yield ratio of high-strength steel with a tensile strength of 610 MPa or more.

[0005] On the other hand, precipitation-hardened steels using Nb, Ti, V, etc. improve yield strength by precipitating fine carbides in ferrite. Precipitation-hardened steels increase the yield ratio without deteriorating workability, making them a suitable strengthening mechanism for high-strength steel sheets with a tensile strength of 610 MPa or more, which have excellent crashworthiness and workability. Patent Documents 1 and 2 disclose methods for improving the formability and yield ratio of steel sheets by introducing non-recrystallized ferrite and adding Ti or Nb. Precipitation-hardened steels using Ti or Nb and non-recrystallized ferrite directly strengthen ferrite, which is effective in increasing yield strength without significantly increasing tensile strength.

[0006] However, the techniques described in Patent Documents 1 and 2 do not have an appropriate unrecrystallized ferrite fraction, and therefore are unable to adequately achieve both excellent formability and a high yield ratio.

[0007] The technology described in Patent Document 3 has an appropriate level of area fraction of unrecrystallized ferrite, but the balance of tensile strength, elongation, hole expandability, and yield ratio is not appropriate. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-114523 [Patent Document 2] Japanese Patent Application Publication No. 2017-002333 [Patent Document 3] Japanese Patent Application Publication No. 2017-002332 Summary of the Invention [Problem to be solved by the invention]

[0009] One embodiment of the present invention is to provide a steel sheet and a method for manufacturing the same.

[0010] An embodiment of the present invention provides a high strength steel sheet having excellent formability and a high yield ratio, and a method for manufacturing the same.

[0011] The object of the present invention is not limited to the above-mentioned contents, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the general contents of this specification. [Means for solving the problem]

[0012] According to one embodiment of the present invention, the composition is, by 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, the balance being Fe and other unavoidable impurities, and the composition contains one or more selected from Ti, Nb, and V in an amount of 0.22% or less; The microstructure contains, by area, 80 to 99% recrystallized ferrite and 1 to 20% cementite; The square of the tensile strength (TS) and the square root of the elongation ratio (El) (TS 2 ×√EL) is 1.8 to 2.3 × 10 6 MPa 2 % 0.5 It is possible to provide a steel sheet having the above formula:

[0013] The steel plate may further contain, by weight percent, one or more selected from 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] One or more elements selected from the above Ti, Nb and V may be contained in an amount of 0.01 to 0.22%.

[0015] The steel plate 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 one embodiment of the present invention, the composition is, by 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, the balance being Fe and other unavoidable impurities, and the composition contains one or more selected from Ti, Nb, and V in an amount of 0.22% or less; The microstructure contains, by area, 80 to 99% recrystallized ferrite and 1 to 20% cementite; The square of the tensile strength (TS) and the square root of the hole expansion ratio (HER) (TS 2 ×√HER) is 2.5 to 3.8 × 10 6 MPa 2 % 0.5 It is possible to provide a steel sheet having the above formula:

[0017] The steel plate may further contain, by weight percent, one or more selected from 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] One or more elements selected from the above Ti, Nb and V may be contained in an amount of 0.01 to 0.22%.

[0019] The steel plate 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 one embodiment of the present invention, the composition is, by 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, the balance being Fe and other unavoidable impurities, and the composition contains one or more selected from Ti, Nb, and V in an amount of 0.22% or less; The microstructure contains, by area, 80 to 99% recrystallized ferrite and 1 to 20% cementite; The square of the tensile strength (TS) and the square root of the elongation ratio (El) (TS 2 ×√EL) is 1.8 to 2.3 × 10 6 MPa 2 % 0.5 It is possible to provide a steel sheet having the above formula:

[0021] The steel plate may further contain, by weight percent, one or more selected from 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] One or more elements selected from the above Ti, Nb and V may be contained in an amount of 0.01 to 0.22%.

[0023] The steel sheet has a tensile strength (TS) of 610 MPa or more, a yield ratio (YR) of 0.8 to 0.95, and a product of the square of the tensile strength (TS) and the square root of the hole expansion ratio (HER) (TS 2 ×√HER) is 2.5 to 3.8 × 10 6 MPa 2 % 0.5 It can be.

[0024] According to another embodiment of the present invention, a step of reheating a steel slab containing, by 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, the balance being Fe and other unavoidable impurities, and containing one or more 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 in the range of 25 to 300°C; a heat treatment step of heating the coiled steel sheet to a temperature range of 650 to 800°C and maintaining the temperature for 600 to 1700 seconds; cold rolling the heat-treated steel sheet; a step of primarily heating the cold-rolled steel sheet to a temperature range of 720 to 880°C, maintaining the temperature for 50 seconds or more, and primarily cooling the cold-rolled steel sheet to a temperature range of 600 to 760°C at an average cooling rate of 1°C / s or more; Secondarily cooling the primarily cooled steel sheet to a temperature range of 520 to 620°C at an average cooling rate of 2°C / s or more, and maintaining the temperature for 20 seconds or more; and tertiary cooling the steel sheet after the secondary cooling and maintaining to a temperature range of 420 to 520°C at an average cooling rate of 2°C / s or more, and maintaining the temperature for 20 seconds or more.

[0025] The steel slab may further contain, by weight, one or more selected from 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] One or more elements selected from the above Ti, Nb and V may be contained in an amount of 0.01 to 0.22%.

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

[0028] After the hot rolling step, the sheet can be cooled to the coiling temperature at an average cooling rate of 10°C / s or more.

[0029] After the heat treatment step, the method may further include pickling the steel sheet.

[0030] After the tertiary cooling and maintaining step, the method may further include a step of plating the steel sheet. [Effects of the Invention]

[0031] According to one embodiment of the present invention, a steel sheet and a manufacturing method thereof can be provided.

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

[0033] According to one embodiment of the present invention, it is possible to provide a high-strength steel sheet that can be used for various applications including automotive parts and has excellent formability such as ductility and hole expandability, and a manufacturing method thereof. DETAILED DESCRIPTION OF THE INVENTION

[0034] Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments are provided to further explain the present invention to those of ordinary skill in the art to which the present invention pertains.

[0035] The present inventors have confirmed that high-strength steel sheets having excellent formability and a high yield ratio can be manufactured by optimizing the alloying components and microstructure of steel materials, and have completed the present invention.

[0036] The present invention will be described in detail below.

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

[0038] In the present invention, unless otherwise specified, the percentage representing the content of each element is based on weight.

[0039] A steel sheet according to one embodiment of the present invention contains, by weight percent, 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, the balance being Fe and other unavoidable impurities, and may contain one or more selected from Ti, Nb, and V in an amount of 0.22% or less.

[0040] Carbon (C): 0.04~0.25% Carbon (C) is an essential element for forming precipitates together with Ti, Nb, or V in the ferrite phase to impart strength to the steel sheet. If the carbon (C) content is less than 0.04%, it may be difficult to ensure the desired level of strength. According to one embodiment of the present invention, it may be contained in an amount of 0.05% or more. On the other hand, if the carbon (C) content exceeds 0.25%, it may be difficult to ensure the weld strength of the weld. According to one embodiment, the upper limit of the carbon (C) content may be limited to 0.24%.

[0041] Silicon (Si): 0.7% or less Silicon (Si) is an element that has the effect of improving strength through solid solution strengthening, strengthening ferrite, homogenizing the structure, and improving workability. It is also an element necessary for deoxidation during steelmaking. If the silicon (Si) content exceeds 0.7%, problems such as uncoated areas may occur during the plating process, and the weldability of the steel sheet may be reduced. According to one embodiment of the present invention, silicon (Si) may be contained in an amount of 0.68% or less to further improve weldability as needed. On the other hand, in order to strengthen ferrite and homogenize the structure, the lower limit of the silicon content may be limited to 0.01%.

[0042] Manganese (Mn): 0.46-1.8% Manganese (Mn) is an element useful for simultaneously increasing strength and ductility. If the manganese (Mn) content is less than 0.46%, it is difficult to ensure the above-mentioned effects. According to one embodiment of the present invention, manganese (Mn) can be contained in an amount of 0.47% or more to further improve strength and ductility, as needed. On the other hand, if the manganese (Mn) content exceeds 1.8%, the formation of a low-temperature transformation phase from austenite to martensite or bainite is promoted, which may result in a decrease in the yield ratio of the steel sheet. According to one embodiment, the upper limit of the manganese (Mn) content can be limited to 1.78% as needed.

[0043] Aluminum (Al): 0.7% or less Aluminum (Al) is an element that combines with oxygen in steel to deoxidize it. Similarly to Si, it also strengthens ferrite, homogenizes the structure, and improves workability. If the aluminum (Al) content exceeds 0.7%, it may cause problems with plating defects, such as unplated areas, during the plating process and may also reduce the weldability of the steel sheet. According to one embodiment of the present invention, the upper limit of the Al content may be limited to 0.68% if necessary to more effectively ensure plating and weldability. Meanwhile, the lower limit of the Al content may be limited to 0.01% to strengthen ferrite and homogenize the structure.

[0044] Phosphorus (P): 0.05% or less Phosphorus (P) is an element that is contained as an impurity in steel and deteriorates impact toughness. Therefore, the phosphorus (P) content can be controlled to 0.05% or less. However, 0% is excluded to take into account the portion that is inevitably added during the manufacturing process.

[0045] Sulfur (S): 0.03% or less Sulfur (S) is an element contained in steel as an impurity and forms MnS in the steel sheet, which deteriorates ductility. Therefore, the sulfur (S) content is preferably controlled to 0.03% or less. However, 0% is excluded in consideration of the portion that is inevitably added during the manufacturing process.

[0046] Nitrogen (N): 0.03% or less Nitrogen (N) is an element that is contained as an impurity in steel and forms nitrides during continuous casting, causing cracks in the slab. Therefore, it is preferable to control the nitrogen (N) content to 0.03% or less. However, 0% is excluded to take into account the amount of nitrogen that is inevitably added during the manufacturing process.

[0047] One or more selected from titanium (Ti), niobium (Nb), and vanadium (V) 0.22% or less Titanium (Ti), niobium (Nb), and vanadium (V) are important elements that form precipitates in steel sheets. They can be added to improve the strength and impact toughness of steel sheets. In one embodiment of the present invention, the total content of these elements may be 0.01% or more. If the content of one or more of titanium (Ti), niobium (Nb), and vanadium (V) exceeds 0.22%, excessive precipitates may be formed, resulting in excessive formation of unrecrystallized ferrite, which may result in excessive performance effects and increased manufacturing costs. In one embodiment of the present invention, the content may be limited to 0.20% or less. In one embodiment of the present invention, 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] In addition to the above-described composition, the steel material of the present invention may contain the remaining iron (Fe) and inevitable impurities. The inevitable impurities cannot be excluded because they may be unintentionally mixed in during the normal manufacturing process. Since such impurities are known to any engineer in the field of normal steel manufacturing, the contents of all of them will not be specifically mentioned in this specification.

[0049] The steel sheet according to one embodiment of the present invention may further include, by weight percent, one or more selected from 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.

[0050] Chromium (Cr): 0.8% or less, Molybdenum (Mo): 0.8% or less Chromium (Cr) and molybdenum (Mo) are elements that inhibit the decomposition of austenite during alloying and, like Mn, stabilize austenite. If the chromium (Cr) or molybdenum (Mo) content exceeds 0.8%, the low-temperature transformation phase of martensite or bainite is promoted, which may result in a decrease in the yield ratio of the steel sheet.

[0051] Copper (Cu): 0.8% or less, Nickel (Ni): 0.8% or less Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. Furthermore, copper (Cu) and nickel (Ni) concentrate on the steel sheet surface to prevent the penetration of hydrogen that migrates into the steel sheet, thereby inhibiting delayed hydrogen fracture. If the copper (Cu) or nickel (Ni) content exceeds 0.8%, it may not only result in excessively poor performance but also increase manufacturing costs.

[0052] Boron (B): 0.005% or less Boron (B) is an element that improves hardenability, increases strength, and inhibits nucleation of grain boundaries. If the boron (B) content exceeds 0.005%, not only will excessive property effects be observed, but manufacturing costs may also increase.

[0053] Calcium (Ca): 0.05% or less, Magnesium (Mg): 0.05% or less, Rare Earth Elements (REM) excluding Yttrium (Y): 0.05% or less 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) improve the ductility of steel sheets by spheroidizing sulfides. If the content of rare earth elements (REM) excluding calcium (Ca), magnesium (Mg), and yttrium (Y) exceeds 0.05%, it may not only result in excessive property effects but also increase manufacturing costs.

[0054] Tungsten (W): 0.5% or less, Zirconium (Zr): 0.5% or less Tungsten (W) and zirconium (Zr) are elements that improve hardenability and increase the strength of steel sheets. If the content of tungsten (W) or zirconium (Zr) exceeds 0.5%, it may not only have excessive property effects but also cause an increase in manufacturing costs.

[0055] Antimony (Sb): 0.5% or less, Tin (Sn): 0.5% or less Antimony (Sb) and tin (Sn) are elements that improve the plating wettability and plating adhesion of steel sheets. If the content of antimony (Sb) or tin (Sn) exceeds 0.5%, the brittleness of the steel sheet increases, and cracks may occur during hot or cold working.

[0056] Yttrium (Y): 0.2% or less, Hafnium (Hf): 0.2% or less Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel sheets. If the content of yttrium (Y) or hafnium (Hf) exceeds 0.2%, the ductility of the steel sheet may deteriorate.

[0057] The microstructure of the steel of the present invention will now be described in detail.

[0058] In the present invention, unless otherwise specified, the percentage representing the fraction of the microstructure is based on the area.

[0059] The microstructure of the steel sheet according to one embodiment of the present invention may contain, in area %, 80 to 99% recrystallized ferrite and 1 to 20% cementite.

[0060] In the present invention, the microstructure can be observed using a scanning electron microscope (SEM) after nital etching of the steel material. After nital etching, a structure with no irregularities on the surface of the test piece is determined to be ferrite, and a structure with a spherical or lamellar structure is determined to be cementite.

[0061] On the other hand, unrecrystallized ferrite containing many dislocations has crystal misorientation within the grains. Therefore, after measuring the crystal orientation of ferrite using FESEM-EBSD, the unrecrystallized ferrite in the ferrite can be distinguished using the KAM (Kernel Average Misorientation) method. That is, in the present invention, the proposed ferrite can mean recrystallized ferrite, and in one embodiment of the present invention, the ferrite can contain 80 to 99% recrystallized ferrite.

[0062] In the present invention, ferrite can be contained in an amount of 80% or more to ensure appropriate strength and ductility. On the other hand, if the area fraction of ferrite exceeds 99%, there is a possibility that a problem will occur in which the desired strength of the steel sheet cannot be ensured.

[0063] Cementite can be contained in an amount of 1% or more to ensure the strength of the steel sheet. On the other hand, if the area fraction of cementite exceeds 20%, there is a possibility that the ductility and hole expandability of the steel sheet cannot be ensured.

[0064] In addition, the balance may contain unavoidable structures, for example, low-temperature transformation structures such as bainite and martensite.

[0065] The method for producing a steel sheet according to the present invention will be described in detail below.

[0066] The steel sheet according to one embodiment of the present invention can be produced by reheating a steel slab satisfying the above-mentioned alloy composition, hot rolling, coiling, heat treating, cold rolling, continuous annealing, and cooling.

[0067] reheating A steel slab satisfying the alloy composition of the present invention can be reheated to a temperature range of 1000 to 1350°C.

[0068] If the reheating temperature is less than 1000°C, there is a risk that hot rolling will be performed in a temperature range below the finish rolling temperature proposed in the present invention, whereas if the reheating temperature exceeds 1350°C, there is a risk that the steel will reach its melting point and melt.

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

[0070] If the finish rolling temperature is less than 800°C, the high strength of the steel slab may place a heavy burden on the hot rolling mill, whereas if the finish rolling temperature is more than 1000°C, the crystal grains of the hot-rolled steel sheet may become coarse, which may deteriorate the physical properties of the high-strength steel sheet.

[0071] Winding The hot-rolled steel sheet can be coiled at a temperature in the range of 25 to 300°C.

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

[0073] On the other hand, when the main phase of the coiled hot-rolled steel sheet is a martensite low-temperature transformation phase, a high-strength steel sheet having an excellent balance of tensile strength, elongation, hole expandability, and yield ratio can be produced after annealing heat treatment of the cold-rolled steel sheet. Therefore, in the present invention, the coiling temperature of the hot-rolled steel sheet can be limited to 25 to 300°C. If the coiling temperature exceeds 300°C, it is difficult to produce a high-strength steel sheet that does not contain unrecrystallized ferrite and has an excellent balance of tensile strength, elongation, hole expandability, and yield ratio by subjecting the cold-rolled steel sheet to annealing heat treatment in a continuous annealing line or continuous hot-dip galvanizing line. On the other hand, if the temperature is less than 25°C, workability may deteriorate, and cold rolling ability may be impaired.

[0074] Heat Treatment The coiled steel sheet can be subjected to a heat treatment in which the steel sheet is heated to a temperature range of 650 to 800°C and maintained at that temperature for 600 to 1700 seconds.

[0075] When heat treatment is performed at high temperatures for a short time, precipitates are easily formed, which can improve the yield ratio. During coiling, unprecipitated Ti, Nb, and V are reprecipitated in the steel sheet during heat treatment at the appropriate temperature and time, which contributes to optimizing the precipitates in the steel sheet.

[0076] If the heat treatment temperature is less than 650°C or less than 600 seconds, it may not be easy to optimize the precipitation of the heat-treated steel sheet, while if the heat treatment conditions are more than 800°C or more than 1700 seconds, it may not be easy to optimize the precipitation of the heat-treated steel sheet.

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

[0078] In the present invention, the conditions of the cold rolling step that determine the thickness of the final steel sheet are not particularly limited, but the cumulative reduction rate during cold rolling is preferably 30 to 90%. If the cumulative reduction rate during cold rolling exceeds 90%, the high strength of the steel sheet may make it difficult to perform cold rolling in a short time.

[0079] In one embodiment of the present invention, the method may further include pickling the steel sheet before cold rolling. The pickling conditions are not particularly limited, and conventional conditions may be applied.

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

[0081] If the heating temperature is less than 720°C, there is a risk of non-recrystallized ferrite being generated, whereas if the heating temperature exceeds 880°C, there is a risk of the yield ratio of the steel sheet being reduced.

[0082] If the maintenance time after primary heating is less than 50 seconds, the yield ratio and the desired physical properties of the steel sheet may be reduced due to insufficient heat treatment time. In the present invention, the upper limit of the maintenance time may be limited to 200 seconds, taking into consideration the durability and limitations of the production equipment and the production rate.

[0083] If the cooling end temperature during primary cooling is less than 600°C, the cementite fraction will exceed 20%, which may result in a decrease in the desired physical properties. On the other hand, if the cooling end temperature exceeds 760°C, the desired levels of strength and ductility cannot be achieved.

[0084] If the average cooling rate during the primary cooling is less than 1°C / s, the desired physical properties may be reduced.

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

[0086] If the cooling end temperature during secondary cooling is less than 520°C, there is a problem that the yield ratio, tensile strength, and hole expandability of the steel sheet cannot be ensured at the desired levels at a low heat treatment temperature. On the other hand, if the cooling end temperature exceeds 620°C, the cementite fraction exceeds 20%, which may reduce the strength and ductility of the steel sheet.

[0087] If the holding time after secondary cooling is less than 20 seconds, the heat treatment time is insufficient and the desired level of physical properties may not be achieved. In the present invention, the upper limit of the holding time can be limited to 300 seconds, taking into consideration the durability and limitations of the production equipment and the production rate.

[0088] If the average cooling rate during secondary cooling is less than 2°C / s, the cementite fraction will exceed 20%, which may result in a deterioration in physical properties.

[0089] tertiary cooling The steel sheet that has been secondarily cooled and maintained may be thirdly cooled to a temperature range of 420 to 520°C at an average cooling rate of 2°C / s or more, and maintained for 20 seconds or more.

[0090] If the cooling end temperature during tertiary cooling is less than 420°C, the desired physical properties of the steel sheet cannot be ensured at a low heat treatment temperature, whereas if the cooling end temperature exceeds 520°C, the yield ratio, tensile strength, and hole expandability of the steel sheet may be reduced.

[0091] If the holding time after tertiary cooling is less than 20 seconds, the heat treatment time is insufficient and the desired levels of strength and ductility cannot be adequately secured. In the present invention, the upper limit of the holding time can be limited to 200 seconds, taking into consideration the durability and limitations of the production equipment and the production rate.

[0092] In the present invention, the steel sheet that has been subjected to the tertiary cooling and maintained thereafter may be cooled to room temperature. The cooling conditions after the tertiary cooling are not particularly limited, but air cooling may be used as an example.

[0093] plating According to an embodiment of the present invention, the steel sheet that has been tertiary cooled and maintained may be cooled after being coated.

[0094] The steel sheet produced according to the present invention can be plated to produce a plated steel sheet. In the present invention, hot-dip galvanizing, electrogalvanizing, or hot-dip galvanizing can be performed to impart corrosion resistance. The plating conditions are not particularly limited, but plating can be performed under normal conditions applicable in the same technical field.

[0095] The steel sheet of the present invention thus manufactured has a tensile strength (TS) of 610 MPa or more, a yield ratio (YR) of 0.8 to 0.95, and a product of the square of the tensile strength and the square root of the elongation ratio (TS 2 ×√EL) is 1.8 to 2.3 × 10 6 MPa 2 % 0.5 and the product of the square of the tensile strength and the square root of the hole expansion ratio (TS 2×√HER) is 2.5 to 3.8 × 10 6 MPa 2 % 0.5 It is possible to ensure excellent balance of properties such as strength, elongation rate, hole expandability, and yield ratio. [Example]

[0096] The present invention will be described in more detail with reference to the following examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0097] (Example) A 100 mm thick steel slab having the chemical composition shown in Table 1 below was manufactured, heated to 1200°C, and then hot rolled at a finish rolling temperature of 900°C. The hot-rolled steel sheet was cooled at an average cooling rate of 30°C / s and coiled at the coiling temperature shown in Table 2 to manufacture a 3 mm thick steel sheet. The steel sheet was then heated and maintained under the heat treatment conditions shown in Table 2. The heat-treated steel sheet was pickled to remove surface scale and then cold-rolled to a thickness of 1.5 mm. The steel sheet was then heated and maintained at the heating temperature shown in Table 2, and cooled under the conditions of primary cooling, secondary cooling, and tertiary cooling. The heating rate during heating was 10°C / s, and after tertiary cooling, the steel sheet was cooled to room temperature at 10°C / s.

[0098] [Table 1]

[0099] [Table 2]

[0100] Table 3 below shows the microstructure and physical properties of the steel sheets that were observed and measured. The microstructure of the steel sheets was observed using a scanning electron microscope (SEM) after nital etching of the polished cross-section of the test specimen. After nital etching, the smooth structure on the surface of the test specimen was identified as recrystallized ferrite, while the structure with a globular or lamellar structure was identified as cementite. Unrecrystallized ferrite, which contains many dislocations, has crystal misorientation within the particles. Therefore, the crystal orientation of ferrite was measured using FESEM-EBSD, and the unrecrystallized ferrite within the ferrite was distinguished using the KAM (Kernel Average Misorientation) method.

[0101] The physical properties of the steel sheets were evaluated by tensile tests and hole expansion tests. The tensile tests were carried out using test pieces taken in accordance with JIS No. 5 standard, with the 0° direction relative to the rolling direction of the rolled sheet material as the reference. The yield ratio (YR), the product of the square of the tensile strength and the square root of the elongation ratio (TS 2 ×√EL) was calculated. Yield ratio (YR) is the value obtained by dividing yield strength (YS) by tensile strength (TS). In the hole expansion test, a punched hole with a diameter of 10 mm (die inner diameter 10.3 mm, clearance 12.5%) was formed by compressing and expanding it at 20 mm / min in the direction where the burr of the punched hole faces outward using a conical punch with an apex angle of 60°. The hole expansion ratio (HER) was then calculated using the following formula. This gave the product of the square of the tensile strength and the square root of the hole expansion ratio (TS 2 ×√HER) was calculated.

[0102] [formula] HER(%)={(D-D0) / D0}×100 (In the formula, D is the hole diameter (mm) when the crack penetrates the plate thickness, and D0 is the initial hole diameter (mm).)

[0103] [Table 3]

[0104] As shown in Table 3, in the case of the invention examples that satisfy the alloy composition and manufacturing conditions of the present invention, the characteristics of the microstructure proposed in the present invention are satisfied, and the physical properties targeted by the present invention are also ensured.

[0105] On the other hand, in Comparative Examples 1 and 2, the coiling temperature exceeded the temperature range proposed by the present invention. The coiling temperature was high, and unrecrystallized ferrite was present in the cold-rolled steel sheet after heat treatment, making it impossible to ensure the desired yield ratio.

[0106] Comparative Examples 3 and 4 are examples in which the heat treatment temperature was outside the range of the present invention. In Comparative Example 3, the heat treatment temperature was too high, making precipitation difficult. As a result, it was difficult to ensure the desired physical properties. In Comparative Example 4, the heat treatment temperature was below the range of the present invention, making it difficult to optimize precipitation, and therefore the desired strength and ductility could not be ensured.

[0107] In Comparative Examples 5 and 6, the heat treatment time was outside the range of the present invention. In Comparative Examples 5 and 6, the heat treatment time was either too long or not sufficient, and precipitation was not easy, so the desired physical properties of the present invention could not be achieved.

[0108] In Comparative Examples 7 and 8, the primary heating and maintenance temperatures were outside the temperature ranges proposed by the present invention. In Comparative Example 7, the heating temperature was too high, and the area fraction of recrystallized ferrite did not reach the value proposed by the present invention, resulting in a poor yield ratio. In Comparative Example 8, the heating temperature was too low, and unrecrystallized ferrite was present, making it impossible to ensure the desired strength and ductility.

[0109] In Comparative Example 9, the maintenance time after the primary heating was insufficient, and unrecrystallized ferrite was formed, resulting in failure to secure the desired strength and elongation.

[0110] Comparative Examples 10 and 11 are cases where the cooling end temperature during primary cooling proposed in the present invention is not met. In Comparative Example 10, the cooling end temperature during primary cooling was exceeded, resulting in deterioration of physical properties. In Comparative Example 11, the cooling end temperature during primary cooling was too low, resulting in excessive formation of cementite, and as a result, the desired strength and elongation could not be achieved.

[0111] In Comparative Example 12, the average cooling rate during secondary cooling was low and the cementite fraction was excessive, resulting in deterioration of physical properties.

[0112] In Comparative Examples 13 and 14, the cooling end temperature during secondary cooling was outside the range of the present invention. In Comparative Example 13, the cooling end temperature during secondary cooling was exceeded, resulting in an excessive cementite fraction and making it impossible to ensure the desired physical properties. In Comparative Example 14, the secondary cooling end temperature was low, resulting in a yield ratio outside the desired range and also in deterioration of strength and elongation.

[0113] In Comparative Example 15, the maintenance time after secondary cooling did not reach the range proposed in the present invention, and the desired physical properties could not be secured.

[0114] Comparative Examples 16 and 17 are examples in which the cooling end temperature during tertiary cooling was outside the range of the present invention. In Comparative Example 16, the cooling end temperature during tertiary cooling was excessively high, and unrecrystallized ferrite was formed. As a result, the recrystallized ferrite fraction was not reached, the yield ratio exceeded the proposed range, and the desired strength and ductility could not be ensured. In Comparative Example 17, the cooling end temperature during tertiary cooling was not reached, and it was difficult to ensure the desired strength and ductility.

[0115] In Comparative Example 18, the maintenance time after the tertiary cooling was short, and the desired physical properties and elongation ratio could not be ensured.

[0116] Comparative Examples 19 and 20 are cases where the carbon content is outside the range of the present invention. Comparative Example 19 is a case where the carbon content is not reached, and the yield ratio is insufficient, resulting in reduced strength and elongation. Comparative Example 20 is a case where the carbon content is excessive, and non-recrystallized ferrite is formed. As a result, the recrystallized ferrite fraction is insufficient, and the desired physical properties cannot be secured.

[0117] In Comparative Example 21, the silicon content exceeded the range proposed by the present invention, and the desired strength and elongation could not be ensured.

[0118] Comparative Examples 22 and 23 are examples in which the manganese content was outside the range of the present invention. Comparative Example 22 had an insufficient manganese content, and the desired strength and ductility could not be ensured. Comparative Example 23 had an excessive manganese content, and unrecrystallized ferrite was formed, exceeding the proposed physical property levels.

[0119] Comparative Example 24 had an excessive aluminum content and did not satisfy the physical properties proposed by the present invention.

[0120] In Comparative Examples 25 to 28, the total content of titanium, niobium, and vanadium exceeded the range proposed by the present invention, and unrecrystallized ferrite was formed, resulting in insufficient recrystallized ferrite, and as a result, the yield ratio exceeded the target level, and the strength and elongation were also excessive.

[0121] Although the present invention has been described in detail with reference to the above embodiments, other embodiments are possible, and the spirit and scope of the claims set forth below should not be limited to the embodiments.

Claims

1. In weight percent, 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, the balance being Fe and other inevitable impurities, and containing one or more selected from Ti, Nb, and V in an amount of 0.22% or less; The microstructure comprises, by area percent, 80 to 99% recrystallized ferrite and 1 to 20% cementite; The square of the tensile strength (TS) and the square root of the elongation ratio (El) (TS ２ ×√EL) is 1.8 to 2.3 × 10 ６ MPa ２ % ０．５ That is, steel plate.

2. 2. The steel plate according to claim 1, further comprising, by weight%, one or more selected from 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 plate according to claim 1, containing one or more selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.

4. The steel plate according to claim 1, wherein the steel plate has a tensile strength (TS) of 610 MPa or more and a yield ratio (YR) of 0.8 to 0.

95.

5. In weight percent, 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, the balance being Fe and other inevitable impurities, and containing one or more selected from Ti, Nb, and V in an amount of 0.22% or less; The microstructure comprises, by area percent, 80 to 99% recrystallized ferrite and 1 to 20% cementite; The square of the tensile strength (TS) and the square root of the hole expansion ratio (HER) (TS ２ ×√HER) is 2.5 to 3.8 × 10 ６ MPa ２ % ０．５ That is, steel plate.

6. 6. The steel plate according to claim 5, further comprising, by weight%, one or more selected from 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 plate according to claim 5, containing one or more selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.

8. The steel plate according to claim 5, wherein the steel plate has a tensile strength (TS) of 610 MPa or more and a yield ratio (YR) of 0.8 to 0.

95.

9. a step of reheating a steel slab containing, by 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, the balance being Fe and other unavoidable impurities, and containing one or more 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 in the range of 25 to 300°C; a heat treatment step of heating the coiled steel sheet to a temperature range of 650 to 800°C and maintaining the temperature for 600 to 1700 seconds; cold rolling the heat-treated steel sheet; primary heating the cold-rolled steel sheet to a temperature range of 720 to 880°C, maintaining the temperature for 50 seconds or more, and primary cooling to a temperature range of 600 to 760°C at an average cooling rate of 1°C / s or more; Secondarily cooling the primarily cooled steel sheet to a temperature range of 520 to 620°C at an average cooling rate of 2°C / s or more, and maintaining the temperature for 20 seconds or more; and tertiary cooling the steel sheet after the secondary cooling and maintaining at a temperature in the range of 420 to 520°C at an average cooling rate of 2°C / s or more, and maintaining the temperature for 20 seconds or more.

10. 10. The method for producing a steel plate according to claim 9, wherein the steel slab further contains, by weight, one or more selected from 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 producing a steel plate according to claim 9, wherein the steel plate contains one or more selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.

12. The reheating step is performed at a temperature in the range of 1000 to 1350°C, The hot rolling step is performed at a finish rolling temperature of 800 to 1000°C, The method of claim 9, wherein the cold rolling is performed at a reduction rate of 30% or more.

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

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

15. The method for manufacturing a steel sheet according to claim 9, further comprising the step of plating the steel sheet after the tertiary cooling and maintaining step.

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