Steel sheet and method for manufacturing same

Abstract

A steel sheet and a method for manufacturing the same are provided. The steel sheet according to the present invention comprises, in wt%, 0.03 to 0.18% of C, 0 to 0.70% of Si, 0.25 to 1.80% of Mn, 0 to 0.70% of Al, 0.050% or less of P, 0.0300% or less of S, 0.0300% or less of N, 0 to 0.08% of Ti, 0 to 0.06% of Nb, 0 to 0.07% of V, 0.03 to 0.10% of Ti + Nb + V, 0 to 0.80% of Cr, 0 to 0.80% of Mo, 0 to 0.80% of Cu, 0 to 0.80% of Ni, 0 to 0.0050% of B, 0 to 0.050% of Ca, 0 to 0.050% of REM other than Y, 0 to 0.050% of Mg, 0 to 0.50% of W, 0 to 0.50% of Zr, 0 to 0.50% of Sb, 0 to

Description

Technical Field

[0001] The present invention relates to a steel sheet that can be used for various purposes including components such as automobiles, home appliances, and steel structures. In particular, it relates to a steel sheet having excellent balance among ductility, formability, and yield ratio, and a method for manufacturing the same. Background Art

[0002] Cold-rolled steel sheets or coated steel sheets for automobiles, home appliances, steel structures, etc. require excellent ductility and formability as well as sufficient yield ratio.

[0003] As methods for increasing the yield strength, there are solid-solution strengthened steel sheets and precipitation strengthened steel sheets.

[0004] Solid-solution strengthened steel is a steel sheet in which solid-solution strengthening elements (Mn, Si, Cr, etc.) are dissolved in a ferrite phase having excellent formability to increase the yield strength. However, Si or Cr is an element that easily forms oxides on the surface of the steel sheet in a continuous annealing production line or a continuous hot-dip alloy coating production line. In addition, Mn is an element that promotes low-temperature phase transformation (bainite or martensite), and low-temperature phase transformation has the characteristic of reducing the yield strength. Therefore, as a method for increasing the yield ratio of a steel sheet having a tensile strength of 410 MPa or more, solid-solution strengthened steel with a large amount of Mn, Si, Cr added is not suitable.

[0005] Precipitation strengthened steel using Nb, Ti, V, etc. is a steel sheet that increases the yield strength by precipitating fine carbides in ferrite. Precipitation strengthened steel can increase the yield ratio without reducing the workability, and thus is a strengthening mechanism suitable for a steel sheet having excellent workability and a tensile strength of 410 MPa or more.

[0006] As a technology for improving the formability and yield ratio of steel sheets, methods of introducing unrecrystallized ferrite and using the addition of Ti or Nb are disclosed in Patent Document 1 and Patent Document 2. Precipitation strengthening using Ti or Nb and unrecrystallized ferrite effectively increase the yield strength without significantly increasing the tensile strength by directly strengthening ferrite.

[0007] In the manufacturing method described in Patent Document 1, a part of the yield ratio (YR) satisfies 0.7 or 0.85, or a part of the product of the square of the tensile strength and the square root of the hole expansion ratio (TS 2 ×√HER) satisfies 1.4 - 3.0×10 6 (MPa) 2 %. 0.5 . However, the product of the square of the tensile strength and the square root of the elongation (TS 2 ×√EL) cannot satisfy 0.7 - 1.7×10 6 (MPa) 2 %. 0.5 .

[0008] In addition, in the manufacturing method described in Patent Document 2, the product of the square of a part of the tensile strength and the square root of the hole expansion rate (TS 2 ×√HER) satisfies 1.4 - 3.0×10 6 (MPa) 2 % 0.5 , or the product of the square of the tensile strength and the square root of the elongation rate (TS 2 ×√EL) satisfies 0.7 - 1.7×10 6 (MPa) 2 % 0.5 . However, the yield ratio (YR) does not satisfy 0.7 to 0.85.

[0009] This may mean that, in order to manufacture a steel sheet having an excellent balance of ductility, formability, and yield ratio, it is not suitable to introduce unrecrystallized ferrite of the steel sheet, and thus there has been a continuous need for technical development in this regard.

[0010] [Prior Art Documents]

[0011] [Patent Documents]

[0012] (Patent Document 1) Japanese Patent Laid-Open Gazette No. 2009-114523

[0013] (Patent Document 2) Japanese Patent Laid-Open Gazette No. 2017-002333 Summary of the Invention

[0014] (I) Technical Problems to be Solved

[0015] Therefore, an object of the present invention is to provide a cold-rolled steel sheet or an alloy-coated steel sheet that does not contain unrecrystallized ferrite of the steel sheet and has an excellent balance of ductility, formability, and yield ratio. That is, an object of the present invention is to provide a steel sheet having an excellent balance of YR, TS 2 ×√EL and TS 2 ×√HER and a manufacturing method thereof.

[0016] In addition, the technical problems to be achieved by the present invention are not limited to the above technical problems, and those skilled in the technical field to which the present invention pertains can clearly understand other technical problems not described from the following description.

[0017] (II) Technical Solutions

[0018] One aspect of the present invention relates to a steel plate which, by weight %, comprises: C: 0.03 - 0.18%, Si: 0 - 0.70%, Mn: 0.25 - 1.80%, Al: 0 - 0.70%, P: below 0.050%, S: below 0.0300%, N: below 0.0300%, Ti: 0 - 0.08%, Nb: 0 - 0.06%, V: 0 - 0.07%, Ti + Nb + V: 0.03 - 0.10%, Cr: 0 - 0.80%, Mo: 0 - 0.80%, Cu: 0 - 0.80%, Ni: 0 - 0.80%, B: 0 - 0.0050%, Ca: 0 - 0.050%, REM other than Y: 0 - 0.050%, Mg: 0 - 0.050%, W: 0 - 0.50%, Zr: 0 - 0.50%, Sb: 0 - 0.50%, Sn: 0 - 0.50%, Y: 0 - 0.20%, Hf: 0 - 0.20%, the balance being Fe and other inevitable impurities. By area %, the steel plate has a steel plate microstructure comprising 1 - 15% of pearlite containing cementite, residual ferrite, and inevitably introduced structures, and the yield ratio (YR) of the steel plate is 0.70 to 0.85. The product of the square of the tensile strength of the steel plate and the square root of the elongation rate (TS 2 ×√EL) satisfies 0.7×10 6 -1.7×10 6 (MPa) 2 % 0.5 , and the product of the square of the tensile strength of the steel plate and the square root of the hole expansion rate (TS 2 ×√HER) satisfies 1.4×10 6 -3.0×10 6 (MPa) 2 % 0.5 .

[0019] Preferably, the C is contained in the range of 0.04 - 0.17% by weight %.

[0020] Preferably, the Mn is contained in the range of 0.27 - 1.78% by weight %.

[0021] Preferably, by area %, it has a steel plate microstructure comprising 3 - 13% of pearlite containing cementite, residual ferrite, and inevitably introduced structures.

[0022] The yield ratio (YR = YS / TS) of the value obtained by dividing the yield strength (YS) of the steel plate by the tensile strength (TS) preferably satisfies 0.71 to 0.84.

[0023] The product of the square of the tensile strength of the steel plate and the square root of the elongation rate (TS 2 ×√EL) preferably satisfies 0.71×106 -1.69×10 6 (MPa) 2 % 0.5 。

[0024] The product of the square of the tensile strength of the steel plate and the square root of the hole expansion rate (TS 2 ×√HER) preferably satisfies 1.43×10 6 -2.93×10 6 (MPa) 2 % 0.5 。

[0025] In addition, another aspect of the present invention relates to a method for manufacturing a steel plate, the manufacturing method comprising the following steps: heating an ingot or slab having the above composition to a temperature range of 1000 - 1350 °C; hot-rolling the heated ingot or slab at a finish rolling temperature of 800 - 1000 °C to manufacture a hot-rolled steel plate; cooling the manufactured hot-rolled steel plate and then coiling it in a temperature range of 300 - 600 °C; heat-treating the coiled hot-rolled steel plate in a temperature range of 650 - 800 °C for 600 - 1700 seconds, and then cold-rolling it at a reduction rate of 30 - 90%; heating the cold-rolled steel plate after rolling in a temperature range of 740 - 860 °C and holding it for more than 50 seconds at one time; cooling the cold-rolled steel plate after the one-time holding at an average cooling rate of 5 °C / second or less to a temperature range of 600 - 760 °C; and cooling the cold-rolled steel plate after the one-time cooling at an average cooling rate of more than 5 °C / second and 20 °C / second or less to a temperature range of 450 - 550 °C, and then holding it for more than 50 seconds in this temperature range and then cooling it to room temperature.

[0026] (III) Beneficial Effects

[0027] As described above, the present invention can effectively provide a cold-rolled steel plate or an alloy-coated steel plate that does not contain unrecrystallized ferrite in the steel plate and has excellent balance of ductility, formability, and yield ratio, and a manufacturing method thereof. Best Embodiment

[0028] Hereinafter, the present invention will be described.

[0029] The present inventors have confirmed that in order to manufacture a steel plate having excellent formability and a high yield ratio, an optimal fine grain fraction, addition of components for realizing it, and an optimal manufacturing process are required.

[0030] In particular, it has been confirmed that in order to improve the YR, TS 2 ×√EL, TS 2×√HER, it is important to optimize the ferrite fraction. Specifically, when the recrystallized ferrite fraction does not meet 85 - 99%, it is impossible to obtain a YR of 0.7 to 0.85 or 0.7×10 6 -1.7×10 6 (MPa) 2 % 0.5 of TS 2 ×√EL or 1.4×10 6 -3×10 6 (MPa) 2 % 0.5 of TS 2 ×√HER, and the present invention is proposed.

[0031] This steel plate with excellent balance of ductility, formability and yield ratio of the present invention contains, by weight%, C: 0.03 - 0.18%, Si: 0 - 0.70%, Mn: 0.25 - 1.80%, Al: 0 - 0.70%, P: 0.050% or less, S: 0.0300% or less, N: 0.0300% or less, Ti: 0 - 0.08%, Nb: 0 - 0.06%, V: 0 - 0.07%, Ti + Nb + V: 0.03 - 0.10%, Cr: 0 - 0.80%, Mo: 0 - 0.80%, Cu: 0 - 0.80%, Ni: 0 - 0.80%, B: 0 - 0.0050%, Ca: 0 - 0.050%, REM other than Y: 0 - 0.050%, Mg: 0 - 0.050%, W: 0 - 0.50%, Zr: 0 - 0.50%, Sb: 0 - 0.50%, Sn: 0 - 0.50%, Y: 0 - 0.20%, Hf: 0 - 0.20%, the balance being Fe and other inevitable impurities. By area%, the steel plate has a steel plate microstructure containing 1 - 15% of pearlite containing cementite, residual ferrite and inevitably introduced structures, and the yield ratio (YR) of the steel plate is 0.7 to 0.85, and the product of the square of the tensile strength of the steel plate and the square root of the elongation (TS 2 ×√EL) satisfies 0.7×10 6 -1.7×10 6 (MPa) 2 % 0.5 , and the product of the square of the tensile strength of the steel plate and the square root of the hole expansion rate (TS 2 ×√HER) satisfies 1.4×10 6 -3×10 6 (MPa) 2 % 0.5 .

[0032] Hereinafter, the composition of the steel plate provided in the present invention will be described in detail. At this time, unless otherwise specifically stated, the content of each component is expressed in weight %.

[0033] C: 0.03 - 0.18%

[0034] Carbon (C) is an essential element that forms precipitates together with Ti, Nb, or V in the ferrite phase to impart strength to the steel plate. When the addition amount of C is less than 0.03%, it is difficult to ensure a tensile strength of 410 MPa or more. On the other hand, when the addition amount of C exceeds 0.18%, it is difficult to ensure the welding strength of the welded part. Therefore, in the present invention, the content of C is preferably controlled within the range of 0.03 - 0.18%. More preferably, the C content is controlled within the range of 0.04 - 0.17%.

[0035] Si: 0 - 0.70%

[0036] Silicon (Si) is an element that has the effect of increasing strength through solid solution strengthening, is an element that strengthens ferrite, homogenizes the structure, and improves workability. In addition, the Si is an element required for deoxidation during steelmaking. When the addition amount of Si exceeds 0.70%, plating defects such as non-plating occur in the plating process, and the weldability of the steel plate decreases. Therefore, in the present invention, the content of Si is preferably controlled within the range of 0 - 0.70%, and more preferably, it is controlled within the range of 0 - 0.50%.

[0037] Mn: 0.25 - 1.80%

[0038] Manganese (Mn) is a useful element that improves both strength and ductility. When the addition amount of Mn is 0.25% or more, its effect is exhibited, but when the addition amount of Mn exceeds 1.80%, it promotes the low-temperature phase transformation from austenite to martensite or bainite, resulting in a decrease in the yield ratio of the steel plate. Therefore, in the present invention, the content of Mn is preferably controlled within 0.25 - 1.80%. More preferably, the Mn content is controlled within the range of 0.27 - 1.78%.

[0039] Al: 0 - 0.70%

[0040] Aluminum (Al) is an element that combines with oxygen in the steel to play a role in deoxidation. In addition, like Si, the Al is an element that strengthens ferrite, homogenizes the structure, and improves workability. When the addition amount of Al exceeds 0.70%, plating defects such as non-plating occur in the plating process, and the weldability of the steel plate decreases. Therefore, in the present invention, the content of Al is preferably controlled within the range of 0 - 0.70%, and more preferably, it is controlled within the range of 0 - 0.50%.

[0041] P: 0.050% or less

[0042] P is an element that is contained as an impurity and deteriorates the impact toughness. Therefore, in the present invention, the content of the P is preferably controlled below 0.050%. Limiting the P content to below 0.020%, and further preferably limiting it to below 0.015%.

[0043] S: Below 0.0300%

[0044] S is an element that is contained as an impurity and forms MnS in the steel plate, deteriorating the ductility. Therefore, in the present invention, the content of the S is preferably controlled below 0.0300% by weight. Limiting the S content to below 0.0100%, and further preferably limiting it to below 0.0050%.

[0045] N: Below 0.0300%

[0046] N is an element that is contained as an impurity and forms nitrides during the continuous casting process, resulting in cracks in the slab. Therefore, in the present invention, the content of the N is preferably controlled below 0.0300% by weight. Limiting the N content to below 0.0100%, and further preferably limiting it to below 0.0050%.

[0047] Ti: 0 - 0.08%, Nb: 0 - 0.06%, V: 0 - 0.07%,

[0048] Ti + Nb + V: 0.03 - 0.10%

[0049] Ti, Nb, and V are important elements for forming precipitates in the steel plate and are elements that can be added to improve the strength and impact toughness of the steel plate. In the present invention, it is preferred to control the contents of these elements in the ranges of Ti: 0 - 0.08%, Nb: 0 - 0.06%, and V: 0 - 0.07% respectively. When the Ti, Nb, and V respectively exceed the upper limits of their contents, excessive precipitates are formed and unrecrystallized ferrite is generated, which may lead to excessive characteristic effects and an increase in manufacturing costs.

[0050] Preferably, in the present invention, the content of one or more of the Ti, Nb, and V (Ti + Nb + V) is controlled in the range of 0.03 - 0.10%. When the Ti + Nb + V content is less than 0.03%, it is difficult to expect the effects of the added component elements. When the Ti + Nb + V content exceeds 0.10%, due to the formation of excessive precipitates and the generation of unrecrystallized ferrite, this may lead to excessive characteristic effects and an increase in manufacturing costs.

[0051] Cr: 0 - 0.80% and Mo: 0 - 0.80%

[0052] Cr and Mo are optional components of the present invention. During alloying treatment, they inhibit the decomposition of austenite and, like Mn, are elements that stabilize austenite. When the content of each of Cr and Mo exceeds 0.80%, it promotes the low-temperature phase transformation of martensite or bainite, thereby reducing the yield ratio of the steel plate. Therefore, the content of each of Cr and Mo is preferably controlled below 0.80%, and more preferably, controlled below 0.50%.

[0053] Cu: 0 - 0.80%, Ni: 0 - 0.80%

[0054] Cu and Ni are optional components of the present invention and are elements that stabilize austenite and inhibit corrosion. In addition, Cu and Ni are enriched on the surface of the steel plate, preventing the penetration of hydrogen that has moved into the steel plate, and thus also having the effect of inhibiting hydrogen-induced delayed fracture. In the present invention, when the content of each of Cu and Ni exceeds 0.80%, it results in excessive characteristic effects and an increase in manufacturing cost. Therefore, in the present invention, the content of each of Cu and Ni is preferably controlled below 0.80%, and more preferably, controlled below 0.50%.

[0055] B: 0 - 0.0050%

[0056] B is an optional component of the present invention and is an element that improves hardenability, increases strength, and inhibits nucleation at grain boundaries. When the content of B exceeds 0.0050%, it results in excessive characteristic effects and an increase in manufacturing cost. Therefore, in the present invention, the content of B is preferably controlled below 0.0050%.

[0057] Ca: 0 - 0.050%, Mg: 0 - 0.050% and REM other than Y: 0 - 0.050%

[0058] REM refers to a total of 17 elements including Sc, Y, and lanthanide elements. Ca, Mg, and REM other than Y are optional components of the present invention and are elements that improve the ductility of the steel plate by spheroidizing sulfides. When the content of each of Ca, Mg, and REM other than Y exceeds 0.050%, it results in excessive characteristic effects and an increase in manufacturing cost. Therefore, in the present invention, the content of each of Ca, Mg, and REM other than Y is preferably controlled below 0.050%, and more preferably, controlled below 0.010%.

[0059] W: 0 - 0.50%, Zr: 0 - 0.50%

[0060] W and Zr are optional components of the present invention and are elements that increase the strength of the steel plate by improving hardenability. When the content of each of W and Zr exceeds 0.50%, it causes excessive characteristic effects and an increase in manufacturing cost. Therefore, in the present invention, the content of each of W and Zr is preferably controlled below 0.50%, and more preferably, controlled below 0.20%.

[0061] Sb: 0 - 0.50%, Sn: 0 - 0.50%

[0062] Sb and Sn are optional components of the present invention and are elements that improve the plating wettability and plating adhesion of the steel plate. When the content of each of Sb and Sn exceeds 0.50%, the brittleness of the steel plate increases, and cracks may occur during hot working or cold working. Therefore, in the present invention, the content of one or more of Sb and Sn is preferably controlled below 0.50% by weight, and more preferably, controlled below 0.20%.

[0063] Y: 0 - 0.20%, Hf: 0 - 0.20%

[0064] Y and Hf are optional components of the present invention and are elements that improve the corrosion resistance of the steel plate. When the content of each of Y and Hf exceeds 0.20%, the ductility of the steel plate may deteriorate. Therefore, in the present invention, the content of each of Y and Hf is preferably controlled below 0.20%, and more preferably, controlled below 0.10%.

[0065] In addition to the above composition, the balance preferably contains Fe and inevitable impurities, and the addition of other compositions is not excluded for the steel of the present invention. These inevitable impurities may undesirably mix in from raw materials or the surrounding environment during the normal steel manufacturing process, so it is impossible to exclude these impurities. As long as those skilled in the art of normal steel manufacturing can understand the inevitable impurities.

[0066] Furthermore, by area%, the present invention may have a steel plate microstructure including 1 - 15% of pearlite containing cementite, residual ferrite, and inevitably introduced structures. When the pearlite fraction is less than 1% or exceeds 15%, the yield ratio, elongation, or hole expansion ratio of the steel plate may decrease. This will ultimately result in the inability to meet the YR of 0.70 to 0.85, 0.7×10 6 -1.7×10 6 (MPa) 2 % 0.5 of TS 2 ×√EL or 1.4×10 6 -3.0×10 6 (MPa) 2 % 0.5 of TS2 ×√HER, so it is not preferred.

[0067] More preferably, in terms of area %, the steel sheet microstructure has pearlite containing cementite, residual ferrite, and unavoidably introduced structures, accounting for 3-13%.

[0068] In addition, in the steel sheet of the present invention having the said microstructure fraction, for the steel sheet with a tensile strength of 410 MPa or more (preferably 410-580 MPa), the yield ratio (YR) can satisfy 0.70 to 0.85, and the product of the square of the tensile strength and the square root of the elongation (TS 2 ×√EL) can satisfy 0.7×10 6 -1.7×10 6 (MPa) 2 % 0.5 , and the product of the square of the tensile strength and the square root of the hole expansion rate (TS 2 ×√HER) can satisfy 1.4×10 6 -3×10 6 (MPa) 2 % 0.5 .

[0069] More preferably, the yield ratio (YR = YS / TS) is controlled within 0.71 to 0.84.

[0070] In addition, the product of the square of the tensile strength and the square root of the elongation (TS 2 ×√EL) of the steel sheet more preferably satisfies 0.71×10 6 -1.69×10 6 (MPa) 2 % 0.5 .

[0071] In addition, the product of the square of the tensile strength and the square root of the hole expansion rate (TS 2 ×√HER) of the steel sheet more preferably satisfies 1.43×10 6 -2.93×10 6 (MPa) 2 % 0.5 .

[0072] Next, a manufacturing method of a steel sheet according to another embodiment of the present invention will be described in detail.

[0073] The manufacturing method of the steel plate of the present invention includes the following steps: heating an ingot or slab having the above composition to a temperature of 1000 - 1350°C; hot rolling the heated ingot or slab at a finish rolling temperature of 800 - 1000°C to manufacture a hot rolled steel plate; cooling the manufactured hot rolled steel plate and coiling it within a temperature range of 300 - 600°C; heat treating the coiled hot rolled steel plate within a temperature range of 650 - 800°C for 600 - 1700 seconds, and then cold rolling it with a reduction rate of 30 - 90%; heating the cold rolled steel plate after rolling within a temperature range of 740 - 860°C and holding it for more than 50 seconds; cooling the cold rolled steel plate after the first holding at an average cooling rate of 5°C / second or less to a temperature range of 600 - 760°C; and cooling the cold rolled steel plate after the first cooling at an average cooling rate of more than 5°C / second and 20°C / second or less to a temperature range of 450 - 550°C, and then holding it for more than 50 seconds within this temperature range and then cooling it to room temperature.

[0074] Heating

[0075] In the present invention, first, an ingot or slab having the above composition is heated to a temperature range of 1000 - 1350°C.

[0076] The present invention does not particularly limit the melting process for manufacturing the ingot or slab, but preferably limits the heating temperature to the range of 1000 - 1350°C. This is because when the heating temperature is lower than 1000°C, hot rolling may be carried out below the finish rolling temperature range, and when the heating temperature exceeds 1350°C, it may reach the melting point of the steel and cause it to melt.

[0077] Hot finish rolling

[0078] Next, in the present invention, the heated ingot or slab is hot rolled at a finish rolling temperature of 800 - 1000°C to manufacture a hot rolled steel plate.

[0079] When the finish rolling temperature is lower than 800°C, the high strength of the steel may impose a greater burden on the hot rolling mill. On the other hand, when the finish rolling temperature exceeds 1000°C, the grains of the hot rolled steel plate become coarse, and the physical properties of the steel plate may deteriorate.

[0080] Preferably, hot finish rolling is carried out within a temperature range of 830 - 950°C.

[0081] Coiling after cooling

[0082] In addition, in the present invention, the manufactured hot rolled steel plate is cooled and then coiled within a temperature range of 300 - 600°C.

[0083] After the hot finish rolling, the hot-rolled steel sheet is cooled to refine the grains of the hot-rolled steel sheet. At this time, the average cooling rate is preferably 10 °C / second or more.

[0084] In addition, since the hot-rolled steel sheet with a low-temperature phase transformation or scale formation to the inside of the steel sheet where the main phase is martensite or bainite is not easy to coil or pickling, the coiling temperature of the hot-rolled steel sheet is preferably 300 - 600 °C.

[0085] When the coiling temperature is lower than 300 °C, the main phase of the hot-rolled steel sheet is composed of a low-temperature phase transformation with high strength, so the hot-rolled steel sheet may be difficult to coil. On the other hand, when the coiling temperature exceeds 600 °C, the scale generated on the surface of the hot-rolled steel sheet penetrates into the inside of the hot-rolled steel sheet, which may make pickling difficult.

[0086] Preferably, coiling is carried out in the temperature range of 350 - 550 °C.

[0087] Precipitate formation annealing treatment followed by cold rolling

[0088] Next, in the present invention, the coiled hot-rolled steel sheet is heat-treated in the temperature range of 650 - 800 °C for 600 - 1700 seconds, and then cold-rolled with a reduction rate of 30 - 90%.

[0089] The inventors of the present invention confirmed that it is easy to optimize the precipitates of the hot-rolled steel sheet when the hot-rolled steel sheet is subjected to short-time annealing treatment at an appropriate high temperature. This is because Ti, Nb or V that did not precipitate during the coiling process of the hot-rolling process reprecipitates on the hot-rolled steel sheet during the annealing treatment at an appropriate temperature and time, which helps to optimize the precipitates of the steel sheet. That is, the hot-rolled steel sheet in the present invention needs to be subjected to short-time annealing treatment at an appropriate high temperature. This is because the annealing treatment at an appropriate temperature and time can promote the formation of precipitates of the hot-rolled steel sheet, thereby ultimately improving the yield ratio of the final product.

[0090] Therefore, in the present invention, the annealing treatment of the hot-rolled steel sheet is preferably carried out in the temperature range of 650 - 800 °C for 600 - 1700 seconds. When the heat treatment conditions are lower than 650 °C or less than 600 seconds, it may be difficult to optimize the precipitates of the annealed steel sheet. On the other hand, when the heat treatment conditions exceed 800 °C or exceed 1700 seconds, the annealed steel sheet may not be easily precipitated. Therefore, there may be a problem that the desired yield ratio of the final steel sheet cannot be obtained.

[0091] Preferably, the annealing treatment is carried out in the temperature range of 680 - 750 °C.

[0092] Next, in the present invention, the hot-rolled steel sheet after the annealing heat treatment can also be subjected to a normal pickling treatment. In addition, the pickled hot-rolled steel sheet is preferably cold-rolled at a cumulative reduction rate of 30-90%. When the cumulative cold-rolling reduction rate exceeds 90%, it may be difficult to perform cold rolling in a short time due to the high strength of the steel sheet.

[0093] The cold-rolled steel sheet according to the present invention can be made into an uncoated cold-rolled steel sheet through an annealing process, or a coated steel sheet can be manufactured through a coating process to impart corrosion resistance. Coating can apply coating methods such as hot-dip galvanizing, electro-galvanizing, or hot-dip aluminizing.

[0094] One-time holding

[0095] In addition, in the present invention, the cold-rolled steel sheet after rolling is heated in a temperature range of 740-860 °C and held for more than 50 seconds at one time.

[0096] When the one-time holding temperature is lower than 740 °C, unrecrystallized ferrite is generated, which will reduce the YR, TS 2 ×√EL and TS 2 ×√HER. In addition, when the one-time holding temperature exceeds 860 °C, the pearlite fraction containing cementite exceeds 15%, thereby reducing the YR, TS 2 ×√EL and TS 2 ×√HER.

[0097] In addition, when the one-time holding time is less than 50 seconds, the heat treatment time is insufficient, which may reduce the YR, TS 2 ×√EL and TS 2 ×√HER.

[0098] One-time cooling

[0099] The cold-rolled steel sheet after the one-time holding is cooled at an average cooling rate of 5 °C / second or less for one time to a temperature range of 600-760 °C.

[0100] When the cooling termination temperature is lower than 600 °C, the cooling termination temperature is relatively low, so TS is reduced 2 ×√EL and TS 2 ×√HER. In addition, when the cooling termination temperature exceeds 760 °C, the cooling termination temperature is relatively high, which may reduce TS 2 ×√EL and TS 2 ×√HER.

[0101] In addition, when the one-time cooling rate exceeds 5 °C / second, there may be problems with equipment specifications.

[0102] Secondary holding after secondary cooling

[0103] Subsequently, in the present invention, the cold-rolled steel sheet that has undergone the primary cooling is subjected to secondary cooling at an average cooling rate exceeding 5°C / second and not exceeding 20°C / second, cooled to a temperature range of 450 - 550°C, and then held at this temperature range for more than 50 seconds and then cooled to room temperature.

[0104] When the secondary average cooling rate is 5°C / second or less, the pearlite fraction containing cementite exceeds 15%, which may reduce the TS 2 ×√EL and TS 2 ×√HER. When the secondary average cooling rate exceeds 20°C / second, equipment specification problems may occur.

[0105] In addition, when the secondary holding temperature is lower than 450°C, the lower heat treatment temperature may reduce the TS of the steel sheet 2 ×√EL and TS 2 ×√HER. In addition, when the secondary holding temperature exceeds 550°C, the YR and TS of the steel sheet may be reduced 2 ×√EL and TS 2 ×√HER.

[0106] In addition, when the secondary holding time is less than 50 seconds, the heat treatment time is insufficient, which may reduce the TS of the steel sheet 2 ×√EL and TS 2 ×√HER.

[0107] As described above, by subsequently cooling the steel sheet after secondary holding to room temperature, a steel sheet with an excellent balance of YR, TS 2 ×√EL, TS 2 ×√HER can be effectively manufactured. Detailed Embodiments

[0108] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are only used to illustrate the present invention for more detailed description, and are not used to limit the scope of the rights of the present invention. It is clearly pointed out that the scope of the rights of the present invention is determined by the matters recorded in the claims and the matters reasonably deduced therefrom.

[0109] (Example)

[0110] A slab with a thickness of 100 mm and having a composition system as shown in Table 1 below is manufactured. The slab is heated at 1200 °C and then hot-rolled at a finish rolling temperature of 900 °C to produce a hot-rolled steel sheet with a thickness of 3 mm. Next, the hot-rolled steel sheet is cooled at an average cooling rate of 30 °C / second and coiled at the coiling temperature of the hot-rolled steel sheet shown in Table 2 below. Then, as shown in Table 2 below, the coiled hot-rolled steel sheet is heated to the annealing temperature of the hot-rolled steel sheet and held for a specified annealing time, and then cooled to room temperature. The steel sheet subjected to the annealing heat treatment is pickled to remove the scale on the surface and then cold-rolled to a thickness of 1.5 mm. The cold-rolled steel sheet is heated to the primary holding temperature shown in Table 2 below and held at the primary holding temperature for the primary holding time. In addition, the cold-rolled steel sheet subjected to the primary holding is cooled to the primary cooling end temperature shown in Table 3 below at a primary average cooling rate. Next, the cold-rolled steel sheet subjected to the primary cooling is cooled to the secondary holding temperature at a secondary average cooling rate shown in Table 3 below, then held for the secondary holding time, and then cooled to room temperature at a tertiary average cooling rate shown in Table 3 below.

[0111] The microstructure of each of the steel sheets manufactured as described above is measured and shown in Table 4 below. For recrystallized ferrite, unrecrystallized ferrite, cementite, and low-temperature transformation structures (bainite, martensite) in the microstructure of the steel sheet, after the polished specimen cross-section is etched with nitric acid ethanol, the microstructure is observed by SEM. After etching with nitric acid ethanol, the structure with no unevenness on the specimen surface is judged as ferrite, and the structure with a spherical or layered structure is judged as cementite. The unrecrystallized ferrite containing a large number of dislocations has an intragranular crystal orientation difference. Therefore, after measuring the crystal orientation of ferrite by FESEM-EBSD, the unrecrystallized ferrite in ferrite is distinguished by the Kernel Average Misorientation (KAM) method.

[0112] In addition, the mechanical and physical properties of the manufactured steel sheets are measured respectively and shown in Table 5 below. The physical property evaluation of the steel sheet is carried out by tensile test and flanging test. The tensile test is evaluated using a test piece collected according to JIS No. 5 standard with respect to the 0° direction of the rolling direction of the rolled sheet, and YR and TS 2 ×√EL are determined. The flanging test is carried out by pressing and expanding a conical punch with a vertex angle of 60° at a speed of 20 mm / minute in a 10 mmØ punch hole (the inner diameter of the die is 10.3 mm and the gap is 12.5%) in the direction of the burr outside the punch hole.

[0113] Flanging rate: HER(%) = {(D - D0) / D0} × 100

[0114] D: The hole diameter (mm) when the crack penetrates the plate thickness

[0115] D0: Initial aperture (mm)

[0116] [Table 1]

[0117]

[0118] The surplus components in Table 1 are Fe and inevitable impurities.

[0119] [Table 2]

[0120]

[0121]

[0122] [Table 3]

[0123]

[0124]

[0125] [Table 4]

[0126]

[0127]

[0128] [Table 5]

[0129]

[0130]

[0131] As shown in Tables 1 to 4, it can be seen that Examples 1, 15 to 31 that satisfy the alloy composition, manufacturing process, and fine microstructure conditions of the present invention all satisfy that the yield ratio (YR) of the steel plate is 0.7 to 0.85, and the product of the square of the tensile strength and the square root of the elongation rate (TS 2 ×√EL) is 0.7×10 6 -1.7×10 6 (MPa) 2 % 0.5 , and the product of the square of the tensile strength and the square root of the hole expansion rate (TS 2 ×√HER) is 1.4×10 6 -3×10 6 (MPa) 2 % 0.5 , so it can be confirmed that a steel plate with excellent balance of ductility, formability, and yield ratio can be manufactured.

[0132] In addition, in Comparative Example 2, since the annealing temperature of the hot-rolled steel plate is high, it is not easy for the steel plate to precipitate. As a result, TS2 ×√EL is less than 0.7 × 10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER is less than 1.4 × 10 6 (MPa) 2 % 0.5 。

[0133] In Comparative Example 3, since the annealing temperature of the hot-rolled steel sheet was low, it was not easy to optimize the precipitation of the steel sheet. As a result, TS 2 ×√EL is less than 0.7 × 10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER is less than 1.4 × 10 6 (MPa) 2 % 0.5 。

[0134] In Comparative Example 4, since the annealing time of the hot-rolled steel sheet was long, it was not easy to precipitate the steel sheet. As a result, TS 2 ×√EL is less than 0.7 × 10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER is less than 1.4 × 10 6 (MPa) 2 % 0.5 。

[0135] In Comparative Example 5, since the annealing time of the hot-rolled steel sheet was short, it was not easy to optimize the precipitation of the steel sheet. As a result, TS 2 ×√EL is less than 0.7 × 10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER is less than 1.4 × 10 6 (MPa) 2 % 0.5 。

[0136] In Comparative Example 6, since the primary holding temperature was high, the pearlite fraction containing cementite in the steel sheet exceeded 15%. As a result, YR was less than 0.7, TS 2 ×√EL is less than 0.7 × 10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER is less than 1.4 × 10 6 (MPa) 2 % 0.5 。

[0137] In Comparative Example 7, since the holding temperature was low during the first stage, unrecrystallized ferrite was formed in the produced steel sheet. As a result, YR exceeded 0.85, TS 2 ×√EL exceeded 1.7×10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER exceeded 3.0×10 6 (MPa) 2 % 0.5 。

[0138] In Comparative Example 8, since the holding time was short during the first stage, unrecrystallized ferrite was formed in the produced steel sheet. As a result, YR exceeded 0.85, TS 2 ×√EL exceeded 1.7×10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER exceeded 3.0×10 6 (MPa) 2 % 0.5 。

[0139] In Comparative Example 9, since the termination temperature of the first cooling was high, TS 2 ×√EL was less than 0.7×10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER was less than 1.4×10 6 (MPa) 2 % 0.5 。

[0140] In Comparative Example 10, since the termination temperature of the first cooling was low, TS 2 ×√EL was less than 0.7×10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER was less than 1.4×10 6 (MPa) 2 % 0.5 。

[0141] In Comparative Example 11, since the secondary average cooling rate was low, the pearlite fraction containing cementite exceeded 15%. As a result, TS 2 ×√EL was less than 0.7×10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER was less than 1.4×10 6 (MPa)2 % 0.5 。

[0142] In Comparative Example 12, due to the relatively high secondary holding temperature, YR was less than 0.7, and TS 2 ×√EL was less than 0.7×10 6 (MPa) 2 % 0.5 , TS 2 ×√HER was less than 1.4×10 6 (MPa) 2 % 0.5 。

[0143] In Comparative Example 13, due to the relatively low secondary holding temperature, TS 2 ×√EL was less than 0.7×10 6 (MPa) 2 % 0.5 , TS 2 ×√HER was less than 1.4×10 6 (MPa) 2 % 0.5 。

[0144] In Comparative Example 14, due to the relatively short secondary holding time, TS 2 ×√EL was less than 0.7×10 6 (MPa) 2 % 0.5 , TS 2 ×√HER was less than 1.4×10 6 (MPa) 2 % 0.5 。

[0145] In Comparative Example 32, due to the relatively low C content, YR was less than 0.7, and TS 2 ×√EL was less than 0.7×10 6 (MPa) 2 % 0.5 , TS 2 ×√HER was less than 1.4×10 6 (MPa) 2 % 0.5 。

[0146] In Comparative Example 33, due to the relatively high C content, unrecrystallized ferrite was formed, and TS 2 ×√EL exceeded 1.7×10 6 (MPa) 2 % 0.5 , TS 2 ×√HER exceeded 3.0×10 6 (MPa) 2 %0.5 .

[0147] In Comparative Example 34, due to the high Si content, embrittlement increased, and TS 2 ×√EL was less than 0.7×10 6 (MPa) 2 %. 0.5 , and TS 2 ×√HER was less than 1.4×10 6 (MPa) 2 %. 0.5 .

[0148] In Comparative Example 35, due to the low Mn content, TS 2 ×√EL was less than 0.7×10 6 (MPa) 2 %. 0.5 , and TS 2 ×√HER was less than 1.4×10 6 (MPa) 2 %. 0.5 .

[0149] In Comparative Example 36, due to the high Mn content, unrecrystallized ferrite was formed, and TS 2 ×√EL exceeded 1.7×10 6 (MPa) 2 %. 0.5 , and TS 2 ×√HER exceeded 3.0×10 6 (MPa) 2 %. 0.5 .

[0150] In Comparative Example 37, due to the high Al content, TS 2 ×√EL was less than 0.7×10 6 (MPa) 2 %. 0.5 , and TS 2 ×√HER was less than 1.4×10 6 (MPa) 2 %. 0.5 .

[0151] In Comparative Example 38, due to the high Ti content, unrecrystallized ferrite was formed, YR exceeded 0.85, and TS 2 ×√EL exceeded 1.7×10 6 (MPa) 2 %. 0.5 , and TS 2 ×√HER exceeded 3.0×10 6 (MPa) 2 %. 0.5 .

[0152] In Comparative Example 39, due to the high Nb content, unrecrystallized ferrite was formed, YR exceeded 0.85, TS 2 ×√EL exceeded 1.7×10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER exceeded 3.0×10 6 (MPa) 2 % 0.5 。

[0153] In Comparative Example 40, due to the high V content, unrecrystallized ferrite was formed, YR exceeded 0.85, TS 2 ×√EL exceeded 1.7×10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER exceeded 3.0×10 6 (MPa) 2 % 0.5 。

[0154] In Comparative Example 41, due to the high Ti+Nb+V content, unrecrystallized ferrite was formed, YR exceeded 0.85, TS 2 ×√EL exceeded 1.7×10 6 (MPa) 2 % 0.5 ,TS 2 ×√HER exceeded 3.0×10 6 (MPa) 2 % 0.5 。

[0155] The above has been described with reference to the embodiments. However, it should be clear that those skilled in the art can make various modifications and changes to the present invention within the scope of the basic idea of the present invention, and the scope of the rights of the present invention should be interpreted based on the claims.

Claims

1. A steel plate, by weight %, the steel plate comprises: C: 0.03 - 0.18%, Si: 0 - 0.70%, Mn: 0.25 - 1.80%, Al: 0 - 0.70%, P: below 0.050%, S: below 0.0300%, N: below 0.0300%, Ti: 0 - 0.08%, Nb: 0 - 0.06%, V: 0 - 0.07%, Ti + Nb + V: 0.03 - 0.10%, Cr: 0 - 0.80%, Mo: 0 - 0.80%, Cu: 0 - 0.80%, Ni: 0 - 0.80%, B: 0 - 0.0050%, Ca: 0 - 0.050%, REM except Y: 0 - 0.050%, Mg: 0 - 0.050%, W: 0 - 0.50%, Zr: 0 - 0.50%, Sb: 0 - 0.50%, Sn: 0 - 0.50%, Y: 0 - 0.20%, Hf: 0 - 0.20%, the balance being Fe and other inevitable impurities, By area %, the steel plate has a steel plate microstructure comprising 1 - 15% of pearlite containing cementite, residual ferrite, and inevitably introduced structures. And the yield ratio (YR) of the steel plate is 0.70 to 0.85, and the product of the square of the tensile strength of the steel plate and the square root of the elongation rate, i.e., TS 2 ×√EL satisfies 0.7×10 6 -1.7×10 6 (MPa) 2 % 0.5 , and the product of the square of the tensile strength of the steel plate and the square root of the hole expansion rate, i.e., TS 2 ×√HER satisfies 1.4×10 6 -3.0×10 6 (MPa) 2 % 0.5 .

2. The steel plate according to claim 1, wherein, The steel plate contains C in the range of 0.04 - 0.17%.

3. The steel plate according to claim 1, wherein, The steel plate contains Mn in the range of 0.27 - 1.78%.

4. The steel plate according to claim 1, wherein, By area %, the steel plate has a microstructure comprising 3 - 13% of pearlite containing cementite, residual ferrite, and inevitably introduced structures.

5. The steel plate according to claim 1, wherein, The yield ratio of the steel plate, which is the value of the yield strength (YS) divided by the tensile strength (TS), i.e., YR = YS / TS, satisfies 0.71 to 0.

84.

6. The steel plate according to claim 1, wherein, The product of the square of the tensile strength of the steel plate and the square root of the elongation, i.e., TS 2 ×√EL satisfies 0.71×10 6 -1.69×10 6 (MPa) 2 % 0.5 。 7. The steel plate according to claim 1, wherein, The product of the square of the tensile strength of the steel plate and the square root of the hole expansion rate, i.e., TS 2 ×√HER satisfies 1.43×10 6 -2.93×10 6 (MPa) 2 % 0.5 。 8. A method for manufacturing a steel plate, which comprises the following steps: Heating an ingot or slab to a temperature range of 1000 - 1350°C, by weight %, the ingot or slab comprises: C: 0.03 - 0.18%, Si: 0 - 0.70%, Mn: 0.25 - 1.80%, Al: 0 - 0.70%, P: below 0.050%, S: below 0.0300%, N: below 0.0300%, Ti: 0 - 0.08%, Nb: 0 - 0.06%, V: 0 - 0.07%, Ti + Nb + V: 0.03 - 0.10%, Cr: 0 - 0.80%, Mo: 0 - 0.80%, Cu: 0 - 0.80%, Ni: 0 - 0.80%, B: 0 - 0.0050%, Ca: 0 - 0.050%, REM except Y: 0 - 0.050%, Mg: 0 - 0.050%, W: 0 - 0.50%, Zr: 0 - 0.50%, Sb: 0 - 0.50%, Sn: 0 - 0.50%, Y: 0 - 0.20%, Hf: 0 - 0.20%, the balance being Fe and other inevitable impurities; Hot rolling the heated ingot or slab at a finish rolling temperature of 800 - 1000°C to manufacture a hot rolled steel plate; Cooling the manufactured hot rolled steel plate and coiling it in a temperature range of 300 - 600°C; The hot-rolled steel sheet after the winding is heat-treated in a temperature range of 650 - 800 °C for 600 - 1700 seconds, and then cold-rolled with a reduction ratio of 30 - 90%; The cold-rolled steel sheet after rolling is heated in a temperature range of 740 - 860 °C and held for more than 50 seconds at one time; The cold-rolled steel sheet after the one-time holding is cooled for the first time at an average cooling rate of 5 °C / second or less to a temperature range of 600 - 760 °C; and The cold-rolled steel sheet after the first cooling is cooled for the second time at an average cooling rate of more than 5 °C / second and 20 °C / second or less to a temperature range of 450 - 550 °C, and then held for more than 50 seconds in this temperature range and then cooled to room temperature.

9. The manufacturing method of the steel plate according to claim 8, wherein, The ingot or slab contains C in the range of 0.04 - 0.17%.

10. The manufacturing method of the steel plate according to claim 8, wherein, The ingot or slab contains Mn in the range of 0.27 - 1.78%.

11. The manufacturing method of the steel plate according to claim 8, wherein, By area%, the steel sheet cooled to room temperature after the second holding has a steel sheet microstructure including pearlite containing cementite, residual ferrite, and inevitably introduced structures, containing 1 - 15%; And the yield ratio (YR) of the steel plate is 0.70 to 0.85, and the product of the square of the tensile strength of the steel plate and the square root of the elongation rate, i.e., TS 2 ×√EL satisfies 0.7×10 6 -1.7×10 6 (MPa) 2 % 0.5 , and the product of the square of the tensile strength of the steel plate and the square root of the hole expansion rate, i.e., TS 2 ×√HER satisfies 1.4×10 6 -3.0×10 6 (MPa) 2 % 0.5 .

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