Ultrahigh-strength cold-rolled steel sheet having corrosion resistance and method for manufacturing same

Abstract

The present invention provides an ultra-high strength cold-rolled steel sheet having corrosion resistance comprising, in wt%, 0.1% to 0.5% of carbon (C), 0.01% to 2.0% of silicon (Si), 0.1% to 5.0% of manganese (Mn), 0.01% to 2.0% of aluminum (Al), 0% (excluding) to 3.0% of chromium (Cr), 0% (excluding) to 1.0% of molybdenum (Mo), 0.02% to 3.0% of nickel (Ni), 0.02% to 3.0% of copper (Cu), 0.01% to 0.2% of titanium (Ti), 0.01% to 0.1% of niobium (Nb), 0.01% to 1.0% of vanadium (V), 0.001% to 0.005% of boron (B), 0% (excluding) to 0.02% of phosphorus (P), 0% (excluding) to 0.01% of sulfur (S), and the balance being Fe and unavoidable impurities. And the balance being iron (Fe) and other unavoidable impurities, in which the ratio of the content of copper (Cu) divided by the content of nickel (Ni) ([Cu] / [Ni]) ranges from 0.54 to 5.7, and the steel sheet satisfies yield strength (YS) of 1000 MPa or more, tensile strength (TS) of 1100 MPa or more, elongation (EL) of 3% or more, and non-fracture time according to a hydrogen embrittlement test method of 100 hours or more.

Description

Technical Field

[0001] The technical idea of the present invention relates to a steel, and more particularly to an ultra-high strength cold-rolled steel sheet having excellent corrosion resistance and a method for manufacturing the same. Background Art

[0002] In the automotive industry, the demand for the collision safety of vehicle bodies is continuously increasing. As electric vehicles become more common, the number of vehicle components has decreased, but due to the addition of batteries, the weight of the vehicle has increased, which has led to an increased demand for collision safety. Therefore, the ultra-high strength of collision absorption components (such as front bumper beams, side beams, and door impact beams) that contribute to collision safety is also continuously improved. In particular, in the case of martensitic steel, which has the highest strength among cold-rolled steel sheets, due to the increased use of roll forming technology, its application range has expanded, but due to its high strength, there is a problem of delayed fracture. In particular, corrosion in a hydrogen permeation environment is a representative fracture behavior of such delayed fracture. Therefore, many studies have been conducted to improve the corrosion resistance or hydrogen embrittlement resistance of ultra-high strength sheets with a tensile strength of 1 GPa or higher, but it is still insufficient.

[0003] The following attempts have been proposed to improve the corrosion resistance of cold-rolled steel sheets. Regarding hydrogen-induced delayed fracture, a method for improving hydrogen embrittlement resistance by controlling the residual hydrogen content in steel has been proposed, particularly a method for improving hydrogen embrittlement resistance by controlling the hydrogen content to 0.1 ppm or less during a heat treatment process maintained at 350°C to 450°C after cold rolling. In addition, a method for controlling the area fractions of ferrite, upper bainite, and martensite, adding boron (a grain boundary strengthening element), controlling the effective grain size of martensite, and controlling the number of iron-based carbides has been proposed. In addition, a method for improving the hydrogen embrittlement resistance of steel sheets with a strength of 1.1 GPa or higher by controlling the grain size and aspect ratio of ferrite and martensite based on the cold rolling pressure ratio has been proposed. This hydrogen embrittlement is proposed not only as an important research topic in cold-rolled steel sheets for automobiles but also as an important research topic in steels for pressure vessels. A method for delaying the crack propagation rate caused by hydrogen embrittlement by forming a microstructure has been proposed, in which due to the addition of copper, based on the improvement of hydrogen embrittlement resistance, the fraction of the band structure in the microstructure can be reduced. In addition, a method for improving hydrogen embrittlement resistance by adding nickel in an amount of 1 wt% to 4 wt% has been proposed. A method for restricting the ratios of manganese, nickel, and copper to improve the stability of austenite during the manufacturing process of high-strength steel sheets has been proposed. In this case, the addition of nickel and copper is for ensuring the hardenability of steel sheets with a high aluminum content, rather than for preventing hydrogen-induced delayed fracture. As a related technical document, there is Korean Patent Application Publication No. 2012-0144482. Summary of the Invention

[0004] Technical problem

[0005] Therefore, in view of the above problems, the present invention has been completed, and an object of the present invention is to provide an ultra-high strength cold-rolled steel sheet having a tensile strength of 1.1 GPa or more and excellent corrosion resistance and a method for manufacturing the same.

[0006] It will be understood that the technical problem is provided only as an example, and the technical idea of the present invention is not limited thereto.

[0007] Technical solution

[0008] According to one aspect of the present invention, the above object and other objects can be achieved by providing an ultra-high strength cold-rolled steel sheet having corrosion resistance.

[0009] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet contains, by weight%: carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): greater than 0% and less than or equal to 3.0%, molybdenum (Mo): greater than 0% and less than or equal to 1.0%, nickel (Ni): 0.02% to 3.0%, copper (Cu): 0.02% to 3.0%, titanium (Ti): 0.01% to 0.2%, niobium (Nb): 0.01% to 0.1%, vanadium (V): 0.01% to 1.0%, boron (B): 0.001% to 0.005%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the balance contains iron (Fe) and other inevitable impurities, wherein the ratio ([Cu] / [Ni]) of the content of copper (Cu) to the content of nickel (Ni) ranges from 0.54 to 5.7, and the ultra-high strength cold-rolled steel sheet satisfies: yield strength (YS): 1000 MPa or more, tensile strength (TS): 1100 MPa or more, elongation index (EL): 3% or more, and unbroken time based on the hydrogen embrittlement test method: 100 hours or longer.

[0010] According to an embodiment of the present invention, the microstructure of the ultra-high strength cold-rolled steel sheet having corrosion resistance may be selected from ferrite, bainite, and retained austenite, wherein the area fraction of martensite is 95% or more and less than 100%, and the area fraction of the remaining phase is greater than 0% and less than or equal to 5%.

[0011] According to an embodiment of the present invention, the ultra-high strength cold-rolled steel sheet having corrosion resistance may further contain carbides, wherein the carbides have an average size of 100 nm or less and an aspect ratio of 5 or less.

[0012] According to an embodiment of the present invention, the carbide may include at least one of Fe-based carbide, Ti-based carbide, Nb-based carbide, V-based carbide, and Mo-based carbide.

[0013] According to another aspect of the present invention, a method for manufacturing a high-strength cold-rolled steel sheet with corrosion resistance is provided.

[0014] According to an embodiment of the present invention, in the method for manufacturing a high-strength cold-rolled steel sheet with corrosion resistance, the method includes: manufacturing a hot-rolled steel sheet by hot-rolling steel, the steel containing, by weight %: carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): greater than 0% and less than or equal to 3.0%, molybdenum (Mo): greater than 0% and less than or equal to 1.0%, nickel (Ni): 0.02% to 3.0%, copper (Cu): 0.02% to 3.0%, titanium (Ti): 0.01% to 0.2%, niobium (Nb): 0.01% to 0.1%, vanadium (V): 0.01% to 1.0%, boron (B): 0.001% to 0.005%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the balance containing iron (Fe) and other inevitable impurities, wherein the ratio ([Cu] / [Ni]) of the content of copper (Cu) to the content of nickel (Ni) ranges from 0.54 to 5.7; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; annealing the cold-rolled steel sheet by holding it at 800°C to 900°C for 60 seconds to 600 seconds; first cooling the annealed cold-rolled steel sheet to 500°C to 700°C at a cooling rate of 1°C / s to 20°C / s; second cooling the first-cooled cold-rolled steel sheet to a temperature below Mf at a cooling rate of 5°C / s to 100°C / s; and tempering the second-cooled cold-rolled steel sheet at 100°C to 350°C.

[0015] According to an embodiment of the present invention, the manufacturing of the hot-rolled steel sheet may include: reheating the steel with an alloy composition at a reheating temperature of 1150°C to 1300°C; manufacturing a hot-rolled steel sheet by hot-rolling the reheated steel such that hot-rolling is completed at a finishing temperature of 800°C to 1000°C; and coiling the hot-rolled steel sheet at a coiling temperature of 400°C to 700°C.

[0016] According to an embodiment of the present invention, the tempering may be performed at a temperature range higher than 200°C and lower than or equal to 350°C for 60 seconds to 600 seconds.

[0017] According to an embodiment of the present invention, tempering can be carried out in a temperature range of 100°C to 200°C for 3 hours to 20 hours.

[0018] According to another aspect of the present invention, a method for manufacturing a super high strength cold-rolled steel sheet with corrosion resistance is provided, and the method includes plating with molten zinc.

[0019] According to an embodiment of the present invention, in the method for manufacturing a super high strength cold-rolled steel sheet with corrosion resistance, the method includes: manufacturing a hot-rolled steel sheet by hot-rolling a steel, the steel containing by weight%: carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): greater than 0% and less than or equal to 3.0%, molybdenum (Mo): greater than 0% and less than or equal to 1.0%, nickel (Ni): 0.02% to 3.0%, copper (Cu): 0.02% to 3.0%, titanium (Ti): 0.01% to 0.2%, niobium (Nb): 0.01% to 0.1%, vanadium (V): 0.01% to 1.0%, boron (B): 0.001% to 0.005%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the balance contains iron (Fe) and other inevitable impurities, wherein the ratio ([Cu] / [Ni]) of the content of copper (Cu) to the content of nickel (Ni) ranges from 0.54 to 5.7; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; annealing the cold-rolled steel sheet by holding it at 800°C to 900°C for 60 seconds to 600 seconds; first cooling the annealed cold-rolled steel sheet to 500°C to 700°C at a cooling rate of 1°C / s to 20°C / s; second cooling the first-cooled cold-rolled steel sheet to 400°C to 500°C at a cooling rate of 5°C / s to 100°C / s; subjecting the second-cooled cold-rolled steel sheet to molten zinc plating; and tempering the molten zinc-plated cold-rolled steel sheet at 100°C to 350°C.

[0020] According to an embodiment of the present invention, between the molten zinc plating and the tempering, the method may further include alloying the molten zinc-plated cold-rolled steel sheet by heat treatment at 450°C to 600°C.

[0021] According to an embodiment of the present invention, tempering can be carried out in a temperature range higher than 200°C and lower than or equal to 350°C for 60 seconds to 600 seconds.

[0022] According to an embodiment of the present invention, tempering can be carried out in a temperature range of 100°C to 200°C for 3 hours to 20 hours.

[0023] Advantageous effects

[0024] According to the technical idea of the present invention, by controlling the content ratio of copper to nickel (([Cu] / [Ni])) and thus controlling the reheating temperature during the hot rolling process within an appropriate range, a cold-rolled steel sheet with high tensile strength and excellent hydrogen embrittlement resistance can be manufactured. The effects of the present invention are described as examples, and the scope of the present invention is not limited by these effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A process flow chart schematically showing a method for manufacturing a super high strength cold-rolled steel sheet with corrosion resistance according to an embodiment of the present invention.

[0026] Figure 2 Shows the heat treatment history over time of an embodiment of the present invention.

[0027] Figure 3 A process flow chart schematically showing a method for manufacturing a super high strength cold-rolled steel sheet with corrosion resistance by hot-dip coating according to an embodiment of the present invention.

[0028] Figure 4 Shows the heat treatment history over time of an embodiment of the present invention.

[0029] Figure 5 Shows the melting point of the Cu-Ni homogeneous solid solution calculated using the ThermoCalc program.

[0030] Figure 6 A set of photographs showing the fracture state after a hydrogen embrittlement test on a super high strength cold-rolled steel sheet with corrosion resistance according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those skilled in the art, and the following embodiments can be modified in various different forms, but the scope of the technical idea of the present invention is not limited to the following embodiments. On the contrary, the embodiments are provided to make the present invention thorough and complete, and to fully convey the technical idea of the present invention to those skilled in the art. In the specification, the same reference numerals denote the same elements. In addition, various elements and regions in the drawings are schematically drawn. Therefore, the technical idea of the present invention is not limited by the relative sizes or spacings drawn in the accompanying drawings.

[0032] The technical idea of the present invention provides a corrosion-resistant ultra-high strength cold-rolled steel sheet and a method for manufacturing the corrosion-resistant ultra-high strength cold-rolled steel sheet, and improves the hydrogen embrittlement resistance of the corrosion-resistant ultra-high strength cold-rolled steel sheet by controlling the component content and microstructure composition applicable to an ultra-high strength steel sheet for automobiles having a tensile strength of 1100 MPa or higher.

[0033] The ultra-high strength cold-rolled steel sheet having corrosion resistance according to the technical idea of the present invention is described in detail below.

[0034] The ultra-high strength cold-rolled steel sheet having corrosion resistance according to an embodiment of the present invention contains, by weight%: carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): greater than 0% and less than or equal to 3.0%, molybdenum (Mo): greater than 0% and less than or equal to 1.0%, nickel (Ni): 0.02% to 3.0%, copper (Cu): 0.02% to 3.0%, titanium (Ti): 0.01% to 0.2%, niobium (Nb): 0.01% to 0.1%, vanadium (V): 0.01% to 1.0%, boron (B): 0.001% to 0.005%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the balance contains iron (Fe) and other inevitable impurities.

[0035] The functions and contents of each component contained in the ultra-high strength cold-rolled steel sheet having corrosion resistance according to the present invention are described below. Here, the content of each component element means weight% based on the entire steel sheet.

[0036] Carbon (C): 0.1% to 0.5%

[0037] Carbon is added to ensure the strength of the steel sheet and control its microstructure, and the strength can be ensured by increasing the hardness of martensite. When the carbon content is less than 0.1%, it is difficult to achieve the target strength. When the carbon content exceeds 0.5%, the weldability and workability may deteriorate. Therefore, it is desirable to add carbon in an amount of 0.1% to 0.5% of the total weight of the steel sheet.

[0038] Silicon (Si): 0.01% to 2.0%

[0039] Silicon is a ferrite stabilizing element, which can ensure hydrogen embrittlement resistance by suppressing the growth of cementite. When the silicon content is less than 0.01%, the effect produced by the addition of silicon is insufficient. When the silicon content exceeds 2.0%, a large amount of ferrite may be formed, making it impossible to ensure the target strength. Therefore, it is desirable to add silicon in an amount of 0.01% to 2.0% of the total weight of the steel sheet.

[0040] Manganese (Mn): 0.1% to 5.0%

[0041] Manganese has a solid-solution strengthening effect and can contribute to strength improvement by increasing hardenability. When the content of manganese is less than 0.1%, due to insufficient hardenability, it is difficult to ensure strength, and the effect caused by the addition of manganese is insufficient. When the content of manganese exceeds 5.0%, due to the formation of manganese bands and the formation of MnS, the hydrogen embrittlement resistance may decrease. Therefore, it is desirable to add manganese in a content of 0.1% to 5.0% of the total weight of the steel sheet.

[0042] Aluminum (Al): 0.01% to 2.0%

[0043] Aluminum is used as a deoxidizer and can help purify ferrite. When the content of aluminum is less than 0.01%, the effect caused by the addition of aluminum (such as deoxidation effect) is insufficient. When the content of aluminum exceeds 2.0%, AlN may be formed during the slab manufacturing process, which may cause cracks during casting or hot rolling, and a large amount of ferrite may be formed, which may reduce the strength. Therefore, it is desirable to add aluminum in a content of 0.01% to 2.0% of the total weight of the steel sheet.

[0044] Chromium (Cr): greater than 0% and less than or equal to 3.0%

[0045] Chromium (which is a ferrite stabilizer in steel) increases hardenability and can contribute to strength improvement by refining carbides. When the content of chromium exceeds 3.0%, the manufacturing cost is relatively high, and the quenching effect during the cooling process is large, which will cause an increase in strength. Therefore, the elongation index may be relatively reduced, and the laser weldability may deteriorate. Therefore, it is desirable to add chromium in a content greater than 0% and less than or equal to 3.0% of the total weight of the steel sheet.

[0046] Molybdenum (Mo): greater than 0% and less than or equal to 1.0%

[0047] Molybdenum has a solid-solution strengthening effect and can contribute to strength improvement by increasing hardenability. In addition, it can improve the hydrogen embrittlement resistance by refining Ti-based precipitates. When the content of molybdenum exceeds 1.0%, the material cost may increase. Therefore, it is desirable to add molybdenum in a content greater than 0% and less than or equal to 1.0% of the total weight of the steel sheet.

[0048] Copper (Cu): 0.02% to 3.0%

[0049] Copper is a precipitate-forming element that forms carbides or nitrides by combining with carbon (C) and nitrogen (N). Through this precipitation and the grain refinement obtained by suppressing recrystallization and grain growth during precipitation and rolling, the toughness and strength of steel can be improved. In addition, copper can be added to improve hydrogen embrittlement resistance. When the copper content is less than 0.02%, the effects produced by copper addition are insufficient, and delayed fracture may occur. When the copper content exceeds 3.0%, as a high-temperature embrittlement-inducing element, it may cause cracks during the hot rolling process, the rolling load during the rolling process may increase significantly, and the manufacturing cost of the steel may increase. Therefore, it is desirable to add copper in an amount of 0.02% to 3.0% of the total weight of the steel plate.

[0050] Nickel (Ni): 0.02% to 3.0%

[0051] Nickel is a precipitate-forming element that forms carbides or nitrides by combining with carbon (C) and nitrogen (N). Through this precipitation and the grain refinement obtained by suppressing recrystallization and grain growth during rolling, it can improve the toughness and strength of steel. In addition, it can suppress the hot brittleness caused by copper. When the nickel content is less than 0.02%, the effects produced by nickel addition are insufficient. When the nickel content exceeds 3.0%, the rolling load during the rolling process may increase significantly, which may increase the manufacturing cost of the steel. Therefore, it is desirable to add nickel in an amount of 0.02% to 3.0% of the total weight of the steel plate. In particular, it is necessary to add nickel in an appropriate ratio to the copper content to prevent copper from melting during the reheating process (this will be explained in more detail).

[0052] [Cu] / [Ni]: 0.54 to 5.7

[0053] In a high-strength cold-rolled steel sheet with corrosion resistance, the ratio ([Cu] / [Ni]) of the copper (Cu) content to the nickel (Ni) content can be 0.54 to 5.7. Here, [Cu] represents the content (wt%) of copper (Cu), and [Ni] represents the content (wt%) of nickel (Ni). This [Cu] / [Ni] ratio is to prevent liquid Cu from penetrating into the grain boundaries of the steel plate during the reheating step for hot rolling and weakening the grain boundaries.

[0054] The melting point of copper is 1084.6 °C, which is lower than that of iron. When copper exists in the slab, the copper can move to its surface. When the surface temperature of the slab or bar is higher than the melting point of copper, the copper that has moved to the surface may melt and penetrate along the grain boundaries of the steel, resulting in hot brittleness and leading to a reduction in ductility and the appearance of cracks. One way to prevent this hot brittleness is to add nickel to form a homogeneous solid solution of copper and nickel, thereby inhibiting the melting of copper. Therefore, the temperature of the Cu-Ni homogeneous solid solution needs to be at least 1150 °C to prevent liquid copper from penetrating into the steel plate during reheating. In addition, the upper limit of the melting point of the Cu-Ni homogeneous solid solution is set to 1300 °C or lower to inhibit the cost increase caused by nickel addition. The melting point of the Cu-Ni homogeneous solid solution is calculated using the ThermoCalc program, which is shown in Figure 5 . Figure 5 The "Cu-Ni ratio for hot brittleness inhibition" (i.e., [Cu] / [Ni]j) shown is in the range of 0.54 to 5.7.

[0055] Titanium (Ti): 0.01% to 0.2%

[0056] Titanium is a precipitate-forming element that can provide the precipitation and grain refinement effects of TiN and TiC. In particular, through the precipitation of TiN, the nitrogen content in the steel can be reduced, and when added together with boron, the precipitation of BN can be prevented, so the solid solution state of boron (a grain boundary strengthening element) can be maintained. When the titanium content is less than 0.01%, the precipitation of BN may be induced, and the effects of titanium addition are insufficient. If the titanium content exceeds 0.2%, the hydrogen embrittlement resistance may decrease due to the coarsening of TiN precipitates, and it may be difficult to ensure strength due to the decrease in the solubility of carbon in the base material, which may increase the manufacturing cost of the steel. Therefore, it is desirable to add titanium in an amount of 0.01% to 0.2% of the total weight of the steel plate.

[0057] Niobium (Nb): 0.01% to 0.1%

[0058] Niobium is a precipitate-forming element that forms carbides or nitrides by combining with carbon (C) and nitrogen (N), and can improve the toughness and strength of the steel through grain refinement obtained by suppressing this precipitation, recrystallization, and grain growth during the rolling process. When the niobium content is less than 0.01%, there is no grain refinement effect, and the effects of niobium addition are insufficient. When the niobium content exceeds 0.1%, the precipitates may grow, and there may be no strength improvement effect. The rolling load during the rolling process may increase significantly, and the manufacturing cost of the steel may increase. Therefore, it is desirable to add niobium in an amount of 0.01% to 0.1% of the total weight of the steel plate.

[0059] Vanadium (V): 0.01% to 1.0%

[0060] Vanadium is a precipitate-forming element that forms carbides or nitrides by combining with carbon (C) and nitrogen (N), and can improve the toughness and strength of steel through grain refinement obtained by suppressing such precipitation, recrystallization, and grain growth during rolling. When the content of vanadium is less than 0.01%, there is no grain refinement effect, and the effect produced by vanadium addition is insufficient. When the content of vanadium exceeds 1.0%, the precipitates may grow, and the strength improvement effect may not be obtained. In addition, the rolling load during the rolling process may increase significantly, and the manufacturing cost of the steel may increase. Therefore, it is desirable to add vanadium in a content of 0.01% to 1.0% of the total weight of the steel plate.

[0061] Boron (B): 0.001% to 0.005%

[0062] Boron is a grain boundary strengthening element that can improve hydrogen embrittlement resistance when distributed at grain boundaries. When the content of boron is less than 0.001%, the effect produced by boron addition is insufficient. When the content of boron exceeds 0.005%, there is a risk of grain boundary embrittlement due to BN formation. Therefore, it is preferable to add boron in a content of 0.001% to 0.005% of the total weight of the steel plate.

[0063] Phosphorus (P): greater than 0% and less than or equal to 0.02%

[0064] Phosphorus is an impurity contained in the steel manufacturing process. Phosphorus can help improve strength through solid solution strengthening, but when contained in large amounts, low-temperature embrittlement may occur due to grain boundary segregation, and the spot weldability may be reduced. Therefore, it is desirable to limit phosphorus to a content greater than 0% and less than or equal to 0.02% of the total weight of the steel plate.

[0065] Sulfur (S): greater than 0% and less than or equal to 0.01%

[0066] Sulfur is an impurity contained in the steel manufacturing process and can form non-metallic inclusions (such as FeS and MnS), which may reduce toughness, hydrogen embrittlement resistance, and weldability. Therefore, it is desirable to limit sulfur to a content greater than 0% and less than or equal to 0.01% of the total weight of the steel plate.

[0067] The remaining component of the ultra-high strength cold-rolled steel plate with corrosion resistance is iron (Fe). However, due to the possible inevitable mixing of raw materials or accidental impurities in the surrounding environment during the normal steelmaking process, it cannot be excluded. These impurities are known to those skilled in the manufacturing field, and thus are not specifically mentioned in this specification.

[0068] The ultra-high strength cold-rolled steel sheet with corrosion resistance, manufactured by controlling the specific components and their content ranges of the above alloy composition and by the manufacturing method described below, can meet the following requirements: for example, yield strength (YS): 1000 MPa or greater, tensile strength (TS): 1100 MPa or greater, elongation index (EL): 3% or greater, and unbroken time based on the hydrogen embrittlement test method: 100 hours or longer. Additionally, the ultra-high strength cold-rolled steel sheet with corrosion resistance can meet the following requirements: for example, yield strength (YS): 1000 MPa to 1700 MPa, tensile strength (TS): 1100 MPa to 1900 MPa, elongation index (EL): 3% to 9%, and unbroken time based on the hydrogen embrittlement test method: 100 hours to 300 hours.

[0069] The ultra-high strength cold-rolled steel sheet with corrosion resistance has a microstructure with a martensite / tempered martensite area fraction of 95% or greater. Here, "martensite / tempered martensite" means the sum of fresh martensite and tempered martensite, without distinction. For example, the range of the area fraction of martensite / tempered martensite can be 95% or greater and less than 100%, and the remaining phase can be one or more phases selected from ferrite, bainite, and retained austenite, and the area fraction is greater than 0% and less than or equal to 5%. The area fraction represents the area ratio obtained from a microstructure photograph using an image analyzer. Additionally, the area fraction of the microstructure is based on the results of analysis performed using a scanning electron microscope at 1 / 4 of the thickness direction of the steel sheet in a direction perpendicular to the rolling direction.

[0070] Carbides should be present to inhibit delayed fracture caused by hydrogen embrittlement. These carbides can capture and fix hydrogen, such that hydrogen diffusing and penetrating into the steel sheet cannot move freely within the steel sheet. However, when the size of the carbides is too large, embrittlement may occur due to hydrogen collected at the grain boundaries, so it is desirable to limit the size of the carbides.

[0071] Therefore, the ultra-high strength cold-rolled steel sheet with corrosion resistance can contain more carbides. The average size (particle diameter) of the carbides can be, for example, 100 nm or smaller, for example, in the range of 1 nm to 100 nm. The aspect ratio of the carbides can be, for example, 5 or smaller, for example, in the range of 1 to 5. Here, the aspect ratio represents the ratio of the major axis length to the minor axis length of the carbides. When the average size of the carbides exceeds 100 nm, it can be considered that martensite is formed due to over-tempering or insufficient stability of retained austenite.

[0072] The carbide may include cementite and transition carbide, or may include carbide without Fe, and may include at least one of, for example, Fe-based carbide, Ti-based carbide, Nb-based carbide, V-based carbide, and Mo-based carbide. Specifically, the carbide may include, for example, Fe3C, ε-carbide (Fe 2.5 C), η-carbide (Fe2C), (Fe, substitution element) 2～3 (C), and (Ti, Nb, V, Mo)(C, N).

[0073] Hydrogen embrittlement test method

[0074] The hydrogen embrittlement test method for quantitatively measuring the delayed fracture of a corrosion-resistant ultra-high strength cold-rolled steel sheet caused by hydrogen embrittlement in the present invention is as follows.

[0075] Cut the cold-rolled steel sheet using shearing, laser, water jet, grinding, wire (EDM), etc. to prepare specimens with a length of 100 mm to 300 mm and a width of 10 mm to 50 mm. Here, the longitudinal direction of each specimen is manufactured such that it is 0 degrees or 90 degrees with respect to the rolling direction of the material.

[0076] Then, apply stresses of 60%, 70%, 80%, 90%, and 100% of the yield strength to the specimens using the two-point or four-point bending method. Here, the applied stress should vary according to the yield strength of each cold-rolled steel sheet. The goal is to achieve no fracture under conditions of three or more stress application conditions (including the 100% stress condition specimen).

[0077] Immerse the specimens subjected to stress into a hydrochloric acid (HCl) solution with a concentration of 0.01 N to 0.2 N, preferably into a 0.1 N hydrochloric acid solution. Since hydrogen is injected into the interior of the specimens through this immersion, it can be said to be an accelerated test for hydrogen-induced delayed fracture. Keep the immersed specimens for 100 hours or longer. Here, the purpose is to ensure that no fracture occurs under any stress conditions.

[0078] Hereinafter, a method for manufacturing a corrosion-resistant ultra-high strength cold-rolled steel sheet according to the present invention will be described with reference to the accompanying drawings.

[0079] Method for manufacturing a super high strength cold rolled steel sheet with corrosion resistance

[0080] Figure 1 A process flow chart schematically showing a method for manufacturing a corrosion-resistant ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention, and relates to a method for manufacturing an uncoated cold-rolled steel sheet.

[0081] Refer to Figure 1, a method for manufacturing a high-strength cold-rolled steel sheet with corrosion resistance according to an embodiment of the present invention includes a hot-rolled steel sheet manufacturing step (S110), a cold-rolled steel sheet manufacturing step (S120), an annealing step (S130), a first cooling step (S140), a second cooling step (S150), a tempering step (S160), and a third cooling step (S170).

[0082] Hot rolled steel sheet manufacturing step (S110)

[0083] In the hot-rolled steel sheet manufacturing step (S110), steel is prepared which, by weight %, contains carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): greater than 0% and less than or equal to 3.0%, molybdenum (Mo): greater than 0% and less than or equal to 1.0%, nickel (Ni): 0.02% to 3.0%, copper (Cu): 0.02% to 3.0%, titanium (Ti): 0.01% to 0.2%, niobium (Nb): 0.01% to 0.1%, vanadium (V): 0.01% to 1.0%, boron (B): 0.001% to 0.005%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the balance contains iron (Fe) and other inevitable impurities.

[0084] In the manufacturing method according to the present invention, the semi-finished product to be subjected to the hot-rolling process can be, for example, a slab. After obtaining molten steel with a determined composition through a steelmaking process, a slab in a semi-finished state can be obtained through a continuous casting process.

[0085] The steel (such as a slab) is reheated at a reheating temperature (slab reheating temperature, SRT) of 1150°C to 1300°C for 1 hour to 5 hours. Through this reheating, a complete transformation to a single austenite phase can occur, the components segregated during the casting process can be redissolved, and the precipitates can be redissolved, thereby homogenizing the steel and preparing it for hot rolling.

[0086] As shown above, the slab reheating temperature in the present invention is set by considering the melting point of the homogeneous solid solution of the added elements (copper and nickel). The temperature of the Cu-Ni homogeneous solid solution should be 1150°C or higher to prevent liquid copper from penetrating into the steel sheet during reheating. When the reheating temperature is lower than 1150°C, the components segregated during the casting process may not be fully redissolved, and thus may not be evenly distributed.

[0087] In addition, in order to suppress the cost increase caused by nickel addition, the upper limit of the melting point of the Cu-Ni homogeneous solid solution is set to 1300 °C or lower. When the reheating temperature is higher than 1300 °C, the austenite grains may become coarser, which may lead to a decrease in yield strength. In addition, as the reheating temperature increases, problems such as an increase in manufacturing cost and a decrease in productivity may occur due to heating costs, additional time required to match the hot rolling temperature, etc.

[0088] When the reheating time is less than 1 hour, the reduction of the segregation region may be insufficient, and when it exceeds 5 hours, the grain size may increase and the process cost may increase.

[0089] The melting point of copper is 1084.6 °C, which is lower than that of iron. When copper exists in the slab, copper can move to its surface. When the surface temperature of the slab or bar is higher than the melting point of copper, the copper that has moved to the surface may melt and penetrate along the grain boundaries of the steel, resulting in hot brittleness and causing a decrease in ductility and the appearance of cracks. One way to prevent this hot brittleness is to add nickel to form a homogeneous solid solution of copper and nickel, thereby suppressing the melting of copper. Therefore, the temperature of the Cu-Ni solid solution needs to be at least 1150 °C to prevent liquid copper from penetrating into the steel sheet during reheating. In addition, the upper limit of the melting point of the Cu-Ni solid solution is set to 1300 °C or lower to suppress the cost increase caused by nickel addition. The melting point of the Cu-Ni solid solution is calculated using the ThermoCalc program, which is shown in Figure 5 in.

[0090] Then, the reheated steel is first heated to adjust its shape and then hot rolled. The hot rolling can be carried out in sequence with rough rolling and finish milling. Through the hot rolling step, the steel can be formed into hot rolled steel. The hot rolled steel can be a hot rolled steel sheet.

[0091] Rough rolling is the step of rolling the slab to manufacture bars and can be carried out at a temperature within a range of 1000 °C from the end temperature of reheating (1150 °C to 1300 °C).

[0092] Finish milling can be completed at a finish rolling temperature (FRT) of 800 °C to 1000 °C. When the finish rolling temperature is lower than 800 °C, the rolling load may increase rapidly, which may reduce productivity. When the finish rolling temperature is higher than 1000 °C, the grains may become coarser, which may lead to a decrease in the strength of the final steel.

[0093] Then, the hot rolled steel is cooled to a predetermined coiling temperature. The cooling can be air cooling or water cooling and can be carried out at a cooling rate of, for example, 1 °C / s to 100 °C / s. A faster cooling rate can be beneficial for reducing the average grain size. The cooling is preferably carried out to a coiling temperature of, for example, 400 °C to 700 °C, such as 500 °C to 650 °C.

[0094] Then, the hot-rolled steel sheet is coiled at a coiling temperature (CT) of, for example, 400°C to 700°C, for example, 500°C to 650°C. When the coiling temperature is lower than 400°C, the shape of the coiled hot-rolled coil becomes uneven and the strength increases, which may increase the rolling load during the cold rolling process. When the coiling temperature is higher than 700°C, due to the difference in the cooling rate between the center and the edge of the steel sheet, an uneven microstructure may occur, and defects may occur in the continuous process due to surface oxidation, etc. The coiled steel can be cooled to room temperature.

[0095] Cold rolled steel sheet manufacturing step (S120)

[0096] In the cold-rolled steel sheet manufacturing step (S120), the hot-rolled steel sheet is used and cold-rolled to match the thickness of the final steel sheet product. The coiled hot-rolled steel sheet is pickled by using acid cleaning. Then, the pickled hot-rolled steel sheet is cold-rolled at a cold reduction rate of, for example, 35% or more, for example, 35% to 70% to form a cold-rolled steel sheet. The higher the reduction rate, the more the improvement in hydrogen embrittlement resistance may be due to the grain refinement effect. When the cold reduction rate is less than 35%, it is difficult to obtain a uniform microstructure, and since the amount of nuclei generated for recrystallization during annealing is small, the grains may grow excessively during annealing (which will be described below), so that the strength may rapidly decrease. When the cold reduction rate exceeds 70%, the amount of nuclei generated becomes excessive, so that the grains formed by annealing become too fine, which may reduce the ductility and the formability.

[0097] The cold-rolled steel sheet manufactured by cold rolling is subjected to the following heat treatment steps. Figure 2 The heat treatment history of this example over time is shown and will be explained step by step.

[0098] Annealing step (S130)

[0099] In the annealing step (S130), the cold-rolled steel sheet can be heat-treated in a continuous annealing furnace having a general slow cooling section.

[0100] The annealing is carried out as follows: heating at a heating rate of, for example, 1°C / s or more, for example, 1°C / s to 10°C / s at a temperature of, for example, Ac3 or higher, for example, 800°C to 900°C, and then holding the above temperature for, for example, 60 seconds to 600 seconds. When the annealing temperature is lower than 800°C or the holding time is less than 60 seconds, it is difficult to form sufficient austenite, and the ferrite fraction may increase, resulting in a decrease in strength. When the annealing temperature is higher than 900°C or the holding time exceeds 600 seconds, the austenite grain size may become coarser or the productivity may be excessively reduced.

[0101] The Ac3 temperature can be calculated by the following equation.

[0102] Ac3 = 910 - 203×[C] 0.5 - 30[Mn] + 44.7[Si] + 31.5[Mo] - 15.2[Ni]

[0103] Where [C] represents the carbon content (by weight) in the steel, [Mn] represents the manganese content (by weight) in the steel, [Si] represents the silicon content (by weight) in the steel, [Mo] represents the molybdenum content (by weight) in the steel, and [Ni] represents the nickel content (by weight) in the steel.

[0104] First cooling step (S140)

[0105] In the first cooling step (S140), the annealed cold-rolled steel sheet is first cooled at a cooling rate of, for example, 1 °C / s to 20 °C / s to, for example, 500 °C to 700 °C. The cooling can be air cooling or water cooling. The first cooling can be referred to as a slow cooling step.

[0106] Second cooling step (S150)

[0107] In the second cooling step (S150), the cold-rolled steel sheet that has been first cooled is second cooled at a cooling rate of, for example, 5 °C / s or greater, for example, 5 °C / s to 100 °C / s to a temperature, for example, below the martensite transformation end temperature (Mf) (e.g., room temperature (0 °C to 40 °C) to 350 °C). In the second cooling step (S150), when the cooling proceeds to the second cooling end temperature below Mf, most of the austenite transforms into fresh martensite. When the cooling rate is large, the second cooling is advantageous. The second cooling can be referred to as a rapid cooling step. In the second cooling, additional ferrite transformation should be suppressed.

[0108] Tempering step (S160)

[0109] In the tempering step (S160), heating is carried out at a heating rate of 1 °C / s to 50 °C / s, and the temperature is maintained for a predetermined time within the range of 100 °C to 350 °C. By tempering, the formation and growth of transition carbides and the growth of cementite can be carried out, while fresh martensite transforms into tempered martensite.

[0110] When the growth of carbides in the longitudinal direction is excessive, the yield strength and tensile strength of the steel may be reduced, and it may also become a fracture initiation point during hydrogen permeation, making hydrogen embrittlement likely to occur. Therefore, in the tempering step, excessive growth of carbides can be suppressed by appropriately controlling the holding time according to the tempering temperature.

[0111] It must be maintained at a relatively high temperature for a relatively short time to inhibit the growth of cementite. For example, it can be carried out for 60 seconds to 600 seconds in the temperature range of 200°C to 350°C.

[0112] When tempering is carried out at a high temperature, the growth of carbides in the longitudinal direction becomes very active because they grow rapidly, so it may be difficult to control the growth of carbides. On the other hand, in the case of tempering at a relatively low temperature, the heat treatment control for the growth of these carbides is easier, so the excessive growth of carbides can be effectively inhibited, which can be more beneficial to ensuring strength and inhibiting fracture caused by hydrogen embrittlement. For example, it can be carried out for 3 hours to 20 hours in the temperature range of 100°C to 200°C.

[0113] After the tempering is completed, the cold-rolled steel sheet is cooled to room temperature (0°C to 40°C) at a cooling rate of 1°C / s to 100°C / s (S170).

[0114] The cold-rolled steel sheet manufactured by the above method may additionally have a galvanized layer or a galvanized alloy layer formed on its surface by an electroplating method.

[0115] Figure 3 The process flow chart is schematically shown for a method of manufacturing a high-strength hot-dip galvanized cold-rolled steel sheet with corrosion resistance according to an embodiment of the present invention. Figure 4 The heat treatment history of this embodiment over time is shown.

[0116] Reference Figure 3 and Figure 4 According to an embodiment of the present invention, a method of manufacturing a high-strength hot-dip galvanized cold-rolled steel sheet with corrosion resistance includes a hot-rolled steel sheet manufacturing step (S210), a cold-rolled steel sheet manufacturing step (S220), an annealing step (S230), a first cooling step (S240), a second cooling step (S250), a hot-dip galvanizing step (S260), an alloying step (S265), a third cooling step (S270), and a tempering step (S280).

[0117] In this embodiment, the hot-rolled steel sheet manufacturing step (S210), the cold-rolled steel sheet manufacturing step (S220), and the annealing step (S230) are the same as those in the above method of manufacturing an uncoated cold-rolled steel sheet.

[0118] First cooling step (S240)

[0119] In the first cooling step (S240), the annealed cold-rolled steel sheet is first cooled to, for example, 500°C to 700°C at a cooling rate of 1°C / s to 20°C / s and held for 5 seconds to 100 seconds.

[0120] Second cooling step (S250)

[0121] Then, in the second cooling step (S250), the cold-rolled steel sheet is cooled a second time to a temperature range of, for example, 400°C to 500°C at a cooling rate of, for example, 5°C / s or greater, for example, 5°C / s to 100°C / s.

[0122] Hot dip galvanizing step (S260)

[0123] After the second cooling step is completed, a hot-dip galvanizing step (S260) is performed. In the hot-dip galvanizing step (S260), the cold-rolled steel sheet is immersed in a hot-dip galvanizing bath to form a hot-dip galvanized layer. The temperature range of the plating bath is, for example, 400°C to 500°C. Under the plating bath conditions, the hot-dip galvanized layer can be easily formed on the surface of the cold-rolled steel sheet, and the adhesion of the coating can be excellent.

[0124] When no subsequent alloying step (S265) is performed after the hot-dip galvanizing step (S260), the steel sheet that has been plated and removed from the plating bath is subjected to the above-described third cooling step (S270), so austenite transforms into martensite in this step. Then, a tempering step (S280) is performed, followed by cooling at room temperature (S290). The tempering step and subsequent steps are as described above, so their descriptions are omitted to avoid repetition.

[0125] Alloying step (S265)

[0126] If necessary, an alloying step (S265) of alloying the cold-rolled steel sheet on which the hot-dip galvanized layer has been formed by heat treatment can be further performed. For the alloying step (S265), the steel sheet that has been plated and removed from the plating bath is placed in a heat treatment apparatus to perform heat treatment for alloying. The heat treatment for alloying can be performed by: holding it at, for example, 450°C to 600°C for, for example, 5 seconds to 100 seconds. Since the hot-dip galvanized layer grows stably in the heat treatment for alloying under the said conditions, the adhesion of the coating can be excellent. When the heat treatment temperature for alloying is lower than 450°C, alloying may not proceed sufficiently, which may reduce the integrity of the hot-dip galvanized layer. When the heat treatment temperature for alloying exceeds 600°C, material changes may occur when the material enters the duplex temperature range.

[0127] After the heat treatment for alloying is completed, the steel sheet is subjected to the third cooling step (S270) as described above, so austenite transforms into martensite in this step. Then, a tempering step (S280) is performed, followed by cooling at room temperature (S290). The tempering step and subsequent steps are as described above, so their descriptions are omitted to avoid repetition.

[0128] Experimental examples

[0129] The following shows preferred experimental embodiments to assist in understanding the present invention. However, the following experimental embodiments are only for helping to understand the present invention, and the present invention is not limited to the following experimental embodiments. Those skilled in the art of this technology can fully infer from the technology the contents not described herein, so their descriptions will be omitted.

[0130] Steels with the compositions shown in Table 1 and Table 2 below (unit: wt%) were prepared, and cold-rolled steel sheets according to the examples and comparative examples were prepared through specific hot-rolling and cold-rolling processes and heat treatment processes. In Table 1 and Table 2, the balance consists of iron (Fe) and impurities inevitably contained in the steelmaking process. The unit of the content of each component is wt%.

[0131] [Table 1]

[0132] Category C Si Mn Cr Mo Ti Example 1 0.253 0.10 1.83 0.42 0.20 0.045 Example 2 0.249 0.10 1.70 0.40 0.21 0.050 Example 3 0.294 0.39 1.02 0.29 0.10 0.045 Example 4 0.294 0.39 1.02 0.29 0.10 0.045 Example 5 0.326 0.41 1.00 0.30 0.10 0.046 Example 6 0.326 0.41 1.00 0.30 0.10 0.046 Example 7 0.248 0.10 1.00 0.00 0.10 0.034 Example 8 0.248 0.10 1.00 0.00 0.10 0.034 Example 9 0.248 0.09 0.58 0.20 0.10 0.032 Example 10 0.248 0.09 0.58 0.20 0.10 0.032 Example 11 0.246 0.09 1.97 0.38 0.20 0.045 Example 12 0.258 0.11 1.86 0.39 0.19 0.050 Comparative example 1 0.246 0.09 1.97 0.38 0.20 0.062 Comparative example 2 0.258 0.11 1.86 0.39 0.19 0.046 Comparative example 3 0.321 0.39 0.97 0.29 0.00 0.040 Comparative example 4 0.321 0.39 0.97 0.29 0.00 0.040 Comparative example 5 0.244 0.09 0.97 0.00 0.10 0.038 Comparative example 6 0.244 0.09 0.97 0.00 0.10 0.038 Comparative example 7 0.246 0.09 1.97 0.38 0.20 0.062 Comparative example 8 0.258 0.11 1.86 0.39 0.19 0.046

[0133] [Table 2]

[0134] Category Ni Cu Nb V B Cu / Ni Example 1 0.50 0.50 0.001 0.001 0.0029 1.00 Example 2 0.02 0.10 0.002 0.003 0.0028 5.00 Example 3 0.14 0.14 0.030 0.003 0.0022 1.00 Example 4 0.14 0.14 0.030 0.003 0.0022 1.00 Example 5 0.08 0.15 0.001 0.030 0.0023 1.88 Example 6 0.08 0.15 0.001 0.030 0.0023 1.88 Example 7 0.02 0.08 0.003 0.031 0.0019 4.00 Example 8 0.02 0.08 0.003 0.031 0.0019 4.00 Example 9 0.06 0.08 0.001 0.030 0.0020 1.33 Example 10 0.06 0.08 0.001 0.030 0.0020 1.33 Example 11 0.50 0.50 0.001 0.001 0.0029 1.00 Example 12 0.02 0.10 0.002 0.003 0.0028 5.00 Comparative example 1 0.00 0.00 0.003 0.003 0.0025 Not added Comparative example 2 0.02 0.01 0.001 0.003 0.0024 0.50 Comparative example 3 0.00 0.00 0.001 0.100 0.0020 Not added Comparative example 4 0.00 0.00 0.001 0.100 0.0020 Not added Comparative example 5 0.00 0.00 0.003 0.029 0.0025 Not added Comparative example 6 0.00 0.00 0.003 0.029 0.0025 Not added Comparative example 7 0.00 0.00 0.003 0.003 0.0025 Not added Comparative example 8 0.02 0.01 0.001 0.003 0.0024 0.50

[0135] Referring to Table 1 and Table 2, Examples 1 to 10 and Comparative Examples 1 to 6 are cold-rolled steel sheets, and Examples 11 and 12 and Comparative Examples 7 and 8 are alloyed molten zinc steel sheets. The examples satisfy the composition range of the present invention. On the other hand, the comparative examples are different from the examples in that the comparative examples do not contain copper and nickel, or as in Comparative Example 2, the ratio of Cu / Ni is less than the lower limit of the present invention. Table 3 shows the manufacturing process condition values of the ultra-high strength cold-rolled steel sheets with corrosion resistance in the comparative examples and examples.

[0136] [Table 3]

[0137]

[0138]

[0139] All the examples and comparative examples were carried out at a reheating temperature of 1200°C to 1250°C for hot rolling. Table 4 shows the measurement results of the yield strength (YS), tensile strength (TS), elongation index (EL), and fracture time in each of the comparative examples and examples.

[0140] [Table 4]

[0141]

[0142] In the examples, under the condition of applying a load of 100% of the yield strength for 100 hours or longer, no fracture occurred. Therefore, it can be seen that the examples have excellent corrosion resistance against hydrogen embrittlement. On the other hand, under the conditions of applying loads of 90% and 100% of the yield strength, the comparative examples showed fractures in less than 100 hours. This indicates that the comparative examples are prone to hydrogen embrittlement because they do not contain nickel and copper. In addition, from the results of Comparative Example 4 and Comparative Example 8, it can be seen that even when the content ratio of nickel to copper (Cu / Ni) is less than 0.5, hydrogen embrittlement is likely to occur. Furthermore, compared with other examples, Examples 1, 2, 11, and 12 (which were tempered at a relatively low temperature of 150 °C for a longer time of 20,000 seconds or longer) did not show fractures in the hydrogen embrittlement test, even after a long time. Therefore, it can be seen that Examples 1, 2, 11, and 12 have better corrosion resistance against hydrogen embrittlement. As described above, this is because, in the case of low-temperature tempering, it is easier to control the heat treatment for the length growth of carbides, which can suppress fractures caused by hydrogen embrittlement.

[0143] Figure 6 A set of photographs showing the fracture states after a hydrogen embrittlement test on a high-strength cold-rolled steel sheet with corrosion resistance according to an embodiment of the present invention.

[0144] Reference Figure 6 , Comparative Example 2 fractured under loads of 100% and 90% of the yield strength. On the other hand, Examples 1 and 5 did not fracture under loads of 60% to 100% of the yield strength. Therefore, it can be seen that the examples have excellent corrosion resistance against hydrogen embrittlement.

[0145] It is obvious to those skilled in the art to which the technical idea of the present invention pertains that the technical idea of the present invention as described above is not limited to the above-described embodiments and the accompanying drawings, and various alternatives, changes, and variations are possible within the scope of the technical idea of the present invention.

Claims

1. An ultra-high strength cold-rolled steel sheet with corrosion resistance, which contains by weight%: carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): greater than 0% and less than or equal to 3.0%, molybdenum (Mo): greater than 0% and less than or equal to 1.0%, nickel (Ni): 0.02% to 3.0%, copper (Cu): 0.02% to 3.0%, titanium (Ti): 0.01% to 0.2%, niobium (Nb): 0.01% to 0.1%, vanadium (V): 0.01% to 1.0%, boron (B): 0.001% to 0.005%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the balance contains iron (Fe) and other inevitable impurities, Among them, the ratio ([Cu] / [Ni]) of the content of copper (Cu) to the content of nickel (Ni) ranges from 0.54 to 5.7, and the ultra-high strength cold-rolled steel sheet satisfies: yield strength (YS): 1000 MPa or greater, tensile strength (TS): 1100 MPa or greater, elongation index (EL): 3% or greater, and the unbroken time based on the hydrogen embrittlement test method: 100 hours or longer.

2. The ultra-high strength cold-rolled steel sheet according to claim 1, wherein, The microstructure of the ultra-high strength cold-rolled steel sheet with corrosion resistance is selected from ferrite, bainite and retained austenite, wherein the area fraction of martensite is 95% or greater and less than 100%, and the area fraction of the remaining phase is greater than 0% and less than or equal to 5%.

3. The ultra-high strength cold-rolled steel sheet according to claim 1, wherein, The ultra-high strength cold-rolled steel sheet with corrosion resistance further contains carbides, wherein, the carbides have an average size of 100 nm or less and an aspect ratio of 5 or less.

4. The ultra-high strength cold-rolled steel sheet according to claim 3, wherein, The carbides include at least one of Fe-based carbides, Ti-based carbides, Nb-based carbides, V-based carbides and Mo-based carbides.

5. A method for manufacturing an ultra-high strength cold-rolled steel sheet with corrosion resistance, the method includes: A hot-rolled steel sheet is manufactured by hot-rolling steel which, by weight %, contains: carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): greater than 0% and less than or equal to 3.0%, molybdenum (Mo): greater than 0% and less than or equal to 1.0%, nickel (Ni): 0.02% to 3.0%, copper (Cu): 0.02% to 3.0%, titanium (Ti): 0.01% to 0.2%, niobium (Nb): 0.01% to 0.1%, vanadium (V): 0.01% to 1.0%, boron (B): 0.001% to 0.005%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the balance contains iron (Fe) and other inevitable impurities, wherein the ratio ([Cu] / [Ni]) of the content of copper (Cu) to the content of nickel (Ni) ranges from 0.54 to 5.7; A cold-rolled steel sheet is manufactured by cold-rolling the hot-rolled steel sheet; The cold-rolled steel sheet is annealed by holding at 800°C to 900°C for 60 seconds to 600 seconds; The annealed cold-rolled steel sheet is first cooled to 500°C to 700°C at a cooling rate of 1°C / s to 20°C / s; The cold-rolled steel sheet that has been first cooled is second cooled to a temperature below Mf at a cooling rate of 5°C / s to 100°C / s; and The cold-rolled steel sheet that has been second cooled is tempered at 100°C to 350°C.

6. The method according to claim 5, wherein The manufacture of the hot-rolled steel sheet includes: Reheating the steel having the alloy composition at a reheating temperature of 1150°C to 1300°C; Manufacturing the hot-rolled steel sheet by hot-rolling the reheated steel such that the hot-rolling is completed at a finish rolling temperature of 800°C to 1000°C; and Coiling the hot-rolled steel sheet at a coiling temperature of 400°C to 700°C.

7. The method according to claim 5, wherein The tempering is carried out at a temperature range higher than 200°C and less than or equal to 350°C for 60 seconds to 600 seconds.

8. The method according to claim 5, wherein The tempering is carried out at a temperature range of 100°C to 200°C for 3 hours to 20 hours.

9. The method according to claim 5, wherein The corrosion-resistant ultra-high strength cold-rolled steel sheet manufactured by the method satisfies: yield strength (YS): 1000 MPa or greater, tensile strength (TS): 1100 MPa or greater, elongation index (EL): 3% or greater, and the unbroken time based on the hydrogen embrittlement test method: 100 hours or longer, and has such a microstructure that the area fraction of martensite / tempered martensite is 95% or greater and less than 100%, and the area fraction of the remaining phase is greater than 0% and less than or equal to 5%, and the remaining phase is one or more phases selected from ferrite, bainite, and retained austenite.

10. A method for manufacturing a corrosion-resistant ultra-high strength cold-rolled steel sheet, the method comprising: A hot-rolled steel sheet is manufactured by hot-rolling steel, which contains, by weight percentage: carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): greater than 0% and less than or equal to 3.0%, molybdenum (Mo): greater than 0% and less than or equal to 1.0%, nickel (Ni): 0.02% to 3.0%, copper (Cu): 0.02% to 3.0%, titanium (Ti): 0.01% to 0.2%, niobium (Nb): 0.01% to 0.1%, vanadium (V): 0.01% to 1.0%, boron (B): 0.001% to 0.005%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the balance contains iron (Fe) and other inevitable impurities, wherein the ratio ([Cu] / [Ni]) of the content of copper (Cu) to the content of nickel (Ni) ranges from 0.54 to 5.7; A cold-rolled steel sheet is manufactured by cold-rolling the hot-rolled steel sheet; The cold-rolled steel sheet is annealed by holding at 800°C to 900°C for 60 seconds to 600 seconds; The annealed cold-rolled steel sheet is first cooled to 500°C to 700°C at a cooling rate of 1°C / s to 20°C / s; The cold-rolled steel sheet that has been first cooled is second cooled to 400°C to 500°C at a cooling rate of 5°C / s to 100°C / s; The cold-rolled steel sheet that has been second cooled is subjected to hot-dip galvanizing; and The hot-dip galvanized cold-rolled steel sheet is tempered at 100°C to 350°C.

11. According to the method of claim 10, between hot-dip galvanizing and tempering, the method further includes alloying the hot-dip galvanized cold-rolled steel sheet by heat treatment at 450°C to 600°C.

12. The method according to claim 10, wherein The tempering is carried out at a temperature range higher than 200°C and lower than or equal to 350°C for 60 seconds to 600 seconds.

13. The method according to claim 10, wherein, The tempering is carried out at a temperature range of 100°C to 200°C for 3 hours to 20 hours.

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