Corrosion-resistant ultra-high strength cold-rolled steel sheet and its manufacturing method

Abstract

The present invention includes, in weight percent, 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): more than 0% to 3.0%, molybdenum (Mo): more than 0% 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. The present invention provides a corrosion-resistant, ultra-high strength cold-rolled steel sheet having copper (Cu) content of 0.001% to 0.005%, phosphorus (P) content of more than 0% to 0.02%, sulfur (S) content of more than 0% to 0.01%, and the balance being iron (Fe) and other unavoidable impurities, wherein the ratio ([Cu] / [Ni]) obtained by dividing the copper (Cu) content by the nickel (Ni) content is in the range of 0.54 to 5.7, and the steel sheet satisfies the following requirements: yield strength (YS): 1000 MPa or more, tensile strength (TS): 1100 MPa or more, elongation (EL): 3% or more, and time to failure according to the hydrogen embrittlement test method standard: 100 hours or more.

Description

[Technical Field]

[0001] The technical concept of the present invention relates to a steel material, and more particularly to a corrosion-resistant ultra-high strength cold-rolled steel sheet having excellent corrosion resistance and a method for producing the same. [Background technology]

[0002] The automotive industry has consistently demanded increased crashworthiness for vehicle bodies. While the number of automotive parts has decreased with the recent widespread adoption of electric vehicles, the weight of vehicles has increased due to the introduction of batteries, further increasing the need for crashworthiness. Accordingly, efforts are being made to develop ultra-high strength crash components, such as front bumper beams, side seals, and door impact beams, which contribute to crashworthiness. Martensitic steel, which boasts the highest strength among cold-rolled steels, has seen its application expand with the increasing use of roll forming techniques, but its high strength poses the problem of delayed fracture. Corrosion in a hydrogen permeation environment is known to be a typical example of this type of delayed fracture. Therefore, while much research has been conducted to improve the corrosion resistance and hydrogen embrittlement resistance of ultra-high-strength steels with tensile strength standards of 1 GPa or higher, the results are still insufficient.

[0003] The following attempts have been made to improve the corrosion resistance of cold-rolled steel sheets. In relation to hydrogen-induced delayed fracture, one method for increasing hydrogen embrittlement resistance by controlling the residual hydrogen content in steel has been proposed: controlling the hydrogen content to 0.1 ppm or less during heat treatment at 350–450°C after cold rolling. Another method has been proposed: 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 simultaneously controlling the number of iron-based carbides. Another method has been proposed for increasing the hydrogen embrittlement resistance of steel sheets of 1.1 GPa or higher by controlling the grain size and aspect ratio of ferrite and martensite based on the cold rolling reduction ratio. Hydrogen embrittlement is an important research topic not only for automotive cold-rolled steel sheets but also for pressure vessel steels. Based on the increased hydrogen embrittlement resistance achieved by the addition of copper, a method has been proposed in which the proportion of banded structures in the microstructure is reduced to form a microstructure, thereby slowing the propagation speed of cracks caused by hydrogen embrittlement. Another method has been proposed in which nickel is added at 1-4 wt% to increase hydrogen embrittlement resistance. In the manufacture of high-strength steel sheets, a method has been proposed in which the ratios of manganese, nickel, and copper are limited to improve the stability of austenite. In this case, nickel and copper were added to ensure hardenability in steel sheets with high aluminum content, rather than to prevent delayed fracture due to hydrogen. Related prior art documents include Korean Patent Application No. 10-2012-0144482. Summary of the Invention [Problem to be solved by the invention]

[0004] The technical problem to be achieved by the technical idea of ​​the present invention is to provide a corrosion-resistant 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.

[0005] However, these problems are merely examples, and the technical idea of ​​the present invention is not limited to these. [Means for solving the problem]

[0006] According to one aspect of the present invention, there is provided a corrosion-resistant ultra-high strength cold-rolled steel sheet.

[0007] According to one embodiment of the present invention, the ultra-high strength cold rolled steel sheet comprises, in weight percent, 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% to 3.0%, molybdenum (Mo): greater than 0% 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): The copper (Cu) content is 0.01% to 1.0%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0.02%, sulfur (S): more than 0.01%, and the balance is iron (Fe) and other unavoidable impurities. The ratio ([Cu] / [Ni]) obtained by dividing the copper (Cu) content by the nickel (Ni) content is in the range of 0.54 to 5.7, and the steel satisfies the following requirements: yield strength (YS): 1000 MPa or more, tensile strength (TS): 1100 MPa or more, elongation (EL): 3% or more, and time to fracture: 100 hours or more, as specified in the hydrogen embrittlement test method standard.

[0008] According to one embodiment of the present invention, the microstructure of the corrosion-resistant ultra-high strength cold-rolled steel sheet may have an area fraction of martensite in the range of 95% to less than 100%, and the remaining phases may be any one or more selected from ferrite, bainite, and retained austenite, with the area fraction being in the range of more than 0% to 5%.

[0009] According to an embodiment of the present invention, the corrosion-resistant ultra-high strength cold-rolled steel sheet may further include carbides, and the carbides may have an average size of 100 nm or less and an aspect ratio of 5 or less.

[0010] According to an embodiment of the present invention, the carbide may include at least one of an Fe-based carbide, a Ti-based carbide, an Nb-based carbide, a V-based carbide, and an Mo-based carbide.

[0011] According to another aspect of the present invention, there is provided a method for producing a corrosion-resistant ultra-high strength cold rolled steel sheet.

[0012] According to one embodiment of the present invention, a method for producing an ultra-high strength cold rolled steel sheet has, in weight percent, 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% to 3.0%, molybdenum (Mo): greater than 0% to 1.0%, and 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): over 0% to 0.02%, sulfur (S): over 0% to 0.01%, and the balance is iron (Fe) and other unavoidable impurities. The method includes the steps of: hot-rolling a steel material containing copper (Cu) and nickel (Ni), wherein the ratio ([Cu] / [Ni]) of the copper (Cu) content divided by the nickel (Ni) content is in the range of 0.54 to 5.7, to produce a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; annealing the cold-rolled steel sheet by holding it at a temperature of 800°C to 900°C for 60 to 600 seconds; primarily cooling the annealed cold-rolled steel sheet to 500°C to 700°C at a cooling rate of 1°C / sec to 20°C / sec; secondary cooling the primarily-cooled cold-rolled steel sheet to a temperature below Mf at a cooling rate of 5°C / sec to 100°C / sec; and tempering the cold-rolled steel sheet after secondary cooling at a temperature of 100°C to 350°C.

[0013] According to an embodiment of the present invention, the step of producing the hot-rolled steel sheet may include the steps of: reheating a steel material having the alloy composition at a reheating temperature of 1,150°C to 1,300°C; hot-rolling the heated steel material to a finish rolling end temperature of 800°C to 1,000°C to produce a hot-rolled steel sheet; and coiling the hot-rolled steel sheet at a coiling temperature of 400°C to 700°C.

[0014] According to an embodiment of the present invention, the tempering step may be performed at a temperature in the range of more than 200° C. and not more than 350° C. for a period in the range of 60 seconds to 600 seconds.

[0015] According to an embodiment of the present invention, the tempering step may be performed at a temperature range of 100° C. to 200° C. for a period of 3 hours to 20 hours.

[0016] According to yet another aspect of the present invention, there is provided a method for producing a corrosion-resistant ultra-high strength cold-rolled steel sheet, which includes a hot-dip galvanizing step.

[0017] According to one embodiment of the present invention, the method for producing the ultra-high strength cold rolled steel sheet comprises, in weight percent, 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): more than 0% to 3.0%, molybdenum (Mo): more than 0% 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): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, and the balance includes iron (Fe) and other unavoidable impurities, and the content of said copper (Cu) The method may include the steps of: hot-rolling a steel material having a ratio ([Cu] / [Ni]) divided by a nickel (Ni) content in the range of 0.54 to 5.7 to produce a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; annealing the cold-rolled steel sheet by holding it at a temperature of 800°C to 900°C for 60 to 600 seconds; primarily cooling the annealed cold-rolled steel sheet to 500°C to 700°C at a cooling rate of 1°C / sec to 20°C / sec; secondarily cooling the primarily-cooled cold-rolled steel sheet to 400°C to 500°C at a cooling rate of 5°C / sec to 100°C / sec; hot-dip galvanizing the secondarily-cooled cold-rolled steel sheet; and tempering the hot-dip galvanized cold-rolled steel sheet at a temperature of 100°C to 350°C.

[0018] According to one embodiment of the present invention, the method may further include a step of subjecting the hot-dip galvanized cold-rolled steel sheet to an alloying heat treatment at a temperature of 450°C to 600°C between the hot-dip galvanizing step and the tempering step.

[0019] According to an embodiment of the present invention, the tempering step may be performed at a temperature in the range of more than 200° C. and not more than 350° C. for a period in the range of 60 seconds to 600 seconds.

[0020] According to an embodiment of the present invention, the tempering step may be performed at a temperature range of 100° C. to 200° C. for a period of 3 hours to 20 hours. [Effects of the Invention]

[0021] According to the technical concept of the present invention, by controlling the content ratio of copper to nickel (([Cu] / [Ni])) and thereby controlling the reheating temperature during hot rolling within an appropriate range, it is possible to produce a cold-rolled steel sheet having high tensile strength and excellent hydrogen embrittlement resistance. The effects of the present invention described above are given by way of example, and the scope of the present invention is not limited by these effects. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a process flowchart illustrating a method for manufacturing a corrosion-resistant ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention.

[0023] [Figure 2] FIG. 1 is a diagram showing a heat treatment history over time according to an embodiment of the present invention.

[0024] [Figure 3] 1 is a process flowchart illustrating a method for manufacturing a corrosion-resistant ultra-high strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present invention.

[0025] [Figure 4]FIG. 1 is a diagram showing a heat treatment history over time according to an embodiment of the present invention.

[0026] [Figure 5] The melting point of a Cu-Ni complete solid solution was calculated using the ThermoCalc (registered trademark) program.

[0027] [Figure 6] 1 is a photograph showing a fracture state of a corrosion-resistant ultra-high strength cold-rolled steel sheet according to an example of the present invention after a hydrogen embrittlement test. BEST MODE FOR CARRYING OUT THE INVENTION

[0028] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The present embodiments are provided to more completely explain the technical concept of the present invention to those skilled in the art. The following embodiments may be modified into various other forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more complete and complete and to fully convey the technical concept of the present invention to those skilled in the art. Throughout this specification, the same reference numerals refer to the same elements. Furthermore, various elements and regions in the drawings are shown schematically. Therefore, the technical concept of the present invention is not limited by the relative sizes and spacings shown in the accompanying drawings.

[0029] The technical idea of ​​the present invention is to provide a corrosion-resistant ultra-high strength cold-rolled steel sheet and a manufacturing method thereof, in which the content of elements and the composition of the microstructure are controlled to increase resistance to hydrogen embrittlement, so that the steel sheet can be used for ultra-high strength steel sheets for automobiles having a tensile strength of 1100 MPa or more.

[0030] The corrosion-resistant ultra-high strength cold-rolled steel sheet according to the technical concept of the present invention will be described in detail below.

[0031] The corrosion-resistant ultra-high strength cold-rolled steel sheet according to one embodiment of the present invention contains, in weight percent, 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): more than 0% to 3.0%, molybdenum (Mo): more than 0% 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): over 0% to 0.02%, sulfur (S): over 0% to 0.01%, and the balance includes iron (Fe) and other unavoidable impurities.

[0032] The role and content of each component contained in the corrosion-resistant ultra-high strength cold-rolled steel sheet according to the present invention will be described below. Here, the contents of the component elements are all expressed in wt% relative to the total weight of the steel sheet.

[0033] Carbon (C): 0.1%~0.5%

[0034] Carbon is added to ensure the strength of the steel sheet and control the microstructure, and can ensure strength by improving the hardness of martensite. If the carbon content is less than 0.1%, it is difficult to achieve the desired strength. If the carbon content is more than 0.5%, weldability and workability may be reduced. Therefore, it is preferable to add carbon in an amount of 0.1% to 0.5% of the total weight of the steel sheet.

[0035] Silicon (Si): 0.01% to 2.0%

[0036] Silicon is a ferrite stabilizing element and can suppress the growth of cementite to ensure hydrogen embrittlement resistance. If the silicon content is less than 0.01%, the effect of adding silicon is insufficient. If the silicon content is more than 2.0%, a large amount of ferrite is formed, and the target strength may not be ensured. Therefore, it is preferable to add silicon in an amount of 0.01% to 2.0% of the total weight of the steel sheet.

[0037] Manganese (Mn): 0.1% to 5.0%

[0038] Manganese has a solid solution strengthening effect and can contribute to improving strength by increasing hardenability. If the manganese content is less than 0.1%, the hardenability is insufficient, making it difficult to ensure strength, and the effect of adding manganese is insufficient. If the manganese content exceeds 5.0%, hydrogen embrittlement resistance may decrease due to the formation of manganese bands and MnS. Therefore, it is preferable to add manganese in an amount of 0.1% to 5.0% of the total weight of the steel sheet.

[0039] Aluminum (Al): 0.01% to 2.0%

[0040] Aluminum is used as a deoxidizer and can help purify ferrite. If the aluminum content is less than 0.01%, the effect of adding aluminum is insufficient, such as insufficient deoxidizing effect. If the aluminum content is more than 2.0%, AlN is formed during slab production, which can induce cracks during casting or hot rolling, and a large amount of ferrite is formed, which can reduce strength. Therefore, it is preferable to add aluminum in an amount of 0.01% to 2.0% of the total weight of the steel sheet.

[0041] Chromium (Cr): Over 0% to 3.0%

[0042] Chromium is a ferrite stabilizing element in steel and a component that improves hardenability (hardenability). It can refine carbides, thereby contributing to improved strength. It can also increase strength through solid solution strengthening. If the chromium content exceeds 3.0%, the manufacturing cost becomes relatively high, and the hardening effect during cooling is large, leading to an increase in strength, which can result in a relative decrease in elongation and a decrease in laser weldability. Therefore, it is preferable to add chromium in an amount greater than 0% to 3.0% of the total weight of the steel sheet.

[0043] Molybdenum (Mo): Over 0% to 1.0%

[0044] Molybdenum has a solid solution strengthening effect, and can contribute to improving strength by increasing hardenability. It can also improve hydrogen embrittlement resistance by refining Ti-based precipitates. If the molybdenum content exceeds 1.0%, the raw material cost may increase. Therefore, it is preferable to add molybdenum in an amount of more than 0% to 1.0% of the total weight of the steel sheet.

[0045] Copper (Cu): 0.02% to 3.0%

[0046] Copper is a precipitate-forming element that combines with carbon (C) and nitrogen (N) to form carbides or nitrides. This precipitation and rolling suppress recrystallization and grain growth, thereby reducing grain size and improving the toughness and strength of steel. Copper can also be added to increase hydrogen embrittlement resistance. If the copper content is less than 0.02%, the effect of copper addition is insufficient, resulting in delayed fracture. If the copper content exceeds 3.0%, it acts as a hot embrittlement-inducing element, which can cause cracks during hot rolling and significantly increase the rolling load during rolling, thereby increasing the steel production cost. Therefore, copper is preferably added in an amount of 0.02% to 3.0% of the total weight of the steel sheet.

[0047] Nickel (Ni): 0.02% to 3.0%

[0048] Nickel is a precipitate-forming element that combines with carbon (C) and nitrogen (N) to form carbides or nitrides. This precipitation and rolling suppress recrystallization and grain growth, thereby reducing grain size and improving the toughness and strength of steel. It also suppresses hot embrittlement caused by copper. If the nickel content is less than 0.02%, the effect of adding nickel is insufficient. If the nickel content exceeds 3.0%, the rolling load during rolling may increase significantly, increasing the steel production cost. Therefore, nickel is preferably added in an amount of 0.02% to 3.0% of the total weight of the steel sheet. In particular, nickel needs to be added in an appropriate ratio relative to the copper content to prevent copper melting during the reheating process, which will be described in more detail below.

[0049] [Cu] / [Ni]: 0.54~5.7

[0050] In the corrosion-resistant ultra-high strength cold-rolled steel sheet, the ratio of the copper (Cu) content divided by the nickel (Ni) content ([Cu] / [Ni]) may be 0.54 to 5.7. Here, [Cu] means the copper (Cu) content (wt%), and [Ni] means the nickel (Ni) content (wt%). This [Cu] / [Ni] ratio is intended to prevent liquid phase Cu from penetrating into the grain boundaries of the steel sheet and weakening the boundaries during the reheating step for hot rolling.

[0051] Copper has a melting point of 1084.6°C, which is lower than that of iron. When copper is present in a slab, it can migrate to the surface. If the surface temperature of the slab or bar is higher than the melting point of copper, the copper that migrates to the surface melts and penetrates along the grain boundaries of the steel, causing hot shortness, which reduces ductility and leads to cracks. One way to prevent this hot shortness is to add nickel to form a complete copper-nickel solid solution, thereby suppressing copper melting. Therefore, to prevent liquid copper from penetrating the steel sheet during reheating, the temperature of the Cu-Ni complete solid solution must be at least 1150°C. To prevent the cost increase caused by the addition of nickel, the upper melting point of the Cu-Ni complete solid solution was set at 1300°C. The melting point of the Cu-Ni complete solid solution was calculated using the ThermoCalc program and is shown in Figure 5. The "Cu-Ni ratio for suppressing hot shortness" shown in FIG. 5, that is, [Cu] / [Ni], is in the range of 0.54 to 5.7.

[0052] Titanium (Ti): 0.01% to 0.2%

[0053] Titanium is a precipitate-forming element and can provide the effects of TiN and TiC precipitation and grain refinement. In particular, titanium can reduce the nitrogen content within the steel through TiN precipitation. When added together with boron, titanium can prevent BN precipitation, thereby maintaining the solid solution state of boron, a grain boundary strengthening element. If the titanium content is less than 0.01%, BN precipitation may be induced, resulting in insufficient effects of titanium addition. If the titanium content exceeds 0.2%, TiN precipitation may become coarse, reducing hydrogen embrittlement resistance and reducing the solid solubility of carbon in the base material, making it difficult to maintain strength and increasing steel manufacturing costs. Therefore, titanium is preferably added in an amount of 0.01% to 0.2% of the total weight of the steel sheet.

[0054] Niobium (Nb): 0.01% to 0.1%

[0055] Niobium is a precipitate-forming element that combines with carbon (C) and nitrogen (N) to form carbides or nitrides. This precipitation and rolling suppress recrystallization and grain growth, thereby refining grains and improving the toughness and strength of steel. If the niobium content is less than 0.01%, the grain refinement effect is insufficient, and the effect of adding niobium is insufficient. If the niobium content exceeds 0.1%, precipitates grow, resulting in no strength enhancement effect and a significant increase in the rolling load during rolling, which can increase the steel production cost. Therefore, niobium is preferably added in an amount of 0.01% to 0.1% of the total weight of the steel sheet.

[0056] Vanadium (V): 0.01% to 1.0%

[0057] Vanadium is a precipitate-forming element that combines with carbon (C) and nitrogen (N) to form carbides or nitrides. This precipitation and rolling suppress recrystallization and grain growth, thereby refining grains and improving the toughness and strength of steel. If the vanadium content is less than 0.01%, the grain refinement effect is insufficient, and the vanadium addition effect is insufficient. If the vanadium content exceeds 1.0%, precipitates grow, resulting in no strength enhancement effect and a significant increase in the rolling load during rolling, which can increase the steel production cost. Therefore, vanadium is preferably added in an amount of 0.01% to 1.0% of the total weight of the steel sheet.

[0058] Boron (B): 0.001% to 0.005%

[0059] Boron is a grain boundary strengthening element, and when distributed in grain boundaries, it can increase hydrogen embrittlement resistance. If the boron content is less than 0.001%, the effect of adding boron is insufficient. If the boron content is more than 0.005%, grain boundary embrittlement may occur due to the formation of BN. Therefore, boron is preferably added in an amount of 0.001% to 0.005% of the total weight of the steel sheet.

[0060] Phosphorus (P): Over 0% to 0.02%

[0061] Phosphorus is an impurity contained in the steel manufacturing process and can help improve strength through solid solution strengthening. However, if it is contained in large amounts, low-temperature embrittlement can occur due to grain boundary segregation, and spot weldability can be reduced. Therefore, it is preferable to limit the phosphorus content to more than 0% to 0.02% of the total weight of the steel sheet.

[0062] Sulfur (S): More than 0%~0.01%

[0063] Sulfur is an impurity contained in the steel manufacturing process and can form non-metallic inclusions such as FeS, MnS, etc., which can reduce toughness, hydrogen embrittlement resistance, and weldability. Therefore, it is preferable to limit the sulfur content to more than 0% to 0.01% of the total weight of the steel plate.

[0064] The remaining component of the corrosion-resistant ultra-high strength cold-rolled steel sheet is iron (Fe). However, in a typical steelmaking process, unintended impurities are inevitably mixed in from raw materials or the surrounding environment, and these cannot be eliminated. These impurities are known to any engineer of a typical manufacturing process, and therefore, the full details of these impurities will not be specifically mentioned in this specification.

[0065] A corrosion-resistant ultra-high strength cold-rolled steel sheet manufactured by controlling the specific components and content ranges of the alloy composition described above and using the manufacturing method described below can satisfy, for example, a yield strength (YS) of 1000 MPa or more, a tensile strength (TS) of 1100 MPa or more, an elongation (EL) of 3% or more, and a time to fracture according to the hydrogen embrittlement test method of 100 hours or more. Furthermore, the corrosion-resistant ultra-high strength cold-rolled steel sheet can satisfy, for example, a yield strength (YS) of 1000 MPa to 1700 MPa, a tensile strength (TS) of 1100 MPa to 1900 MPa, an elongation (EL) of 3% to 9%, and a time to fracture according to the hydrogen embrittlement test method of 100 hours to 300 hours.

[0066] The corrosion-resistant ultra-high strength cold-rolled steel sheet has a microstructure with an area fraction of martensite / tempered martensite of 95% or more. Here, "martensite / tempered martensite" refers to both fresh martensite and tempered martensite, without distinguishing between them. For example, the area fraction of the martensite / tempered martensite may be in the range of 95% to less than 100%, and the remaining phase may be one or more phases selected from ferrite, bainite, and retained austenite, with an area fraction of more than 0% to 5%. The area fraction refers to an area ratio determined from a microstructure photograph using an image analyzer. The area fraction of the microstructure is based on the results of analyzing a quarter point in the thickness direction of the steel sheet in a direction perpendicular to the rolling direction using a scanning electron microscope.

[0067] The presence of carbides is necessary to suppress delayed fracture due to hydrogen embrittlement. Such carbides diffuse into the steel sheet and serve to capture and fix hydrogen so that the infiltrated hydrogen cannot move freely inside the steel sheet. However, if the size of the carbides is too large, embrittlement due to hydrogen collected at the interface may occur, so it is preferable to limit the size of the carbides.

[0068] Therefore, the corrosion-resistant ultra-high strength cold-rolled steel sheet may further include carbides. The carbides may have an average size (grain size) of, for example, 100 nm or less, for example, in the range of 1 nm to 100 nm. The carbides may have an aspect ratio of, for example, 5 or less, for example, in the range of 1 to 5. Here, the aspect ratio refers to the ratio of the major axis length to the minor axis length of the carbide. If the average size of the carbides exceeds 100 nm, it can be considered that the martensite has been excessively tempered or that the retained austenite has been insufficiently stabilized.

[0069] The carbides may include both cementite and transition carbides, or may include carbides that do not contain Fe, and may include, for example, at least one of Fe-based carbides, Ti-based carbides, Nb-based carbides, V-based carbides, and Mo-based carbides. Specifically, the carbides may include, for example, Fe3C, ε-carbides (Fe 2.5 C), η-carbide (Fe2C), (Fe, substitutional element) 2~3 (C), and (Ti, Nb, V, Mo)(C, N).

[0070] Hydrogen Embrittlement Testing Method

[0071] In the present invention, the hydrogen embrittlement test method for quantitatively measuring delayed fracture due to hydrogen embrittlement of the corrosion-resistant ultra-high strength cold-rolled steel sheet is as follows.

[0072] A cold-rolled steel sheet is cut using a shear, laser, water jet, milling, wire (EDM), or the like to prepare a test piece having a length of 100 mm to 300 mm and a width of 10 mm to 50 mm, with the longitudinal direction of the test piece forming an angle of 0° or 90° with the rolling direction of the material.

[0073] Next, the test specimens are subjected to stresses of 60%, 70%, 80%, 90%, and 100% of the yield strength using a two-point or four-point bending method. The applied stress should be varied depending on the yield strength of each cold-rolled steel sheet. The goal is to ensure that no fracture occurs under three or more stress application conditions, including the 100% stress condition.

[0074] The stressed test specimen is immersed in a hydrochloric acid (HCl) solution with a concentration of 0.01N to 0.2N, preferably a 0.1N solution. This immersion injects hydrogen into the test specimen, which can be considered an accelerated test for hydrogen-induced delayed fracture. The immersed test specimen is maintained for 100 hours or more. The goal is to ensure that no fracture occurs under any of the stress conditions.

[0075] Hereinafter, a method for producing a corrosion-resistant ultra-high strength cold-rolled steel sheet according to the present invention will be described with reference to the accompanying drawings.

[0076] Method for producing corrosion-resistant ultra-high strength cold-rolled steel sheet

[0077] FIG. 1 is a process flow chart that schematically shows a method for producing a corrosion-resistant ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention, and relates to a method for producing an ungalvanized cold-rolled steel sheet.

[0078] Referring to FIG. 1, a method for manufacturing a corrosion-resistant ultra-high strength cold-rolled steel sheet 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 heat treatment step (S130), a first cooling step (S140), a second cooling step (S150), a tempering step (S160), and a third cooling step (S170).

[0079] Hot-rolled steel sheet manufacturing step (S110)

[0080] In the hot-rolled steel sheet manufacturing step (S110), the following components are added in weight percent: 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): over 0% to 3.0%, molybdenum (Mo): over 0% 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): over 0% to 0.02%, sulfur (S): over 0% to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities.

[0081] In the manufacturing method according to the present invention, the semi-finished product to be subjected to the hot rolling process may be, for example, a slab. The semi-finished slab can be obtained through a continuous casting process after molten steel having a predetermined composition is obtained through a steelmaking process.

[0082] The steel material, for example a slab, is reheated for 1 to 5 hours at a slab reheating temperature (SRT) of 1,150°C to 1,300°C. This reheating process transforms the steel into a complete austenite single phase, and re-dissolves elements that segregated during casting and precipitates, thereby homogenizing the steel material and making it suitable for hot rolling.

[0083] As already mentioned above, the slab reheating temperature in the present invention is set taking into consideration the melting point of the complete solid solution of copper and nickel, which are the added elements. The temperature of the Cu-Ni complete solid solution is set to at least 1,150°C so that liquid copper does not penetrate into the steel sheet during reheating. If the reheating temperature is less than 1,150°C, the elements that segregated during casting may not be sufficiently re-dissolved and may not be uniformly distributed.

[0084] In addition, to prevent the cost increase due to the addition of nickel, the upper limit of the melting point of the Cu-Ni complete solid solution is set to 1,300°C. If the reheating temperature exceeds 1,300°C, the austenite grains will become coarse, which may cause a decrease in yield strength. Furthermore, the higher the reheating temperature, the higher the manufacturing cost and the additional time required to adjust the hot rolling temperature, which may result in an increase in manufacturing cost and a decrease in productivity.

[0085] If the reheating time is less than 1 hour, the segregation band may not be reduced sufficiently, and if it exceeds 5 hours, the grain size may increase, which may increase the process cost.

[0086] Copper has a melting point of 1084.6°C, which is lower than that of iron. When copper is present in a slab, it can migrate to the surface. If the surface temperature of a slab or bar is higher than the melting point of copper, the copper that migrates to the surface melts and penetrates along the grain boundaries of the steel, causing hot shortness, which reduces ductility and leads to cracks. One way to prevent this hot shortness is to add nickel to form a complete solid solution of copper and nickel, thereby suppressing copper melting. Therefore, to prevent liquid copper from penetrating the steel sheet during reheating, the temperature of the Cu-Ni solid solution must be at least 1150°C. To prevent the cost increase caused by the addition of nickel, the upper melting point of the Cu-Ni solid solution was set at 1300°C. The melting point of the Cu-Ni solid solution was calculated using ThermoCalc and is shown in Figure 5.

[0087] The reheated steel is then hot-rolled after heating to adjust its shape. The hot-rolling may be performed successively by rough rolling and finish rolling. The hot-rolling step may form the steel into a hot-rolled steel. The hot-rolled steel may be a hot-rolled steel plate.

[0088] The rough rolling is a step of rolling the slab to produce a bar, and can be performed at a temperature ranging from the reheating end temperature (1,150°C to 1,300°C) to 1,000°C.

[0089] The finish rolling may be completed at a finish rolling temperature (FRT) of 800°C to 1,000°C. If the finish rolling temperature is less than 800°C, the rolling load may increase sharply, resulting in reduced productivity. If the finish rolling temperature is more than 1,000°C, the crystal grains may become coarse, resulting in reduced strength of the final steel material.

[0090] Next, the hot-rolled steel material is cooled to a predetermined coiling temperature. The cooling can be either air-cooling or water-cooling, and can be performed at a cooling rate of, for example, 1°C / sec to 100°C / sec. A faster cooling rate can be more advantageous for reducing the average grain size. The cooling is preferably performed to a coiling temperature of, for example, 400°C to 700°C, e.g., 500°C to 650°C.

[0091] The hot-rolled steel sheet is then coiled at a coiling temperature (CT) of, for example, 400°C to 700°C, e.g., 500°C to 650°C. If the coiling temperature is less than 400°C, the shape of the coiled hot-rolled coil may become non-uniform, the strength may increase, and the rolling load during cold rolling may increase. If the coiling temperature exceeds 700°C, a non-uniform microstructure may occur due to a difference in cooling rate between the center and edge of the steel sheet, and defects may occur in subsequent processes due to surface oxidation, etc. The coiled steel material may be cooled to room temperature.

[0092] Cold-rolled steel sheet manufacturing step (S120)

[0093] The cold-rolled steel sheet manufacturing step (S120) is performed to adjust the thickness of the final steel sheet using the hot-rolled steel sheet. The coiled hot-rolled steel sheet is then subjected to a pickling treatment, in which the hot-rolled steel sheet is washed with acid. The pickled hot-rolled steel sheet is then cold-rolled at a cold reduction of, for example, 35% or more, e.g., 35% to 70%, to form a cold-rolled steel sheet. The higher the reduction, the more the grain refinement effect can be improved, resulting in increased resistance to hydrogen embrittlement. If the cold reduction is less than 35%, it is difficult to obtain a uniform microstructure, and the amount of nucleation for recrystallization during annealing is small, which can lead to excessive grain growth during the annealing heat treatment described below, resulting in a rapid decrease in strength. If the cold reduction exceeds 70%, the amount of nucleation becomes excessive, and the grains formed during the annealing heat treatment are excessively fine, which can reduce ductility and formability.

[0094] The cold-rolled steel sheet manufactured by cold rolling undergoes the following heat treatment steps. Figure 2 shows the heat treatment history according to time in this example, and each step will be described with reference to this.

[0095] Annealing heat treatment step (S130)

[0096] In the annealing heat treatment step (S130), the cold-rolled steel sheet can be heat-treated in a continuous annealing furnace having a normal slow cooling section.

[0097] The annealing heat treatment is performed, for example, by heating the steel sheet to a temperature of 800°C to 900°C, for example, at a temperature increase rate of 1°C / second or more, for example, 1°C / second to 10°C / second, for example, at Ac3 temperature or higher, and then holding the temperature for 60 seconds to 600 seconds. If the annealing heat treatment temperature is less than 800°C or the holding time is less than 60 seconds, it is difficult to form sufficient austenite, the ferrite fraction increases, and strength may decrease. If the annealing heat treatment temperature exceeds 900°C or the holding time exceeds 600 seconds, the austenite grain size may become coarse and productivity may be excessively reduced.

[0098] The Ac3 temperature can be calculated by the following formula.

[0099] Ac3=910-203×[C] 0.5 -30[Mn]+44.7[Si]+31.5[Mo]-15.2[Ni]

[0100] Here, [C] is the carbon content (wt%) in the steel, [Mn] is the manganese content (wt%) in the steel, [Si] is the silicon content (wt%) in the steel, [Mo] is the molybdenum content (wt%) in the steel, and [Ni] is the nickel content (wt%) in the steel.

[0101] First cooling step (S140)

[0102] In the primary cooling step (S140), the annealed cold-rolled steel sheet is primarily cooled, for example, at a cooling rate of 1°C / sec to 20°C / sec to a temperature of, for example, 500°C to 700°C. The cooling may be performed by air cooling or water cooling. The primary cooling may be referred to as a slow cooling step.

[0103] Secondary cooling step (S150)

[0104] In the secondary cooling step (S150), the primarily cooled cold-rolled steel sheet is secondarily cooled, for example, at a cooling rate of 5°C / sec or more, for example, 5°C / sec to 100°C / sec, to a temperature below the martensite transformation finish temperature (Mf), for example, from room temperature (0°C to 40°C) to 350°C. In the secondary cooling step (S150), cooling is performed to a secondary cooling finish temperature below Mf, so that most of the austenite transforms to fresh martensite. The faster the cooling rate in the secondary cooling, the more advantageous it is. The secondary cooling can be referred to as a quenching step. Additional ferrite transformation must be suppressed during the secondary cooling.

[0105] Tempering step (S160)

[0106] The tempering step (S160) is a step of heating at a temperature increase rate of 1°C / sec to 50°C / sec and holding at a temperature range of 100°C to 350°C for a predetermined time. The tempering treatment changes fresh martensite to tempered martensite, and the formation and growth of transition carbides and the growth of cementite can proceed.

[0107] If carbides grow excessively in the longitudinal direction, not only will the yield strength and tensile strength of the steel decrease, but they may also act as fracture initiation points during hydrogen penetration, making the steel susceptible to hydrogen embrittlement. Therefore, in the tempering step, the excessive growth of carbides can be suppressed by appropriately controlling the holding time depending on the tempering temperature.

[0108] At high temperatures, it is necessary to suppress the growth of cementite by holding for a relatively short time, for example, for 60 to 600 seconds in the temperature range of more than 200°C and not more than 350°C.

[0109] When tempering is performed at high temperatures, carbide growth in the longitudinal direction becomes very active and rapid, making it difficult to control the growth of carbides. In contrast, when tempering at a relatively low temperature, it is easier to control the heat treatment for carbide growth, so excessive carbide growth can be effectively suppressed, which is more advantageous for ensuring the strength and suppressing fracture due to hydrogen embrittlement. For example, tempering can be performed at a temperature range of 100°C to 200°C for 3 to 20 hours.

[0110] After the tempering treatment is completed, the cold-rolled steel sheet is cooled to room temperature (0°C to 40°C) at a cooling rate of 1°C / sec to 100°C / sec (S170).

[0111] The cold rolled steel sheet manufactured by the above-mentioned method may have a zinc plating layer or a zinc alloy plating layer formed on its surface by an electroplating method.

[0112] Fig. 3 is a process flow chart that schematically illustrates a method for manufacturing a corrosion-resistant ultra-high strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present invention. Fig. 4 shows the heat treatment history over time in this embodiment.

[0113] 3 and 4, a method for manufacturing a corrosion-resistant ultra-high strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present invention includes a hot-rolled steel sheet manufacturing step (S210), a cold-rolled steel sheet manufacturing step (S220), an annealing heat treatment 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).

[0114] In this embodiment, the step of producing a hot-rolled steel sheet (S210), the step of producing a cold-rolled steel sheet (S220), and the step of annealing (S230) are the same as the above-described method of producing an ungalvanized cold-rolled steel sheet.

[0115] Primary cooling step (S240)

[0116] In the primary cooling step (S240), the annealed cold-rolled steel sheet is primarily cooled, for example, at a cooling rate of 1°C / sec to 20°C / sec to a temperature of 500°C to less than 700°C, and held for 5 to 100 seconds.

[0117] Secondary cooling step (S250)

[0118] In the secondary cooling step (S250), the cold-rolled steel sheet is then subjected to secondary cooling, for example, at a cooling rate of 5°C / sec or more, for example, 5°C / sec to 100°C / sec, to a temperature range of, for example, 400°C to 500°C.

[0119] Hot-dip galvanized step (S260)

[0120] After the secondary 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 of the galvanizing bath is, for example, in the range of 400°C to 500°C. Under the conditions of the galvanizing bath, a hot-dip galvanized layer is easily formed on the surface of the cold-rolled steel sheet, and the adhesion of the galvanized layer is excellent.

[0121] If the subsequent alloying step (S265) is not performed after the hot-dip galvanizing step (S260), the steel sheet is immediately subjected to the above-mentioned tertiary cooling step (S270) after it has been completely coated and exits the coating bath, whereby austenite is transformed into martensite. Thereafter, the steel sheet is subjected to a tempering step (S280) and then cooled to room temperature (S290). The steps after the tempering step have already been described above, so a detailed description will be omitted here to avoid redundancy.

[0122] Alloying step (S265)

[0123] If necessary, an alloying step (S265) can be further performed, in which the cold-rolled steel sheet having the hot-dip galvanized layer formed thereon is subjected to an alloying heat treatment. To perform the alloying step (S265), the steel sheet that has been completely coated and exits the coating bath can be introduced into a heat treatment device for the alloying heat treatment. The alloying heat treatment can be performed at a temperature of, for example, 450°C to 600°C, for example, by holding the temperature for, for example, 5 seconds to 100 seconds. When the alloying heat treatment is performed under these conditions, the hot-dip galvanized layer grows stably and exhibits excellent adhesion. If the alloying heat treatment temperature is less than 450°C, alloying may not proceed sufficiently, potentially reducing the integrity of the hot-dip galvanized layer. If the alloying heat treatment temperature is more than 600°C, changes in the material properties may occur as the steel sheet transitions into the two-phase temperature range.

[0124] After the alloying heat treatment is completed, the steel sheet undergoes the above-mentioned tertiary cooling step (S270), in which austenite transforms to martensite. Thereafter, the steel sheet undergoes a tempering step (S280) and is then cooled to room temperature (S290). The tempering step and subsequent steps have already been described above, so a detailed description will be omitted here to avoid redundancy.

[0125] Experimental example

[0126] Below, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are merely provided to aid in understanding the present invention, and the present invention is not limited to these experimental examples. Contents not described here can be fully inferred by those skilled in the art, and therefore, explanations thereof will be omitted.

[0127] Steels having the compositions (unit: wt%) shown in Tables 1 and 2 below were prepared, and cold-rolled steel sheets according to the examples and comparative examples were prepared by subjecting them to prescribed hot rolling, cold rolling, and heat treatment processes. In Tables 1 and 2, the balance consists of iron (Fe) and impurities inevitably contained in the steelmaking process, etc. The content of each component is expressed in wt%.

[0128] [Table 1]

[0129] [Table 2]

[0130] Referring to Tables 1 and 2, Examples 1 to 10 and Comparative Examples 1 to 6 are cold-rolled steel sheets, while Examples 11, 12, 7, and 8 are galvannealed steel sheets. The Examples satisfied the composition ranges of the present invention. On the other hand, the Comparative Examples differ in that they do not contain copper or nickel, or, like Comparative Example 2, the Cu / Ni ratio is lower than the lower limit of the present invention. Table 3 shows the condition values ​​for the manufacturing process of the corrosion-resistant ultra-high strength cold-rolled steel sheets of the Comparative Examples and Examples.

[0131] [Table 3]

[0132] In both the Examples and Comparative Examples, the reheating temperature for hot rolling was set to 1200° C. to 1250° C. Table 4 shows the results of measurements of yield strength (YS), tensile strength (TS), elongation (EL), and time to fracture for the Examples and Comparative Examples.

[0133] [Table 4]

[0134] The Examples did not fracture under conditions where a load of 100% of the yield strength was applied for 100 hours or more. Therefore, the Examples demonstrate excellent corrosion resistance against hydrogen embrittlement. In contrast, the Comparative Examples demonstrated fracture in less than 100 hours under conditions where loads of 90% and 100% of the yield strength were applied. This demonstrates that the Comparative Examples are vulnerable to hydrogen embrittlement due to the absence of nickel and copper. Furthermore, the results of Comparative Examples 4 and 8 demonstrate that the Comparative Examples are vulnerable to hydrogen embrittlement even when the nickel-to-copper content ratio (Cu / Ni) is 0.5 or less. Furthermore, Examples 1, 2, 11, and 12, which were tempered for a long period of time (20,000 seconds or more) at a relatively low temperature of 150°C, did not fracture in the hydrogen embrittlement test, even after a longer period of time, compared to the other Examples. Therefore, Examples 1, 2, 11, and 12 demonstrate even better corrosion resistance against hydrogen embrittlement. This is thought to be because, as already explained, in the case of low-temperature tempering, it is easier to control the heat treatment with respect to the growth of the length of carbides, and therefore fracture due to hydrogen embrittlement can be suppressed.

[0135] FIG. 6 is a photograph showing the fracture state of a corrosion-resistant ultra-high strength cold-rolled steel sheet according to an example of the present invention after undergoing a hydrogen embrittlement test.

[0136] 6, in Comparative Example 2, fracture occurred under loads of 100% and 90% of the yield strength. In contrast, in Examples 1 and 5, fracture did not occur under loads of 60% to 100% of the yield strength. This shows that the Examples have excellent corrosion resistance against hydrogen embrittlement.

[0137] It will be apparent to those skilled in the art to which the technical idea of ​​the present invention pertains that the technical idea of ​​the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and that various substitutions, modifications and changes are possible within the scope of the technical idea of ​​the present invention.

Claims

1. In weight percent, 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): more than 0% to 3.0%, molybdenum (Mo): more than 0% 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): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, and the balance including iron (Fe) and other unavoidable impurities, The ratio ([Cu] / [Ni]) of the copper (Cu) content divided by the nickel (Ni) content is in the range of 0.54 to 5.7; A corrosion-resistant, ultra-high strength cold-rolled steel sheet that satisfies the following requirements: yield strength (YS): 1000 MPa or more, tensile strength (TS): 1100 MPa or more, elongation (EL): 3% or more, and a time to failure according to the hydrogen embrittlement test method standard: 100 hours or more.

2. The microstructure of the corrosion-resistant ultra-high strength cold-rolled steel sheet is 2. The corrosion-resistant ultra-high strength cold-rolled steel sheet according to claim 1, wherein the area fraction of martensite is in the range of 95% to less than 100%, and the remaining phase is any one or more phases selected from ferrite, bainite, and retained austenite, having an area fraction in the range of more than 0 to 5%.

3. The corrosion-resistant ultra-high strength cold-rolled steel sheet further contains carbides, 2. The corrosion-resistant ultra-high strength cold rolled steel sheet according to claim 1, wherein the carbides have an average size of 100 nm or less and an aspect ratio of 5 or less.

4. 4. The corrosion-resistant 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. In weight percent, 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): more than 0% to 3.0%, molybdenum (Mo): more than 0% 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): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, and the balance including iron (Fe) and other unavoidable impurities, hot-rolling the steel material having a ratio ([Cu] / [Ni]) of the copper (Cu) content divided by the nickel (Ni) content in the range of 0.54 to 5.7 to manufacture a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; A step of performing annealing heat treatment by holding the cold-rolled steel sheet at a temperature of 800 ° C. to 900 ° C. for 60 seconds to 600 seconds; A step of primarily cooling the annealed cold-rolled steel sheet to 500°C to 700°C at a cooling rate of 1°C / sec to 20°C / sec; Secondary cooling of the primarily cooled cold-rolled steel sheet at a cooling rate of 5°C / sec to 100°C / sec to a temperature below Mf; and tempering the cold-rolled steel sheet after the secondary cooling at a temperature of 100°C to 350°C.

6. The step of manufacturing the hot-rolled steel sheet includes: Reheating the steel material having the alloy composition at a reheating temperature of 1,150°C to 1,300°C; hot rolling the reheated steel material to a finish rolling end temperature of 800°C to 1,000°C to produce a hot rolled steel sheet; The method for producing a corrosion-resistant ultra-high strength cold-rolled steel sheet according to claim 5, further comprising the step of coiling the hot-rolled steel sheet at a coiling temperature of 400°C to 700°C.

7. The tempering step includes: The method for producing a corrosion-resistant ultrahigh strength cold-rolled steel sheet according to claim 5, wherein the heating is performed for 60 seconds to 600 seconds at a temperature range of more than 200 ° C. and not more than 350 ° C.

8. The tempering step includes: The method for producing a corrosion-resistant ultra-high strength cold-rolled steel sheet according to claim 5, wherein the heating is carried out for a period of 3 to 20 hours at a temperature range of 100°C to 200°C.

9. The corrosion-resistant ultra-high strength cold-rolled steel sheet manufactured by the method for manufacturing the corrosion-resistant ultra-high strength cold-rolled steel sheet has the following features: Yield strength (YS): 1000 MPa or more, tensile strength (TS): 1100 MPa or more, elongation (EL): 3% or more, and hydrogen embrittlement test method standard non-fracture time: 100 hours or more are satisfied, 6. The method for producing a corrosion-resistant ultra-high strength cold-rolled steel sheet according to claim 5, wherein the steel sheet has a microstructure in which the area fraction of martensite / tempered martensite is in the range of 95% to less than 100%, and the remaining phase is any one or more phases selected from ferrite, bainite, and retained austenite, having an area fraction in the range of more than 0 to 5%.

10. In weight percent, 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): over 0% to 3.0%, molybdenum (Mo): over 0% 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): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, and the balance being iron (Fe) and other inevitable impurities, and a ratio ([Cu] / [Ni]) obtained by dividing the copper (Cu) content by the nickel (Ni) content is in the range of 0.54 to 5.7, to manufacture a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; A step of performing annealing heat treatment by holding the cold-rolled steel sheet at a temperature of 800 ° C. to 900 ° C. for 60 seconds to 600 seconds; A step of primarily cooling the annealed cold-rolled steel sheet to 500°C to 700°C at a cooling rate of 1°C / sec to 20°C / sec; Secondary cooling of the primarily cooled cold-rolled steel sheet to 400°C to 500°C at a cooling rate of 5°C / sec to 100°C / sec; hot-dip galvanizing the secondarily cooled cold-rolled steel sheet; and tempering the hot-dip galvanized cold-rolled steel sheet at a temperature of 100°C to 350°C.

11. 11. The method for producing a corrosion-resistant ultra-high strength cold-rolled steel sheet according to claim 10, further comprising a step of subjecting the hot-dip galvanized cold-rolled steel sheet to an alloying heat treatment at a temperature of 450 ° C to 600 ° C between the hot-dip galvanizing step and the tempering step.

12. The tempering step includes: The method for producing a corrosion-resistant ultrahigh strength cold-rolled steel sheet according to claim 10, wherein the heating is performed for 60 seconds to 600 seconds in a temperature range of more than 200 ° C. and not more than 350 ° C.

13. The tempering step includes: The method for producing a corrosion-resistant ultra-high strength cold-rolled steel sheet according to claim 10, wherein the heating is carried out for a range of 3 hours to 20 hours at a temperature range of 100 ° C. to 200 ° C.

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