Ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and method for manufacturing the same

Abstract

This invention provides an ultra-high strength cold-rolled steel sheet with a tensile strength of 1100 MPa or greater and excellent resistance to hydrogen embrittlement, and a method for manufacturing the same. According to one embodiment of the invention, the ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement contains, by weight, 0.1% to 0.5% carbon (C), 0.01% to 2.0% silicon (Si), 0.1% to 5.0% manganese (Mn), 0.01% to 2.0% aluminum (Al), greater than 0% and less than or equal to 3.0% chromium (Cr), greater than 0% and less than or equal to 1.0% molybdenum (Mo), and greater than 0% and less than or equal to 0.4% nickel (…). Ni), 0.05% to 0.4% copper (Cu), 0.01% to 0.2% titanium (Ti), 0.01% to 0.1% niobium (Nb), 0.01% to 1.0% vanadium (V), 0.001% to 0.005% boron (B), greater than 0% and less than or equal to 0.02% phosphorus (P), greater than 0% and less than or equal to 0.01% sulfur (S), and the balance iron (Fe) and other unavoidable impurities, and meet the following requirements: tensile strength (TS) of at least 1100 MPa, elongation (EL) of at least 3%, and hydrogen embrittlement test time without fracture of at least 100 hours.

Description

Technical Field

[0001] The technical concept of this invention relates to a steel material, and more specifically to an ultra-high strength cold-rolled steel sheet with excellent corrosion resistance or hydrogen embrittlement resistance and a method for manufacturing the same. Background Technology

[0002] In the automotive industry, the demand for crash safety of vehicle bodies is constantly increasing. Recently, with the proliferation of electric vehicles, the number of automotive components has decreased; however, the introduction of batteries has increased vehicle weight, further expanding the need for crash safety. Therefore, ultra-high strength crash components that contribute to crash safety (such as front bumper crossbeams, side beams, and door impact beams) are continuously being developed. In particular, with the increased use of roll forming technology, the application of martensitic steel (which has the highest strength among cold-rolled steel sheets) has expanded; however, the problem of delayed fracture due to its high strength exists. Specifically, corrosion in hydrogen-permeable environments is considered representative of this delayed fracture behavior. Therefore, extensive research has been conducted to improve the corrosion resistance or hydrogen embrittlement resistance of ultra-high strength steel materials with tensile strengths of 1 GPa or greater.

[0003] As prior art, there is Korean patent application number 2012-0144482. Summary of the Invention

[0004] Technical issues The technical problem addressed by the present invention is to provide an ultra-high strength cold-rolled steel sheet with a strength of at least 1100 MPa (based on tensile strength) and excellent resistance to hydrogen embrittlement by limiting the content of elements that affect delayed fracture caused by hydrogen embrittlement (e.g., copper (Cu), manganese (Mn), chromium (Cr), silicon (Si), and nickel (Ni)).

[0005] However, this problem is illustrative, and the technical concept of the present invention is not limited thereto.

[0006] Technical solution According to one aspect of the present invention, an ultra-high strength cold-rolled steel sheet having a strength of at least 1100 MPa (based on tensile strength) and excellent resistance to hydrogen embrittlement, and a method thereof for manufacturing the same, are provided.

[0007] According to one embodiment of the present invention, the ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement 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%, and nickel (Ni): greater than 0% and less than or equal to 1.0%. The content of the following components is equal to 0.4%, copper (Cu): 0.05% to 0.4%, 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 being iron (Fe) and unavoidable impurities, and meeting a tensile strength (TS) of 1100 MPa or greater, an elongation (EL) of 3% or greater, and a non-fracture time of 100 hours or longer based on the hydrogen embrittlement test method.

[0008] Furthermore, in ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement, the sum of the weight percentages of silicon and nickel [Si]+[Ni] and the sum of the weight percentages of manganese and chromium [Mn]+[Cr] can satisfy the following relationship: [Si]+[Ni]≤0.85-0.23×([Mn]+[Cr]).

[0009] Furthermore, the microstructure of ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement includes ferrite, tempered bainite and tempered martensite phases, wherein the area fraction of ferrite ranges from 0% to 20%, the area fraction of tempered bainite ranges from 5% to 20%, and the area fraction of tempered martensite ranges from 60% to 100%, provided that the sum of tempered bainite and tempered martensite can satisfy the range of 80% to 100%.

[0010] Furthermore, the ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement further contains carbides, wherein the carbides may have an average size of less than or equal to 100 nm and an aspect ratio of less than or equal to 5.

[0011] In addition, the carbide may include at least one selected from Fe-based carbides, Ti-based carbides, Nb-based carbides, V-based carbides and Mo-based carbides.

[0012] According to one embodiment of the present invention, a method for manufacturing an ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement includes: hot-rolling a steel material to produce a hot-rolled steel sheet, said steel material comprising, 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): greater than 0% and less than or equal to 0.4%, copper (Cu): 0.05% to 0.4%, titanium (Ti): 0.01% to 0.2%, niobium (Nb): 0.01% to 0.1%, vanadium (V): 0.01% to 1.0%, and 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 iron (Fe) and unavoidable impurities; cold rolling hot-rolled steel sheet to produce cold-rolled steel sheet; annealing heat treatment by holding the cold-rolled steel sheet at a temperature ranging from 800°C to 950°C for 10 seconds to 600 seconds; first cooling the annealed cold-rolled steel sheet to 500°C to 700°C at a cooling rate of 0°C / s to 50°C / s; second cooling the first cooled cold-rolled steel sheet to 0°C to 350°C at a cooling rate of 5°C / s or greater; and tempering step of heat treatment of the second cooled cold-rolled steel sheet at a temperature of 100°C to 350°C for 10 seconds or longer.

[0013] Furthermore, the steps for preparing hot-rolled steel sheets may include: reheating the steel material having the above-mentioned alloy composition at a reheating temperature ranging from 1150°C to 1300°C; hot-rolling the reheated steel material to finish rolling at a finishing rolling temperature ranging from 800°C to 1000°C to prepare hot-rolled steel sheets; and cooling the hot-rolled steel sheets at a cooling rate ranging from 1°C / s to 100°C / s, and then coiling them at a coiling temperature ranging from 300°C to 700°C.

[0014] Furthermore, the ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement, manufactured by a method for producing such sheet, meets a tensile strength (TS) of 1100 MPa or greater, an elongation (EL) of 3% or greater, and a non-fracture time of 100 hours or more based on a hydrogen embrittlement test method. The microstructure of the ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement comprises ferrite, tempered bainite, and tempered martensite phases, wherein the area fraction of ferrite ranges from 0% to 20%, the area fraction of tempered bainite ranges from 5% to 20%, and the area fraction of tempered martensite ranges from 60% to 100%, provided that the sum of tempered bainite and tempered martensite meets a range of 80% to 100%.

[0015] Beneficial effects According to the technical concept of the present invention, the resistance to hydrogen embrittlement of cold-rolled steel sheets can be improved by limiting the content of components that affect carbon activity under certain relationships.

[0016] The above-described effects of the invention have been illustrated by examples, and the scope of the invention is not limited by these effects. Attached Figure Description

[0017] Figure 1 A graph showing the carbon activity of the components constituting the ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to an embodiment of the present invention.

[0018] Figure 2 A graph showing the evaluation results of the hydrogen embrittlement test method after the composition of the ultra-high strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance according to the embodiment of the present invention is tested.

[0019] Figure 3 A process flow diagram illustrating a method for manufacturing an ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to an embodiment of the present invention is provided. Detailed Implementation

[0020] Preferred embodiments of the invention will now be described in detail with reference to the accompanying drawings. These embodiments are provided to more fully explain the technical concept of the invention to those skilled in the art. The following embodiments can be modified in various other forms, and the scope of the technical concept of the invention is not limited to the following embodiments. Rather, these embodiments are provided to make the invention more credible and complete, and to fully convey the technical concept of the invention to those skilled in the art. In this specification, the same reference numerals denote the same elements. Furthermore, various elements and regions in the drawings are shown schematically. Therefore, the technical concept of the invention is not limited to the relative dimensions or spacing shown in the accompanying drawings.

[0021] The following methods have been proposed to improve the corrosion resistance or hydrogen embrittlement resistance of cold-rolled steel sheets. One method to improve hydrogen embrittlement resistance by controlling the residual hydrogen content in the steel, which is associated with hydrogen-induced delayed fracture, involves controlling the hydrogen content to 0.1 ppm or less during heat treatment at 350°C to 450°C after cold rolling. Another method has been proposed that involves controlling the area fractions of ferrite, upper bainite, and martensite, adding boron (a grain boundary strengthening element), controlling the effective martensite grain size, and also controlling the amount of Fe-based carbides. Furthermore, a method has been proposed that involves controlling the grain size and aspect ratio of ferrite and martensite based on the cold rolling reduction rate, thereby improving the hydrogen embrittlement resistance of steel sheets with a grade of 1100 MPa or higher. This hydrogen embrittlement has become an important research topic not only for cold-rolled steel sheets used in automotive applications but also for steel materials used in pressure vessels. Methods have been proposed in which the improved resistance to hydrogen embrittlement based on the addition of copper results in a microstructure that reduces the fraction of banded structures in the microstructure, thereby slowing the crack propagation rate caused by hydrogen embrittlement. Furthermore, methods have been proposed in which nickel is added in amounts of 1% to 4% by weight to improve resistance to hydrogen embrittlement. A composition control method has been proposed in which the nickel to copper content ratio in thick-gauge steel materials used in shipbuilding, marine, construction, and civil engineering is limited to Cu / Ni ≤ 0.6. Methods have been proposed in which the ratio of manganese, nickel, and copper is limited to improve the stability of austenite during the manufacture of high-strength steel plates. In this case, the addition of nickel and copper is to ensure the hardenability of the steel plate with high aluminum addition, rather than to prevent hydrogen-induced delayed fracture.

[0022] The technical concept of this invention is to provide an ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement and a method thereof by controlling the alloy composition and microstructure configuration of the ultra-high strength automotive steel sheet with a tensile strength of at least 1100 MPa.

[0023] The following will describe in detail the ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to the technical concept of the present invention.

[0024] According to one embodiment of the invention, an ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement comprises, 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): greater than 0% and less than or equal to 0.4%, copper (Cu): 0.05% to 0.4%, 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 being iron (Fe) and unavoidable impurities.

[0025] In the following, the role of each element contained in the ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to the present invention and its contents will be described. In this regard, the content of each component element refers to a weight percentage based on the total weight of the steel sheet.

[0026] Carbon (C): 0.1% to 0.5% Adding carbon ensures the strength of the steel sheet and controls its microstructure; strength can also be ensured by increasing martensitic hardness. When the carbon content is less than 0.1%, it is difficult to achieve the target strength. When the carbon content exceeds 0.5%, weldability and machinability may deteriorate. Therefore, based on the total weight of the steel sheet, it is preferable to add 0.1% to 0.5% carbon.

[0027] Silicon (Si): 0.01% to 2.0% Silicon is a ferrite-stabilizing element and can inhibit cementite growth, thus ensuring resistance to hydrogen embrittlement. When the silicon content is less than 0.01%, the effect of silicon addition is insufficient. When the silicon content exceeds 2.0%, a large amount of ferrite may form, and the target strength may not be guaranteed. Therefore, based on the total weight of the steel sheet, it is preferable to add 0.01% to 2.0% silicon.

[0028] Manganese (Mn): 0.1% to 5.0% Manganese has a solid solution strengthening effect and can contribute to strength improvement by increasing hardenability. When the manganese content is less than 0.1%, hardenability is insufficient, making it difficult to ensure strength, and the effect of manganese addition is insufficient. When the manganese content exceeds 5.0%, the formation of manganese bands and manganese sulfide (MnS) may worsen resistance to hydrogen embrittlement. Therefore, based on the total weight of the steel plate, it is preferable to add 0.1% to 5.0% of manganese.

[0029] Aluminum (Al): 0.01% to 2.0% Aluminum is used as a deoxidizer and can help purify ferrite. When the aluminum content is less than 0.01%, the effect of aluminum addition is insufficient (including insufficient deoxidation). When the aluminum content exceeds 2.0%, aluminum nitride (AlN) may form during slab manufacturing, which may lead to cracks during casting or hot rolling, and may also form a large amount of ferrite, resulting in reduced strength. Therefore, based on the total weight of the steel sheet, it is preferable to add aluminum in an amount of 0.01% to 2.0%.

[0030] Chromium (Cr): greater than 0% and 3.0% or less Chromium is a ferrite-stabilizing element in steel, a component that improves hardenability, and can contribute to strength improvement by refining carbides. Furthermore, strength can be enhanced through solid solution strengthening. When the chromium content exceeds 3.0%, manufacturing costs become relatively high, and the quenching effect during cooling is significant, leading to a relative decrease in elongation and potentially worsening laser weldability. Therefore, based on the total weight of the steel sheet, it is preferable to add chromium in an amount greater than 0% and less than or equal to 3.0%.

[0031] Molybdenum (Mo): greater than 0% and less than or equal to 1.0% Molybdenum has a solid solution strengthening effect and can contribute to improved strength by increasing hardenability. Furthermore, it can improve resistance to hydrogen embrittlement by refining Ti-based precipitates. Material costs may increase when the molybdenum content exceeds 1.0%. Therefore, based on the total weight of the steel sheet, it is preferable to add more than 0% and less than or equal to 1.0% of molybdenum.

[0032] Nickel (Ni): greater than 0% and less than or equal to 0.4% Nickel is a precipitation-forming element that combines with carbon (C) and nitrogen (N) to form carbides or nitrides. It can improve the toughness and strength of steel through grain refinement resulting from this precipitation and the suppression of recrystallization and grain growth during rolling. Furthermore, it can suppress hot brittleness caused by copper. When the nickel content is less than 0.02%, the effect of adding nickel is insufficient. When the nickel content exceeds 3.0%, the rolling load during rolling may increase significantly, and the manufacturing cost of the steel may increase. In particular, the ratio to copper needs to be controlled to prevent melting during reheating. Therefore, based on the total weight of the steel sheet, it is preferable to add more than 0% and less than or equal to 0.4% nickel.

[0033] Copper (Cu): 0.05% to 0.4% Copper is a precipitation-forming element that combines with carbon (C) and nitrogen (N) to form carbides or nitrides. It can improve the toughness and strength of steel through grain refinement resulting from this precipitation and the inhibition of recrystallization and grain growth during rolling. Furthermore, copper can be added to improve resistance to hydrogen embrittlement. When the copper content is less than 0.05%, the effect of copper addition is insufficient, and delayed fracture may occur. When the copper content exceeds 0.4%, as an element contributing to hot embrittlement, cracks may appear during hot rolling, the rolling load during rolling may increase significantly, and the manufacturing cost of the steel may increase. Therefore, based on the total weight of the steel sheet, it is preferable to add copper in an amount of 0.05% to 0.4%.

[0034] Titanium (Ti): 0.01% to 0.2% Titanium is a precipitate-forming element that can promote the precipitation of titanium nitride (TiN) and titanium carbide (TiC) and refine grain size. Specifically, TiN precipitation can reduce the nitrogen content in steel, and when added together with boron, it can prevent the precipitation of boron nitride (BN), thus retaining boron (a grain boundary strengthening element) in a solid solution state. When the titanium content is less than 0.01%, BN precipitation may be induced, and the effect of titanium addition may be insufficient. When the titanium content exceeds 0.2%, resistance to hydrogen embrittlement may deteriorate due to the coarsening of TiN precipitates, strength may be difficult to ensure due to the reduced solid solubility of carbon in the matrix material, and the manufacturing cost of the steel may increase. Therefore, based on the total weight of the steel sheet, it is preferable to add 0.01% to 0.2% titanium.

[0035] Niobium (Nb): 0.01% to 0.1% Niobium is a precipitate-forming element that combines with carbon (C) and nitrogen (N) to form carbides or nitrides. It can improve the toughness and strength of steel through grain refinement resulting from this precipitation and the inhibition of recrystallization and grain growth during rolling. When the niobium content is less than 0.01%, there is no grain refinement effect, and the effect of niobium addition is insufficient. When the niobium content exceeds 0.1%, precipitates may grow, and there may be no strength-enhancing effect; the rolling load during rolling may increase significantly, and the manufacturing cost of the steel may increase. Therefore, based on the total weight of the steel plate, it is preferable to add 0.01% to 0.1% niobium.

[0036] Vanadium (V): 0.01% to 1.0% Vanadium is a precipitate-forming element that combines with carbon (C) and nitrogen (N) to form carbides or nitrides. It can improve the toughness and strength of steel through grain refinement resulting from this precipitation and the inhibition of recrystallization and grain growth during rolling. When the vanadium content is less than 0.01%, there is no grain refinement effect, and the effect of vanadium addition is insufficient. When the vanadium content exceeds 1.0%, precipitates may grow, and there may be no strength-enhancing effect; the rolling load during rolling may increase significantly, and the manufacturing cost of the steel may increase. Therefore, based on the total weight of the steel plate, it is preferable to add 0.01% to 1.0% vanadium.

[0037] Boron (B): 0.001% to 0.005% Boron is a grain boundary strengthening element, and when distributed at grain boundaries, it can improve resistance to hydrogen embrittlement. When the boron content is less than 0.001%, the effect of boron addition is insufficient. When the boron content exceeds 0.005%, there is a risk of grain boundary embrittlement due to the formation of boron nitride (BN). Therefore, based on the total weight of the steel sheet, it is preferable to add 0.001% to 0.005% boron.

[0038] Phosphorus (P): greater than 0% and less than or equal to 0.02% Phosphorus is an impurity introduced during the steel manufacturing process. Although it can help improve strength through solid solution strengthening, when present in large quantities, it can cause low-temperature embrittlement due to grain boundary segregation and may deteriorate spot weldability. Therefore, based on the total weight of the steel sheet, the phosphorus content is preferably limited to greater than 0% and less than or equal to 0.02%.

[0039] Sulfur (S): greater than 0% and less than or equal to 0.01% Sulfur is an impurity introduced during the steelmaking process and can form non-metallic inclusions (such as FeS and manganese sulfide (MnS)), thereby degrading toughness, resistance to hydrogen embrittlement, and weldability. Therefore, based on the total weight of the steel plate, the sulfur content is preferably limited to greater than 0% and less than or equal to 0.01%.

[0040] The remainder of ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement is iron (Fe). However, these impurities cannot be excluded because they may be unavoidably introduced from raw materials or the surrounding environment during normal steelmaking processes. Since these impurities are known to those skilled in the art of normal manufacturing processes, not all of these details are specifically mentioned in this specification.

[0041] After martensite formation begins, carbon segregates towards dislocations or interfaces during all tempering processes, forming clusters and transition carbides and precipitating cementite. Therefore, carbon activity is a very important factor in the tempering process, and factors affecting it include temperature and composition. In particular, the hydrogen embrittlement resistance of the final product (cold-rolled steel sheet) varies depending on the composition content.

[0042] Figure 1 A graph showing the carbon activity of the constituent elements of an ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to an embodiment of the present invention.

[0043] refer to Figure 1 Among the elements that affect carbon activity, for representative elements that remain in solid solution without precipitation (i.e., Cr, Mn, Ni, and Si), it can be confirmed that Cr and Mn reduce carbon activity, while Ni and Si are elements that increase carbon activity.

[0044] Figure 2 A graph showing the evaluation results of the hydrogen embrittlement test method on the constituent elements of the ultra-high strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance according to an embodiment of the present invention.

[0045] For conditions where the tensile strength of the specimen at the end of the final heat treatment is at least 1.1 GPa, hydrogen embrittlement is assessed by the hydrogen embrittlement test method described below.

[0046] refer to Figure 2 Under the assumption that the Cu content range is limited to greater than or equal to 0.05% and less than or equal to 0.40% by weight, as described above, each axis is set according to the effect of elements on carbon activity; the X-axis represents the sum of the weight percentages of Mn and Cr (which reduce carbon activity), and the Y-axis represents the sum of the weight percentages of Si and Ni (which increase carbon activity). As a result of the evaluation of the hydrogen embrittlement test method, it can be seen that, in terms of the delayed fracture performance caused by hydrogen embrittlement, the sum of the weight percentages of Mn and Cr satisfies a certain condition with the sum of the weight percentages of Si and Ni. According to an embodiment of the present invention, it can be confirmed that the delayed fracture performance caused by hydrogen embrittlement can be defined by the following relationship: [Si] + [Ni] ≤ 0.85 - 0.23 × ([Mn] + [Cr]). That is, the hydrogen embrittlement effect of (Mn + Cr) and (Si + Ni) (which have similar effects on carbon activity) can be confirmed.

[0047] Ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement can be manufactured by controlling the specific components and content range of the alloy composition described above and by the manufacturing method described below. For example, they can meet the following requirements: tensile strength (TS): at least 1100 MPa, elongation (EL): at least 3%, and time without fracture based on hydrogen embrittlement test method: at least 100 hours.

[0048] The microstructure of ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement can include ferrite, tempered bainite, and tempered martensite phases. The area fraction of ferrite can range from 0% to 20%, the area fraction of tempered bainite can range from 5% to 20%, and the area fraction of tempered martensite can range from 60% to 100%, provided that the sum of tempered bainite and tempered martensite satisfies a range of 80% to 100%. Area fraction refers to the area ratio obtained by analyzing microstructure photographs using an image analyzer. Furthermore, the aforementioned area fractions of the microstructure are based on analysis results using a scanning electron microscope at a quarter-point along the thickness direction of the steel sheet in a direction perpendicular to the rolling direction.

[0049] The presence of carbides is necessary to suppress delayed fracture caused by hydrogen embrittlement. These carbides can be used to capture and immobilize hydrogen that has diffused and permeated into the steel plate, thus preventing the free movement of hydrogen within the steel plate. However, if the carbide size is too large, embrittlement due to hydrogen accumulation at the interface may occur; therefore, limiting the size of the carbides is preferable.

[0050] Therefore, ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement can further contain carbides. The average size (particle diameter) of the carbides can be, for example, 100 nm or smaller, in the range of 1 nm to 100 nm. The aspect ratio of the carbides can be, for example, 5 or smaller, in the range of 1 to 5. Here, aspect ratio refers to 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 the martensite has been over-tempered, or that retained austenite has formed due to insufficient stabilization.

[0051] Carbides may include cementite and transition carbides, or may include carbides lacking Fe, and may include at least one of, for example, Fe-based carbides, Ti-based carbides, Nb-based carbides, V-based carbides, and Mo-based carbides. Specifically, carbides may include, for example, Fe3C, ε-carbides (Fe... 2.5 C), η-carbides (Fe₂C), (Fe, substitution element) 2-3 (C) and (Ti, Nb, V, Mo)(C, N).

[0052] Hydrogen embrittlement test method In this invention, the hydrogen embrittlement test method for quantitatively measuring delayed fracture caused by hydrogen embrittlement in ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement is as follows.

[0053] Cold-rolled steel sheets are cut using methods such as shearing, laser cutting, water jetting, grinding, and wire cutting (EDM) to prepare specimens with lengths ranging from 100 mm to 300 mm and widths from 10 mm to 50 mm. The longitudinal direction of the specimen is then manufactured at 0 degrees or 90 degrees relative to the rolling direction of the material.

[0054] Subsequently, stresses of 60%, 70%, 80%, 90%, and 100% of the yield strength are applied to the specimens using either a two-point or four-point bending method. The applied stress must vary according to the yield strength of each cold-rolled steel sheet. Under the stress application conditions, fracture must not occur in the specimens under three or more conditions (including the 100% stress application condition).

[0055] The stressed specimen is immersed in a hydrochloric acid (HCl) solution with a concentration of 0.01 N to 0.2 N, preferably in a 0.1 N hydrochloric acid solution. Since this immersion injects hydrogen into the interior of the specimen, it can be termed an accelerated test for hydrogen-induced delayed fracture. The immersed specimen is held for at least 100 hours. During this time, fracture must not occur even under the aforementioned stress conditions.

[0056] In the following description, a method for manufacturing ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement according to the present invention will be described with reference to the accompanying drawings.

[0057] Method for manufacturing ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement Figure 3 The diagram illustrates a process flow of a method for manufacturing ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement according to an embodiment of the present invention, and relates to a method for manufacturing uncoated cold-rolled steel sheets.

[0058] refer to Figure 3 According to an embodiment of the present invention, a method for manufacturing ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement 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), and a tempering step (S160).

[0059] Hot-rolled steel sheet manufacturing steps (S110) In the hot-rolled steel sheet manufacturing step (S110), a steel material is prepared, which comprises, 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): greater than 0% and less than or equal to 0.4%, copper (Cu): 0.05% to 0.4%, 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 being iron (Fe) and unavoidable impurities.

[0060] In the manufacturing method according to the invention, the semi-finished product subjected to the hot rolling process can be, for example, a slab. A slab in a semi-finished state can be obtained by obtaining molten steel with a predetermined composition through a steelmaking process and then by a continuous casting process.

[0061] The aforementioned steel material (e.g., slab) is reheated for 1 to 5 hours at a reheating temperature (slab reheating temperature, SRT) of 1150°C to 1300°C. This reheating allows for a complete single-phase austenite transformation, and may result in the redissolution of segregated components and precipitates from the casting process, thereby homogenizing the steel and making it suitable for hot rolling. When the reheating temperature is below 1150°C, the segregated components from the casting process may not redissolve sufficiently and may not distribute uniformly. When the reheating temperature exceeds 1300°C, the austenite grains may coarsen, potentially leading to a decrease in yield strength. Furthermore, with increasing reheating temperature, there are issues of increased manufacturing costs and reduced productivity due to heating costs and the additional time required to adjust the hot rolling temperature. When the reheating time is less than 1 hour, the reduction of segregation bands may be insufficient, and when it exceeds 5 hours, the grain size increases, and process costs may increase.

[0062] The reason for limiting the reheating temperature range is explained below.

[0063] 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 has migrated to the surface may melt and penetrate along the grain boundaries of the steel, leading to hot brittleness, which in turn causes reduced ductility and cracking. One way to prevent this hot brittleness is to add nickel to form a complete copper-nickel solid solution, thereby inhibiting the melting of copper. Therefore, to prevent liquid copper from penetrating into the steel sheet during reheating, the temperature of the Cu-Ni solid solution needs to be at least 1150°C or higher. Furthermore, to mitigate the cost increase caused by the addition of nickel, the upper limit of the melting point of the Cu-Ni solid solution is set at 1300°C or lower. The melting point of the Cu-Ni solid solution can be calculated using the ThermoCalc program.

[0064] Subsequently, the reheated steel material is first hot-rolled after heating to achieve shape adjustment. Hot rolling can be carried out continuously through width rolling, roughing rolling, and finishing rolling. Through the hot rolling process, the steel material can be formed into hot-rolled steel material. Hot-rolled steel material can be hot-rolled steel sheet.

[0065] Rough rolling is the step of rolling a slab to produce bars, and it can be carried out in the range of reheating end temperature (1150°C to 1300°C) to 1000°C.

[0066] Finish rolling can be completed at a finishing rolling temperature (FRT) of 800°C to 1000°C. When the finishing rolling temperature is below 800°C, the rolling load may increase sharply, leading to a decrease in productivity. When the finishing rolling temperature exceeds 1000°C, the grains may coarsen, resulting in a decrease in the strength of the final steel material.

[0067] The hot-rolled steel is then cooled to a predetermined coiling temperature. Cooling can be performed by air or water, and at a cooling rate of, for example, 1°C / s to 100°C / s. A faster cooling rate can help reduce the average grain size. Cooling is preferably performed to a coiling temperature, for example, in the range of 400°C to 700°C, such as 500°C to 650°C.

[0068] The hot-rolled steel sheet is then coiled at a coiling temperature (CT) of, for example, 300°C to 700°C, or 500°C to 650°C. When the coiling temperature is below 300°C, the shape of the coiled hot-rolled coil may become uneven, and its strength may increase, leading to an increase in the rolling load during the cold rolling process. When the coiling temperature exceeds 700°C, an uneven microstructure may occur due to the difference in cooling rates between the center and edge portions of the steel sheet, and defects may appear in subsequent processes due to surface oxidation, etc. The coiled steel material can then be cooled to room temperature.

[0069] Cold-rolled steel sheet manufacturing steps (S120) In the cold-rolled steel sheet manufacturing step (S120), hot-rolled steel sheet is used to adjust the thickness of the final produced steel sheet. The coiled hot-rolled steel sheet is pickled (wherein it is cleaned with acid). Subsequently, the pickled hot-rolled steel sheet is cold-rolled at a reduction rate of, for example, at least 35%, or from 35% to 70%, to form a cold-rolled steel sheet. A higher reduction rate can improve resistance to hydrogen embrittlement through grain refinement. When the reduction rate is less than 35%, it is difficult to obtain a uniform microstructure, and due to the limited number of nucleation sites for recrystallization during annealing, grains may overgrow during the annealing heat treatment described below, resulting in a sharp decrease in strength. When the reduction rate exceeds 70%, the number of nucleation sites becomes excessive, and the grains formed by the annealing heat treatment may become too fine, resulting in reduced ductility and poor formability.

[0070] Annealing heat treatment step (S130) In the annealing heat treatment step (S130), the cold-rolled steel sheet can be heat-treated in a continuous annealing furnace with a conventional slow cooling zone.

[0071] Annealing heat treatment is performed by heating at a rate of, for example, at least 1°C / s, or, for example, from 1°C / s to 10°C / s, to a temperature in the range of, for example, Ac3-50°C or higher, such as 800°C to 950°C, and then holding for, for example, 10 seconds to 600 seconds. When the annealing heat treatment temperature is below 800°C or the holding time is less than 10 seconds, it is difficult to form sufficient austenite, and the ferrite fraction may increase, resulting in a decrease in strength. When the annealing heat treatment temperature exceeds 950°C or the holding time exceeds 600 seconds, the austenite grain size may coarsen, or the productivity may be excessively reduced.

[0072] The Ac3 temperature can be calculated using the following formula: Ac3=910-203×[C]^0.5-30[Mn]+44.7[Si]+31.5[Mo]-15.2[Ni] Where [C] is the carbon content (by weight %) in the steel, [Mn] is the manganese content (by weight %) in the steel, [Si] is the silicon content (by weight %) in the steel, [Mo] is the molybdenum content (by weight %) in the steel, and [Ni] is the nickel content (by weight %) in the steel.

[0073] First cooling step (S140) In the first cooling step (S140), the annealed heat-treated cold-rolled steel sheet is cooled for the first time to, for example, 500°C to 700°C at a cooling rate of, for example, 0°C / s to 50°C / s. Cooling can be carried out by air cooling or water cooling. The first cooling step can be referred to as a slow cooling step.

[0074] Second cooling step (S150) In the second cooling step (S150), the cold-rolled steel sheet that has undergone the first cooling is cooled a second time to, for example, below the martensitic transformation completion temperature (Mf) (e.g., in the range of room temperature (0°C to 40°C) to 350°C) at a cooling rate of at least 5°C / s, for example, from 5°C / s to 30°C / s. During the second cooling process, additional ferrite transformation must be suppressed.

[0075] Tempering step (S160) In the tempering step (S160), the cold-rolled steel sheet that has undergone a second cooling is heated at a heating rate of up to 100°C / s and heat-treated, for example, at a temperature ranging from 100°C to 350°C for a holding time of at least 10 seconds.

[0076] This tempering heat treatment promotes the formation and growth of transition carbides and cementite. Tempering heat treatment can be carried out at low temperatures (e.g., 200°C or lower) for a longer period of time, but at high temperatures (e.g., above 200°C), the growth of cementite must be suppressed by holding the treatment for a relatively short time, thereby controlling the reduction in resistance to hydrogen embrittlement.

[0077] Cold-rolled steel sheets can be formed by performing the above steps. The method can also be applied to steel sheets electroplated using uncoated steel materials.

[0078] Experimental Examples Preferred experimental embodiments are shown below to aid in understanding the invention. However, the following experimental embodiments are intended only to aid in understanding the invention, and the invention is not limited to the following experimental embodiments. Those skilled in the art can readily infer from the technical details anything not described herein, and therefore such descriptions are omitted.

[0079] 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 predetermined hot rolling, cold rolling, and heat treatment processes. In Tables 1 and 2, the balance consists of iron (Fe) and impurities inevitably included in the steelmaking process, etc. The content of each component is in weight percentage. At this time, the examples and comparative examples listed in Tables 1 and 2 are limited to steel grades with an added content of 0.05 wt% to 0.40 wt% Cu.

[0080] [Table 1] [Table 2] Referring to Tables 1 and 2, the embodiments satisfy the composition range and relationship of the present invention: [Si] + [Ni] ≤ 0.85 - 0.23 × ([Mn] + [Cr]). In contrast, the comparative examples differ in that they do not satisfy the composition range relationship.

[0081] Table 3 shows the manufacturing method conditions for the ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement in the comparative examples and embodiments, as well as the results of fracture time measurements based on the hydrogen embrittlement test method.

[0082] [Table 3] In the manufacturing methods of the embodiments and comparative examples, all conditions other than those listed in Table 3 (e.g., hot rolling and cold rolling process conditions) were identically manufactured to meet the process conditions of the present invention.

[0083] Referring to Table 3, the examples and comparative examples meet the process conditions of the present invention (annealing heat treatment temperature, second cooling end temperature, tempering temperature and tempering time).

[0084] However, referring to Table 3, the examples did not exhibit fracture under conditions of applying a load of 300% of the yield strength for 100 hours or longer. Therefore, it can be seen that the examples possess excellent resistance to hydrogen embrittlement. In contrast, the comparative examples exhibited fracture in less than 100 hours under conditions of applying a load of 60% to 100% of the yield strength. This demonstrates that the comparative examples are prone to hydrogen embrittlement due to not satisfying the compositional range and relationship of [Si] + [Ni] ≤ 0.85 - 0.23 × ([Mn] + [Cr]) of the present invention.

[0085] When tempering is performed at relatively high temperatures, carbide growth becomes very active in the longitudinal direction, which may reduce the yield strength and tensile strength of the steel. Furthermore, the steel may become susceptible to hydrogen embrittlement because carbides can act as fracture initiation sites during hydrogen permeation. In contrast, when tempering is performed at relatively lower temperatures, heat treatment control of carbide growth is more easily achieved, thus helping to ensure the desired strength and suppress fracture caused by hydrogen embrittlement.

[0086] It will be apparent to those skilled in the art to which the technical concept of the present invention pertains that the technical concept of the present invention, as described above, is not limited to the above-described embodiments and the accompanying drawings, and that various substitutions, alterations, and variations are possible without departing from the technical concept of the present invention.

Claims

1. An ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement, said ultra-high strength cold-rolled steel sheet comprising, by weight percentage: 0.1% to 0.5% carbon (C), 0.01% to 2.0% silicon (Si), 0.1% to 5.0% manganese (Mn), 0.01% to 2.0% aluminum (Al), greater than 0% and less than or equal to 3.0% chromium (Cr), greater than 0% and less than or equal to 1.0% molybdenum (Mo), greater than 0% and less than or equal to 1.0% chromium (Cr), and more than 0% and less than or equal to 1.0% molybdenum (Mo). The composition includes 0.4% nickel (Ni), 0.05% to 0.4% copper (Cu), 0.01% to 0.2% titanium (Ti), 0.01% to 0.1% niobium (Nb), 0.01% to 1.0% vanadium (V), 0.001% to 0.005% boron (B), greater than 0% and less than or equal to 0.02% phosphorus (P), greater than 0% and less than or equal to 0.01% sulfur (S), and the balance iron (Fe) and unavoidable impurities. The cold-rolled steel sheet has a tensile strength (TS) of at least 1100 MPa, an elongation (EL) of at least 3%, and a non-fracture time of at least 100 hours based on a hydrogen embrittlement test method.

2. The ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to claim 1, wherein, The sum of the weight percentages of silicon and nickel, [Si] + [Ni], and the sum of the weight percentages of manganese and chromium, [Mn] + [Cr], satisfy the following relationship: [Si]+[Ni]≤0.85-0.23×([Mn]+[Cr]).

3. The ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to claim 1, wherein, The microstructure of the ultra-high strength cold-rolled steel sheet includes ferrite, tempered bainite, and tempered martensite phases. The area fraction of ferrite ranges from 0% to 20%, the area fraction of tempered bainite ranges from 5% to 20%, and the area fraction of tempered martensite ranges from 60% to 100%. The sum of tempered bainite and tempered martensite meets the range of 80% to 100%.

4. The ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to claim 1, wherein, The cold-rolled steel sheet further contains carbides. The carbide has an average size of no more than 100 nm and an aspect ratio of no more than 5.

5. The ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to claim 4, wherein, The carbide includes at least one selected from Fe-based carbides, Ti-based carbides, Nb-based carbides, V-based carbides, and Mo-based carbides.

6. A method for manufacturing ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement, the method comprising: Hot rolling of steel material to produce hot-rolled steel sheet, said steel material comprising, by weight percentage: 0.1% to 0.5% carbon (C), 0.01% to 2.0% silicon (Si), 0.1% to 5.0% manganese (Mn), 0.01% to 2.0% aluminum (Al), greater than 0% and less than or equal to 3.0% chromium (Cr), greater than 0% and less than or equal to 1.0% molybdenum (Mo), and greater than 0% and less than or equal to 0.4%... Nickel (Ni), 0.05% to 0.4% copper (Cu), 0.01% to 0.2% titanium (Ti), 0.01% to 0.1% niobium (Nb), 0.01% to 1.0% vanadium (V), 0.001% to 0.005% boron (B), greater than 0% and less than or equal to 0.02% phosphorus (P), greater than 0% and less than or equal to 0.01% sulfur (S), and the balance iron (Fe) and unavoidable impurities; Cold-rolled steel sheets are manufactured by cold rolling hot-rolled steel sheets. Annealing heat treatment is performed by holding the cold-rolled steel sheet at a temperature ranging from 800°C to 950°C for a duration of 10 to 600 seconds. The annealed heat-treated cold-rolled steel sheet is first cooled to a temperature ranging from 500°C to 700°C at a cooling rate of 0°C / s to 50°C / s. The cold-rolled steel sheet, after the first cooling, is then subjected to a second cooling at a cooling rate of at least 5°C / s to a temperature ranging from 0°C to 350°C; and A tempering step is performed on the cold-rolled steel sheet that has been heat-treated at a temperature ranging from 100°C to 350°C for a duration of at least 10 seconds after a second cooling.

7. The method for manufacturing ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to claim 6, wherein, The preparation of hot-rolled steel sheets includes: Reheating of steel materials with alloy composition at reheating temperatures ranging from 1150°C to 1300°C; Hot rolling of reheated steel materials, completed at a finishing rolling temperature ranging from 800°C to 1000°C, is used to produce hot-rolled steel sheets; and The hot-rolled steel sheet is cooled at a cooling rate of 1℃ / s to 100℃ / s, and then coiled at a coiling temperature ranging from 300℃ to 700℃.

8. The method for manufacturing ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement according to claim 6, wherein, The ultra-high strength cold-rolled steel sheet manufactured by a method for producing ultra-high strength cold-rolled steel sheets with excellent resistance to hydrogen embrittlement has a tensile strength (TS) of at least 1100 MPa, an elongation (EL) of at least 3%, and a non-fracture time of at least 100 hours based on a hydrogen embrittlement test method. The microstructure of the ultra-high strength cold-rolled steel sheet comprises ferrite, tempered bainite and tempered martensite phases, with the area fraction of ferrite ranging from 0% to 20%, the area fraction of tempered bainite ranging from 5% to 20%, the area fraction of tempered martensite ranging from 60% to 100%, and the sum of tempered bainite and tempered martensite satisfying the range of 80% to 100%.

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