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

Abstract

The present invention provides an ultra-high strength cold-rolled steel sheet having a strength of 1100 MPa or more on the basis of tensile strength and excellent hydrogen embrittlement resistance, and a manufacturing method therefor. According to one embodiment of the present invention, the ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance comprises by wt%, carbon (C): 0.1-0.5%, silicon (Si): 0.01-2.0%, manganese (Mn): 0.1-5.0%, aluminum (Al): 0.01-2.0%, chromium (Cr): more than 0% to 3.0%, molybdenum (Mo): more than 0% to 1.0%, nickel (Ni): more than 0% to 0.4%, copper (Cu): 0.05-0.4%, titanium (Ti): 0.01-0.2%, niobium (Nb): 0.01-0.1%, vanadium (V): 0.01-1.0%, boron (B): 0.001-0.005%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, and the balance of iron (Fe) and other inevitable impurities, and includes a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface, wherein the steel sheet satisfies a tensile strength (TS) of 1100 MPa or more, an elongation (EL) of 3% or more, and a fracture-free time of 100 hours or more on the basis of a hydrogen embrittlement test method.

Description

Ultra-high-strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance and manufacturing method thereof

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

[0002] The automotive industry's demand for crashworthiness has steadily increased. While the recent proliferation of electric vehicles has reduced the number of vehicle parts, the introduction of batteries has increased vehicle weight, further expanding the need for crashworthiness. Consequently, the ultra-high strength of impact components, such as front bumper beams, side sills, and door impact beams, that contribute to crashworthiness are continuously being improved. Martensitic steel, which has the highest strength among cold-rolled steel sheets, has seen its application expanded with the increasing use of roll forming techniques. However, its high strength poses the issue of delayed failure. Corrosion in hydrogen permeation is a known example of this delayed failure. Therefore, much research has been conducted to enhance the corrosion resistance or hydrogen embrittlement resistance of ultra-high strength plates with a tensile strength of 1 GPa or higher, and accordingly, electro-galvanized steel sheets (EG) with high corrosion resistance by electroplating zinc on cold rolled steel sheets have been studied. However, electro-galvanized steel sheets (EG) have the disadvantage of having a manufacturing cost that is 5 to 10 times higher than that of other plating materials.

[0003] Accordingly, we developed galvanized steel sheets (GA: Galva Annealed) with enhanced weldability and corrosion resistance for manufacturing automobile parts.

[0004] Prior art literature includes Korean Patent Application No. 2012-0144482.

[0005] The technical task to be achieved by the technical idea of ​​the present invention is to provide an ultra-high strength cold rolled steel sheet having a tensile strength of 1100 MPa or more and excellent hydrogen embrittlement resistance by limiting the content of elements such as Cu, Mn, Cr, Si, and Ni that affect delayed fracture due to hydrogen embrittlement, and a method for manufacturing the same.

[0006] However, these tasks are exemplary and the technical idea of ​​the present invention is not limited thereto.

[0007] According to one aspect of the present invention, an ultra-high strength cold rolled steel sheet having a tensile strength of 1100 MPa or more and excellent hydrogen embrittlement resistance and a method for manufacturing the same are provided.

[0008] According to one embodiment of the present invention, the ultra-high strength cold rolled steel sheet having excellent hydrogen embrittlement resistance contains, in wt%, 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): more than 0% 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): 0% to 0.02%, sulfur (S): 0% to 0.01%, and the remainder contains iron (Fe) and other unavoidable impurities, and includes a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface, but satisfies 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.

[0009] In addition, the ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance can satisfy the following relationship [Si]+[Ni] ≤ 0.85 - 0.23×([Mn]+[Cr]) in which the sum of the weight % of silicon and nickel [Si]+[Ni] and the sum of the weight % of manganese and chromium [Mn]+[Cr] are as follows.

[0010] In addition, the microstructure of the ultra-high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance includes ferrite, tempered bainite, and tempered martensite phases, and the area fraction of the ferrite is in the range of 0% to 20%, the area fraction of the tempered bainite is in the range of 5% to 20%, and the area fraction of the tempered martensite is in the range of 60% to 100%, but the sum of the tempered bainite and the tempered martensite can satisfy the range of 80% to 100%.

[0011] In addition, the ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance 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.

[0012] Additionally, the carbide may include at least one of Fe-based carbide, Ti-based carbide, Nb-based carbide, V-based carbide, and Mo-based carbide.

[0013] According to one embodiment of the present invention, the method for manufacturing the ultra-high strength cold rolled steel sheet having excellent hydrogen embrittlement resistance comprises, in wt%, 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): more than 0% 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 A step of hot-rolling a steel material containing 0.005%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities, to manufacture a hot-rolled steel sheet; A step of cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; A step of annealing the cold-rolled steel sheet by maintaining it at a temperature of 800°C to 950°C for 60 seconds to 600 seconds; A step of first cooling the annealed cold-rolled steel sheet at a cooling rate of 0°C / sec to 50°C / sec; A step of heat-treating the first-cooled cold-rolled steel sheet by maintaining it at a temperature of 450°C to less than 600°C for 5 seconds to 100 seconds and simultaneously hot-dip galvanizing it; A step of secondarily cooling the hot-dip galvanized cold-rolled steel sheet from 0°C to 350°C at a cooling rate of 1°C / sec or more; It includes a tempering step of heat-treating the secondarily cooled cold-rolled steel sheet at a temperature of 100°C to 350°C for a time of 10 seconds or more.

[0014] In addition, a step of alloying heat treatment of the above-mentioned hot-dip galvanized cold-rolled steel sheet may be further included.

[0015] In addition, the ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance manufactured by the method for manufacturing the ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance satisfies tensile strength (TS): 1100 MPa or more, elongation (EL): 3% or more, and non-fracture time based on hydrogen embrittlement test method: 100 hours or more, and the microstructure of the ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance includes ferrite, tempered bainite, and tempered martensite phases, and the area fraction of the ferrite is in the range of 0% to 20%, the area fraction of the tempered bainite is in the range of 5% to 20%, and the area fraction of the tempered martensite is in the range of 60% to 100%, and the sum of the tempered bainite and the tempered martensite can satisfy the range of 80% to 100%.

[0016] According to the technical idea of ​​the present invention, the hydrogen embrittlement resistance of cold-rolled steel sheets can be improved by limiting the content of components that affect the activity of carbon under a certain relationship.

[0017] The effects of the present invention described above are illustrative, and the scope of the present invention is not limited by these effects.

[0018] Figure 1 is a graph showing the carbon activity of components forming an ultra-high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance according to an embodiment of the present invention.

[0019] Figure 2 is a graph showing the evaluation results after performing a hydrogen embrittlement test on components forming an ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance according to an embodiment of the present invention.

[0020] Figure 3 is a process flow diagram schematically showing a method for manufacturing an ultra-high strength cold-rolled plate having excellent hydrogen embrittlement resistance according to an embodiment of the present invention.

[0021] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Embodiments of the present invention are provided to more completely explain the technical idea of ​​the present invention to those skilled in the art. The following embodiments may be modified in various different forms, and the scope of the technical idea of ​​the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely convey the technical idea of ​​the present invention to those skilled in the art. Like reference numerals throughout this specification denote like elements. Furthermore, various elements and areas in the drawings are schematically drawn. Therefore, the technical idea of ​​the present invention is not limited by the relative sizes or intervals drawn in the attached drawings.

[0022] The following attempts have been proposed to improve the corrosion resistance and hydrogen embrittlement resistance of cold-rolled steel sheets. Regarding delayed fracture caused by hydrogen, a method has been proposed to increase hydrogen embrittlement resistance by controlling the residual hydrogen content within the steel. For example, a method has been proposed to increase hydrogen embrittlement resistance by controlling the hydrogen content to 0.1 ppm or less during the heat treatment process maintained at 350 to 450°C after cold rolling. Furthermore, methods have been proposed to control the area fractions of ferrite, upper bainite, and martensite, add boron, a grain-boundary strengthening element, control the effective martensite grain size, and control the number of iron-based carbides. Furthermore, a method has been proposed to increase the hydrogen embrittlement resistance of steel sheets graded 1100 MPa 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 cold-rolled steel sheets for automobiles but also for steels for pressure vessels. Based on the increase in hydrogen embrittlement resistance due to the addition of copper, a method was proposed to form a microstructure with a lower fraction of band structures in the microstructure to delay the propagation of cracks caused by hydrogen embrittlement. In addition, a method was proposed to increase hydrogen embrittlement resistance by adding nickel at 1 to 4 wt%. A method for controlling the content of nickel and copper in ultra-thick steels for the shipbuilding, marine, architectural, and civil engineering industries by limiting the content ratio of nickel to Cu / Ni ≤ 0.6 was proposed. A method was proposed to limit the ratio of manganese, nickel, and copper to improve the stability of austenite in the production of high-strength steel plates. In this case, nickel and copper were added for the purpose of securing hardenability in high-aluminum steel plates rather than to prevent delayed fracture by hydrogen.

[0023] The technical idea of ​​the present invention is to provide an ultra-high strength cold rolled steel sheet having excellent hydrogen embrittlement resistance by controlling the component content and microstructure composition for application to an ultra-high strength steel sheet for automobiles having a tensile strength of 1100 MPa or more, and a method for manufacturing the same.

[0024] Hereinafter, a high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance according to the technical idea of ​​the present invention will be described in detail.

[0025] According to one embodiment of the present invention, an ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance contains, in wt%, 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): more than 0% 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): 0% to 0.02%, Sulfur (S): 0% to 0.01%, and the remainder includes iron (Fe) and other unavoidable impurities.

[0026] Hereinafter, the role and content of each component included in the ultra-high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance according to the present invention will be described. Here, the content of each component element all refers to the weight % of the entire steel sheet.

[0027] Carbon (C): 0.1% ~ 0.5%

[0028] Carbon is added to steel sheets to secure strength and control the microstructure, and can be achieved by improving martensite hardness. If the carbon content is less than 0.1%, it is difficult to achieve the target strength. If the carbon content exceeds 0.5%, weldability and workability may deteriorate. Therefore, it is preferable to add carbon at a level of 0.1% to 0.5% of the total weight of the steel sheet.

[0029] Silicon (Si): 0.01% to 2.0%

[0030] Silicon is a ferrite-stabilizing element that inhibits the growth of cementite, thereby ensuring hydrogen embrittlement resistance. If the silicon content is less than 0.01%, the effect of silicon addition is insufficient. If the silicon content exceeds 2.0%, excessive ferrite formation may occur, preventing the target strength from being achieved. Therefore, it is preferable to add silicon in an amount of 0.01% to 2.0% of the total weight of the steel sheet.

[0031] Manganese (Mn): 0.1% to 5.0%

[0032] Manganese has a solid-solution strengthening effect and can contribute to strength enhancement by increasing hardenability. When the manganese content is less than 0.1%, hardenability is insufficient, making it difficult to secure strength, and the effect of manganese addition is insufficient. When the manganese content exceeds 5.0%, hydrogen embrittlement resistance may be reduced 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.

[0033] Aluminum (Al): 0.01% to 2.0%

[0034] Aluminum is used as a deoxidizer and can help purify ferrite. When the aluminum content is less than 0.01%, the deoxidation effect is insufficient, and the effect of aluminum addition is insufficient. When the aluminum content exceeds 2.0%, AlN may form during slab manufacturing, which may cause cracks during casting or hot rolling, and a large amount of ferrite may form, reducing 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.

[0035] Chromium (Cr): 0% to 3.0%

[0036] Chromium is a ferrite-stabilizing element in steel and improves hardenability. It can contribute to strength enhancement by refining carbides. Furthermore, it can increase strength through solid solution strengthening. When the chromium content exceeds 3.0%, manufacturing costs are relatively high, and the quenching effect during cooling is large, leading to increased strength. This, in turn, can lead to a relative decrease in elongation and deterioration in laser weldability. Therefore, it is preferable to add chromium in an amount exceeding 0% to 3.0% of the total weight of the steel sheet.

[0037] Molybdenum (Mo): 0% to 1.0%

[0038] Molybdenum has a solid-solution strengthening effect and can contribute to increased strength by enhancing hardenability. Furthermore, it can improve hydrogen embrittlement resistance by refining Ti-based precipitates. If the molybdenum content exceeds 1.0%, the material cost may increase. Therefore, it is preferable to add molybdenum in an amount exceeding 0% to 1.0% of the total weight of the steel sheet.

[0039] Nickel (Ni): >0% ~ 0.4%

[0040] Nickel is a precipitate-forming element that forms carbides or nitrides by combining with carbon (C) and nitrogen (N). Through this precipitation and grain refinement by suppressing recrystallization and grain growth during rolling, it can improve the toughness and strength of steel. In addition, it can suppress hot embrittlement caused by copper. When the nickel content is less than 0.02%, the effect of nickel addition is insufficient. When the nickel content exceeds 3.0%, the rolling load may increase significantly during rolling, which may increase the manufacturing cost of steel. In particular, it is necessary to control the ratio with copper to prevent melting during the reheating process. Therefore, it is preferable to add nickel in an amount exceeding 0% to 0.4% of the total weight of the steel sheet.

[0041] Copper (Cu): 0.05% ~ 0.4%

[0042] Copper is a precipitate-forming element that forms carbides or nitrides by combining with carbon (C) and nitrogen (N). It can improve the toughness and strength of steel through grain refinement by suppressing precipitation and recrystallization and grain growth during rolling. It can also be added to increase hydrogen embrittlement resistance. If the copper content is less than 0.05%, the effect of copper addition is insufficient, and delayed fracture may occur. If the copper content exceeds 0.4%, it is an element that induces high-temperature embrittlement, which may cause cracks during hot rolling, significantly increase the rolling load during rolling, and increase the manufacturing cost of steel. Therefore, it is preferable to add copper in an amount of 0.05% to 0.4% of the total weight of the steel sheet.

[0043] Titanium (Ti): 0.01% ~ 0.2%

[0044] Titanium is a precipitate-forming element that can provide precipitation of TiN and TiC and grain refinement effects. In particular, the nitrogen content inside the steel can be reduced through TiN precipitation, and when added together with boron, it can prevent BN precipitation, thereby maintaining the solid solution state of boron, a grain boundary strengthening element. When the titanium content is less than 0.01%, BN precipitation may be induced, and the effect of titanium addition is insufficient. When the titanium content exceeds 0.2%, hydrogen embrittlement resistance may be reduced due to coarsening of TiN precipitation, and it may be difficult to secure strength by reducing the carbon solid solution in the base material, and it may increase the manufacturing cost of the steel. Therefore, it is preferable to add titanium in an amount of 0.01% to 0.2% of the total weight of the steel sheet.

[0045] Niobium (Nb): 0.01% ~ 0.1%

[0046] Niobium is a precipitate-forming element that forms carbides or nitrides by combining with carbon (C) and nitrogen (N). Through this precipitation and grain refinement by suppressing recrystallization and grain growth during rolling, it can improve the toughness and strength of steel. When the niobium content is less than 0.01%, the grain refinement effect is absent, and the niobium addition effect is insufficient. When the niobium content exceeds 0.1%, the precipitates may grow, resulting in no strength-enhancing effect, and the rolling load may significantly increase during rolling, which may increase the manufacturing cost of the steel. Therefore, it is preferable to add niobium in an amount of 0.01% to 0.1% of the total weight of the steel sheet.

[0047] Vanadium (V): 0.01% to 1.0%

[0048] Vanadium is a precipitate-forming element that combines with carbon (C) and nitrogen (N) to form carbides or nitrides. Through this precipitation and grain refinement by suppressing recrystallization and grain growth during rolling, it can improve the toughness and strength of steel. When the vanadium content is less than 0.01%, the grain refinement effect is absent, and the vanadium addition effect is insufficient. When the vanadium content exceeds 1.0%, the precipitates may grow, resulting in no strength-enhancing effect, and the rolling load may significantly increase during rolling, which may increase the manufacturing cost of the steel. Therefore, it is preferable to add vanadium in an amount of 0.01% to 1.0% of the total weight of the steel sheet.

[0049] Boron (B): 0.001% ~ 0.005%

[0050] Boron is a grain-boundary strengthening element that can increase resistance to hydrogen embrittlement when distributed at grain boundaries. If the boron content is less than 0.001%, the effect of adding boron is insufficient. If the boron content exceeds 0.005%, there is a risk of grain-boundary embrittlement due to BN formation. Therefore, it is preferable to add boron at a level of 0.001% to 0.005% of the total weight of the steel sheet.

[0051] Phosphorus (P): 0% to 0.02%

[0052] Phosphorus is an impurity contained during the steel manufacturing process. While it can help improve strength through solid solution strengthening, high levels can cause low-temperature embrittlement due to grain boundary segregation and deteriorate spot weldability. Therefore, it is recommended to limit the phosphorus content to between 0% and 0.02% of the total weight of the steel sheet.

[0053] Sulfur (S): Over 0% ~ 0.01%

[0054] Sulfur is an impurity contained during the steel manufacturing process. It can form non-metallic inclusions such as FeS and MnS, which can reduce toughness, hydrogen embrittlement resistance, and weldability. Therefore, it is desirable to limit the sulfur content to between 0% and 0.01% of the total weight of the steel sheet.

[0055] The remaining component of the ultra-high-strength cold-rolled steel sheet, which boasts excellent hydrogen embrittlement resistance, is iron (Fe). However, during the typical steelmaking process, unintended impurities from raw materials or the surrounding environment can inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the typical manufacturing process, their full content is not specifically addressed in this specification.

[0056] During all tempering processes after the initiation of martensite formation, carbon is involved in dislocation or interface segregation, clustering and transition carbide formation, and cementite precipitation. Therefore, carbon activity is a very important factor in the tempering process, and factors such as temperature and composition affect it. In particular, the hydrogen embrittlement resistance of the final product, the cold-rolled steel sheet, varies depending on the composition.

[0057] Figure 1 is a graph showing the carbon activity of components forming an ultra-high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance according to an embodiment of the present invention.

[0058] Referring to Figure 1, it can be confirmed that Cr and Mn are elements that lower the activity of carbon, while Ni and Si are elements that increase the activity of carbon, with Cr, Mn, Ni, and Si being representative elements that do not precipitate and remain in a solid solution state among the elements that affect the activity of carbon.

[0059] Figure 2 is a graph showing the evaluation results after performing a hydrogen embrittlement test on components forming an ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance according to an embodiment of the present invention.

[0060] Hydrogen embrittlement was evaluated using the hydrogen embrittlement test method described below for conditions where the tensile strength of the final heat treatment-finished test specimen was 1.1 GPa or higher.

[0061] Referring to Fig. 2, assuming that the Cu content range is limited to 0.05 wt% or more and 0.40 wt% or less as described above, each axis is set according to the effect of the element on the activity of carbon, and the X-axis represents the sum of the weight% of Mn and Cr, which reduce the activity of carbon, and the Y-axis represents the sum of the weight% of Si and Ni, which increase the activity of carbon. As a result of the evaluation of the hydrogen embrittlement test method, it can be seen that a certain condition is satisfied between the sum of the weight% of Mn and Cr and the sum of the weight% of Si and Ni in terms of the delayed fracture performance due to hydrogen embrittlement. According to an embodiment of the present invention, it can be confirmed that the delayed fracture performance due to hydrogen embrittlement can be defined as the following relationship, [Si]+[Ni] ≤ 0.85 - 0.23Х([Mn]+[Cr]). That is, the effect of hydrogen embrittlement can be confirmed for (Mn+Cr) and (Si+Ni), which have similar effects on the activity of carbon.

[0062] An ultra-high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance, manufactured by controlling the specific components of the alloy composition described above and the content range thereof and using the manufacturing method described below, can satisfy, for example, a tensile strength (TS): 1100 MPa or more, an elongation (EL): 3% or more, and a hydrogen embrittlement test method-based non-fracture time: 100 hours or more.

[0063] The microstructure of the above ultra-high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance may include ferrite, tempered bainite, and tempered martensite phases. The area fraction of the ferrite is in the range of 0% to 20%, the area fraction of the tempered bainite is in the range of 5% to 20%, and the area fraction of the tempered martensite is in the range of 60% to 100%, and the sum of the tempered bainite and the tempered martensite may satisfy the range of 80% to 100%. The above area fraction refers to the area ratio derived from the microstructure photograph using an image analyzer. In addition, the area fraction of the above microstructure is based on the result of analyzing a 1 / 4 point in the thickness direction of the steel sheet in a direction perpendicular to the rolling direction using a scanning electron microscope.

[0064] To suppress delayed fracture due to hydrogen embrittlement, carbides are essential. These carbides can trap and immobilize hydrogen that has diffused into the steel sheet, preventing it from freely moving within the steel sheet. However, if the carbides are too large, hydrogen collected at the interface can cause embrittlement, so it is desirable to limit the size of the carbides.

[0065] Accordingly, the ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance may further include carbides. The carbides may have an average size (grain diameter) 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 carbides. When the average size of the carbides exceeds 100 nm, it can be considered that martensite is formed due to excessive tempering or insufficient stabilization of retained austenite.

[0066] The carbide may include both cementite and transition carbide, or may include carbide without Fe, for example, may include at least one of Fe-based carbide, Ti-based carbide, Nb-based carbide, V-based carbide, and Mo-based carbide. Specifically, the carbide may include, for example, Fe3C, ε-carbide (Fe 2.5 C), η-carbide (Fe2C), (Fe, substitutional element) 2~3 (C), and may include at least one of (Ti, Nb, V, Mo)(C, N).

[0067] Hydrogen embrittlement test method

[0068] In the present invention, the hydrogen embrittlement test method for quantitatively measuring delayed fracture due to hydrogen embrittlement of the ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance is as follows.

[0069] Cold rolled steel plates are cut using shear, laser, water jet, milling, wire (EDM), etc. to prepare specimens with a length of 100 mm to 300 mm and a width of 10 mm to 50 mm. At this time, the longitudinal direction of the specimen is manufactured so that it is 0 degrees or 90 degrees to the rolling direction of the material.

[0070] Next, the above specimen is subjected to stresses of 60%, 70%, 80%, 90%, and 100% of the yield strength using a 2-point or 4-point bending method. At this time, the applied stress should be varied according to the yield strength of each cold-rolled steel sheet. Under the above stress application conditions, no fracture should occur under three or more conditions, including the 100% stress condition specimen.

[0071] The stressed specimen is immersed in a hydrochloric acid (HCl) solution with a concentration of 0.01 N to 0.2 N, preferably a 0.1 N hydrochloric acid solution. This immersion injects hydrogen into the interior of the specimen, so it can be considered an accelerated test for delayed fracture by hydrogen. The immersed specimen is maintained for at least 100 hours. At this time, fracture should not occur under any of the above stress conditions.

[0072] Hereinafter, a method for manufacturing an ultra-high strength cold rolled steel sheet having excellent hydrogen embrittlement resistance according to the present invention will be described with reference to the attached drawings.

[0073] Manufacturing method of ultra-high-strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance

[0074] Figure 3 is a process flow diagram schematically showing a method for manufacturing an ultra-high strength cold-rolled plate having excellent hydrogen embrittlement resistance according to an embodiment of the present invention.

[0075] Referring to FIG. 3, a method for manufacturing an ultra-high strength cold-rolled plate having excellent hydrogen embrittlement resistance according to an embodiment of the present invention includes a hot-rolled steel plate manufacturing step (S110), a cold-rolled steel plate manufacturing step (S120), an annealing heat treatment step (S130), a first cooling step (S141), a hot-dip galvanizing step (S145), an alloying step (S151), a second cooling step (S161), and a tempering step (S170).

[0076] Hot rolled steel sheet manufacturing stage (S110)

[0077] In the hot-rolled steel sheet manufacturing step (S110), in weight %, carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): more than 0% to 3.0%, molybdenum (Mo): more than 0% to 1.0%, nickel (Ni): more than 0% 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): more than 0% to Prepare steel containing 0.02%, sulfur (S): 0% to 0.01%, and the remainder iron (Fe) and other unavoidable impurities.

[0078] In the manufacturing method according to the present invention, the semi-finished product subject to the hot rolling process may be, for example, a slab. The slab in semi-finished form can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0079] The above steel, for example, a slab plate, is reheated at a Slab Reheating Temperature (SRT) of 1,150°C to 1,300°C for 1 to 5 hours. Through this reheating, it is completely transformed into a single austenite phase, and the components segregated during casting are re-dissolved and the precipitates are re-dissolved, thereby homogenizing the steel and making it ready for hot rolling. If the reheating temperature is lower than 1,150°C, the components segregated during casting may not be sufficiently re-dissolved and may not be evenly distributed. If the reheating temperature exceeds 1,300°C, the austenite grains may coarsen, which may cause a decrease in the yield strength. In addition, as the reheating temperature increases, there are problems such as increased manufacturing costs and decreased productivity due to heating costs and additional time required to match the hot rolling temperature. If the reheating time is less than 1 hour, the reduction of segregation zone may not be sufficient, and if it exceeds 5 hours, the grain size may increase and the process cost may increase.

[0080] The reasons for limiting the above reheating temperature range are as follows.

[0081] Copper has a melting point of 1084.6℃, which is relatively low compared to iron. When copper exists within 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 can melt and infiltrate along the grain boundaries of the steel, causing hot shortness, which reduces ductility and causes cracks. One way to prevent this hot shortness is to add nickel to form a solid solution of copper and nickel, thereby inhibiting copper melting. Therefore, the temperature of the Cu-Ni solid solution must be at least 1150℃ to prevent liquid copper from infiltrating the steel sheet during reheating. Furthermore, to suppress the increased cost associated with nickel addition, the upper limit of the melting point of the Cu-Ni solid solution is set at 1300℃ or lower. The melting point of the Cu-Ni solid solution can be calculated using the ThermoCalc program.

[0082] Next, the reheated steel is first heated to adjust its shape and then hot-rolled. The hot-rolling can be performed sequentially through width rolling, rough rolling, and finish rolling. Through the hot-rolling step, the steel can be formed into hot-rolled steel. The hot-rolled steel can be a hot-rolled steel sheet.

[0083] The above-mentioned rough rolling is a step of rolling the slab to manufacture a bar, and can be performed in a range of 1000°C from the above-mentioned reheating end temperature (1,150°C to 1,300°C).

[0084] The above-described rolling process may be completed at a finish rolling temperature (FRT) of 800°C to 1,000°C. If the finish rolling temperature is lower than 800°C, the rolling load may increase rapidly, resulting in a decrease in productivity. If the finish rolling temperature exceeds 1,000°C, the grains may become coarser, resulting in a decrease in the strength of the final steel.

[0085] Next, the hot-rolled steel is cooled to a predetermined coiling temperature. The cooling can be performed by 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 beneficial for reducing the average grain size. The cooling is preferably performed to a coiling temperature of, for example, 400°C to 700°C, or, for example, 500°C to 650°C.

[0086] Next, the hot-rolled steel sheet is coiled at a coiling temperature (CT) of, for example, 300°C to 700°C, for example, 500°C to 650°C. If the coiling temperature is less than 300°C, the shape of the coiled hot-rolled coil may become non-uniform, and the strength may increase, which may increase the rolling load during cold rolling. If the coiling temperature exceeds 700°C, a non-uniform microstructure may occur due to a difference in cooling speed between the center and edge of the steel sheet, and defects may occur in the subsequent process due to surface oxidation, etc. The coiled steel material may be cooled to room temperature.

[0087] Cold rolled steel sheet manufacturing stage (S120)

[0088] In the cold-rolled steel sheet manufacturing step (S120), the hot-rolled steel sheet is used to adjust the thickness of the final steel sheet. The coiled hot-rolled steel sheet is pickled with acid. Next, the pickled hot-rolled steel sheet is cold-rolled at a cold reduction ratio of, for example, 35% or more, for example, 35% to 70%, to form a cold-rolled steel sheet. The higher the reduction ratio, the higher the hydrogen embrittlement resistance due to the grain refinement effect. If the cold reduction ratio is less than 35%, it is difficult to obtain a uniform microstructure, and since the amount of nuclei generated for recrystallization during annealing is small, the grains may grow excessively during the annealing heat treatment described later, which may rapidly reduce the strength. If the cold reduction ratio exceeds 70%, the amount of nuclei generated is excessively large, and the grains formed by the annealing heat treatment may be too fine, which may reduce ductility and deteriorate formability.

[0089] Annealing heat treatment step (S130)

[0090] 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.

[0091] The above annealing heat treatment is performed by heating to a temperature higher than, for example, Ac3-50 temperature, for example, 800°C to 950°C, at a heating rate of, for example, 1°C / sec or more, for example, 1°C / sec to 10°C / sec, and then maintaining the temperature for, for example, 10 to 600 seconds. If the above annealing heat treatment temperature is less than 800°C or the maintaining 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. If the above annealing heat treatment temperature exceeds 950°C or the maintaining time exceeds 600 seconds, the size of the austenite grains may become coarser or the productivity may be excessively reduced.

[0092] The above Ac3 temperature can be calculated by the following formula.

[0093] Ac3 = 910 - 203×[C] 0.5 -30[Mn] +44.7[Si] +31.5[Mo] -15.2[Ni]

[0094] Here, [C] is the carbon content (weight %) in the steel, [Mn] is the manganese content (weight %) in the steel, [Si] is the silicon content (weight %) in the steel, [Mo] is the molybdenum content (weight %) in the steel, and [Ni] is the nickel content (weight %) in the steel.

[0095] 1st cooling stage (S141)

[0096] In the first cooling step (S141), the annealed heat-treated steel plate can be cooled at a cooling rate of, for example, 0°C / sec to 50°C / sec.

[0097] Hot dip galvanizing step (S145)

[0098] Afterwards, heat treatment is performed, for example, at a temperature of less than 450°C to 600°C, for example, by maintaining it for 5 to 100 seconds. At this time, simultaneously with the heat treatment, a hot-dip galvanizing step (S145) is performed in which the cold-rolled steel sheet is immersed in a molten zinc plating bath under the above-described temperature conditions, for example, less than 450°C to 600°C, to form a hot-dip galvanized layer. The temperature of the plating bath may vary depending on the type and ratio of alloy elements for forming the plating layer and the composition of the cold-rolled steel sheet, and may be, for example, less than 450°C to 600°C. That is, after the cold-rolled steel sheet is heated to a temperature of less than 450°C to 600°C, it is immersed in the plating bath and maintained for, for example, 5 to 100 seconds. Under the plating bath conditions, a hot-dip galvanized layer can be easily formed on the surface of the cold-rolled steel sheet, and the adhesion of the plating layer can be excellent.

[0099] If the subsequent alloying step (S151) is not performed after the hot-dip galvanizing step (S145), the steel sheet that has completed plating and exited the plating bath is immediately subjected to a second cooling step (S161) under the same process conditions, thereby transforming austenite into martensite in this step. Thereafter, a tempering step (S170) is performed, and since the tempering heat treatment step has already been described above, its description is omitted here to avoid duplication.

[0100] Alloying stage (S151)

[0101] If necessary, an alloying step (S151) for alloying heat treatment of the cold-rolled steel sheet on which the above-described hot-dip galvanized layer has been formed may be further performed. In order for the above-described alloying step (S151) to be performed, the steel sheet that has been plated and exited the plating bath is placed in a heat treatment device to perform the alloying heat treatment, thereby stably growing the hot-dip galvanized layer and ensuring excellent adhesion of the plating layer.

[0102] Secondary cooling stage (S161)

[0103] In the secondary cooling step (S161), the cold-rolled steel sheet is then subjected to secondary cooling, for example, from room temperature (0°C to 40°C) to 350°C, at a cooling rate of, for example, 1°C / second or more.

[0104] Tempering stage (S170)

[0105] In the tempering step (S170), the secondarily cooled cold-rolled steel sheet is heated at a heating rate of 100°C / sec or less, and heat-treated at a temperature of, for example, 100°C to 350°C for a holding time of, for example, 10 seconds or more.

[0106] Such tempering heat treatment can promote the formation and growth of transition carbides and the growth of cementite. The tempering heat treatment can be performed for a long time at low temperatures, for example, below 200°C, but at high temperatures, for example, above 200°C, the tempering heat treatment should be performed for a relatively short time to suppress the growth of cementite, thereby controlling the decrease in hydrogen embrittlement resistance.

[0107] By performing the above-described steps, the ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance according to the embodiment of the present invention may be a galvanized steel sheet (GA: Galva Annealed).

[0108] Galvanized steel sheet (GA: Galva Annealed) is a product developed to improve weldability and provide good paintability with simple pretreatment equipment. It is a hot-dip galvanized steel sheet with a zinc and iron plating layer formed through special heat treatment.

[0109] Conventional hot-dip galvanized steel sheets (GI: Galvanized Iron) or electro-galvanized steel sheets (EG: Electro Galvanized) develop cracks when immersed due to the large amount of hydrogen that penetrates during the plating process, whereas hot-dip galvanized steel sheets (GA: Galva Annealed) have the characteristic that the amount of hydrogen that penetrates when immersed in a molten zinc plating bath to form a molten zinc plating layer during the plating process is significantly lower, and thus cracks do not develop during immersion.

[0110] Accordingly, hot-dip galvanized steel (GA) has excellent plating adhesion, so it has less plating peeling during bending and folding and has excellent corrosion resistance.

[0111]

[0112] Experimental example

[0113] Below, preferred experimental examples are presented to aid understanding of the present invention. However, these examples are provided solely to aid understanding of the present invention and are not intended to limit the present invention. Any details not described herein are technically feasible for those skilled in the art, and therefore, their description will be omitted.

[0114] Steel having the composition (unit: wt%) shown in Tables 1 and 2 below was prepared, and cold-rolled steel sheets according to examples and comparative examples were prepared through predetermined hot-rolling and cold-rolling processes and heat treatment processes. The remainder in Tables 1 and 2 consists of iron (Fe) and impurities inevitably contained during the steelmaking process, etc. The unit of content of each component is wt%. In this case, the examples and comparative examples described in Tables 1 and 2 are all limited to steel types in which Cu is added in a content of 0.05 to 0.40 wt%.

[0115] Classification CSiMnCrMoNiCuExample 10.2440.1071.9190.4150.2100.0990.093Example 20.1950.0601.5060.3330.3870.0220.190Comparative Example 10.2630.5921.4820.4090.2080.1150.088Comparative Example 20.2850.4121.0120.1460.0000.1990.088

[0116] ClassificationTiNbVBSi+NiMn+Cr0.85 - 0.23Х([Mn]+[Cr])Relationship satisfactionExample 10.0380.0010.0010.00260.2062.3340.313OKExample 20.0310.022 0.000 0.00290.0821.8390.427OKComparative Example 10.0310.0210.010 0.00270.7071.8910.415NGComparative Example 20.0270.030 0.000 0.00210.6111.1580.584NG

[0117] Referring to Tables 1 and 2, the examples satisfy the composition range and relationship [Si]+[Ni] ≤ 0.85 - 0.23×([Mn]+[Cr]) of the present invention. On the other hand, the comparative examples differ in that they do not satisfy the above relationship between the composition ranges.

[0118] Table 3 shows the manufacturing process conditions of ultra-high-strength cold-rolled steel sheets with excellent hydrogen embrittlement resistance in comparative examples and examples, and the results of measuring the fracture time based on the hydrogen embrittlement test method.

[0119] Annealing heat treatment temperature (℃)Second cooling end temperature (℃)Tempering temperature (℃)Tempering time (sec)Hydrogen embrittlement test standard fracture time (hr)[Applied stress, MPa]Example 1840Room temperature15043,200No fracture [100%]Example 2860120250120No fracture [100%]Comparative example 1860Room temperature18021,60072 [100%]Comparative example 286015025012072 [100%]

[0120] In the manufacturing process of the examples and comparative examples, conditions other than those described in Table 3, for example, hot rolling and cold rolling process conditions, were all manufactured identically to satisfy the process conditions of the present invention.

[0121] Referring to Table 3, the examples and comparative examples satisfied the process conditions of the present invention in terms of annealing heat treatment temperature, secondary cooling end temperature, tempering temperature, and tempering time.

[0122] However, referring to Table 3, no fracture occurred in the examples under the condition that a load of 300% of the yield strength was applied for more than 100 hours. Therefore, it can be seen that the examples have excellent resistance to hydrogen embrittlement. On the other hand, the comparative examples suffered fracture in less than 100 hours under the condition that a load of 60% to 100% of the yield strength was applied. This can be seen that the comparative examples are vulnerable to hydrogen embrittlement because they do not satisfy the composition range of the present invention and the relationship [Si]+[Ni] ≤ 0.85 - 0.23×([Mn]+[Cr]).

[0123] When the tempering is performed at a relatively high temperature, the carbides grow very actively in the longitudinal direction, so the yield strength and tensile strength of the steel may decrease, and the carbides may act as a starting point for fracture when hydrogen penetrates, so the steel may become susceptible to hydrogen embrittlement. On the other hand, when the tempering is performed at a relatively low temperature, it is easier to control the heat treatment for the growth of the carbides, so it is easy to secure the desired strength and also to suppress fracture due to hydrogen embrittlement.

[0124] It will be apparent to a person 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 attached drawings, and that various substitutions, modifications, and changes are possible within a scope that does not depart from the technical idea of ​​the present invention.

Claims

1. In weight%, carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): more than 0% to 3.0%, molybdenum (Mo): more than 0% to 1.0%, nickel (Ni): more than 0% 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): more than 0% to 0.02%, sulfur (S): 0% to 0.01%, and the remainder contains iron (Fe) and other unavoidable impurities. Including a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface, Tensile strength (TS): 1100 MPa or more, elongation (EL): 3% or more, and hydrogen embrittlement test method standard non-breakage time: 100 hours or more, Ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance.

2. In paragraph 1, The above ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance is An ultra-high strength cold rolled steel sheet having excellent hydrogen embrittlement resistance, wherein the sum of the weight % of the silicon and nickel [Si]+[Ni] and the sum of the weight % of the manganese and chromium [Mn]+[Cr] satisfy the following relationship. [Si]+[Ni] ≤ 0.85 - 0.23×([Mn]+[Cr]) 3. In paragraph 1, The microstructure of the above ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance includes ferrite, tempered bainite and tempered martensite phases. The area fraction of the above ferrite is in the range of 0% to 20%, the area fraction of the above tempered bainite is in the range of 5% to 20%, and the area fraction of the above tempered martensite is in the range of 60% to 100%. The sum of the above tempered bainite and tempered martensite satisfies the range of 80% to 100%. Ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance.

4. In paragraph 1, The above ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance further contains carbides, The above carbide has an average size of less than 100 nm and an aspect ratio of less than 5. Ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance.

5. In paragraph 4, The above carbide comprises at least one of Fe-based carbide, Ti-based carbide, Nb-based carbide, V-based carbide, and Mo-based carbide. Ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance.

6. In weight%, carbon (C): 0.1% to 0.5%, silicon (Si): 0.01% to 2.0%, manganese (Mn): 0.1% to 5.0%, aluminum (Al): 0.01% to 2.0%, chromium (Cr): more than 0% to 3.0%, molybdenum (Mo): more than 0% to 1.0%, nickel (Ni): more than 0% 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): more than 0% to 0.02%, sulfur (S): A step of manufacturing a hot-rolled steel sheet by hot-rolling a steel material containing more than 0% to 0.01% of iron (Fe) and the remainder being unavoidable impurities; A step of manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet; A step of annealing the cold rolled steel sheet by maintaining it at a temperature of 800°C to 950°C for 60 to 600 seconds; A step of first cooling the above-mentioned annealed cold rolled steel sheet at a cooling rate of 0℃ / sec to 50℃ / sec; A step of heat-treating the first-cooled cold-rolled plate by maintaining it at a temperature of 450°C to 600°C for 5 to 100 seconds and simultaneously performing molten zinc plating; A step of secondary cooling the above-mentioned hot-dip galvanized cold-rolled steel sheet from 0℃ to 350℃ at a cooling rate of 1℃ / sec or more; Including a tempering step of heat-treating the secondarily cooled cold-rolled steel sheet at a temperature of 100°C to 350°C for 10 seconds or longer, The above heat treatment step A method for manufacturing ultra-high strength cold rolled steel sheets having excellent hydrogen embrittlement resistance.

7. In paragraph 6, A step of alloying and heat-treating the above-mentioned hot-dip galvanized cold-rolled steel sheet; further comprising; A method for manufacturing ultra-high strength cold rolled steel sheets having excellent hydrogen embrittlement resistance.

8. In paragraph 6, The ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance manufactured by the above method for manufacturing the ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance is Tensile strength (TS): 1100 MPa or more, elongation (EL): 3% or more, and hydrogen embrittlement test method standard non-breakage time: 100 hours or more, The microstructure of the above ultra-high strength cold rolled steel sheet having excellent hydrogen embrittlement resistance includes ferrite, tempered bainite and tempered martensite phases, and the area fraction of the ferrite is in the range of 0% to 20%, the area fraction of the tempered bainite is in the range of 5% to 20%, and the area fraction of the tempered martensite is in the range of 60% to 100%, and the sum of the tempered bainite and the tempered martensite satisfies the range of 80% to 100%. A method for manufacturing ultra-high strength cold rolled steel sheets having excellent hydrogen embrittlement resistance.

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