Ultra-high strength galvanized steel sheet and method for manufacturing the same

Abstract

This invention provides an ultra-high strength galvanized steel sheet, comprising: a steel sheet and a coating on the steel sheet, the steel sheet consisting of an inner layer and a surface layer on the inner layer, wherein the surface layer has a thickness of 10 μm to 50 μm from the surface of the steel sheet, the inner layer comprising, by weight, 0.1% to 0.3% C, 1.0% to 2.0% Si, 2.5% to 3.5% Mn, greater than 0% and not greater than 0.02% P, greater than 0% and not greater than 0.01% S, and the balance iron (Fe), the area fraction of tempered martensite in the final microstructure of the inner layer being at least 50%, and the surface layer having a lower carbon content than the inner layer, and comprising, by weight, 1.0% to 2.0% Si and 2.5% to 3.5% Mn. The surface layer contains 0% to 0.02% P, 0% to 0.01% S, and the balance iron (Fe). The area fraction of tempered martensite in the final microstructure of the surface layer is 10% or less, and when grain boundaries with an orientation difference of 15° to 180° in the final microstructure of the steel sheet are classified as large-angle grain boundaries, the ratio of the area fraction of large-angle grain boundaries on the surface to the area fraction of large-angle grain boundaries at a point measuring 1 / 4 of the steel sheet thickness from the surface is 1.2 or greater and less than 1.4.

Description

Technical Field

[0001] This invention relates to a steel plate and its manufacturing method, and more specifically to an ultra-high strength coated steel plate and its manufacturing method. Background Technology

[0002] The automotive industry has consistently focused on improving fuel efficiency and reducing weight to address the demands of the times, such as resource depletion, rapid global warming, and high oil prices. Furthermore, the increasing number of regulations related to passenger safety is driving a growing demand for ultra-high-strength steel. In addition, various improvements are needed for coated steel sheets with excellent sacrificial corrosion resistance (where zinc, with its lower potential, preferentially dissolves when exposed to corrosive environments, thus preventing corrosion of the steel).

[0003] For example, there is an increasing need for improvements to address the problem of liquid metal embrittlement (LME), in which the plating melts during resistance spot welding in automotive assembly lines, and the molten zinc penetrates into the interface of residual austenite present in the surface layer of the steel sheet, leading to embrittlement.

[0004] Specifically, during the spot welding process of ultra-high strength hot-dip galvanized steel sheets, in the case of galvanized steel (GI), the melting point of the coating is as low as 420°C. Even in the case of galvanized annealed steel sheets, liquid zinc may form due to peritectic reactions near 880°C. The formed liquid zinc may penetrate along the grain boundaries of the matrix material in the region where the welding electrode generates a load at high temperatures, and the strength of the steel sheet (which is the matrix material) may decrease rapidly due to heat. Around LME cracks, an austenite-to-αFe(Zn) phase transformation occurs at high temperatures due to zinc diffusion towards the matrix material side, and the αFe(Zn) phase further accelerates embrittlement. The sensitivity increases with the amount of austenite phase. The frequency of liquid metal embrittlement (LME) may increase with the increase of silicon content in the material or martensitic microstructure.

[0005] As a relevant prior art document, Korean Patent Application No. 20200075949A is known. Summary of the Invention

[0006] Technical issues This invention provides an ultra-high strength coated steel sheet with improved liquid metal embrittlement and its manufacturing method.

[0007] However, the aforementioned problems are exemplary, and the technical ideas of the present invention are not limited thereto.

[0008] Technical solution According to one aspect of the present invention, an ultra-high strength coated steel sheet is provided. The ultra-high strength coated steel sheet comprises: a steel sheet and a coating on the steel sheet, the steel sheet comprising an inner layer portion and a surface layer portion on the inner layer portion, wherein the surface layer portion has a thickness of 10 μm to 50 μm measured from the surface of the steel sheet, wherein the inner layer portion comprises, by weight %, C: 0.1% to 0.3%, Si: 1.0% to 2.0%, Mn: 2.5% to 3.5%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities, wherein the area fraction of tempered martensite in the final microstructure of the inner layer portion is 50% or greater, wherein the carbon content of the surface layer portion is lower than that of the inner layer portion, and the surface layer portion comprises, by weight %, Si: 1.0% to 2.0%, Mn: 2.5% to 3.5% 0.5%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, balance iron (Fe) and other unavoidable impurities, wherein the area fraction of tempered martensite in the final microstructure of the surface layer is 10% or less, and wherein when grain boundaries with orientation difference angles of 15° to 180° in the final microstructure of the steel sheet are classified as large-angle grain boundaries, the ratio of the area fraction of large-angle grain boundaries at the surface to the area fraction of large-angle grain boundaries at a point corresponding to one-quarter of the thickness (t) of the steel sheet from the surface is 1.2 or greater and less than 1.4.

[0009] Coated steel sheets may have a yield strength (YS) greater than or equal to 850 MPa and less than or equal to 1070 MPa, a tensile strength (TS) greater than or equal to 1180 MPa and less than 1470 MPa, and an elongation of 14% or greater.

[0010] In ultra-high strength coated steel sheets, the carbon content of the surface layer can be 10% or less of the carbon content of the inner layer.

[0011] In ultra-high strength coated steel sheets, the final microstructure of the inner layer may comprise, in area fraction, 50% to 75% tempered martensite, 5% to 25% ferrite, and 10% to 30% retained austenite, and the final microstructure of the surface layer may comprise, in area fraction, greater than 0% and not greater than 10% tempered martensite, and greater than or equal to 90% and less than 100% ferrite.

[0012] When spot welding is performed on coated steel sheets under conditions of a 6 mm welding electrode tip diameter, an electrode force of 3.5 kN, and a welding current of 6.0 kA to 7.5 kA, the maximum length of liquid metal embrittlement (LME) cracks in the steel sheet can be 50 μm or less.

[0013] According to another aspect of the present invention, a method for manufacturing ultra-high strength coated steel sheet is provided. The method includes: a first step of providing a steel sheet comprising an inner layer portion and a surface layer portion located on the inner layer portion; and a second step of forming a coating on the steel sheet, wherein the inner layer portion comprises, by weight %, C: 0.1% to 0.3%, Si: 1.0% to 2.0%, Mn: 2.5% to 3.5%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities, wherein the area fraction of tempered martensite in the final microstructure of the inner layer portion is 50% or greater, and wherein the carbon content of the surface layer portion is lower than that of the inner layer portion, the surface layer portion comprising, by weight %, 0.1% to 0.3%, Si: 1.0% to 2.0%, Mn: 2.5% to 3.5%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, ... The material comprises, by weight %: Si: 1.0% to 2.0%, Mn: 2.5% to 3.5%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities, wherein the area fraction of tempered martensite in the final microstructure of the surface layer portion is 10% or less, and wherein when grain boundaries with orientation difference angles of 15° to 180° in the final microstructure of the steel sheet are classified as large-angle grain boundaries, the ratio of the area fraction of large-angle grain boundaries at the surface to the area fraction of large-angle grain boundaries at a point corresponding to one-quarter of the steel sheet thickness (t) from the surface is 1.2 or greater and less than 1.4.

[0014] Coated steel sheets may have a yield strength greater than or equal to 850 MPa and less than or equal to 1070 MPa, a tensile strength greater than or equal to 1180 MPa and less than 1470 MPa, and an elongation of 14% or greater.

[0015] In a method for manufacturing ultra-high strength coated steel sheets, a first step may sequentially include: providing steel material comprising, by weight percent: C: 0.1% to 0.3%, Si: 1.0% to 2.0%, Mn: 2.5% to 3.5%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities; hot-rolling the steel material; cold-rolling the hot-rolled steel material; annealing the cold-rolled steel material in an annealing furnace; slowly cooling the annealed steel material at a first cooling rate; rapidly cooling the slowly cooled steel material at a second cooling rate greater than the first cooling rate; and reheating the rapidly cooled steel material, wherein in the annealing step, the partial pressure of H2O in the annealing furnace is controlled to be greater than or equal to 0.001 atm and less than 0.023. The atm annealing temperature range is controlled at a lower limit of 30°C lower than the Ac3 temperature of the inner layer and at an upper limit of the annealing temperature range of the surface layer. The slow cooling step is performed at a first cooling rate of 1°C / s to 10°C / s and a cooling end temperature of greater than or equal to 600°C and less than 800°C. The rapid cooling step is performed at a second cooling rate of 30°C / s to 100°C / s and a cooling end temperature of greater than or equal to 150°C and less than 300°C. The reheating step is performed at a temperature of greater than 350°C and less than 500°C and a holding time of 60 seconds or less.

[0016] In a method for manufacturing ultra-high strength coated steel sheets, the carbon content of the surface layer portion is 10% or less of the carbon content of the inner layer portion, and the surface layer portion may have a thickness of 10 μm to 50 μm from the surface of the steel sheet.

[0017] In a method for manufacturing ultra-high strength coated steel sheets, after an annealing step, the steel sheet is divided into an inner layer portion having different carbon contents and a surface layer portion located on the inner layer portion. The microstructure of the inner layer portion may contain, in area fractions, 75% to 95% austenite and 5% to 25% austenite, and the microstructure of the surface layer portion may contain, in area fractions, greater than 0% and not greater than 10% austenite and greater than or equal to 90% and less than 100% austenite. After a slow cooling step, the microstructure of the inner layer portion may contain, in area fractions, 75% to 95% austenite and 5% to 25% austenite, and the microstructure of the surface layer portion may contain, in area fractions, greater than 0% and not greater than 10% austenite and greater than or equal to 90% and less than 100% austenite. Less than 100%; after the rapid cooling step, the microstructure of the inner layer portion may, in area fraction, contain martensite: 50% to 75%, ferrite: 5% to 25%, and retained austenite: 10% to 30%, and the microstructure of the surface layer portion may, in area fraction, contain martensite: greater than 0% and not greater than 10%, and ferrite: greater than or equal to 90% and less than 100%; and after the reheating step, the final microstructure of the inner layer portion may, in area fraction, contain tempered martensite: 50% to 75%, ferrite: 5% to 25%, and retained austenite: 10% to 30%, and the final microstructure of the surface layer portion may, in area fraction, contain tempered martensite: greater than 0% and not greater than 10%, and ferrite: greater than or equal to 90% and less than 100%.

[0018] In the method for manufacturing ultra-high strength coated steel sheets, the hot rolling step can be carried out under conditions of a slab reheating temperature of 1150°C to 1250°C, a finishing rolling temperature of 850°C to 1000°C, and a coiling temperature of 500°C to 700°C, and the cold rolling step can be carried out with a reduction rate of 40% to 60%.

[0019] Beneficial effects According to the present invention, an ultra-high strength coated steel sheet with excellent weldability and a method for manufacturing the same can be provided.

[0020] The above-described effects of the invention have been exemplarily described, and the scope of the invention is not limited by these effects. Attached Figure Description

[0021] Figure 1 A flowchart illustrating a method for manufacturing ultra-high strength coated steel sheets according to an embodiment of the present invention is provided.

[0022] Figure 2The figure illustrates the subsequent heat treatment steps (annealing, cooling, and reheating) applied to cold-rolled steel sheets in a method for manufacturing ultra-high strength coated steel sheets with excellent weldability according to an embodiment of the invention.

[0023] Figure 3 The view is shown schematically of the process of forming an ultra-high strength coated steel sheet in a method for manufacturing an ultra-high strength coated steel sheet with excellent weldability according to an embodiment of the present invention.

[0024] Figure 4 A view illustrating, schematically, an overview of the decarburization reaction in a method for manufacturing ultra-high strength coated steel sheet according to an embodiment of the present invention.

[0025] Figure 5 A photograph showing the state of liquid metal embrittlement cracks in a coated steel sheet, which is a comparative embodiment of the present invention.

[0026] Figure 6 To illustrate the first experimental embodiment of the invention, the view is provided by using the EBSD analysis method at the surface of the steel plate and at points corresponding to one-quarter of the steel plate thickness (t) measured from the surface in the final microstructure to classify grain boundaries into large-angle grain boundaries (when the orientation difference angle in the final microstructure is 15° to 180°) and small-angle grain boundaries (when the orientation difference angle is less than 15°).

[0027] Figure 7 This is a view showing the results of applying the spot welding process in the second experimental embodiment.

[0028] Figure 8 The square root A of the annealing time (A) in the annealing furnace shown in the third experimental embodiment. 1 / 2 A view of the product of the natural logarithm of the reciprocal of the water concentration (B), ln(1 / B).

[0029] Figure 9 A view showing the thickness of the decarburized layer formed in the surface layer of the steel plate according to the annealing time (A) and moisture concentration (B) in the annealing furnace according to the third experimental embodiment.

[0030] Figure 10 A view showing the incidence of liquid metal embrittlement (LME) in the annealing furnace according to the third experimental embodiment, with annealing time (A) and moisture concentration (B). Detailed Implementation

[0031] Preferred embodiments of the invention will be described in detail below 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. Various other modifications may be made to the following embodiments, and the scope of the technical concept of the invention is not limited to these embodiments. Rather, these embodiments are provided to make this application 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.

[0032] Figure 1 A flowchart illustrating a method for manufacturing ultra-high strength coated steel sheets according to an embodiment of the present invention is provided.

[0033] refer to Figure 1 According to an embodiment of the present invention, a method for manufacturing ultra-high strength coated steel sheet includes: providing steel material (S10), hot rolling the steel material to form a hot-rolled steel sheet (S20), cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet (S30), annealing the cold-rolled steel sheet (S40), slowly cooling the annealed steel sheet at a first cooling rate (S50), rapidly cooling the slowly cooled steel sheet at a second cooling rate greater than the first cooling rate (S60), reheating the cooled steel sheet (S70), and galvanizing the steel sheet (S80).

[0034] According to the technical concept of the present invention, in the step (S40) of annealing the cold-rolled steel sheet in an annealing furnace, the dew point temperature of the annealing furnace is controlled within a predetermined high dew point range relative to the prior art, thereby inducing a decarburization reaction in the surface layer portion including the surface of the steel sheet within the predetermined range, resulting in different carbon contents in the surface layer portion and the inner layer portion. Therefore, the Ac3 temperatures of the surface layer portion and the inner layer portion become different from each other, causing the phase transformation behavior of the surface layer portion and the inner layer portion to proceed differently even within the predetermined annealing temperature range. As a result, the martensite fraction in the final microstructure of the surface layer portion and the inner layer portion exhibits different characteristics, thereby minimizing or suppressing the possibility of liquid metal embrittlement (LME) (whereby the coating melts and penetrates to the surface of the steel sheet during welding, leading to brittleness), while providing an ultra-high strength coated steel sheet.

[0035] The following describes in detail a method for manufacturing ultra-high strength coated steel sheets with excellent weldability according to embodiments of the present invention.

[0036] Provide steel materials (S10) Recently, there has been increased interest in lightweight car bodies using ultra-high strength steel in the automotive industry to simultaneously meet crash safety and fuel efficiency regulations. Within the steel industry, research has been actively conducted on the development of ultra-high strength steel materials to address this demand from automakers. Q&P (Quenching and Partitioning) heat treatment technology (developed to simultaneously ensure high strength and high ductility in automotive steel materials) is one such technology. It suppresses the formation of carbide precipitates from carbon expelled from the martensitic structure during quenching and enables carbon to diffuse into the retained austenitic structure through partitioning. Through this carbon re-diffusion, the retained austenitic structure is stabilized even at room temperature, ultimately ensuring both the high ductility resulting from the retained austenitic structure and the high strength resulting from the martensitic structure.

[0037] Compared to ordinary steel, this Q&P steel sheet contains a large amount of silicon (Si), which interferes with the movement of iron (Fe) atoms, thereby inhibiting the formation of carbide precipitates in the structure. In addition, in order to increase the volume fraction of the stabilized residual austenite structure and thus improve TRIP (transformation-induced plasticity) behavior, Q&P steel sheet contains a large amount of austenite-stabilizing alloying elements (such as carbon (C) and manganese (Mn)).

[0038] At the same time, it is obvious that the technical concept of the present invention can be applied to the above-mentioned Q&P steel plate, but the technical concept of the present invention can be widely applied to various steel plates, and is not limited to Q&P steel plates.

[0039] In the following, exemplary roles and contents of the components contained in the steel sheet (base material) constituting the ultra-high strength coated steel sheet according to an embodiment of the present invention will be described. In this context, all contents of the component elements refer to weight%.

[0040] Carbon (C): 0.1% to 0.3% Carbon is the most important alloying element in steelmaking, and its primary purpose is basic strengthening and austenite stabilization. A high carbon concentration in austenite improves its stability and helps ensure an appropriate amount of austenite, thereby improving material properties. When the carbon content is less than 0.1%, it is difficult to ensure the desired yield strength and elongation. On the other hand, when the carbon content is greater than 0.3%, weldability may decrease due to the increased carbon equivalent. Therefore, based on the total weight of the steel plate, the carbon content is preferably between 0.1% and 0.3%.

[0041] Silicon (Si): 1.0% to 2.0% Silicon is an element that inhibits the formation of carbides (such as Fe3C) in ferrite and increases the activity of carbon, thereby increasing the diffusion rate of austenite. Silicon is also a well-known ferrite stabilizing element and is considered an element that increases the ferrite fraction during cooling to improve ductility. When the silicon content is less than 1.0%, the effect of adding silicon is insufficient. On the other hand, when the silicon content is greater than 2.0%, oxides (SiO2) will form on the surface of the steel sheet during processing, resulting in poor coating properties due to poor wettability in the relevant areas. Therefore, based on the total weight of the steel sheet, the silicon content is preferably 1.0% to 2.0%.

[0042] Manganese (Mn): 2.5% to 3.5% Manganese is an austenite-stabilizing element, and with the addition of manganese, the Ms temperature (the martensitic transformation initiation temperature) gradually decreases, thereby increasing the residual austenite fraction during continuous annealing heat treatment. When the manganese content is less than 2.5%, the effect of adding manganese is insufficient. On the other hand, when the manganese content is greater than 3.5%, weldability is significantly reduced due to the increased carbon equivalent, and oxides (MnO) are formed on the surface of the steel sheet during processing, resulting in poor plating properties due to poor wettability in the relevant areas. Therefore, based on the total weight of the steel sheet, the manganese content is preferably 2.5% to 3.5%.

[0043] Phosphorus (P): greater than 0% and not greater than 0.02% Phosphorus can play a similar role to silicon in steel. However, when phosphorus is added in an amount greater than 0.02% based on the total weight of the steel sheet, the weldability of the steel sheet may decrease and its brittleness may increase, leading to a deterioration of material properties. Therefore, the phosphorus content is preferably limited to greater than 0% and not greater than 0.02% based on the total weight of the steel sheet.

[0044] Sulfur (S): greater than 0% and not greater than 0.01% Sulfur is an unavoidable element in steelmaking. Sulfur impairs the toughness and weldability of steel and, by combining with manganese (Mn) to form MnS, deteriorates the steel's corrosion resistance and impact properties. Therefore, based on the total weight of the steel plate, the sulfur content is preferably limited to greater than 0% and not greater than 0.01%.

[0045] The remaining component of the steel sheet (which is the base material for ultra-high strength coated steel sheet) is iron (Fe). However, in conventional steelmaking processes, unintended impurities may inevitably be introduced from raw materials or the surrounding environment, and therefore cannot be eliminated. Since these impurities are understood by those skilled in the art of general manufacturing processes, not all of their details are specifically described in this specification.

[0046] In addition to the above-mentioned components, the ultra-high strength coated steel sheet according to embodiments of the present invention may optionally contain any combination of the following components.

[0047] Boron (B): greater than 0% and not greater than 0.001% Boron can be selectively added to steel sheets and can act as a grain boundary strengthening element. When added in amounts greater than 0.001%, nitrides (e.g., BN) may form, thereby reducing high-temperature ductility. Therefore, based on the total weight of the matrix material, boron can be added in amounts greater than 0% and not greater than 0.001%.

[0048] Titanium (Ti): greater than 0% and not greater than 0.05% Titanium is a major element that precipitates in steel in the form of carbides. In this invention, titanium ensures the stability of residual austenite and improves strength by refining the initial austenite grains resulting from precipitate formation, and provides precipitation hardening due to ferrite grain refinement and the presence of precipitates in ferrite. That is, titanium is an element that precipitates as TiC or TiN by combining with carbon (C) or nitrogen (N), or an element that improves the strength of steel sheets through solid solution strengthening in iron (Fe). For example, titanium is effective in improving strength by forming carbonitrides or sulfides, and is an element that can suppress the formation of boron nitride (BN) by precipitating as titanium nitride (TiN) by combining with nitrogen. When titanium is added in an amount greater than 0.05% throughout the matrix material, manufacturing costs increase, excessive precipitates are formed in the ferrite phase, resulting in excessive precipitation strengthening, and the elongation of the steel sheet may decrease. Furthermore, low-temperature toughness and weldability may deteriorate due to the large amount of precipitates. Therefore, the content is limited to 0.05% or less.

[0049] Chromium (Cr): greater than 0% and not greater than 1.0% Chromium is an element with high hardenability and is added to improve strength through transformation strengthening. However, when chromium is present in amounts greater than 1.0 wt%, it forms structures such as upper bainite and leads to overall inhomogeneity, thereby reducing toughness. Therefore, the chromium content is preferably controlled to 1.0 wt% or less.

[0050] Molybdenum (Mo): greater than 0% and not greater than 1.0% Molybdenum is an element with a higher hardenability than chromium and is added to improve strength through transformation strengthening. When the molybdenum content is greater than 1.0 wt%, a large amount of hard second phase (e.g., martensite / austenite (MA)) will form within the carbon content range of this invention, thereby reducing toughness. Therefore, the molybdenum content is preferably controlled to 1.0 wt% or less.

[0051] Nickel (Ni): greater than 0% and not greater than 1.0% Nickel stabilizes austenite and can help improve the hardenability of steel. However, a nickel content greater than 1.0% is undesirable because it increases the manufacturing cost of the steel. Therefore, nickel is preferably added in an amount of 1.0% or less, based on the total weight of the base material.

[0052] In the manufacturing method according to the invention, the semi-finished product subjected to hot rolling and cold rolling can be, exemplarily, a slab. After obtaining molten steel with a predetermined composition through a steelmaking process, a slab in a semi-finished state can be obtained through a continuous casting process.

[0053] Hot rolling step (S20) The hot rolling process is applied to the steel material to perform the step of forming hot-rolled steel sheet (S20). Since the steel material is a high-alloy steel, in order to ensure productivity, edge cracks and rolling loads must be minimized as much as possible. Therefore, the finishing rolling temperature and coiling temperature can be set in a high-temperature range.

[0054] For example, steel can be reheated at a slab reheating temperature (SRT) ranging from 1150°C to 1250°C. This reheating allows for the redissolution of components that segregated during casting and the redissolution of precipitates. Below 1150°C, a rapid increase in hot rolling load may occur. On the other hand, above 1250°C, charging and unloading the furnace may be difficult due to slab warping, and the strength of the final steel sheet may be compromised due to coarsening of the initial austenite grains. The reheating temperature can vary depending on the steel material.

[0055] The reheated steel is then hot-rolled, and this hot rolling can be performed, for example, at a finishing roll exit temperature (FDT) of 850°C to 1000°C. When the finishing roll temperature is above 1000°C, the quality of the steel sheet may deteriorate due to the formation of surface oxide scale. When the finishing roll temperature is below 850°C, it may lead to an increase in rolling load and a decrease in productivity. The finishing roll exit temperature can vary depending on the steel material.

[0056] Subsequently, after cooling the hot-rolled steel material at a cooling rate of 10°C / s to 30°C / s, coiling is performed, for example, at a coiling temperature (CT) ranging from 500°C to 700°C. The coiling temperature can vary depending on the steel material. When the coiling temperature is above 700°C, an undesirable internal oxide layer may be formed in the hot-rolled steel sheet or the coiled hot-rolled coil. Because the coiled hot-rolled coil thus exhibits variations in internal oxidation, it may be difficult to uniformly control the thickness of the internal oxide layer. When the coiling temperature is below 500°C, an undesirable low-temperature structure may form.

[0057] Softening heat treatment step (S25) In the method for manufacturing ultra-high strength coated steel sheets with excellent weldability according to an embodiment of the present invention, a softening heat treatment step and a pickling step can be performed sequentially after the hot rolling process and before the cold rolling process. In the softening heat treatment step, the hot-rolled steel sheet is softened, thereby mitigating the problems of reduced load and shape defects in the subsequent cold rolling process. That is, by softening the hot-rolled steel sheet, the softening heat treatment ensures cold-rollability, thereby improving the efficiency of cold rolling operations. When the hot-rolled steel sheet has high strength, problems such as thickness fluctuations and shape defects may occur during cold rolling. However, since the softening heat treatment process for the ultra-high strength steel according to the present invention is applied to hot-rolled coils with residual oxide scale rather than cold-rolled coils, it is necessary to respond to the changes in surface properties caused by the high-temperature reaction of the oxide scale during the softening heat treatment process.

[0058] Typically, in the case of ultra-high strength steel materials containing large amounts of Si, Mn, etc., it is known that internal oxides are formed along the grain boundaries of the steel sheet at high temperatures, together with the oxide scale. Since the main component of the oxide layer formed by the internal oxides is Fe, its pickling ability is poor. Therefore, even with pickling times equivalent to those for typical hot-rolled steel sheets, the internal oxide layer may not be completely removed, requiring a longer pickling time and resulting in reduced productivity. This internal oxidation occurs when easily oxidized elements (such as Si and Mn) are highly reactive and are present under specific oxygen partial pressure conditions. Therefore, when hot-rolled coils with residual oxide scale are heat-treated in a high-temperature reducing gas atmosphere, additional internal oxides are generated due to the oxygen produced during the oxide scale reduction reaction.

[0059] Specifically, after the softening heat treatment, internal oxides (not observed in hot-rolled materials) are formed unevenly throughout the coil, and the behavior of scale reduction and internal oxide formation may vary depending on the location within the coiled sheet. In the case of the outer coil portion of the softened heat-treated sheet, the growth of internal oxides mainly caused by the hydrogen reduction reaction of the scale is observed, while in the case of the inner coil portion, the growth of internal oxides caused by the eutectoid reaction of the scale (4FeO->4Fe+2O2) is observed. This difference in the growth behavior of internal oxides is attributed to the difference in the permeability of reducing gases depending on the location within the coiled sheet, and it is understood that during the softening heat treatment, oxygen generated in the hydrogen reduction reaction and eutectoid reaction of the scale diffuses into the matrix material and serves as a material for the internal oxidation reaction. Since it is preferable to form an internal oxide layer as uniformly as possible throughout the steel sheet, it is preferable to suppress the internal oxide layer as much as possible during the coiling step and form the internal oxide layer during the softening heat treatment.

[0060] In view of the above, in the method for manufacturing ultra-high strength cold-rolled steel sheet according to the present invention, the coiling temperature is controlled at 500°C to 700°C, and the softening heat treatment temperature is controlled at 500°C to 650°C.

[0061] The softening heat treatment can be performed in a batch annealing furnace (BAF) with the hot-rolled steel sheet in a coiled state, and can be carried out in a hydrogen atmosphere. Under these process conditions, the hot-rolled steel sheet undergoing the softening heat treatment can be softened to ensure cold rollability. Furthermore, the thickness of the internal oxide layer formed by the softening heat treatment is a predetermined thickness or less (e.g., 10 μm or less), thus simultaneously ensuring subsequent pickling capability.

[0062] When softening heat treatment is applied at temperatures below 500°C, the martensite formed after hot rolling does not undergo recrystallization and only tempering, resulting in the formation of supersaturated carbon in the structure, which then spheroidizes into cementite (θ). In this case, martensite embrittlement may occur, potentially leading to safety hazards (e.g., strip fracture) during cold rolling. In other words, when softening heat treatment is performed, excessive austenite is formed, and martensite forms during cooling, making it difficult to effectively exhibit a reduction in strength.

[0063] Furthermore, when softening heat treatment is applied at temperatures above 650°C, the internal oxide layer formed by the softening heat treatment exceeds the predetermined thickness (e.g., greater than 10 μm), making it difficult to ensure subsequent pickling capability. Additionally, when softening heat treatment is applied at temperatures above 650°C, excessive austenite is formed, and martensite forms during cooling, making it impossible to effectively exhibit strength reduction.

[0064] Furthermore, in the method for manufacturing ultra-high strength coated steel sheets with excellent weldability according to embodiments of the present invention, the application of a softening heat treatment step can be selected based on the steel grade or target strength. For example, a softening heat treatment step can be performed when the target tensile strength after cold rolling / annealing is 1180 MPa or greater, and it can be omitted when the target tensile strength after cold rolling / annealing is 980 MPa or less. The softening heat treatment time can be from 1 hour to 12 hours.

[0065] In the pickling process, after softening heat treatment, the hot-rolled steel sheet can be pickled using acid pickling. By pickling the hot-rolled steel sheet, at least a portion of the internal oxide layer can be removed. The pickling process can be carried out, for example, using hydrochloric acid with a concentration of 5% to 15% at a temperature of 70°C to 90°C for 20 to 40 seconds. Furthermore, the inhibitor concentration can be 0.1% to 0.5%.

[0066] Cold rolling step (S30) The cold rolling process is applied to hot-rolled steel sheet to form cold-rolled steel sheet (S30). Cold rolling is performed using hot-rolled material to match the thickness of the final produced steel sheet.

[0067] In the cold rolling step (S30), the pickled hot-rolled steel sheet can be cold-rolled at an average reduction rate of, for example, 40% to 60%, and thus a cold-rolled steel sheet can be manufactured. The microstructure of the cold-rolled steel sheet has the elongated shape of the hot-rolled steel sheet, and the microstructure of the final produced steel sheet is determined in subsequent heat treatment.

[0068] Figure 2 The diagram illustrates the subsequent heat treatment steps (annealing, cooling, and reheating) applied to cold-rolled steel sheets in a method for manufacturing ultra-high-strength coated steel sheets with excellent weldability according to an embodiment of the invention. Figure 3 A view illustrating the process of forming ultra-high strength coated steel sheets in the method. Figure 3 The construction shown in (a) corresponds to Figure 1 S30 in the step shown, Figure 3 The construction shown in (b) corresponds to Figure 1 Steps S40 to S70 shown, and Figure 3 The construction shown in (c) corresponds to Figure 1 S80 in the steps shown.

[0069] Annealing step (S40) refer to Figure 2 and Figure 3 Cold-rolled steel sheets are annealed in a continuous annealing furnace with a slow cooling section.

[0070] In the annealing step (S40), the dew point temperature of the atmosphere gas in the annealing furnace is controlled to be between -20°C and +20°C. The atmosphere gas in the annealing furnace can be, for example, a mixture of 90% nitrogen and 10% oxygen. By injecting pure H2O into the annealing furnace using a humidifier attached to the outer wall of the annealing furnace, the partial pressure of H2O in the annealing furnace can be controlled to be greater than or equal to 0.001 atm and less than 0.023 atm. Furthermore, the dew point of the heat treatment atmosphere gas can be controlled to be between -20°C and +20°C. Through annealing heat treatment with the above-described partial pressure of H2O in the annealing furnace, the cold-rolled steel sheet constituting the ultra-high strength coated steel sheet of the present invention undergoes a decarburization reaction on the surface of the cold-rolled steel sheet and is ultimately formed into a steel sheet comprising an inner layer portion and a surface layer portion located on the inner layer portion. Figure 3 (a) shows A0 corresponding to cold-rolled steel sheet. Figure 3(b) shows A1 corresponding to the inner layer portion constituting the steel sheet, and A2 corresponding to the surface layer portion constituting the steel sheet. The alloy composition of the cold-rolled steel sheet (A0) is the same as that of the inner layer portion (A1) constituting the steel sheet, but the carbon content is different from that of the surface layer portion (A2) constituting the steel sheet. That is, when the partial pressure of H2O in the annealing furnace is maintained at greater than or equal to 0.001 atm and less than 0.023 atm, it is possible to achieve an inner layer portion (A1) and a surface layer portion (A2) with different compositions within the composition range defined in this patent.

[0071] Figure 4 A view illustrating, schematically, an overview of the decarburization reaction in a method for manufacturing ultra-high strength coated steel sheet according to an embodiment of the present invention.

[0072] Even when the annealing temperature of the annealing step is set in the region for austenite phase formation as described above, when the partial pressure of H2O in the annealing furnace is 0.001 atm or greater, carbon (which is an austenite-stabilizing element) is oxidized at the surface of the cold-rolled steel sheet (A0) and volatilizes as carbon monoxide, thus undergoing a decarburization reaction through a continuous reaction. The reaction can be represented by the following chemical formula 1.

[0073] Chemical Formula 1 C(s) + H₂O(g) -> CO(g) + H₂(g) As the decarburization reaction continues, the carbon in the surface layer is depleted, and since the austenite in the surface layer has already transformed into ferrite even during the annealing step, no austenite will form in the surface layer during subsequent cooling / reheating, ensuring a predominantly ferrite single-phase structure in the final stage at room temperature. Due to the ferrite single-phase structure formed in the surface layer, austenite sensitive to liquid metal embrittlement (LME) cracking is avoided, thereby enabling a wider range of weldable currents and improved weld strength.

[0074] Meanwhile, in order to prevent liquid metal embrittlement (LME) cracking in the surface layer portion (A2), the thickness of the surface layer portion (A2) constituting the steel plate needs to be at least 10 μm or greater. When the thickness of the surface layer portion (A2) is less than 10 μm, it is difficult to prevent liquid metal embrittlement (LME) cracking in the surface layer portion (A2).

[0075] When the partial pressure of H2O in the annealing furnace is less than 0.001 atm, the alloy composition of the inner layer (A1) constituting the steel plate cannot be significantly different from the alloy composition of the surface layer (A2), or the surface layer (A2) is formed with a thickness of less than 10 μm, which may not be able to suppress LME cracks.

[0076] When the H2O partial pressure is 0.023 atm or greater, the thickness of the surface layer portion (A2) constituting the steel sheet becomes too large (e.g., greater than 50 μm), which may not be able to ensure the stable material properties of the ultra-high strength coated steel sheet.

[0077] Specifically, the inner layer (A1) may contain, by weight percent, C: 0.1% to 0.3%, Si: 1.0% to 2.0%, Mn: 2.5% to 3.5%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities. The surface layer (A2) may have a thickness of 10 μm to 50 μm from the surface of the steel sheet, and its carbon content may be less than that of the inner layer. It may also contain, by weight percent, Si: 1.0% to 2.0%, Mn: 2.5% to 3.5%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities. For example, the carbon content of the surface layer (A2) may be 10% or less of the carbon content of the inner layer (A1).

[0078] Typically, the Ac3 temperature of steel is approximated by the following Equation 1. In Equation 1, [C], [Mn], [Si], and [Cr] correspond to the weight % values ​​of carbon, manganese, silicon, and chromium (which are alloying components of steel), respectively.

[0079] Equation 1 Ac3 (℃)≒950-370[C]-27.4[Mn]+27.3[Si]-6.35[Cr] As described above, since the carbon content of the inner layer (A1) and the surface layer (A2) constituting the steel sheet differs from each other, the Ac3 temperature of the inner layer (A1) and the Ac3 temperature of the surface layer (A2) also differ from each other. Therefore, even at a certain annealing temperature, the phase transformation behavior of the inner layer (A1) and the surface layer (A2) may proceed differently, and the martensite fraction in the final microstructure of the inner layer (A1) and the surface layer (A2) may also differ. The technical significance of the difference in martensite fraction is that it can minimize or suppress the possibility of liquid metal embrittlement (LME) (where the coating melts and penetrates to the surface of the steel sheet during the welding process, resulting in brittleness), while providing an ultra-high strength coated steel sheet.

[0080] In the annealing step (S40), the lower limit of the annealing temperature range can be controlled to be a temperature 30°C lower than the Ac3 temperature of the inner layer portion (A1), and the upper limit of the annealing temperature range can be controlled to be the Ac3 temperature of the surface layer portion (A2). When the highest temperature of the annealing furnace is lower than the Ac3 temperature of the inner layer portion (A1) by 30°C (=Ac3-30°C), it is difficult to construct the final microstructure of the inner layer portion (A1) with an area fraction of tempered martensite of 50% or greater, thus making it difficult to ensure a strength of 1.2 GPa or higher. On the other hand, when the highest temperature of the annealing furnace exceeds the Ac3 temperature of the surface layer portion (A2), the area fraction of tempered martensite in the final microstructure of the surface layer portion (A2) exceeds 10%, thus making the LME properties brittle.

[0081] Annealing heat treatment that satisfies the aforementioned annealing temperature can be performed, for example, at a temperature corresponding to the two-phase region of austenite and ferrite. By performing heat treatment in the two-phase region, an appropriate fraction of ferrite can be ensured, and ideal ferrite, tempered martensite, and retained austenite in the final microstructure can be achieved, thereby obtaining the target material properties of the steel sheet.

[0082] After the annealing step, the steel sheet is divided into an inner layer (A1) and a surface layer (A2) that is in contact with the inner layer (A1) and has a different carbon content from each other. The microstructure of the inner layer (A1) may contain, in area fraction, austenite: 75% to 95% and ferrite: 5% to 25%, and the microstructure of the surface layer (A2) may contain, in area fraction, austenite: greater than 0% and not greater than 10%, and ferrite: greater than or equal to 90% and less than 100%.

[0083] Annealing temperature and annealing time affect the austenite grain size and can therefore have a significant impact on the strength of the steel sheet. For annealing heat treatment, the steel sheet is heated at a heating rate of, for example, 1 °C / s or greater, such as a range from 1 °C / s to 10 °C / s. When the heating rate is less than 1 °C / s, a longer time is required to reach the target annealing temperature, resulting in reduced production efficiency and potentially coarser grain size.

[0084] Simultaneously, as the annealing time (the time for annealing heat treatment) increases, similar to the annealing heat treatment temperature, it affects the coarsening caused by austenite grain growth. By controlling the annealing time (A) used for annealing heat treatment and the moisture concentration (B) in the annealing furnace, the possibility of liquid metal embrittlement (LME) (where the coating melts and penetrates to the surface of the steel sheet during welding, leading to brittleness) can be suppressed, and the thickness of the decarburized layer formed on the surface layer of the steel sheet can be controlled. Specifically, the thickness can be controlled based on the square root of the annealing time, A. 1 / 2The annealing time (A) and moisture concentration (B) in the annealing furnace are controlled by multiplying the product of the natural logarithm of the reciprocal of the moisture concentration, ln(1 / B). For example, the annealing step (S40) can be carried out under the condition that the annealing time (A) and the moisture concentration (B) in the annealing furnace satisfy the following equation 2.

[0085] Equation 2 (A) 1 / 2 ×ln(1 / B)≤K (where K is a predetermined constant) As an example of a method for manufacturing ultra-high strength coated steel sheets with excellent weldability, the annealing heat treatment can be controlled such that the moisture concentration (B) in the annealing furnace increases as the annealing time (A) decreases. As another example, the annealing heat treatment can be controlled such that the annealing time (A) increases as the moisture concentration (B) in the annealing furnace decreases. According to this configuration, the possibility of liquid metal embrittlement (LME) (where the coating melts and penetrates to the surface of the steel sheet during welding, leading to brittleness) can be suppressed, and the thickness of the surface layer portion (A2) formed in the surface layer of the steel sheet can be 10 μm or greater. The above is not limited to the understanding that LME is mitigated when the surface layer hardness decreases due to surface decarburization. Instead, based on the understanding that it is difficult to optimize the decarburized layer using only the dew point in the annealing furnace, the present invention provides a technical concept that comprehensively considers annealing time and dew point according to annealing temperature, thereby ensuring optimal decarburization conditions.

[0086] First cooling step and second cooling step (S50, S60) The annealed cold-rolled steel sheet undergoes multi-stage cooling. Specifically, the annealed steel sheet may undergo a first cooling step (S50) and a second cooling step (S60).

[0087] The first cooling step (S50), which cools the steel sheet to a first cooling end temperature greater than or equal to 600°C and less than 800°C at an average cooling rate of 1°C / s to 10°C / s, is a slow cooling step. The plasticity of the final microstructure can be ensured by attempting to ensure a predetermined amount of ferrite in the final microstructure during the heat treatment process. If a rapid quenching process is performed at a cooling rate of 30°C / s without a slow cooling step after the annealing step, the strip, which has expanded at high temperature, shrinks rapidly, making it difficult to control the shape of the product. Therefore, the first cooling step (S50) is introduced to first correct the shape of the strip in the relevant section before proceeding with the subsequent rapid cooling process. When the cooling end temperature of the slow cooling is below 600°C, ferrite transformation may occur in an undesirable amount, and the strength may therefore decrease. After the slow cooling step (S50), the microstructure of the inner layer portion (A1) may contain austenite of 75% to 95% and ferrite of 5% to 25% by area fraction, and the microstructure of the surface layer portion (A2) may contain austenite of greater than 0% and not greater than 10% and ferrite of greater than or equal to 90% and less than 100% by area fraction.

[0088] Subsequently, a second cooling step (S60) is performed to cool the steel sheet to a second cooling end temperature greater than or equal to 150°C and less than 300°C at an average cooling rate of 30°C / s or greater, for example, 30°C / s to 100°C / s. This is a rapid cooling step. By controlling the rapid cooling end temperature, austenite in the microstructure after slow cooling can be transformed into martensite, thereby helping to ensure the final material properties. An average cooling rate of 30°C / s or greater is required to suppress phase transformations that may occur during rapid cooling. After the rapid cooling step (S60), the microstructure of the inner layer portion (A1) may contain, in area fractions, martensite: 50% to 75%, ferrite: 5% to 25%, and retained austenite: 10% to 30%, and the microstructure of the surface layer portion (A2) may contain, in area fractions, martensite: greater than 0% and not greater than 10%, and ferrite: greater than or equal to 90% and less than 100%.

[0089] Subsequently, the cold-rolled steel sheet undergoing a second cooling can be held at a second cooling end temperature of 150°C or higher and less than 300°C for, for example, a period ranging from 1 to 100 seconds. During the holding time following rapid cooling, the temperature of the steel can initially be homogenized. The second cooling end temperature can be a temperature between the martensitic transformation initiation temperature (Ms) and the martensitic transformation end temperature (Mf).

[0090] Reheating step (S70) Cold-rolled steel sheets that have undergone multi-stage cooling can be reheated at heating rates, for example, ranging from 1°C / s to 10°C / s, and held at temperatures, for example, greater than 350°C and less than 500°C, for 60 seconds or less, to perform partition heat treatment. The partition heat treatment temperature can be higher than the martensitic transformation initiation temperature (Ms). The reheating process aims to ensure strength and elongation through the carbon concentration in the retained austenite and the tempering of the martensite during the process, and ultimately maintain the final microstructure.

[0091] When the partitioning heat treatment temperature is 350°C or lower, the partitioning effect may be insufficient. When the partitioning heat treatment temperature is 500°C or higher, the carbides may coarsen and the strength may decrease. The effect of the holding time of the partitioning heat treatment may not be as significant as the effect of the partitioning temperature. When the holding time of the partitioning heat treatment exceeds 60 seconds, the heat treatment efficiency decreases and the carbide size increases, which may reduce the strength. The partitioning heat treatment step can be performed immediately after multi-stage cooling, or it can be performed after holding at room temperature for several minutes or longer.

[0092] Without coating, after the partitioning heat treatment step is completed, the material is cooled to room temperature (e.g., a temperature ranging from 0°C to 40°C). During this step, when cooling is performed rapidly, a portion of the microstructure may undergo a phase transformation into martensite.

[0093] The aforementioned multi-stage cooling steps (S50, S60) and reheating step (S70) correspond to the developed Q&P (quenching and partitioning) heat treatment steps to simultaneously ensure high strength and high ductility of the steel. Q&P heat treatment suppresses the formation of carbide precipitates from the martensitic structure during quenching and enables carbon to diffuse into the retained austenite structure through partitioning. Through carbon re-diffusion, the retained austenite structure is stabilized even at room temperature, ultimately ensuring both the high ductility resulting from the retained austenite structure and the high strength resulting from the martensitic structure.

[0094] After the reheating step (S70), the final microstructure of the inner layer portion (A1) may contain, in area fraction, 50% to 75% tempered martensite, 5% to 25% ferrite, and 10% to 30% retained austenite, and the final microstructure of the surface layer portion (A2) may contain, in area fraction, greater than 0% and not greater than 10% tempered martensite, and greater than or equal to 90% and less than 100% ferrite. Even if retained austenite is present in the final microstructure of the surface layer portion (A2) after the reheating step (S70), its area fraction is controlled to be 1% or less.

[0095] Galvanizing process (S80) A hot-dip galvanized steel sheet (GI) can be formed by immersing and coating annealed cold-rolled steel sheet in a molten plating bath, and a galvanized annealed steel sheet (GA) can be formed by alloying heat treatment of cold-rolled steel sheet or hot-dip galvanized steel sheet with a hot-dip coating. In the case of the coated material, after the partitioning heat treatment step, the material can be directly immersed in the plating bath without cooling to room temperature.

[0096] Meanwhile, the plating bath can be a molten zinc plating bath, and in this case, the hot-dip galvanized steel sheet can be a hot-dip galvanized steel sheet. Specifically, the inlet temperature of the plating bath can be, for example, 460°C, and the composition of the plating bath can be a zinc plating bath containing 0.11% to 0.22% by weight of aluminum and saturated Fe. Alternatively, the plating bath can be a Zn-Mg-Al plating bath. Meanwhile, the alloying heat treatment temperature can be from 450°C to 600°C. The coating weight on both sides can be 40 g / m². 2 Up to 200 g / m 2 Furthermore, the coating thickness can range from 10 μm to 30 μm.

[0097] Meanwhile, in a variant embodiment of the present invention, instead of the GI process or the GA process, an electro-galvanized steel sheet process can also be applied.

[0098] refer to Figure 3(c) The ultra-high strength coated steel sheet with excellent weldability according to the embodiment of the present invention, achieved by performing the above steps, is a coated steel sheet (100), the coated steel sheet (100) comprising: a steel sheet (110) and a coating (120) on the steel sheet (100), the steel sheet (110) comprising an inner layer portion (A1) and a surface layer portion (A2) on the inner layer portion (A1), wherein the inner layer portion (A1) comprises, by weight %: C: 0.1% to 0.3%, Si: 1.0% to 2.0%, Mn: 2.5% to 3.5%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, with the balance being... Iron (Fe) and other unavoidable impurities, wherein the area fraction of tempered martensite in the final microstructure of the inner layer portion (A1) is 50% or greater, and the carbon content of the surface layer portion (A2) is lower than that of the inner layer portion, wherein the surface layer portion (A2) contains, by weight %, Si: 1.0% to 2.0%, Mn: 2.5% to 3.5%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities, wherein the area fraction of tempered martensite in the final microstructure of the surface layer portion (A2) is 10% or less, and wherein the coated steel sheet (100) has a yield strength greater than or equal to 850 MPa and less than or equal to 1070 MPa, a tensile strength greater than or equal to 1180 MPa and less than 1470 MPa and an elongation of 14% or greater.

[0099] In the ultra-high strength coated steel sheet (100), the carbon content of the surface layer portion (A2) can be 10% or less of the carbon content of the inner layer portion (A1), and the surface layer portion (A2) can have a thickness of 10 μm to 50 μm from the surface of the steel sheet (110). The surface of the steel sheet (110) forms the boundary between the steel sheet (110) and the coating (120).

[0100] In the ultra-high strength coated steel sheet (100), the final microstructure of the inner layer (A1) may contain, in area fraction, 50% to 75% tempered martensite, 5% to 25% ferrite, and 10% to 30% retained austenite, and the final microstructure of the surface layer (A2) may contain, in area fraction, greater than 0% and not greater than 10% tempered martensite, and greater than or equal to 90% and less than 100% ferrite. Even if retained austenite is present in the final microstructure of the surface layer (A2), the area fraction of retained austenite is controlled to be 1% or less. Therefore, the final microstructure of the surface layer (A2) may contain a retained austenite with an area fraction of 0% to 1%.

[0101] The ultra-high strength coated steel sheet (100) according to the technical concept of the present invention is characterized in that when grain boundaries with an orientation difference angle of 15° to 180° in the final microstructure of the steel sheet (110) are classified as large-angle grain boundaries and grain boundaries with an orientation difference angle of less than 15° are classified as small-angle grain boundaries, the ratio (=A / B) of the area fraction (A) of the large-angle grain boundaries at the surface of the steel sheet to the area fraction (B) of the large-angle grain boundaries at a point corresponding to one-quarter of the thickness (t) of the steel sheet from the surface is 1.2 or greater and less than 1.4.

[0102] In the final microstructure, grain boundaries are the interfaces between one grain and another adjacent grain. During the phase transformation of the microstructure, the crystal orientation of the grains changes, and the degree of relative change in crystal orientation can be assessed by the orientation difference angle of the grain boundaries. Using the EBSD analysis method, grain boundaries with an orientation difference angle of 15° to 180° in the final microstructure can be classified as large-angle grain boundaries, and grain boundaries with an orientation difference angle less than 15° can be classified as small-angle grain boundaries.

[0103] The area fraction (A) of large-angle grain boundaries at the surface of the steel plate can be the ratio of large-angle grain boundaries among all grain boundaries at the surface of the steel plate. Furthermore, the ratio can be evaluated as the average of values ​​measured over a finite region of arbitrary area. For example, the ratio can be evaluated as the average of the area fractions of large-angle grain boundaries measured at each of five equally spaced points on a straight line connecting one end to the other along the length of the steel plate surface, in regions each having a horizontal length of 1 mm and a vertical length of 1 mm. However, the technical concept of the present invention is not limited to the exemplary evaluation method described above.

[0104] Meanwhile, the point corresponding to one-quarter of the steel plate thickness (t) measured from the steel plate surface is the point corresponding to the midpoint in the thickness direction of the region between the steel plate surface and the center of the steel plate. The area fraction (B) of the large-angle grain boundary at the point corresponding to one-quarter of the steel plate thickness (t) measured from the steel plate surface can be the ratio of the large-angle grain boundary to all grain boundaries in the planar region corresponding to the point corresponding to one-quarter of the steel plate thickness (t) measured from the steel plate surface. This ratio can be evaluated as the average of values ​​measured in a region with a finite arbitrary area. For example, the ratio can be evaluated as the average of the area fractions of large-angle grain boundaries measured at each of five equally spaced points on a straight line connecting one end to the other in the length direction at the point corresponding to one-quarter of the steel plate thickness (t) measured from the steel plate surface, in regions each having a horizontal length of 1 mm and a vertical length of 1 mm. However, the technical concept of the present invention is not limited to the exemplary evaluation method described above.

[0105] When the ratio (HLR) of the area fraction (A) of large-angle grain boundaries at the surface of the steel plate to the area fraction (B) of large-angle grain boundaries at a point corresponding to one-quarter of the steel plate thickness (t) from the surface of the steel plate is less than 1.2, the LME phenomenon occurs (where molten zinc penetrates through the large-angle grain boundaries during spot welding), and when the ratio is 1.4 or greater, the martensite fraction in the surface layer is insufficient, thus the material properties become unsatisfactory.

[0106] When spot welding is performed on a clad steel sheet with a welding electrode tip diameter of 6 mm, an electrode force of 3.5 kN, and a welding current of 6.0 kA to 7.5 kA, the maximum length of liquid metal embrittlement (LME) cracks in the steel sheet (110) can be 50 μm or less. Specifically, the applicable welding current range for the clad steel sheet (100) is 6.0 kA to 7.5 kA, and it is characterized in that the probability of liquid metal embrittlement (LME) (wherein the coating (120) melts and penetrates into the surface of the steel sheet (110), resulting in brittleness) occurring during the welding of the clad steel sheet (100) is 0%.

[0107] Figure 5 A photograph showing the state of liquid metal embrittlement cracks in a coated steel sheet, which is a comparative embodiment of the present invention.

[0108] When spot welding ordinary coated steel sheets is performed on an automotive assembly line, the coating melts, and molten zinc can penetrate to the interface of residual austenite present in the surface layer of the coated steel sheet, leading to liquid metal embrittlement (LME). Due to LME, LME cracks form, and the weld strength decreases rapidly, thus narrowing the weldable current range. In other words, during resistance spot welding of ultra-high strength coated steel sheets, in the case of galvanized steel (GI), the melting point of the coating is very low (i.e., 420°C). Even in the case of galvanized annealed steel, molten zinc forms due to peritectic reactions near 880°C. This molten zinc penetrates along the grain boundaries of the base material in the area where the welding electrode generates a load at high temperatures, thus potentially causing a rapid decrease in the strength of the base material due to heat.

[0109] refer to Figure 5 It can be confirmed that austenite undergoes a phase transformation into αFe(Zn) at high temperatures due to zinc diffusion into the matrix material near the LME crack. Since αFe(Zn) is a highly brittle material, this phase transformation further accelerates the brittleness of the steel sheet. Therefore, the frequency of liquid metal embrittlement (LME) becomes more sensitive with increasing retained austenite, and avoiding the presence of austenite in the surface layer has been confirmed as a measure that can improve weldability.

[0110] According to the technical concept of the present invention, in the annealing step (S40) of annealing a cold-rolled steel sheet having the composition range proposed in the present invention in an annealing furnace, the partial pressure of H2O in the annealing furnace is controlled within a predetermined range, thereby inducing a decarburization reaction in the surface layer portion including the surface of the steel sheet within the predetermined range, resulting in different carbon contents in the surface layer portion and the inner layer portion. Therefore, the Ac3 temperatures of the surface layer portion and the inner layer portion become different from each other, and thus the phase transformation behavior (e.g., the fraction of austenite) of the surface layer portion and the inner layer portion proceeds differently even within the predetermined annealing temperature range. As a result, the fraction of martensite in the final microstructure of the surface layer portion and the inner layer portion behaves differently, thereby minimizing or suppressing the possibility of liquid metal embrittlement (LME) (whereby the coating melts and penetrates to the surface of the steel sheet during welding, leading to brittleness), while providing an ultra-high strength coated steel sheet.

[0111] As stated above, when manufacturing coated steel sheets with a tensile strength grade of 1.2 GPa or higher, it is preferable to configure the silicon content to 0.5% by weight or less, or to control the martensite fraction to 10% or less. However, when the silicon content is controlled to 0.5% by weight or less, the strength and elongation of the steel sheet can be ensured by adjusting the content of other components, but there is a problem of reduced economic efficiency and increased production costs due to the addition of multiple other elements. However, since LME cracks generated during spot welding occur in the coating and surface portions of the steel sheet, it is understood that if the martensite in the base material is fixed at a sufficient fraction to ensure product strength, and the martensite fraction at the surface is controlled to 10% or less, material properties and LME resistance can be ensured even with a higher silicon content.

[0112] Experimental Examples Experimental examples are now provided to facilitate understanding of the invention. However, the following experimental examples are only for facilitating understanding of the invention, and the invention is not limited to the following experimental examples.

[0113] First Experimental Example Table 1 shows the composition (unit: weight %) of the steel sheet constituting the ultra-high strength coated steel sheet according to the first experimental embodiment of the present invention, and Table 2 shows the process conditions of the ultra-high strength coated steel sheet according to the first experimental embodiment of the present invention. The composition in Table 1 refers to the composition of the cold-rolled steel sheet, and further indicates the composition of the inner layer portion constituting the steel sheet.

[0114] Table 1 Table 2 Steels having the composition (unit: wt%) shown in Table 1 were prepared, and hot-rolled steel sheets were produced by a predetermined hot-rolling process. The remainder consisted of iron (Fe) and other unavoidable impurities. The examples and comparative examples had the same alloy composition. Referring to Table 2, the hot rolling process was performed under the following process conditions: reheating temperature: 1200°C, finishing rolling temperature: 900°C, and coiling temperature: 600°C. After the annealing process, the following process conditions were applied: slow cooling temperature: 700°C, rapid cooling temperature: 250°C, and reheating temperature: 460°C. In the first experimental example, a maximum annealing temperature of 850°C was applied.

[0115] Table 3 In Table 3, H2O partial pressure refers to the partial pressure of H2O in the annealing furnace during the annealing process. Quarter-thickness large-angle grain boundary fraction refers to the ratio of large-angle grain boundaries in a planar region corresponding to a point located at a quarter-thickness (t) of the steel sheet from the surface; specifically, it is the ratio (B) of large-angle grain boundaries among all grain boundaries in the planar region corresponding to the point at the point at the surface. Surface large-angle ratio refers to the area fraction of large-angle grain boundaries at the surface; specifically, it is the ratio (A) of large-angle grain boundaries among all grain boundaries at the steel sheet surface. HLR refers to the ratio (A) of the area fraction of large-angle grain boundaries at the steel sheet surface to the area fraction (B) of large-angle grain boundaries at the point corresponding to a quarter-thickness (t) of the steel sheet from the surface (=A / B). Yield strength, tensile strength, elongation, and maximum LME length are material property values ​​realized in coated steel sheets. Figure 6 To illustrate the first experimental embodiment, the view is provided by using the EBSD analysis method to classify grain boundaries into large-angle grain boundaries (when the orientation difference angle in the final microstructure is 15° to 180°) and small-angle grain boundaries (when the orientation difference angle is less than 15°) at the surface of the steel plate and at points corresponding to one-quarter of the steel plate thickness (t) from the surface in the final microstructure.

[0116] Refer to Table 3 and Figure 6 When the ratio (=A / B) of the area fraction (A) of large-angle grain boundaries at the surface of the steel plate to the area fraction (B) of large-angle grain boundaries at a point corresponding to one-quarter of the thickness (t) of the steel plate from the surface is 1.2 or greater and less than 1.4 (Example), it can be confirmed that the coated steel plate meets the material properties of yield strength greater than or equal to 850 MPa and less than or equal to 1070 MPa, tensile strength greater than or equal to 1180 MPa and less than 1470 MPa and elongation of 14% or greater, and is not prone to liquid metal embrittlement (LME).

[0117] In contrast, when the partial pressure of H2O in the annealing furnace is less than 0.001 atm and the ratio of the area fraction (A) of the large-angle grain boundaries at the surface of the steel plate to the area fraction (B) of the large-angle grain boundaries at a point corresponding to one-quarter of the steel plate thickness (t) from the surface (HLR; A / B) is less than 1.2, it can be confirmed that the coated steel plate is prone to liquid metal embrittlement (LME) (where molten zinc penetrates through the large-angle grain boundaries during spot welding).

[0118] Meanwhile, when the H2O partial pressure is 0.023 atm or greater and the ratio (HLR; A / B) of the area fraction (A) of large-angle grain boundaries at the surface of the steel plate to the area fraction (B) of large-angle grain boundaries at a point corresponding to one-quarter of the steel plate thickness (t) from the surface exceeds 1.4, it can be confirmed that the martensite fraction in the surface layer is insufficient, and therefore the material properties are not satisfactory, because the coated steel plate does not meet the requirements of a yield strength greater than or equal to 850 MPa and less than or equal to 1070 MPa and a tensile strength greater than or equal to 1180 MPa and less than 1470 MPa.

[0119] Second Experimental Example Table 1 discloses the composition (in weight %) of the steel sheet constituting the ultra-high strength coated steel sheet according to the second experimental embodiment, and Table 2 discloses the process conditions (except for the annealing process).

[0120] Table 4 shows the annealing process conditions, as well as the thickness and composition of the surface layer portion, in the method for manufacturing the ultra-high strength coated steel sheet according to the second experimental embodiment. In Table 4, the components are expressed in weight percent (%), and the temperature is expressed in °C.

[0121] Table 4 Table 5 shows the final microstructure fraction and material properties of the ultra-high strength coated steel sheet according to the second experimental embodiment. In Table 5, the microstructure fraction is expressed as an area fraction (%), TM refers to tempered martensite, F refers to ferrite, and Ra refers to retained austenite.

[0122] Table 5 Referring to Tables 4 and 5, in the second experimental embodiment, Examples 1 and 2 satisfy the following conditions: during the annealing step, the dew point temperature in the annealing furnace is controlled to be between -20°C and +20°C, and the annealing temperature is within the range of 30°C lower than the Ac3 temperature of the inner layer (which is the lower limit of the annealing temperature range) to the Ac3 temperature of the surface layer (which is the upper limit of the annealing temperature range). Therefore, the thickness of the surface layer is within the range of 10 μm to 50 μm, and the carbon content of the surface layer is lower than that of the inner layer (see Table 1). Specifically, the carbon content of the surface layer is 10% or less of that of the inner layer. The material properties of the clad steel sheet satisfy a yield strength greater than or equal to 850 MPa and less than or equal to 1070 MPa, a tensile strength greater than or equal to 1180 MPa and less than 1470 MPa, and an elongation of 14% or greater. The final microstructure of the inner layer, by area fraction, comprises tempered martensite: 50% to 75%, ferrite: 5% to 25%, and retained austenite: 10% to 30%. The final microstructure of the surface layer, by area fraction, comprises tempered martensite: greater than 0% and not greater than 10%, and ferrite: greater than or equal to 90% and less than 100%. It can also be confirmed that although retained austenite exists in a negligible amount in the microstructure of the surface layer, its area fraction ratio is less than 1%. In contrast, referring to Comparative Examples 1 to 4 in the second experimental embodiment, it can be confirmed that during the annealing step, when the dew point temperature in the annealing furnace is below -20°C, the thickness of the surface layer portion does not meet the requirement of being less than 10 μm to 50 μm, and the composition of the surface layer portion and the inner layer portion is not different from each other; specifically, it can be confirmed that the carbon content of the surface layer portion is not different from the carbon content of the inner layer portion, therefore, it may be impossible to suppress liquid metal embrittlement (LME) cracking. Furthermore, in Comparative Example 1 of the second experimental embodiment, since the highest annealing temperature does not exceed a temperature 30°C lower than the Ac3 temperature of the inner layer portion, the area fraction of tempered martensite in the inner layer portion does not exceed 50%, therefore, it is difficult to ensure a tensile strength of 1.2 GPa or greater.

[0123] Furthermore, in comparative examples 3 and 4 of the second experimental embodiment, the highest annealing temperature exceeded the Ac3 temperature of the surface layer portion. Therefore, it can be confirmed that the area fraction of tempered martensite in the final microstructure of the surface layer portion exceeds 10%, the yield strength exceeds the design range, and the material is prone to liquid metal embrittlement (LME).

[0124] Referring to Comparative Example 5 of the second experimental embodiment, even when the dew point temperature in the annealing furnace is controlled to be between -20°C and +20°C, when the annealing temperature is lower than "the temperature 30°C lower than the Ac3 temperature of the inner layer" (which is the lower limit of the annealing temperature range), the tensile strength does not meet the requirement and is lower than "the range of greater than or equal to 1180 MPa and less than 1470 MPa", making it difficult to ensure a strength of 1.2 GPa or greater.

[0125] Referring to Comparative Example 6 of the second experimental example, even when the dew point temperature in the annealing furnace is controlled to be between -20°C and +20°C, when the annealing temperature exceeds the Ac3 temperature of the surface layer portion (which is the upper limit of the annealing temperature range), it can be confirmed that the area fraction of tempered martensite in the final microstructure of the surface layer portion exceeds 10%, and the material is prone to liquid metal embrittlement (LME).

[0126] Referring to Comparative Examples 7 to 10 of the second experimental embodiment, when the dew point temperature in the annealing furnace exceeds 20°C during the annealing step, it can be confirmed that the thickness of the surface layer portion exceeds and therefore does not meet the range of 10 μm to 50 μm, making it impossible to ensure stable material properties. Specifically, the tensile strength is lower than "greater than or equal to 1180 MPa and less than 1470 MPa", or the elongation is lower than "14% or greater".

[0127] Furthermore, in Comparative Example 10 of the second experimental embodiment, since the highest annealing temperature exceeds the Ac3 temperature of the surface layer portion, it can be confirmed that the area fraction of tempered martensite in the final microstructure of the surface layer portion exceeds 10%, and the material is prone to liquid metal embrittlement (LME).

[0128] Then, after obtaining the coated steel sheet by applying the composition in Table 1 and the process conditions in Table 2 as described above, spot welding is applied. Welding is performed at a current 0.5 kA lower than the conditions where spatter (splashing) occurs, and an LME assessment is conducted. Spot welding of automotive sheet materials typically begins by ensuring a sufficient weld nugget over the widest possible current range. At lower currents, a sufficient weld nugget size cannot be formed, and the robustness (strength, toughness, etc.) of the weld cannot be ensured. At higher currents, spatter (splashing) occurs. Here, spatter refers to the phenomenon where molten metal (which is the base material that melts through resistance heat during spot welding) overflows outside the weld nugget.

[0129] Table 6 shows the conditions of the spot welding process in the second experimental embodiment, and Figure 7 The results of applying the spot welding process in the second experimental embodiment are shown. Figure 7In this context, dew point refers to the dew point based on the moisture concentration in the annealing furnace. Normal dew point refers to a dew point below -45°C (e.g., -50°C), while high dew point refers to a dew point at 0°C.

[0130] In the second experimental embodiment, the evaluation method for liquid metal embrittlement (LME) is as follows. First, two identical fabric layers are prepared and tilted at a 5-degree angle. Then, the maximum welding current that will not cause spatter is applied. The cross-section of the weld material is observed, and the maximum depth at which a crack occurs is determined as the maximum LME depth. Generally, it is known that LME cracks impair the strength of the weld when they exceed 5% of the material thickness. In this experimental embodiment, 1 mm of material is used, and cracks exceeding 50 μm are determined to be defective in the LME evaluation.

[0131] Table 6 Refer to Table 6 and Figure 7 In the method for manufacturing the coated steel sheet according to a comparative embodiment of the present invention, annealing is performed at a dew point below -45°C, and the applicable welding current range is 6.0 kA to 6.5 kA. In contrast, in the method for manufacturing the ultra-high strength coated steel sheet with excellent weldability according to an embodiment of the present invention, annealing is performed at a dew point of 0°C, and it can be confirmed that the applicable welding current range is 6.0 kA to 7.5 kA. Therefore, it can be confirmed that in the method for manufacturing the ultra-high strength coated steel sheet with excellent weldability according to an embodiment of the present invention, the surface austenitic phase that leads to LME cracking in the welded portion is suppressed, thereby ensuring strength even under existing welding conditions, expanding the overall applicable welding current range, and suppressing defects in the spot welding process.

[0132] Third Experimental Example Table 9 shows that, in the third experimental embodiment of the present invention, the thickness of the decarburized layer and whether liquid metal embrittlement (LME) occurs depend on the annealing time and annealing moisture content of the galvanized annealed steel sheet obtained by applying the composition in Table 7 and the process conditions in Table 8. In the third experimental embodiment, the alloying temperature of the galvanizing treatment is 530°C.

[0133] Table 7 Table 8 Table 9 Figure 8 To show the square root A of the annealing time in Experimental Example 3 of the present invention 1 / 2A table showing the product of the product of ln(1 / B), obtained by taking the natural logarithm of the reciprocal of the moisture concentration in the annealing furnace. Figure 9 A table showing the thickness of the decarburized layer formed on the surface layer of the steel plate according to the annealing time (A) and the moisture concentration (B) in the annealing furnace in Experimental Example 3 of the present invention, and Figure 10 A table showing the incidence of liquid metal embrittlement (LME) in Experimental Example 3 of the present invention, based on annealing time (A) and moisture concentration in the annealing furnace (B). For reference, in Figure 8 In this table, annealing time (A) is expressed in seconds (s), and moisture concentration (B) is expressed in ppm. See Table 7 and... Figures 8 to 10 When the annealing heat treatment step is performed under the condition that the annealing time (A) and the moisture concentration (B) in the annealing furnace satisfy the following Equation 3 (Examples 2 to 6 of Experimental Example 3), it can be determined that i) the probability of liquid metal embrittlement (LME) (where the zinc coating melts and penetrates to the surface of the steel sheet, resulting in embrittlement) when the coated steel sheet is welded is 0%; and ii) the thickness of the decarburized layer formed on the surface layer of the steel sheet is 10 μm or greater.

[0134] Equation 3 (A) 1 / 2 ×ln(1 / B)≤-65 In contrast, when the annealing step is performed under conditions where the annealing time (A) and the moisture concentration (B) in the annealing furnace do not satisfy Equation 3 above (Comparative Examples 1 to 6 of the Third Experimental Example), it can be determined that i) the possibility of liquid metal embrittlement (LME) (where the zinc coating melts and penetrates to the surface of the steel sheet during the welding process of the coated steel sheet, resulting in embrittlement) occurs at a meaningful level; and ii) the thickness of the decarburized layer formed on the surface layer of the steel sheet is less than 10 μm.

[0135] In other words, it can be confirmed that in the method for manufacturing ultra-high strength coated steel sheets with excellent weldability, the annealing heat treatment can be controlled to satisfy the following: as the annealing time (A) becomes shorter, the moisture concentration (B) in the annealing furnace becomes higher, and as the moisture concentration (B) in the annealing furnace becomes lower, the annealing time (A) becomes longer.

[0136] Therefore, in the method for manufacturing ultra-high strength coated steel sheet with excellent weldability according to one aspect of the invention, during the annealing of the cold-rolled steel sheet in an annealing furnace, it can be understood that the method can be based on the square root A of the annealing time. 1 / 2 The product of the natural logarithm of the reciprocal of the moisture concentration, ln(1 / B), controls the annealing time (A) and the moisture concentration (B) in the annealing furnace.

[0137] As mentioned above, the first parameter disclosed in Equation 3 (expressed as the square root of the annealing time, A) 1 / 2 The product of the natural logarithm of the reciprocal of the moisture concentration, ln(1 / B), is considered technically significant as a factor capable of simultaneously and effectively controlling the thickness of the decarburized layer and the incidence of liquid metal embrittlement (LME) in ultra-high strength coated steel sheets. For example, in Comparative Examples 3, 6, 2, and 5 of the third experimental embodiment, the value of the square root of the annealing time (A) is... 1 / 2 While both parameters are identical, the thickness of the decarburized layer varies significantly depending on the natural logarithm of the reciprocal of the moisture concentration (B), ln(1 / B). Therefore, it can be confirmed that neither annealing time (A) alone nor moisture concentration (B) alone can effectively control both the thickness of the decarburized layer and the incidence of liquid metal embrittlement (LME) simultaneously. In this respect, the aforementioned first parameter has a better effect and causal relationship, and is not merely a different way of representing known physical properties, thus it can be considered to have technical significance.

[0138] Furthermore, referring to Table 9, it can be confirmed that as the dew point increases and the annealing time becomes longer, the amount of decarburization increases, and the ability to avoid liquid metal embrittlement (LME) becomes excellent. To improve liquid metal embrittlement (LME), it may be necessary to ensure that the decarburized layer thickness is 10 μm or greater.

[0139] Although the invention has been described above with reference to embodiments thereof, various modifications and alterations can be made by those skilled in the art. Such modifications and alterations are to be construed as long as they do not depart from the scope of the invention. Therefore, the scope of the invention should be defined by the claims set forth below.

Claims

1. An ultra-high strength coated steel sheet, the ultra-high strength coated steel sheet comprising: A steel plate, comprising an inner layer portion and a surface layer portion, wherein the surface layer portion is located on the inner layer portion; and A coating, the coating being located on the steel plate, The inner layer portion comprises, by weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 2.5% to 3.5%, phosphorus (P): greater than 0% and not greater than 0.02%, sulfur (S): greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities, and the final microstructure of the inner layer portion having an area fraction of tempered martensite of 50% or greater. The surface layer portion has a lower carbon content than the inner layer portion, and the surface layer portion contains silicon (Si) by weight%. 1.0% to 2.0%, manganese (Mn): 2.5% to 3.5%, phosphorus (P): greater than 0% and not greater than 0.02%, sulfur (S): greater than 0% and not greater than 0.01%, balance iron (Fe) and other unavoidable impurities, and the area fraction of tempered martensite in the final microstructure of the surface layer portion is 10% or less, and in, When grain boundaries with orientation difference angles of 15° to 180° in the final microstructure of a steel plate are classified as large-angle grain boundaries, the ratio of the area fraction of large-angle grain boundaries at the surface to the area fraction of large-angle grain boundaries at a point corresponding to one-quarter of the steel plate thickness (t) from the surface is 1.2 or greater and less than 1.

4.

2. The ultra-high strength coated steel sheet according to claim 1, wherein, The surface layer portion has a thickness of 10 μm to 50 μm measured from the surface of the steel plate.

3. The ultra-high strength coated steel sheet according to claim 1, wherein, The galvanized steel sheet has a yield strength greater than or equal to 850 MPa and less than or equal to 1070 MPa, a tensile strength greater than or equal to 1180 MPa and less than 1470 MPa, and an elongation of 14% or greater.

4. The ultra-high strength coated steel sheet according to claim 1, in, The carbon content of the surface layer is 10% or less of the carbon content of the inner layer.

5. The ultra-high strength coated steel sheet according to claim 1, in, The final microstructure of the inner layer comprises, in area fraction, 50% to 75% tempered martensite, 5% to 25% ferrite, and 10% to 30% retained austenite, and The final microstructure of the surface layer portion comprises, in area fraction, tempered martensite: greater than 0% and not greater than 10%, and ferrite: greater than or equal to 90% and less than 100%.

6. The ultra-high strength coated steel sheet according to claim 1, wherein, When spot welding is performed on coated steel sheets with a welding electrode tip diameter of 6 mm, an electrode force of 3.5 kN, and a welding current of 6.0 kA to 7.5 kA, the maximum length of liquid metal embrittlement (LME) cracks in the steel sheet is 50 μm or less.

7. A method for manufacturing ultra-high strength coated steel sheet, the method comprising: The first step of providing a steel plate, the steel plate comprising an inner layer portion and a surface layer portion, the surface layer portion being located on the inner layer portion; and The second step of forming a coating on the steel plate The inner layer portion comprises, by weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 2.5% to 3.5%, phosphorus (P): greater than 0% and not greater than 0.02%, sulfur (S): greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities, and the final microstructure of the inner layer portion having an area fraction of tempered martensite of 50% or greater. The surface layer portion has a lower carbon content than the inner layer portion, and the surface layer portion comprises, by weight percent, silicon (Si): 1.0% to 2.0%, manganese (Mn): 2.5% to 3.5%, phosphorus (P): greater than 0% and not greater than 0.02%, sulfur (S): greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities. Furthermore, the final microstructure of the surface layer portion has a tempered martensite area fraction of 10% or less. Specifically, when grain boundaries with orientation difference angles of 15° to 180° in the final microstructure of the steel plate are classified as large-angle grain boundaries, the ratio of the area fraction of large-angle grain boundaries at the surface to the area fraction of large-angle grain boundaries at a point corresponding to one-quarter of the steel plate thickness (t) from the surface is 1.2 or greater and less than 1.

4.

8. The method according to claim 7, wherein, The galvanized steel sheet has a yield strength greater than or equal to 850 MPa and less than or equal to 1070 MPa, a tensile strength greater than or equal to 1180 MPa and less than 1470 MPa, and an elongation of 14% or greater.

9. The method according to claim 7, wherein, The first step includes, in sequence: Provide steel material comprising, by weight %: carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 2.5% to 3.5%, phosphorus (P): greater than 0% and not greater than 0.02%, sulfur (S): greater than 0% and not greater than 0.01%, with the balance being iron (Fe) and other unavoidable impurities; The steel material is hot-rolled; Hot-rolled steel materials are subjected to softening heat treatment and pickling. Cold rolling of hot-rolled steel materials; The cold-rolled steel material is annealed in an annealing furnace; The annealed steel is slowly cooled at a first cooling rate; the slowly cooled steel is rapidly cooled at a second cooling rate greater than the first cooling rate; and the rapidly cooled steel is reheated. In the annealing step, the partial pressure of H2O in the annealing furnace is controlled to be greater than or equal to 0.001 atm and less than 0.023 atm. The lower limit of the annealing temperature range is controlled to be 30°C lower than the Ac3 temperature of the inner layer, and the upper limit of the annealing temperature range is controlled to be the Ac3 temperature of the surface layer. The slow cooling step is carried out at a first cooling rate of 1°C / s to 10°C / s and a cooling termination temperature of greater than or equal to 600°C and less than 800°C. The rapid cooling step is carried out at a second cooling rate of 30°C / s to 100°C / s and a cooling end temperature greater than or equal to 150°C and less than 300°C. The reheating step is carried out at a temperature greater than 350°C and less than 500°C and a holding time of 60 seconds or less.

10. The method according to claim 7, wherein, The carbon content of the surface layer portion is 10% or less of the carbon content of the inner layer portion, and the surface layer portion has a thickness of 10 μm to 50 μm from the surface of the steel plate.

11. The method according to claim 9, wherein, Following the annealing step, the steel sheet is divided into an inner layer portion having different carbon contents and a surface layer portion located on top of the inner layer portion. The microstructure of the inner layer portion comprises, by area fraction, 75% to 95% austenite and 5% to 25% ferrite, and the microstructure of the surface layer portion comprises, by area fraction, greater than 0% and not greater than 10% austenite and greater than or equal to 90% and less than 100% ferrite. Following the slow cooling step, the microstructure of the inner layer comprises, by area fraction, 75% to 95% austenite and 5% to 25% ferrite, and the microstructure of the surface layer comprises, by area fraction, greater than 0% and not greater than 10% austenite and greater than or equal to 90% and less than 100% ferrite. Following the rapid cooling step, the microstructure of the inner layer comprises, by area fraction, 50% to 75% martensite, 5% to 25% ferrite, and 10% to 30% retained austenite, and the microstructure of the surface layer comprises, by area fraction, greater than 0% and not greater than 10% martensite, and greater than or equal to 90% and less than 100% ferrite. Wherein, after the reheating step, the final microstructure of the inner layer portion comprises, in area fraction, 50% to 75% tempered martensite, 5% to 25% ferrite, and 10% to 30% retained austenite, and the final microstructure of the surface layer portion comprises, in area fraction, greater than 0% and not greater than 10% tempered martensite, and greater than or equal to 90% and less than 100% ferrite.

12. The method according to claim 7, wherein, The hot rolling process is carried out at a reheating temperature of 1150°C to 1250°C, a finishing temperature of 850°C to 1000°C, and a coiling temperature of 500°C to 700°C. The softening heat treatment step for hot-rolled steel materials includes performing softening heat treatment at a temperature of 500°C to 650°C.

13. The method according to claim 7, wherein, The cold rolling process is carried out with a reduction rate of 40% to 60%.

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