Ultra-high-strength galvanized steel sheet and manufacturing method thereor

Abstract

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

Description

Ultra-high-strength galvanized steel sheet and its manufacturing method

[0001] The present invention relates to a steel plate and a method for manufacturing the same, and more particularly, to an ultra-high strength galvanized steel plate and a method for manufacturing the same.

[0002] The automotive industry is focusing on improving fuel efficiency and reducing vehicle weight to meet the demands of the times, including resource depletion, rapid global warming, and high oil prices. Furthermore, increasing regulations on passenger safety are driving the demand for ultra-high-strength steels. Furthermore, various improvements are needed for galvanized steel sheets, which exhibit superior sacrificial corrosion resistance and can proactively release low-potential zinc when exposed to corrosive environments, thereby preventing corrosion.

[0003] For example, there is a growing demand to improve the liquid metal embrittlement (LME) problem, which occurs when the plating layer melts during resistance spot welding in automobile assembly lines and molten zinc penetrates the interface of the retained austenite present on the surface of the steel sheet, causing brittleness.

[0004] Specifically, when spot welding ultra-high-strength steel hot-dip galvanized steel sheets, the melting point of the coating layer is very low at 420℃ in the case of hot-dip galvanized steel sheets, and in the case of alloyed galvanized steel sheets, liquid zinc can be formed due to the peritectic reaction around 880℃. The formed liquid zinc can penetrate along the grain boundaries of the base material in the area where the load by the welding electrode occurs at high temperatures, which can rapidly reduce the strength of the base material, the steel sheet. The zinc that diffuses toward the base material around the liquid metal embrittlement crack causes a phase transformation from austenite to αFe(Zn) at high temperatures, and the embrittlement is further accelerated by the αFe(Zn) phase, and the greater the austenite phase, the more sensitive it becomes. The frequency of liquid metal embrittlement (LME) occurrence can increase as the silicon content or martensitic microstructure of the material increases.

[0005] Related prior art literature includes Korean Patent Publication No. 20200075949A.

[0006] The problem to be solved by the present invention is to provide an ultra-high strength plated steel sheet with improved liquid metal embrittlement phenomenon and a method for manufacturing the same.

[0007] However, these tasks are exemplary and the technical idea of ​​the present invention is not limited thereto.

[0008] According to one aspect of the present invention, an ultra-high strength plated steel sheet is provided. The ultra-high strength plated steel sheet comprises a steel sheet comprising an inner layer and a surface layer on the inner layer; And a plated steel sheet including a plated layer on the steel sheet; wherein the surface portion has a thickness of 10 ㎛ or more and 50 ㎛ or less from the surface of the steel sheet, and the inner layer portion contains, in wt%, carbon (C): 0.1 to 0.3%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 and 0.01% or less, and the remainder includes iron (Fe) and other inevitable impurities, and the final microstructure of the inner layer portion has an area fraction of tempered martensite of 50% or more, and the surface portion has a carbon content lower than that of the inner layer portion, and in wt%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 It contains 0.01% or less, and the remainder includes iron (Fe) and other inevitable impurities, and the final microstructure of the surface layer has an area fraction of tempered martensite of 10% or less, and when the angle of the misorientation direction of the grain boundary in the final microstructure of the steel plate is 15° or more and 180° or less, and is classified as a high-angle grain boundary, it is characterized in that the ratio of the area fraction of the high-angle grain boundary on the surface and the area fraction of the high-angle grain boundary at a point 1 / 4 of the steel plate thickness (t) from the surface is 1.2 or more and less than 1.4.

[0009] The above-mentioned galvanized steel sheet has a yield strength of 850 MPa or more and 1070 MPa or less, a tensile strength of 1180 MPa or more and less than 1470 MPa, and an elongation of 14% or more.

[0010] In the above ultra-high strength galvanized steel sheet, the carbon content of the surface layer may be 10% or less of the carbon content of the inner layer.

[0011] In the above ultra-high-strength galvanized steel sheet, the final microstructure of the inner layer may include, in terms of area fraction, tempered martensite: 50 to 75%, ferrite: 5 to 25%, and retained austenite: 10 to 30%, and the final microstructure of the surface layer may include, in terms of area fraction, tempered martensite: more than 0% but less than 10%, and ferrite: 90% or more and less than 100%.

[0012] When spot welding is performed on the above-mentioned plated steel sheet under the conditions of a welding electrode tip diameter of 6 mm, a pressure of 3.5 kN, and a welding current of 6.0 kA to 7.5 kA, the maximum length of a liquid metal embrittlement (LME) crack in the steel sheet may be 50 μm or less.

[0013] A method for manufacturing an ultra-high strength galvanized steel sheet according to another aspect of the present invention is provided. The method for manufacturing the ultra-high strength galvanized steel sheet comprises the steps of: a first step of providing a steel sheet comprising an inner layer and a surface layer on the inner layer; And a second step of forming a plating layer on the steel sheet; a method for manufacturing a plated steel sheet, comprising: the inner layer contains, in wt%, carbon (C): 0.1 to 0.3%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): 0 to 0.02% or less, sulfur (S): 0 to 0.01% or less, and the remainder includes iron (Fe) and other inevitable impurities; the final microstructure of the inner layer has an area fraction of tempered martensite of 50% or more, and the surface layer has a carbon content lower than that of the inner layer, and contains, in wt%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): 0 to 0.02% or less, sulfur (S): 0 to 0.01% or less, and the remainder includes iron (Fe) and other inevitable impurities; It includes impurities, and the final microstructure of the surface layer has an area fraction of tempered martensite of 10% or less, and when the angle of the misorientation direction of the grain boundary in the final microstructure of the steel plate is 15° or more and 180° or less, and is classified as a high-angle grain boundary, the ratio of the area fraction of the high-angle grain boundary on the surface to the area fraction of the high-angle grain boundary at a point 1 / 4 of the steel plate thickness (t) from the surface is characterized by being 1.2 or more and less than 1.4.

[0014] The above-mentioned galvanized steel sheet may have a yield strength of 850 MPa or more and 1070 MPa or less, a tensile strength of 1180 MPa or more and less than 1470 MPa, and an elongation of 14% or more.

[0015] In the above method for manufacturing the ultra-high strength galvanized steel sheet, the first step comprises the steps of: providing a steel material containing, in wt%, carbon (C): 0.1 to 0.3%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): 0 to 0.02% or less, sulfur (S): 0 to 0.01% or less, and the remainder including 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 a step of reheating the rapidly cooled steel; sequentially including; In the step of annealing heat treatment, the H2O partial pressure in the annealing furnace is controlled to be 0.001 atm or more and less than 0.023 atm, the lower limit of the annealing temperature range is 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, and the slow cooling step is performed under the conditions that the first cooling rate is 1 to 10°C / s and the cooling end temperature is 600°C or more and less than 800°C, the rapidly cooling step is performed under the conditions that the second cooling rate is 30 to 100°C / s and the cooling end temperature is 150°C or more and less than 300°C, and the reheating step can be performed under the conditions that the temperature is more than 350°C and less than 500°C, and the holding time is within 60 seconds.

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

[0017] In the method for manufacturing the above ultra-high strength plated steel sheet, after the annealing heat treatment step, the steel sheet is divided into an inner layer having different carbon contents and a surface layer on the inner layer, and the microstructure of the inner layer includes, in terms of area fraction, austenite: 75 to 95% and ferrite: 5 to 25%, and the microstructure of the surface layer includes, in terms of area fraction, austenite: more than 0% but less than 10% and ferrite: 90% or more but less than 100%, and after the slow cooling step, the microstructure of the inner layer includes, in terms of area fraction, austenite: 75 to 95% and ferrite: 5 to 25%, and the microstructure of the surface layer includes, in terms of area fraction, austenite: more than 0% but less than 10% and ferrite: 90% or more but less than 100%, and after the rapid cooling step, the microstructure of the inner layer includes, The surface layer includes, in terms of area fraction, martensite: 50 to 75%, ferrite: 5 to 25%, and retained austenite: 10 to 30%, and the microstructure of the surface layer includes, in terms of area fraction, martensite: more than 0% but less than 10%, and ferrite: 90% or more and less than 100%, and the final microstructure of the inner layer after the reheating step includes, in terms of area fraction, tempered martensite: 50 to 75%, ferrite: 5 to 25%, and retained austenite: 10 to 30%, and the surface layer may include, in terms of area fraction, tempered martensite: more than 0% but less than 10%, and ferrite: 90% or more and less than 100%.

[0018] In the above method for manufacturing the ultra-high strength galvanized steel sheet, the hot rolling step is performed under the conditions of a reheating temperature of 1150 to 1250°C, a finishing rolling temperature of 850 to 1000°C, and a coiling temperature of 500 to 700°C, and the cold rolling step can be performed under the conditions of a reduction ratio of 40 to 60%.

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

[0020] The effects of the present invention described above are illustrative, and the scope of the present invention is not limited by these effects.

[0021] FIG. 1 is a flowchart illustrating a method for manufacturing an ultra-high-strength galvanized steel sheet according to one embodiment of the present invention.

[0022] FIG. 2 is a graph illustrating a subsequent heat treatment (annealing, cooling, reheating) step applied to a cold-rolled steel sheet in a method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability according to one embodiment of the present invention.

[0023] FIG. 3 is a drawing schematically illustrating a process of forming an ultra-high strength plated steel sheet in a method for manufacturing an ultra-high strength plated steel sheet with excellent weldability according to one embodiment of the present invention.

[0024] FIG. 4 is a drawing schematically illustrating a decarburization reaction in a method for manufacturing an ultra-high-strength galvanized steel sheet according to one embodiment of the present invention.

[0025] Figure 5 is a photograph showing the appearance of liquid metal embrittlement cracks occurring in a plated steel sheet as a comparative example of the present invention.

[0026] Figure 6 is a drawing showing the final microstructure of the steel plate surface and a point 1 / 4 of the steel plate thickness (t) from the steel plate surface in the first experimental example of the present invention. Using the EBSD analysis method, cases where the misorientation direction of the grain boundary in the final microstructure is 15° or more and 180° or less are classified as high-angle grain boundaries, and cases where it is less than 15° are classified as low-angle grain boundaries.

[0027] Figure 7 is a drawing showing the result of applying the spot welding process in the second experimental example.

[0028] Figure 8 is the positive square root (A) of the annealing time (A) in the third experimental example of the present invention. 1 / 2 ) and the product of the natural logarithm of the reciprocal value of the moisture concentration (B) in the annealing furnace (ln(1 / B)).

[0029] Figure 9 is a drawing showing the thickness of the decarburized layer formed on the surface of a steel plate according to the annealing time (A) and the moisture concentration (B) in the annealing furnace in the third experimental example of the present invention.

[0030] Figure 10 is a diagram showing the occurrence rate of liquid metal embrittlement (LME) according to the annealing time (A) and moisture concentration (B) in the annealing furnace in the third experimental example of the present invention.

[0031] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Embodiments of the present invention are provided to more completely explain the technical idea of ​​the present invention to those skilled in the art. The following embodiments may be modified in various different forms, and the scope of the technical idea of ​​the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely convey the technical idea of ​​the present invention to those skilled in the art. Like reference numerals throughout this specification denote like elements. Furthermore, various elements and areas in the drawings are schematically drawn. Therefore, the technical idea of ​​the present invention is not limited by the relative sizes or intervals drawn in the attached drawings.

[0032] FIG. 1 is a flowchart illustrating a method for manufacturing an ultra-high-strength galvanized steel sheet according to one embodiment of the present invention.

[0033] Referring to FIG. 1, a method for manufacturing an ultra-high-strength galvanized steel sheet according to an embodiment of the present invention includes a step of providing a steel material (S10), a step of hot-rolling the steel material to form a hot-rolled steel sheet (S20), a step of cold-rolling the hot-rolled steel sheet to form a cold-rolled steel sheet (S30), a step of annealing the cold-rolled steel sheet (S40), a step of slowly cooling the annealed steel sheet at a first cooling rate (S50); a step of rapidly cooling the slowly cooled steel sheet at a second cooling rate greater than the first cooling rate (S60); a step of reheating the cooled steel sheet (S70); and a step of performing a zinc plating treatment on the steel sheet (S80).

[0034] According to the method for manufacturing an ultra-high strength coated steel sheet having excellent weldability according to the technical idea of ​​the present invention, in the step (S40) of annealing a cold rolled steel sheet in an annealing furnace, the dew point temperature in the annealing furnace is controlled to a relatively high dew point range compared to the prior art, thereby inducing a decarburization reaction in the surface layer including the surface of the steel sheet within a predetermined range, thereby implementing different carbon contents in the surface layer and the inner layer, and accordingly, the Ac3 temperatures of the surface layer and the inner layer are different, so that the phase transformation patterns of the surface layer and the inner layer progress differently even within a predetermined annealing temperature range, thereby differently expressing the fraction of martensite in the final microstructure of the surface layer and the inner layer, thereby minimizing or suppressing the possibility of liquid metal embrittlement (LME), in which the coating layer melts and penetrates the surface of the steel sheet during welding, causing brittleness, while at the same time providing an ultra-high strength coated steel sheet.

[0035] Hereinafter, a method for manufacturing an ultra-high strength galvanized steel sheet with excellent weldability according to one embodiment of the present invention will be described in detail.

[0036] Steel provision stage (S10)

[0037] In recent years, the automotive industry has seen a growing interest in lightweight vehicle bodies utilizing ultra-high-strength steels to simultaneously meet crashworthiness and fuel efficiency regulations. The steel industry is actively researching ultra-high-strength steels to meet these demands from automotive customers. The Quenching and Partitioning (Q&P) heat treatment technology, developed to simultaneously secure high strength and high ductility in automotive steels, suppresses the formation of carbide precipitates from carbon released from martensite during quenching and promotes the diffusion of carbon into the retained austenite structure through partitioning. This carbon redistribution stabilizes the retained austenite structure even at room temperature, ultimately ensuring high ductility from the retained austenite structure and high strength from the martensite structure.

[0038] These Q&P steel plates contain a large amount of silicon (Si) compared to general steel to inhibit the movement of iron (Fe) atoms and suppress the formation of carbide precipitates within the structure. They also contain a large amount of austenite-stabilizing alloying elements such as carbon (C) and manganese (Mn) to increase the volume fraction of stabilized retained austenite structure and improve TRIP (Transformation-Induced Plasticity) behavior.

[0039] Meanwhile, it is obvious that the technical idea of ​​the present invention can be applied to the above-described Q&P steel plate, but it is also obvious that the technical idea of ​​the present invention can be widely applied to various steel plates, and is not limited to the application only to the Q&P steel plate.

[0040] Hereinafter, the roles and contents of exemplary components included in the steel sheet (base material) constituting the ultra-high-strength plated steel sheet according to one embodiment of the present invention will be described. In this case, the contents of the component elements all refer to weight %.

[0041] Carbon (C): 0.1~0.3%

[0042] Carbon is the most important alloying element in steelmaking, primarily serving to strengthen steel and stabilize austenite. A high carbon concentration within austenite enhances austenite stability, facilitating the acquisition of adequate austenite for material improvement. Carbon content below 0.1% makes it difficult to achieve the desired yield strength and elongation. Carbon content exceeding 0.3% can lead to a decrease in weldability due to the increased carbon equivalent. Therefore, a carbon content of 0.1% to 0.3% of the total weight of the steel sheet is recommended.

[0043] Silicon (Si): 1.0~2.0%

[0044] Silicon is an element that inhibits the formation of carbides (e.g., Fe3C) within ferrite and increases the activity of carbon, thereby increasing the diffusion rate of austenite. Silicon is also well known as a ferrite stabilizing element, and is known to be an element that increases ductility by increasing the ferrite fraction during cooling. If the silicon content is less than 1.0%, the effect of silicon addition is insufficient. If the silicon content exceeds 2.0%, oxides (SiO2) are formed on the surface of the steel sheet during processing, which may result in a decrease in plating properties due to poor wettability in that area. Therefore, the silicon content is preferably 1.0 to 2.0% of the total weight of the steel sheet.

[0045] Manganese (Mn): 2.5~3.5%

[0046] Manganese is an austenite stabilizing element. As manganese is added, the martensite transformation start temperature, Ms, gradually decreases, which can have the effect of increasing the retained austenite fraction during the continuous annealing heat treatment process. If the manganese content is less than 2.5%, the effect of manganese addition is insufficient. If the manganese content exceeds 3.5%, the carbon equivalent increases, which significantly reduces weldability, and during the process, oxides (MnO) are formed on the surface of the steel sheet, which can lead to a decrease in plating properties due to poor wettability in the corresponding part. Therefore, the manganese content is preferably 2.5% to 3.5% of the total weight of the steel sheet.

[0047] Phosphorus (P): 0% or more to 0.02% or less

[0048] Phosphorus can play a role similar to silicon in steel. However, if phosphorus is added in amounts exceeding 0.02% of the total weight of the steel sheet, it can reduce weldability and increase brittleness, resulting in material deterioration. Therefore, it is recommended to limit the phosphorus content to between 0% and 0.02% of the total weight of the steel sheet.

[0049] Sulfur (S): 0% or more to 0.01% or less

[0050] Sulfur is an unavoidable element in steel manufacturing. It impairs the steel's toughness and weldability, and combines with manganese (Mn) to form MnS, thereby reducing its corrosion resistance and impact properties. Therefore, it is recommended that the sulfur content be limited to between 0% and 0.01% of the total steel sheet weight.

[0051] The remaining component of the steel sheet, the base material for the above ultra-high-strength galvanized steel sheet, is iron (Fe). However, during the typical steelmaking process, unintended impurities from raw materials or the surrounding environment can inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the typical manufacturing process, their full content is not specifically addressed in this specification.

[0052] Meanwhile, the ultra-high strength galvanized steel sheet according to one embodiment of the present invention may optionally further contain any combination of the following components in addition to the above-described composition.

[0053] Boron (B): 0 to 0.001%

[0054] Boron can be optionally added to steel sheets and can function as a grain boundary strengthening element. If boron is added in amounts exceeding 0.001%, it can form nitrides such as BN, thereby reducing high-temperature ductility. Therefore, boron can be added in an amount of more than 0% and less than 0.001% of the total weight of the base material.

[0055] Titanium (Ti): 0% or more but less than or equal to 0.05%

[0056] Titanium (Ti) element is a major element that precipitates in the form of carbides in steel, and the purpose of the present invention is to secure the stability of residual austenite and improve the strength through initial austenite grain refinement due to the formation of precipitates, and to refine ferrite grains and precipitation hardening due to the presence of precipitates in ferrite. That is, these are elements that improve the strength of steel sheets by precipitating in the form of TiC or TiN when combined with carbon (C) or nitrogen (N), or by solid solution strengthening in iron (Fe). For example, titanium (Ti) is an element that is effective in improving the strength by forming carbonitrides or sulfides, and can suppress the formation of boron nitride (BN) by combining with nitrogen to precipitate as titanium nitride (TiN). If titanium is added in excess of 0.05% of the total base material, not only will the manufacturing cost increase, but excessive precipitates will be generated in the ferrite phase, which may cause excessive precipitation strengthening and reduce the elongation of the steel sheet. In addition, the large amount of precipitates may cause a decrease in low-temperature toughness and weldability, so it is limited to 0.05% or less.

[0057] Chromium (Cr): 0 to 1.0%

[0058] Chromium (Cr) is an element with high hardenability and is added to increase strength through transformation strengthening. However, if the chromium (Cr) content exceeds 1.0 wt%, a structure such as upper bainite is formed, which causes the overall structure to become uneven and thus reduces toughness. Therefore, it is preferable to control the content to 1.0 wt% or less.

[0059] Molybdenum (Mo): 0 to 1.0%

[0060] Molybdenum (Mo) is an element with greater hardenability than chromium (Cr) and is added to increase strength through transformation strengthening. When the carbon (C) content exceeds 1.0 wt% within the range of the present invention, a large amount of hard secondary phases such as martensite / austenite (MA) phases are formed, resulting in a decrease in toughness. Therefore, it is preferable to control the content to 1.0 wt% or less.

[0061] Nickel (Ni): 0 to 1.0%

[0062] Nickel can help stabilize austenite and increase the hardenability of steel. However, nickel content exceeding 1.0% increases the manufacturing cost of the steel, which is undesirable. Therefore, nickel should be added at a level of 1.0% or less of the total weight of the base metal.

[0063] In the manufacturing method according to the present invention, the semi-finished product subject to the hot rolling and cold rolling processes may be, for example, a slab. The slab in semi-finished form can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0064] Hot rolling stage (S20)

[0065] A step (S20) of forming a hot-rolled steel sheet by applying a hot rolling process to the above steel material is performed. Since the above steel material is a high-alloy steel, edge cracking and rolling load need to be minimized to ensure mass production, so the rolling finishing temperature and coiling temperature can be set to a high temperature range.

[0066] The above steel is reheated at a reheating temperature (Slab Reheating Temperature, SRT) ranging from, for example, 1150°C to 1250°C. This reheating can cause re-dissolution of components segregated during casting and re-dissolution of precipitates. If the reheating temperature is lower than 1150°C, a problem of a rapid increase in hot rolling load may occur. If the reheating temperature exceeds 1250°C, slab warpage may make it difficult to charge and discharge from the furnace, and coarsening of initial austenite grains may make it difficult to secure the strength of the final produced steel sheet. The reheating temperature may vary depending on the steel.

[0067] Next, the reheated steel is hot-rolled, and hot-rolling can be performed at a finish delivery temperature (FDT) of, for example, 850°C to 1000°C. If the finish delivery temperature exceeds 1000°C, there is a concern that the quality of the steel sheet may deteriorate due to the occurrence of surface scale on the steel sheet. In addition, if the finish delivery temperature is lower than 850°C, it may cause an increase in rolling load and a decrease in productivity. The finish delivery temperature may vary depending on the steel sheet.

[0068] Next, the hot-rolled steel is cooled at a cooling rate of 10 to 30°C / s and then coiled at a coiling temperature (CT) ranging from, for example, 500°C to 700°C. The coiling temperature may vary depending on the steel. If the coiling temperature exceeds 700°C, an undesirable internal oxidation layer may occur in the hot-rolled steel sheet or the coiled hot-rolled coil. Since the internal oxidation of the hot-rolled coil thus coiled has deviations, it may be difficult to uniformly control the thickness of the internal oxidation layer. If the coiling temperature is less than 500°C, an undesirable low-temperature structure may be formed.

[0069] Softening heat treatment stage (S25)

[0070] In a method for manufacturing an ultra-high strength galvanized steel sheet with excellent weldability according to one embodiment of the present invention, a softening heat treatment step and a pickling step can be sequentially performed after a hot rolling process and before a cold rolling process. In the softening heat treatment step, the hot-rolled steel sheet is subjected to a softening heat treatment to soften the material, thereby alleviating problems with the reduction ratio load and shape defects during subsequent cold rolling. In other words, the softening heat treatment softens the hot-rolled steel sheet through the softening heat treatment to improve the efficiency of the cold rolling operation, thereby ensuring cold-rollability. If the strength of the hot-rolled steel sheet is high, problems such as thickness hunting and shape defects may occur during cold rolling. However, since the ultra-high strength steel softening heat treatment process according to the present invention is applied to a hot-rolled coil with scale remaining, not a cold-rolled coil, a countermeasure for changes in surface properties due to the high-temperature reaction of the scale during the softening heat treatment is required.

[0071] It is known that ultra-high-strength steels, which generally contain large amounts of silicon and manganese, generate internal oxides along the grain boundaries of the steel sheet along with scale at high temperatures. The oxide layer formed by internal oxidation has poor pickling properties because the main component of the matrix is ​​iron. Therefore, the internal oxide layer cannot be completely removed with the same pickling time as a general hot-rolled steel sheet, and a long pickling time is required, which causes a problem of reduced productivity. This internal oxidation occurs when the activity of easily oxidized elements, such as silicon and manganese, is high and exists under specific oxygen partial pressure conditions. Therefore, when a hot-rolled coil with remaining scale is heat-treated in a high-temperature reducing gas atmosphere, additional internal oxidation occurs due to the oxygen generated during the scale reduction reaction.

[0072] Specifically, internal oxides, which were not observed in the hot-rolled steel, developed unevenly over the entire coil length after the softening heat treatment, and the scale reduction and internal oxidation development behaviors may differ depending on the location within the coil. In the outer winding of the softening heat-treated coil, internal oxidation growth due to scale hydrogen reduction reaction was mainly observed, and in the inner winding of the coil, internal oxidation growth due to scale eutectoid reaction (4FeO → 4Fe + 2O2) was observed. This difference in internal oxidation growth behavior is believed to be due to the difference in the easiness of penetration of the reaction gas depending on the coil location, and it was understood that the oxygen generated from the scale hydrogen reduction and eutectoid reactions during the softening heat treatment diffuses into the base metal and acts as an internal oxidation reaction material. Since it is desirable that the internal oxide layer is formed as uniformly as possible throughout the steel sheet, it is desirable to suppress the internal oxide layer during the coiling step as much as possible and form the internal oxide layer during the softening heat treatment.

[0073] Considering these aspects, in the method for manufacturing an ultra-high-strength cold-rolled steel sheet of the present invention, the coiling temperature is controlled to 500°C to 700°C, and the softening heat treatment temperature is controlled to 500°C to 650°C.

[0074] The above softening heat treatment can be performed in a batch annealing furnace (BAF) while the hot-rolled steel sheet is coiled, and can be performed in a hydrogen atmosphere. The hot-rolled steel sheet that has undergone the softening heat treatment under the above process conditions can secure cold-rolling properties due to the softening of the material. In addition, since the internal oxide layer formed by the softening heat treatment has a predetermined thickness (e.g., a thickness of 10 μm or less), subsequent pickling properties can be secured at the same time.

[0075] When softening heat treatment is applied at a temperature below 500℃, the martensite formed after hot rolling does not recrystallize, but only undergoes tempering, so that supersaturated carbon within the structure forms and spheroidizes in the form of cementite (θ). In this case, the brittleness of martensite may occur, which may lead to safety accidents such as sheet fracture during cold rolling. In other words, if an excessive amount of austenite is formed during softening heat treatment, martensite may be formed during cooling, preventing the strength reduction from being effectively realized.

[0076] In addition, when the softening heat treatment is applied at a temperature exceeding 650℃, the internal oxide layer formed by the softening heat treatment exceeds a predetermined thickness (for example, exceeding a thickness of 10㎛), making it difficult to secure subsequent acid washability. In addition, when the softening heat treatment is applied at a temperature exceeding 650℃, austenite is excessively formed, and martensite is formed during cooling, so the strength reduction is not effectively realized.

[0077] Meanwhile, in the method for manufacturing an ultra-high strength plated steel sheet with excellent weldability according to one embodiment of the present invention, the application of the softening heat treatment step may be optional depending on the steel type or target strength. For example, if the target tensile strength after cold rolling / annealing is 1180 MPa or higher, the softening heat treatment step may be performed, and if the target tensile strength after cold rolling / annealing is 980 MPa (or lower), the softening heat treatment step may not be performed. The time for performing the softening heat treatment may be 1 to 12 hours.

[0078] In the pickling treatment step, after performing the softening heat treatment, a pickling treatment may be performed to clean the hot-rolled steel sheet with acid. By pickling the hot-rolled steel sheet, at least a portion of the internal oxide layer may be removed. The pickling treatment may be performed, for example, at a temperature of 70°C to 90°C, with a hydrochloric acid concentration of 5% to 15%, for example, 20 seconds to 40 seconds. In addition, the pickling treatment may have an inhibitor concentration of 0.1 to 0.5%.

[0079] Cold rolling stage (S30)

[0080] A step (S30) of forming a cold rolled steel sheet by applying a cold rolling process to the above hot rolled steel sheet is performed. In the case of cold rolling, it is performed to match the thickness of the final produced steel sheet using hot rolled material.

[0081] In the cold rolling step (S30), the hot-rolled steel sheet subjected to the pickling treatment can be cold rolled at an average reduction ratio of, for example, 40% to 60%, thereby producing a cold-rolled steel sheet. The microstructure of the cold-rolled steel sheet has an elongated shape of the hot-rolled steel sheet, and the microstructure of the final steel sheet is determined through subsequent heat treatment.

[0082] FIG. 2 is a graph illustrating a subsequent heat treatment (annealing, cooling, reheating) step applied to a cold-rolled steel sheet in a method for manufacturing an ultra-high-strength plated steel sheet with excellent weldability according to an embodiment of the present invention, and FIG. 3 is a diagram schematically illustrating a process for forming an ultra-high-strength plated steel sheet in a method for manufacturing an ultra-high-strength plated steel sheet with excellent weldability according to an embodiment of the present invention. The configuration illustrated in (a) of FIG. 3 corresponds to S30 among the steps illustrated in FIG. 1, the configuration illustrated in (b) of FIG. 3 corresponds to S40 to S70 among the steps illustrated in FIG. 1, and the configuration illustrated in (c) of FIG. 3 corresponds to S80 among the steps illustrated in FIG. 1.

[0083] Annealing heat treatment step (S40)

[0084] Referring to FIGS. 2 and 3, the cold-rolled steel sheet is annealed in a continuous annealing furnace having a slow cooling section.

[0085] In the annealing heat treatment step (S40), the dew point temperature of the atmospheric gas within the annealing furnace is controlled to be -20°C or higher and +20°C or lower. The atmospheric gas within the annealing furnace may be, for example, a mixed gas of 90% nitrogen and 10% oxygen. By using a humidifier attached to the outer wall of the annealing furnace to inject pure H2O into the annealing furnace, the H2O partial pressure within the annealing furnace can be controlled to be 0.001 atm or higher and less than 0.023 atm. Furthermore, the dew point (dew point) of the heat treatment atmospheric gas can be managed to be -20°C or higher and +20°C or lower. Through the annealing heat treatment having the H2O partial pressure within the annealing furnace described above, the cold-rolled steel sheet constituting the ultra-high-strength plated steel sheet of the present invention undergoes a decarburization reaction on the surface of the cold-rolled steel sheet, which is ultimately formed into a steel sheet comprising an inner layer portion and a surface layer portion on the inner layer portion. A0 illustrated in (a) of Fig. 3 corresponds to the cold-rolled steel sheet, A1 illustrated in (b) of Fig. 3 corresponds to the inner layer constituting the steel sheet, and A2 corresponds to the surface layer constituting the steel sheet. The alloy composition of the cold-rolled steel sheet (A0) is the same as the alloy composition of the inner layer (A1) constituting the steel sheet, but is different in carbon content from the alloy composition of the surface layer (A2) constituting the steel sheet. That is, when the H2O partial pressure in the annealing furnace is managed to be 0.001 atm or more and less than 0.023 atm, the inner layer (A1) and the surface layer (A2) having different components can be implemented within the component range restricted by the present patent.

[0086] FIG. 4 is a drawing schematically illustrating a decarburization reaction in a method for manufacturing an ultra-high-strength galvanized steel sheet according to one embodiment of the present invention.

[0087] Even if the annealing temperature of the above-described annealing step is set to a region where the austenite phase is formed, if the H2O partial pressure within the annealing furnace is 0.001 atm or higher, carbon, which is an austenite stabilizing element, is oxidized on the surface of the cold-rolled steel sheet (A0) and volatilized in the form of carbon monoxide, so that a decarburization reaction may proceed through a continuous reaction. This reaction can be expressed by the following chemical formula 1.

[0088] <Chemical Formula 1>

[0089] C(s) + H2O(g) → CO(g) + H2(g)

[0090] As decarburization continues, carbon is depleted in the surface layer, and if the surface austenite has already transformed into ferrite during the annealing stage, no austenite will form in the surface layer during subsequent cooling / reheating processes, and a single-phase ferrite structure will remain, even at room temperature in the final stage. This single-phase ferrite structure formed in the surface layer avoids the austenite that is susceptible to liquid metal embrittlement (LME) cracking, thereby expanding the weld current range and increasing weld strength.

[0091] Meanwhile, in order to prevent liquid metal embrittlement (LME) cracks in the surface layer (A2), the thickness of the surface layer (A2) constituting the steel plate must be at least 10 ㎛. If the thickness of the surface layer (A2) is less than 10 ㎛, it is difficult to prevent liquid metal embrittlement (LME) cracks in the surface layer (A2).

[0092] When the H2O partial pressure in the annealing furnace is less than 0.001 atm, the alloy composition of the inner layer (A1) and the surface layer (A2) constituting the steel plate is not significantly different, or the thickness of the surface layer (A2) is formed to be less than 10 ㎛, so liquid metal embrittlement (LME) cracks cannot be suppressed.

[0093] When the H2O partial pressure is 0.023 atm or more, the thickness of the surface layer (A2) constituting the steel plate becomes excessively thick (e.g., more than 50 ㎛), making it impossible to secure a stable material for the ultra-high-strength galvanized steel plate.

[0094] Specifically, the inner layer (A1) contains, in wt%, carbon (C): 0.1 to 0.3%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): 0 to 0.02% or less, sulfur (S): 0 to 0.01% or less, and may include the remainder iron (Fe) and other unavoidable impurities. The surface layer (A2) may have a thickness of 10 ㎛ to 50 ㎛ from the surface of the steel plate, and has a carbon content lower than the carbon content of the inner layer, and contains, in wt%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): 0 to 0.02% or less, sulfur (S): 0 to 0.01% or less, and may include the remainder iron (Fe) and other unavoidable impurities. For example, the surface layer (A2) may have a carbon content of 10% or less of the carbon content of the inner layer (A1).

[0095] In general, the Ac3 temperature of steel is approximated by the following Equation 1. In Equation 1, [C], [Mn], [Si], and [Cr] correspond to the weight percent values ​​of carbon, manganese, silicon, and chromium, which are alloy components of steel, respectively.

[0096] <Formula 1>

[0097] Ac3(℃)≒ 950 - 370[C] - 27.4[Mn] + 27.3[Si] - 6.35[Cr]

[0098] As described above, since the carbon contents of the inner layer (A1) and the surface layer (A2) constituting the steel plate are different, the Ac3 temperature of the inner layer (A1) and the Ac3 temperature of the surface layer (A2) are different. Therefore, under any specific annealing temperature, the phase transformation patterns of the inner layer (A1) and the surface layer (A2) may progress differently, and the fractions of martensite may be expressed differently in the final microstructures of the inner layer (A1) and the surface layer (A2). This difference in the fraction of martensite has a technological significance in that it can minimize or suppress the possibility of liquid metal embrittlement (LME), which occurs when the plating layer melts and penetrates the surface of the steel plate during welding, causing brittleness, while at the same time providing an ultra-high-strength plating steel plate.

[0099] In the annealing heat treatment step (S40), the lower limit of the annealing temperature range can be controlled to be a temperature 30℃ lower than the Ac3 temperature of the inner layer (A1), and the upper limit of the annealing temperature range can be controlled to be the Ac3 temperature of the surface layer (A2). If the maximum temperature of the annealing furnace is lower than the Ac3 temperature of the inner layer (A1) constituting the steel plate by 30℃ (=Ac3-30℃), it is difficult to form a tempered martensite area ratio of 50% or more in the final microstructure of the inner layer (A1) constituting the steel plate, making it difficult to secure a strength of 1.2 GPa or higher. On the other hand, if the maximum temperature of the annealing furnace exceeds the Ac3 temperature of the surface layer (A2) constituting the steel plate, the tempered martensite area ratio of the final microstructure of the surface layer (A2) constituting the steel plate exceeds 10%, making the liquid metal embrittlement (LME) characteristic vulnerable.

[0100] Annealing heat treatment satisfying the above-described annealing temperature may be performed, for example, at a temperature corresponding to the ideal range of austenite and ferrite. By performing heat treatment within this ideal range, an appropriate fraction of ferrite is secured, thereby realizing ideal ferrite, tempered martensite, and retained austenite in the final microstructure, thereby achieving the target material properties of the steel sheet.

[0101] After the above annealing heat treatment step, the steel plate is divided into an inner layer (A1) having different carbon contents and a surface layer (A2) positioned in contact with the inner layer (A1), and the microstructure of the inner layer (A1) may include, in terms of area fraction, austenite: 75 to 95% and ferrite: 5 to 25%, and the microstructure of the surface layer (A2) may include, in terms of area fraction, austenite: more than 0% and less than 10%, and ferrite: 90% or more and less than 100%.

[0102] The annealing heat treatment temperature and time affect the austenite grain size, and thus, can have a significant impact on the strength of the steel sheet. To perform the annealing heat treatment, heating is performed at a heating rate of, for example, 1°C / s or higher, for example, in the range of 1°C / s to 10°C / s. If the heating rate is less than 1°C / s, it takes a long time to reach the target annealing heat treatment temperature, which reduces production efficiency and may cause the grain size to increase.

[0103] Meanwhile, as the annealing time, which is the time for performing the annealing heat treatment, increases, it affects the coarsening due to the growth of austenite grains, just like the annealing heat treatment temperature. By controlling the annealing time (A) for performing the annealing heat treatment and the moisture concentration (B) in the annealing furnace, the possibility of liquid metal embrittlement (LME), which causes brittleness by melting the plating layer and penetrating the surface of the steel sheet during welding on the plated steel sheet, can be suppressed, and the thickness of the decarburization layer formed on the surface of the steel sheet can be controlled. Specifically, the annealing time (A) for performing the annealing heat treatment and the moisture concentration (B) in the annealing furnace are the positive square root of the annealing time (A 1 / 2) and the product of the natural logarithm of the reciprocal value of the moisture concentration (ln(1 / B)). For example, the annealing heat treatment step (S40) can be performed under the condition that the annealing time (A) and the moisture concentration (B) in the annealing furnace satisfy the following formula 2.

[0104] <Formula 2>

[0105] (A) 1 / 2 × ln(1 / B) ≤ K (where K is a given constant value)

[0106] As an example of a method for manufacturing the above-mentioned ultra-high strength plated steel sheet with excellent weldability, the annealing heat treatment can be controlled so that the shorter the annealing time (A) for performing the annealing heat treatment, the higher the moisture concentration (B) in the annealing furnace. As another example of a method for manufacturing the above-mentioned ultra-high strength plated steel sheet with excellent weldability, the annealing heat treatment can be controlled so that the longer the annealing time (A) for performing the annealing heat treatment, the lower the moisture concentration (B) in the annealing furnace. According to this configuration, the possibility of liquid metal embrittlement (LME), in which the plating layer melts and penetrates the surface of the steel sheet to cause brittleness during welding of the plated steel sheet, can be suppressed, and the thickness of the surface layer (A2) formed on the surface of the steel sheet can be 10 ㎛ or more. The above-mentioned content is not limited to the superficial understanding that liquid metal embrittlement (LME) is alleviated when the hardness of the surface layer is lowered due to surface decarburization, and it can provide technical ideas from an integrated consideration of annealing time and dew point according to annealing temperature to secure optimal decarburization conditions, recognizing the problem that it is difficult to optimize the decarburization layer based on the dew point in the annealing furnace alone.

[0107] Primary and secondary cooling stages (S50, S60)

[0108] The above annealed and heat-treated cold-rolled steel sheet is cooled in multiple stages. Specifically, a first cooling step (S50) and a second cooling step (S60) of the annealed and heat-treated steel sheet can be performed.

[0109] The step (S50) of first cooling the steel plate at an average cooling rate of 1 to 10°C / s to a first cooling end temperature of 600°C or more and less than 800°C is a slow cooling step, which attempts to secure a certain amount of ferrite in the final microstructure during the heat treatment process, thereby ensuring the plasticity of the final microstructure. If the slow cooling step is not performed after the annealing process and rapid cooling is performed at a cooling rate of 30°C / s, the plate expanded at high temperature rapidly shrinks, making it difficult to control the shape of the product. Therefore, a first cooling step (S50) is introduced to first correct the shape of the plate in the relevant section, and a subsequent rapid cooling process is performed. If the cooling end temperature of the slow cooling is less than 600°C, ferrite transformation may occur in an undesirable amount, which may result in a decrease in strength. After the above-mentioned slow cooling step (S50), the microstructure of the inner layer (A1) may include, in terms of area fraction, austenite: 75 to 95%, ferrite: 5 to 25%, and the microstructure of the surface layer (A2) may include, in terms of area fraction, austenite: more than 0% but less than 10%, ferrite: 90% or more and less than 100%.

[0110] Continuing, the step (S60) of secondary cooling the steel plate to a secondary cooling end temperature of 150°C or more and less than 300°C at an average cooling rate of 30°C / s or more, for example, 30 to 100°C / s, is a rapid cooling step, which can easily secure the final material by transforming austenite in the microstructure after the slow cooling into martensite through rapid cooling end temperature control, and requires an average cooling rate of 30°C / s or more to suppress phase transformation that may occur during the rapid cooling process. After the above-mentioned rapid cooling step (S60), the microstructure of the inner layer (A1) may include, in terms of area fraction, martensite: 50 to 75%, ferrite: 5 to 25%, and retained austenite: 10 to 30%, and the microstructure of the surface layer (A2) may include, in terms of area fraction, martensite: more than 0% but less than 10%, and ferrite: 90% or more and less than 100%.

[0111] Next, the second-cooled cold-rolled steel sheet can be maintained at the second cooling end temperature of 150°C or higher and less than 300°C for a time ranging from, for example, 1 second to 100 seconds. During this holding time after rapid cooling, temperature homogenization of the steel can initially occur. The second cooling end temperature is the martensite transformation start temperature (M s ) and martensite transformation completion temperature (M f ) can be between .

[0112] Reheating step (S70)

[0113] The above multi-stage cooled cold rolled steel sheet can be reheated at a heating rate ranging from 1°C / s to 10°C / s, for example, and partitioning heat treatment can be performed by maintaining the temperature at a temperature exceeding 350°C and less than 500°C for a time of less than 60 seconds. The partitioning heat treatment temperature is the martensite transformation start temperature (M s) may be higher. The purpose of the above reheating process is to secure strength and elongation through carbon enrichment and martensite tempering in the retained austenite during the process, and finally to maintain the final microstructure composition.

[0114] 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 size of the carbide may become coarser, resulting in a decrease in strength. The partitioning heat treatment holding time may have less of an effect than the partitioning temperature. When the partitioning heat treatment holding time exceeds 60 seconds, the heat treatment efficiency may decrease, the size of the carbide may increase, resulting in a decrease in strength. The partitioning heat treatment step may be performed immediately after the multi-stage cooling, or after maintaining the temperature at room temperature for several minutes or more.

[0115] For non-plated materials, after the partitioning heat treatment step is completed, the material is cooled to room temperature, for example, to a temperature in the range of 0°C to 40°C. If rapid cooling is performed at this stage, a portion of the microstructure may undergo a phase transformation to martensite.

[0116] The above-mentioned multi-stage cooling steps (S50, S60) and reheating step (S70) correspond to the Q&P (Quenching and Partitioning) heat treatment steps developed to simultaneously secure high strength and high ductility of steel materials. It is a technology that suppresses the formation of carbide precipitates of carbon released from the martensite structure during quenching and allows carbon to diffuse into the retained austenite structure through partitioning. Through the re-diffusion of carbon, the retained austenite structure is stabilized even at room temperature, and ultimately, high ductility by the retained austenite structure and high strength by the martensite structure can be secured.

[0117] After the reheating step (S70), the final microstructure of the inner layer (A1) may include, in terms of area fraction, tempered martensite: 50 to 75%, ferrite: 5 to 25%, and retained austenite: 10 to 30%, and the final microstructure of the surface layer (A2) may include, in terms of area fraction, tempered martensite: more than 0% and less than 10%, and ferrite: 90% or more and less than 100%. Even if retained austenite exists in the final microstructure of the surface layer (A2) after the reheating step (S70), the ratio is managed to be less than 1% in terms of area fraction.

[0118] Zinc plating treatment step (S80)

[0119] A step (GI) of immersing the above-mentioned annealed heat-treated cold-rolled steel sheet in a molten plating bath to form a molten plating steel sheet can be performed, and a step (GA) of alloying the cold-rolled steel sheet and the molten plating steel sheet on which the molten plating layer has been formed to form an alloyed molten plating steel sheet can be further performed. In the case of the plating material, after the above-mentioned partitioning heat treatment step is performed, it can be directly introduced into the plating bath without being cooled to room temperature.

[0120] Meanwhile, the plating bath may be a hot-dip galvanizing bath, and in this case, the hot-dip galvanized steel sheet may be a hot-dip galvanized steel sheet. Specifically, the entry temperature of the plating bath may be, for example, 460°C, and the composition of the plating bath may be a zinc plating bath containing 0.11 to 0.22 wt% of aluminum and saturated Fe. Alternatively, the plating bath may be a Zn-Mg-Al plating bath. Meanwhile, the alloying heat treatment temperature may be 450 to 600°C. The plating deposition amount is 40 to 200 g / m on both sides. 2 , and the plating layer thickness can be 10 to 30㎛.

[0121] Meanwhile, in a modified embodiment of the present invention, unlike the GI process or the GA process, an electroplating steel sheet process may be applied.

[0122] An ultra-high strength galvanized steel sheet having excellent weldability according to one embodiment of the present invention implemented by performing the above-described steps is, with reference to (c) of FIG. 3, a steel sheet (110) composed of an inner layer (A1) and a surface layer (A2) on the inner layer (A1); And a plated steel sheet (100) including a plating layer (120) on the steel sheet (100); wherein the inner layer (A1) contains, in wt%, carbon (C): 0.1 to 0.3%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): 0 to 0.02% or less, sulfur (S): 0 to 0.01% or less, and the remainder includes iron (Fe) and other inevitable impurities, and the final microstructure of the inner layer (A1) has an area fraction of tempered martensite of 50% or more, and the surface layer (A2) has a carbon content lower than that of the inner layer, and contains, in wt%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): 0 to 0.02% or less, sulfur (S): 0 It contains less than 0.01% of excess, and the remainder includes iron (Fe) and other inevitable impurities, and the final microstructure of the surface layer (A2) has an area fraction of tempered martensite of 10% or less, and the plated steel sheet (100) has a yield strength of 850 MPa or more and 1070 MPa or less, a tensile strength of 1180 MPa or more and less than 1470 MPa, and an elongation of 14% or more.

[0123] In the above ultra-high strength galvanized steel plate (100), the surface layer (A2) may have a carbon content of 10% or less of the carbon content of the inner layer (A1), and the surface layer (A2) may have a thickness of 10 µm or more and 50 µm or less from the surface of the steel plate (110). The surface of the steel plate (110) constitutes a boundary between the steel plate (110) and the galvanized layer (120).

[0124] In the above ultra-high-strength galvanized steel sheet (100), the final microstructure of the inner layer (A1) may include, in terms of area fraction, tempered martensite: 50 to 75%, ferrite: 5 to 25%, and retained austenite: 10 to 30%, and the final microstructure of the surface layer (A2) may include, in terms of area fraction, tempered martensite: more than 0% and less than 10%, and ferrite: 90% or more and less than 100%. Even if retained austenite exists in the final microstructure of the surface layer (A2), the proportion of retained austenite is managed to be 1% or less in terms of area fraction. Therefore, the final microstructure of the surface layer (A2) may include retained austenite in terms of area fraction of 0% or more and 1% or less.

[0125] The ultra-high-strength galvanized steel sheet (100) according to the technical idea of ​​the present invention is characterized in that when the angle of the misorientation direction of the grain boundary in the final microstructure of the steel sheet (110) is 15° or more and 180° or less, it is classified as a high-angle grain boundary, and when it is less than 15°, it is classified as a low-angle grain boundary, the ratio (=A / B) of the area fraction (A) of the high-angle grain boundary on the surface of the steel sheet and the area fraction (B) of the high-angle grain boundary at a point 1 / 4 of the steel sheet thickness (t) from the surface of the steel sheet is 1.2 or more and less than 1.4.

[0126] A grain boundary within a final microstructure is the boundary between one grain and another adjacent grain. During the phase transformation process of the microstructure, the crystal orientation of the grain becomes distorted, and the degree to which the crystal orientation of the grain is relatively distorted can be evaluated by the angle of the misorientation direction of the grain boundary. Using the EBSD analysis method, if the misorientation direction of the grain boundary within the final microstructure is 15° or more and 180° or less, it can be classified as a high-angle grain boundary, and if it is less than 15°, it can be classified as a low-angle grain boundary.

[0127] The area fraction (A) of high-angle grain boundaries on the surface of a steel plate may be the ratio of high-angle grain boundaries to the total grain boundaries on the surface of the steel plate. Meanwhile, the ratio may be evaluated as the average of values ​​measured in an area having a limited arbitrary area. For example, the ratio may be evaluated as the average of the area fractions of high-angle grain boundaries measured in an area of ​​1 mm in width and 1 mm in length at each of the 5-division points after dividing a straight line connecting one end to the other in the longitudinal direction of the surface of the steel plate into 5 equal intervals. However, the technical idea of ​​the present invention is not limited to the above-described exemplary evaluation method.

[0128] Meanwhile, the point 1 / 4 of the sheet thickness (t) from the steel plate surface is the point corresponding to the midpoint in the thickness direction in the area between the steel plate surface and the center of the steel plate, and the area fraction (B) of the high-angle grain boundary at the point 1 / 4 of the sheet thickness (t) from the steel plate surface may be the ratio of the high-angle grain boundary to the total grain boundary in the plane area corresponding to the point 1 / 4 of the sheet thickness (t) from the steel plate surface. Meanwhile, the ratio may be evaluated as the average of the values ​​measured in an area having a limited arbitrary area. For example, the straight line connecting one end to the other end in the length direction at the point 1 / 4 of the sheet thickness (t) from the steel plate surface may be divided into five equal intervals, and then the area fraction of the high-angle grain boundary measured in an area of ​​1 mm in width and 1 mm in length at each of the five divided points may be evaluated. However, the technical idea of ​​the present invention is not limited to the above-described exemplary evaluation method.

[0129] If the ratio (HLR) of the area fraction (A) of high-angle grain boundaries on the steel plate surface and the area fraction (B) of high-angle grain boundaries at a point 1 / 4 of the steel plate thickness (t) from the steel plate surface is less than 1.2, the LME phenomenon occurs in which molten zinc (Zn) penetrates through the high-angle grain boundaries during spot welding. If it is 1.4 or more, the martensite fraction in the surface layer is insufficient, resulting in an unsatisfactory material quality.

[0130] When spot welding is performed on the above-mentioned plated steel sheet (100) under the conditions of a welding electrode tip diameter of 6 mm, a pressing force of 3.5 kN, and a welding current of 6.0 kA to 7.5 kA, the maximum length of a liquid metal embrittlement (LME) crack in the steel sheet (110) may be 50 μm or less. Specifically, the plated steel sheet (100) is characterized in that the applicable welding current range is 6.0 kA to 7.5 kA, and the possibility of occurrence of liquid metal embrittlement (LME), in which the plating layer (120) melts during welding on the plated steel sheet (100) and penetrates the surface of the steel sheet (110) to cause embrittlement, is 0%.

[0131] Figure 5 is a photograph showing the appearance of liquid metal embrittlement cracks occurring in a plated steel sheet as a comparative example of the present invention.

[0132] When spot welding is performed on a typical galvanized steel sheet in an automobile assembly line, the plating layer may melt, causing the molten zinc to penetrate the interface of the residual austenite present on the surface of the galvanized steel sheet, which can induce liquid metal embrittlement (LME). This LME phenomenon causes liquid metal embrittlement cracks (LME cracks), which rapidly reduces the weld strength and thus the weld current range. In other words, when spot welding ultra-high-strength steel galvanized steel sheets (UHSS), the melting point of the plating layer is very low at 420℃ in the case of hot-dip galvanized steel sheets, and in the case of alloyed galvanized steel sheets, molten zinc is formed due to the peritectic reaction at around 880℃. The formed molten zinc penetrates along the grain boundaries of the base metal in the area where the load from the welding electrode is generated at high temperatures, rapidly reducing the strength of the base metal.

[0133] Referring to Fig. 5, it can be confirmed that austenite transforms into αFe(Zn) at high temperatures due to zinc diffusing toward the parent material around the liquid metal embrittlement (LME) crack. Since αFe(Zn) is a highly brittle material, the embrittlement of the steel plate is further accelerated by this phase transformation. Therefore, the frequency of occurrence of liquid metal embrittlement (LME) is sensitive to the amount of retained austenite, and it has been confirmed that a method of avoiding austenite present in the surface layer can improve weldability.

[0134] According to the method for manufacturing an ultra-high strength plated steel sheet having excellent weldability according to the technical idea of ​​the present invention, in the step (S40) of annealing a cold rolled steel sheet having a 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 to induce a decarburization reaction in a surface layer including the surface of the steel sheet within a predetermined range, thereby implementing different carbon contents in the surface layer and the inner layer, and accordingly, the Ac3 temperatures of the surface layer and the inner layer are different, so that the phase transformation patterns (for example, the fraction of austenite) of the surface layer and the inner layer progress differently even within a predetermined annealing temperature range, thereby differently expressing the fraction of martensite in the final microstructure of the surface layer and the inner layer, thereby minimizing or suppressing the possibility of liquid metal embrittlement (LME), in which the plated layer melts and penetrates the surface of the steel sheet during welding, causing brittleness, while at the same time providing an ultra-high strength plated steel sheet.

[0135] According to the above, when manufacturing a plated steel sheet with a tensile strength of 1.2 GPa or higher, it is desirable to configure the silicon content to be 0.5 wt% or less or control the proportion of martensite to 10% or less. However, when manufacturing with a silicon content of 0.5 wt% or less, the strength and elongation of the steel sheet can be secured by adjusting the contents of other components, but there is a problem that the economic feasibility is worsened because the production cost increases due to the addition of a large amount of other elements. However, since the liquid metal embrittlement (LME) cracks that occur during spot welding occur in the plating and surface of the steel sheet, it can be understood that both material properties and liquid metal embrittlement (LME) resistance can be secured even if the silicon content is high if the martensite of the base material is secured in a proportion sufficient to secure the strength of the product and only the proportion of martensite on the surface can be controlled to 10% or less.

[0136] Experimental example

[0137] Below, preferred experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are provided solely to aid understanding of the present invention, and the present invention is not limited to the following experimental examples.

[0138] Experimental Example 1

[0139] Table 1 shows the composition (unit: weight%) of the steel sheet constituting the ultra-high-strength galvanized steel sheet according to the first experimental example of the present invention, and Table 2 shows the process conditions of the ultra-high-strength galvanized steel sheet according to the first experimental example of the present invention. The composition in Table 1 is the composition of the cold-rolled steel sheet, and furthermore, refers to the composition of the inner layer constituting the steel sheet.

[0140] IngredientsCMnSiPSFeContent0.181.82.80.0150.003Bal.

[0141] Hot rolling process reheating temperature: 1200℃, finishing rolling temperature: 900℃, coiling temperature: 600℃, heat treatment process annealing temperature: 700℃, rapid cooling temperature: 250℃, reheating temperature: 460℃, plating bath temperature: 460℃

[0142] Steel having the composition (unit: weight%) of Table 1 above is prepared, and hot-rolled steel sheets manufactured through a predetermined hot-rolling process are prepared. The remainder is iron (Fe) and other unavoidable impurities. Both examples and comparative examples have the same alloy composition. Referring to Table 2, the hot-rolling process applied the process conditions of reheating temperature: 1200℃, finish rolling temperature: 900℃, and coiling temperature: 600℃. After the annealing process, the process conditions of slow cooling temperature: 700℃, rapid cooling temperature: 250℃, and reheating temperature: 460℃ were applied. In the first experimental example, the process conditions of the maximum annealing temperature of 850℃ were applied.

[0143] H2O partial pressure (atm) Surface elevation ratio (A) 1 / 4 elevation ratio (B) HLR (A / B) Yield strength (MPa) Tensile strength (MPa) Elongation (%) LME Maximum length (㎛) Classification 0.000564650.98949.621285.224.48230Comparative example 0.000866651.02940.311272.624.24150Comparative example 0.001280651.23931126024≤30Example 0.002188651.35921.691247.423.76≤30Example 0.00391651.40837.9113421.6≤30Comparative example 0.00495651.46791.35107120.4≤30Comparative example

[0144] In Table 3, the H2O partial pressure item is the H2O partial pressure in the annealing furnace in the annealing heat treatment step, the 1 / 4 elevation ratio item means the ratio of the area fraction of high-angle grain boundaries at a point 1 / 4 of the steel sheet thickness (t) from the surface, specifically, the ratio (B) of high-angle grain boundaries among the total grain boundaries in a plane area corresponding to a point 1 / 4 of the steel sheet thickness (t) from the steel sheet surface, the surface elevation ratio item means the area fraction of high-angle grain boundaries at the surface, specifically, the ratio (A) of high-angle grain boundaries among the total grain boundaries at the steel sheet surface, the HLR item is the ratio (=A / B) of the area fraction (A) of high-angle grain boundaries at the steel sheet surface and the area fraction (B) of high-angle grain boundaries at a point 1 / 4 of the steel sheet thickness (t) from the steel sheet surface, and the yield strength, tensile strength, elongation, and LME maximum length are property values ​​implemented in the coated steel sheet. FIG. 6 is a graph showing the surface and the surface of the steel sheet in the first experimental example of the present invention. This is a drawing in which the EBSD analysis method is used to classify the final microstructure at a point 1 / 4 of the steel sheet thickness (t) from the steel sheet surface into a high-angle grain boundary if the misorientation direction of the grain boundary in the final microstructure is 15° or more and 180° or less, and into a low-angle grain boundary if it is less than 15°.

[0145] Referring to Table 3 and Fig. 6, when the ratio (=A / B) of the area fraction (A) of high-angle grain boundaries on the surface of the steel plate and the area fraction (B) of high-angle grain boundaries at a point 1 / 4 of the steel plate thickness (t) from the surface of the steel plate is 1.2 or more and less than 1.4 (Example), it can be confirmed that the coated steel plate satisfies the following properties: yield strength: 850 MPa or more and 1070 MPa or less, tensile strength: 1180 MPa or more and less than 1470 MPa, elongation: 14% or more, and is not susceptible to liquid metal embrittlement (LME).

[0146] In contrast, when the H2O partial pressure in the annealing furnace is less than 0.001 atm and the ratio of the area fraction (A) of high-angle grain boundaries on the steel plate surface to the area fraction (B) of high-angle grain boundaries at a point 1 / 4 of the steel plate thickness (t) from the steel plate surface (HLR; A / B) is less than 1.2, it can be confirmed that the steel is susceptible to liquid metal embrittlement (LME) phenomenon in which molten zinc (Zn) penetrates through high-angle grain boundaries during spot welding.

[0147] Meanwhile, if the H2O partial pressure is 0.023 atm or more and the ratio (HLR; A / B) of the area fraction (A) of high-angle grain boundaries on the steel plate surface and the area fraction (B) of high-angle grain boundaries at a point 1 / 4 of the steel plate thickness (t) from the steel plate surface exceeds 1.4, it can be confirmed that the material is inadequate because the martensite fraction in the surface layer is insufficient and the range of yield strength: 850 MPa or more and 1070 MPa or less, tensile strength: 1180 MPa or more and less than 1470 MPa is not satisfied.

[0148] Experimental Example 2

[0149] The composition (unit: weight%) of the steel sheet constituting the ultra-high strength plated steel sheet according to the second experimental example of the present invention is disclosed in Table 1, and the process conditions (excluding the annealing process) of the ultra-high strength plated steel sheet according to the second experimental example of the present invention are disclosed in Table 2.

[0150] Table 4 shows the annealing process conditions and the thickness and components of the surface layer in the method for manufacturing an ultra-high-strength galvanized steel sheet according to the second experimental example of the present invention. The component unit in Table 4 is weight %, and the temperature unit is ℃.

[0151] Dew point Surface thickness (㎛) Surface component Ac3 (surface) Ac3 (inner) Annealing maximum temperature CMnSi Comparative example 1 - 25℃ < 100.182.81.8855.8855.8800 Comparative example 2 - 25℃ < 100.182.81.8855.8855.8830 Comparative example 3 - 25℃ < 100.182.81.8855.8855.8860 Comparative example 4 - 25℃ < 100.182.81.8855.8855.8930 Comparative example 5 + 5℃ 300.012.81.8918.7855.8800 Example 1 + 5℃ 300.012.81.8918 .7855.8830Example 2+5℃300.012.81.8918.7855.8860Comparative Example 6+5℃300.012.81.8918.7855.8930Comparative Example 7+21℃1000.0012.81.8922.05855.8800Comparative Example 8+21℃1000.0012.81.8922.05855.8830Comparative Example 9+21℃1000.0012.81.8922.05855.8860Comparative Example 10+21℃1000.0012.81.8922.05855.8930

[0152] Table 5 shows the final microstructure fraction and physical properties of the ultra-high-strength galvanized steel sheet according to the second experimental example of the present invention. In Table 5, the unit of the microstructure fraction is area fraction (%), TM means tempered martensite, F means ferrite, and Ra means retained austenite.

[0153] Surface tissue fractionInner tissue fractionYield strength(MPa)Tensile strength(MPa)Elongation(%)LME maximum length(㎛)TMFRaTMFRaComparative example149.732.018.349.732.018.3850116020200Comparative example263.915.420.763.915.420.7980132014230Comparative example371.014.015.071.014.015.011 00140014240Comparative Example 470.314.115.670.314.115.6108913867.9240Comparative Example 57.391.71.049.732.018.37991090.418.8≤30Example 17.592.00.563.915.420.7921125 016≤30Example 26.093.50.571.014.015.01034131615≤30Comparative Example 687.71.211.170.314.115.6107813727.8150Comparative Example 73.296.30.549.732.018.3639.2872.3215.≤ 30Comparison Example 82.996.60.563.915.420.7736.8100011.2≤30Comparison Example 91.398.20.571.014.015.0827.21052.86.4≤30Comparison Example 1087.73.58.870.314.115.6106713587.7100

[0154] Referring to Table 4 and Table 5, in the second experimental example, it can be confirmed that in the annealing heat treatment step of Examples 1 and 2, the dew point temperature inside the annealing furnace is controlled to be -20°C or higher and +20°C or lower, and the annealing temperature satisfies the range from a temperature 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. Accordingly, the thickness of the surface layer satisfies the range of 10㎛ or more and 50㎛ or less, the carbon content of the surface layer is lower than the carbon content of the inner layer (see Table 1), and specifically, the carbon content of the surface layer satisfies 10% or less of the carbon content of the inner layer, and the physical properties of the coated steel sheet satisfies yield strength: 850MPa or more and 1070MPa or less, tensile strength: 1180MPa or more and less than 1470MPa, elongation: 14% or more, and the final microstructure of the inner layer includes, in terms of area fraction, tempered martensite: 50 to 75%, ferrite: 5 to 25%, and retained austenite: 10 to 30%, and the final microstructure of the surface layer includes, in terms of area fraction, tempered martensite: more than 0% and 10% or less, and ferrite: 90% or more and less than 100%. It can be confirmed that a small amount of retained austenite exists in the microstructure of the surface layer, but the proportion is less than 1% in terms of area fraction. In contrast, referring to Comparative Examples 1 to 4 in the second experimental example, in the step of annealing heat treatment, when the dew point temperature inside the annealing furnace is lower than -20℃, the thickness of the surface layer does not satisfy the range of 10㎛ to 50㎛ but falls below it, and the composition of the surface layer and the inner layer are not different. Specifically, it can be confirmed that the carbon content of the surface layer is not different from that of the inner layer, and accordingly, it can be confirmed that liquid metal embrittlement (LME) cracks cannot be suppressed. Furthermore, in Comparative Example 1 of the second experimental example, the maximum annealing temperature is not higher than a temperature 30℃ lower than the Ac3 temperature of the inner layer, so the area fraction of tempered martensite in the inner layer does not exceed 50%, 1.It is difficult to secure a tensile strength of 2 GPa or higher.

[0155] In addition, in Comparative Examples 3 and 4 of the second experimental example, the maximum annealing temperature exceeds the Ac3 temperature of the surface layer, so it can be confirmed that the area fraction of tempered martensite in the final microstructure of the surface layer exceeds 10%, so the yield strength exceeds the design range and is vulnerable to liquid metal embrittlement (LME).

[0156] Referring to Comparative Example 5 of the second experimental example, even if the dew point temperature in the annealing furnace is controlled to be -20℃ or higher and +20℃ or lower, if the annealing temperature is lower than the Ac3 temperature of the inner layer, which is the lower limit of the annealing temperature range, the tensile strength falls below the range of 1180MPa or higher and 1470MPa, making it difficult to secure a strength of 1.2GPa or higher.

[0157] Referring to Comparative Example 6 of the second experimental example, even if the dew point temperature in the annealing furnace is controlled to be -20℃ or higher and +20℃ or lower, if the annealing temperature exceeds the Ac3 temperature of the surface layer, 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 exceeds 10%, making it vulnerable to liquid metal embrittlement (LME).

[0158] Referring to Comparative Examples 7 to 10 of the second experimental example, it can be confirmed that in the annealing heat treatment step, when the dew point temperature inside the annealing furnace exceeds 20℃, the thickness of the surface layer does not satisfy the range of 10㎛ to 50㎛ and thus does not secure a stable material. Specifically, the tensile strength falls below the range of 1180MPa to 1470MPa or the elongation falls below the range of 14% or more.

[0159] Furthermore, in Comparative Example 10 of the second experimental example, the maximum annealing temperature exceeds the Ac3 temperature of the surface layer, so it can be confirmed that the area fraction of tempered martensite in the final microstructure of the surface layer exceeds 10%, making it vulnerable to liquid metal embrittlement (LME).

[0160] Next, a galvanized steel sheet was created by applying the composition in Table 1 and the process conditions in Table 2, and then the spot welding process was applied. The welding current was 0.5 kA lower than the spatter (expulsion) generation condition, and the LME evaluation was performed. Spot welding, which is commonly performed on automotive panels, begins with securing a sufficient nugget over as wide a current range as possible. If the current is low, a sufficient nugget size is not formed, and the weld integrity (strength, toughness), etc. are not secured, and if the current is high, the spatter (expulsion) phenomenon occurs. Here, spatter refers to the phenomenon in which the molten metal, which is melted by the resistance heat during spot welding, escapes out of the nugget.

[0161] Table 6 is a table showing the conditions of the spot welding process in the second experimental example, and Fig. 7 is a drawing showing the results of applying the spot welding process in the second experimental example. In Fig. 7, the dew point refers to the dew point according to the moisture concentration in the annealing furnace, and the general dew point refers to a dew point below -45°C (e.g., -50°C), and the high dew point refers to a dew point of 0°C.

[0162] In this second experimental example, the liquid metal embrittlement (LME) evaluation method was as follows. First, two layers of the same material were prepared, tilted by 5 degrees, and the maximum welding current that did not cause spatter was applied. The cross-section of the welded material was observed, and the maximum depth of cracking was determined as the maximum LME depth. It is generally known that liquid metal embrittlement (LME) cracks that exceed 5% of the material thickness compromise the integrity of the weld. This experimental example used 1 mm material, and cracks exceeding 50 ㎛ were determined as defective in the LME evaluation.

[0163] Weld Control Electrode tip pressure No. of pulses Welding time Holding time DCФ6mm3.5kN1350ms100ms

[0164] Referring to Table 6 and FIG. 7, in the method for manufacturing a plated steel sheet according to a comparative example of the present invention, annealing was performed at a dew point of less than -45°C, and the applicable welding current range was 6.0 kA to 6.5 kA, whereas in the method for manufacturing an ultra-high-strength plated steel sheet with excellent weldability according to an embodiment of the present invention, annealing was performed at a dew point of 0°C, and the applicable welding current range was 6.0 kA to 7.5 kA. Accordingly, in the method for manufacturing an ultra-high-strength plated steel sheet with excellent weldability according to an embodiment of the present invention, it was confirmed that the surface austenite phase, which is the cause of LME cracks in the weld zone, is suppressed, thereby securing strength even under existing welding conditions and expanding the total weldable range, thereby making it possible to suppress defects in the spot welding process.

[0165] Experimental Example 3

[0166] Table 9 shows the decarburization layer thickness and occurrence of liquid metal embrittlement (LME) according to annealing time and annealing moisture in an alloyed hot-dip galvanized steel sheet produced by applying the composition of Table 7 and the process conditions of Table 8 as a third experimental example of the present invention. In the third experimental example, the alloying temperature for the zinc plating treatment was 530°C.

[0167] Ingredients: CSiMnPSFe Content: 0.152.02.50.010.003Bal

[0168] Hot rolling process reheating temperature: 1200℃, finishing rolling temperature: 900℃, coiling temperature: 600℃, heat treatment process annealing temperature: 850℃, slow cooling temperature: 700℃, rapid cooling temperature: 250℃, reheating temperature: 460℃

[0169] Annealing temperature, Annealing time, Cold rolled annealing atmosphere, Cold rolled material, Welding, Hydrogen concentration, Furnace dew point, Furnace moisture concentration, Decarburization layer thickness, LME occurrence rate, Comparative example 1, 850℃, 60s, 5 ~ 7%, -50℃, 40ppm, 0㎛, 100%, Comparative example 2, 850℃, 60s, 5 ~ 7%, -60℃, 10ppm, 0㎛, 100%, Comparative example 3, 850℃, 90s, 5 ~ 7%, -50℃, 40ppm, 0㎛, 100%, Comparative example 4, 850℃, 130s, 5 ~ 7%, -50℃, 40ppm, 5㎛, 80%, Comparative example 5, 850℃, 60s, 5 ~ 7%, -30℃, 375ppm, 0㎛, 100%, Comparative example 6, 850℃, 90s, 5 ~ 7%, -30℃, 375ppm, 7㎛, 30%, Example 2, 850℃, 90s, 5 ~ 7%-10℃2600ppm22㎛0%Example 3850℃60s5 ~ 7%+5℃8500ppm24㎛0%Example 4850℃130s5 ~ 7%-10℃2600ppm30㎛0%Example 5850℃90s5 ~ 7%+5℃8500ppm31㎛0%Example 6850℃130s5 ~ 7%+5℃8500ppm34㎛0%

[0170] Figure 8 is the positive square root (A) of the annealing time (A) in the third experimental example of the present invention. 1 / 2 ) and the reciprocal value of the moisture concentration (B) in the annealing furnace. FIG. 9 is a table showing the thickness of the decarburization layer formed on the surface of the steel sheet according to the annealing time (A) and the moisture concentration (B) in the annealing furnace in the third experimental example of the present invention. FIG. 10 is a table showing the occurrence rate of liquid metal embrittlement (LME) according to the annealing time (A) and the moisture concentration (B) in the annealing furnace in the third experimental example of the present invention. For reference, in Fig. 8, the unit of the annealing time (A) is seconds (s), and the unit of the moisture concentration (B) is ppm. Referring to Table 7 and Figs. 8 to 10, when the annealing heat treatment step is performed under the conditions in which the annealing time (A) and the moisture concentration (B) in the annealing furnace satisfy the following equation 3 (Examples 2 to 6 of the third experimental example), i) the possibility of liquid metal embrittlement (LME), in which the zinc-plated layer melts during welding on the plated steel sheet and penetrates the surface of the steel sheet to cause brittleness, is 0%, and ii) the thickness of the decarburization layer formed on the surface of the steel sheet is 10 ㎛ or more.

[0171] <Formula 3>

[0172] (A) 1 / 2 × ln(1 / B) ≤ -65

[0173] In contrast, when the annealing heat treatment step is performed under conditions where the annealing time (A) and the moisture concentration (B) in the annealing furnace do not satisfy the above formula 3 (Comparative Examples 1 to 6 of the third experimental example), i) the possibility of liquid metal embrittlement (LME), in which the zinc plating layer melts during welding on the plated steel sheet and penetrates the surface of the steel sheet, causing brittleness, appears to be significant, and ii) the thickness of the decarburization layer formed on the surface of the steel sheet is confirmed to be less than 10㎛.

[0174] That is, in the method for manufacturing the above-described ultra-high strength galvanized steel sheet with excellent weldability, it can be confirmed that the annealing heat treatment can be controlled so that the moisture concentration (B) in the annealing furnace increases as the annealing time (A) for performing the annealing heat treatment becomes shorter, and the annealing heat treatment can be controlled so that the moisture concentration (B) in the annealing furnace decreases as the annealing time (A) for performing the annealing heat treatment becomes longer.

[0175] Accordingly, in the method for manufacturing an ultra-high strength galvanized steel sheet with excellent weldability according to one aspect of the present invention, when performing annealing heat treatment on a cold rolled steel sheet in an annealing furnace, the annealing time (A) for performing the annealing heat treatment and the moisture concentration (B) in the annealing furnace are the square root of the positive annealing time (A 1 / 2 ) and the product of the natural logarithm of the reciprocal value of the moisture concentration (ln(1 / B)).

[0176] As we have seen, the positive square root of the annealing time (A) is 1 / 2) and the reciprocal value of moisture concentration (B) and the natural logarithm (ln(1 / B)) of the first parameter disclosed in the above formula 3 is a factor that can effectively control the decarburization layer thickness and liquid metal embrittlement (LME) occurrence rate in ultra-high-strength galvanized steel sheets at the same time, and it can be said that it has technical significance. For example, in Comparative Examples 3, 6, Example 2, and Example 5 of the third experimental example, the positive square root (A) of the annealing time (A 1 / 2 ) is the same, but since the decarburization layer thickness varies greatly depending on the natural logarithm of the reciprocal value of the moisture concentration (B) (ln(1 / B)), it is not possible to effectively control both the decarburization layer thickness and the liquid metal embrittlement (LME) occurrence rate simply by the annealing time (A) or the moisture concentration (B). In this respect, the first parameter described above has a better effect and causal relationship, and is not simply a different way of expressing a known property, so it can be said to have technical significance.

[0177] Meanwhile, referring to Table 9, it can be seen that the higher the dew point and the longer the annealing time, the greater the decarburization amount and the better the ability to avoid liquid metal embrittlement (LME). To improve LME, it may be necessary to secure a decarburization layer thickness of 10 µm or more.

[0178] While the above description focuses on specific embodiments of the present invention, those skilled in the art will appreciate that various modifications and variations can be made. As long as such modifications and variations do not depart from the scope of the present invention, they are considered to be within the scope of the present invention. Therefore, the scope of the present invention should be determined by the claims set forth below.

Claims

1. A plated steel sheet comprising a steel sheet comprising an inner layer and a surface layer on the inner layer; and a plating layer on the steel sheet; The inner layer contains, in weight %, carbon (C): 0.1 to 0.3%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 and 0.01% or less, and the remainder includes iron (Fe) and other inevitable impurities, and the final microstructure of the inner layer has an area fraction of tempered martensite of 50% or more, The surface layer has a lower carbon content than the inner layer, and contains, in wt%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): 0 to 0.02% or less, sulfur (S): 0 to 0.01% or less, and the remainder includes iron (Fe) and other inevitable impurities, and the final microstructure of the surface layer has an area fraction of tempered martensite of 10% or less. When the angle of the misorientation direction of the grain boundary in the final microstructure of the steel plate is 15° or more and 180° or less, it is classified as a high-angle grain boundary, and the ratio of the area fraction of the high-angle grain boundary on the surface and the area fraction of the high-angle grain boundary at a point 1 / 4 of the steel plate thickness (t) from the surface is characterized by being 1.2 or more and less than 1.

4. Ultra high strength galvanized steel plate.

2. In paragraph 1, The above surface layer has a thickness of 10 ㎛ or more and 50 ㎛ or less from the surface of the steel plate. Ultra high strength galvanized steel plate.

3. In paragraph 1, The above-mentioned galvanized steel sheet is characterized by a yield strength of 850 MPa or more and 1070 MPa or less, a tensile strength of 1180 MPa or more and less than 1470 MPa, and an elongation of 14% or more. Ultra high strength galvanized steel plate.

4. In paragraph 1, The surface layer is characterized in that the carbon content is 10% or less of the carbon content of the inner layer. Ultra high strength galvanized steel plate.

5. In paragraph 1, The final microstructure of the inner layer includes, in area fraction, tempered martensite: 50 to 75%, ferrite: 5 to 25%, and retained austenite: 10 to 30%. The final microstructure of the surface layer includes, in area fraction, tempered martensite: more than 0% and less than 10%, ferrite: more than 90% and less than 100%. Ultra high strength galvanized steel plate.

6. In paragraph 1, When spot welding is performed on the above-mentioned galvanized steel plate under the conditions of a welding electrode tip diameter of 6 mm, a pressure of 3.5 kN, and a welding current of 6.0 kA to 7.5 kA, the maximum length of a liquid metal embrittlement (LME) crack in the steel plate is 50 μm or less. Ultra high strength galvanized steel plate.

7. A method for manufacturing a plated steel sheet, comprising: a first step of providing a steel sheet comprising an inner layer and a surface layer on the inner layer; and a second step of forming a plated layer on the steel sheet; The inner layer contains, in weight %, carbon (C): 0.1 to 0.3%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 and 0.01% or less, and the remainder includes iron (Fe) and other inevitable impurities, and the final microstructure of the inner layer has an area fraction of tempered martensite of 50% or more, The surface layer has a lower carbon content than the inner layer, and contains, in wt%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): 0 to 0.02% or less, sulfur (S): 0 to 0.01% or less, and the remainder includes iron (Fe) and other inevitable impurities, and the final microstructure of the surface layer has an area fraction of tempered martensite of 10% or less. When the angle of the misorientation direction of the grain boundary in the final microstructure of the steel plate is 15° or more and 180° or less, it is classified as a high-angle grain boundary, and the ratio of the area fraction of the high-angle grain boundary on the surface and the area fraction of the high-angle grain boundary at a point 1 / 4 of the steel plate thickness (t) from the surface is characterized by being 1.2 or more and less than 1.

4. Method for manufacturing ultra-high strength galvanized steel sheet.

8. In paragraph 7, The above-mentioned galvanized steel sheet is characterized by a yield strength of 850 MPa or more and 1070 MPa or less, a tensile strength of 1180 MPa or more and less than 1470 MPa, and an elongation of 14% or more. Method for manufacturing ultra-high strength galvanized steel sheet.

9. In paragraph 7, The first step sequentially includes the steps of: providing a steel material containing, in wt%, carbon (C): 0.1 to 0.3%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 2.5 to 3.5%, phosphorus (P): 0 to 0.02% or less, sulfur (S): 0 to 0.01% or less, and the remainder iron (Fe) and other inevitable impurities; hot rolling the steel material; softening and pickling the hot-rolled steel material; cold rolling the hot-rolled steel material; annealing and heat-treating 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; In the above annealing heat treatment step, the H2O partial pressure in the annealing furnace is controlled to be 0.001 atm or more and less than 0.023 atm, the lower limit of the annealing temperature range is 30℃ 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 above cooling step is performed under the conditions of the first cooling rate: 1 to 10°C / s, and the cooling end temperature: 600°C or higher and less than 800°C. The above rapid cooling step is performed under the conditions of the second cooling speed: 30 to 100°C / s, and the cooling end temperature: 150°C or higher and less than 300°C. The above reheating step is characterized in that it is performed at a temperature exceeding 350℃ and less than 500℃ and a holding time of less than 60 seconds. Method for manufacturing ultra-high strength galvanized steel sheet.

10. In paragraph 7, The surface layer is characterized in that the carbon content is 10% or less of the carbon content of the inner layer and has a thickness of 10 ㎛ or more and 50 ㎛ or less from the surface of the steel plate. Method for manufacturing ultra-high strength galvanized steel sheet.

11. In paragraph 9, After the above annealing heat treatment step, the steel plate is divided into an inner layer having different carbon contents and a surface layer on the inner layer, and the microstructure of the inner layer includes, in terms of area fraction, austenite: 75 to 95% and ferrite: 5 to 25%, and the microstructure of the surface layer includes, in terms of area fraction, austenite: more than 0% and less than 10%, and ferrite: 90% or more and less than 100%. After the above-mentioned slow cooling step, the microstructure of the inner layer includes, in terms of area fraction, austenite: 75 to 95%, ferrite: 5 to 25%, and the microstructure of the surface layer includes, in terms of area fraction, austenite: more than 0% but less than 10%, and ferrite: 90% or more and less than 100%. After the above-mentioned rapid cooling step, the microstructure of the inner layer includes, in terms of area fraction, martensite: 50 to 75%, ferrite: 5 to 25%, and retained austenite: 10 to 30%, and the microstructure of the surface layer includes, in terms of area fraction, martensite: more than 0% but less than 10%, and ferrite: 90% or more and less than 100%. After the reheating step, the final microstructure of the inner layer includes, in terms of area fraction, tempered martensite: 50 to 75%, ferrite: 5 to 25%, and retained austenite: 10 to 30%, and the final microstructure of the surface layer includes, in terms of area fraction, tempered martensite: more than 0% but less than 10%, and ferrite: 90% or more and less than 100%. Method for manufacturing ultra-high strength galvanized steel sheet.

12. In paragraph 7, The above hot rolling step is performed under the conditions of reheating temperature: 1150 to 1250℃, finishing rolling temperature: 850 to 1000℃, and coiling temperature: 500 to 700℃. The step of softening heat treating the hot-rolled steel includes a step of softening heat treating at a temperature of 500°C to 650°C. Method for manufacturing ultra-high strength galvanized steel sheet.

13. In paragraph 7, The above cold rolling step is performed under the condition of a reduction ratio of 40 to 60%. Method for manufacturing ultra-high strength galvanized steel sheet.

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