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

Abstract

The present invention includes a step of annealing and heat-treating a cold-rolled steel sheet in an annealing furnace, and the annealing time (A) for performing the annealing heat treatment and the moisture concentration (B) in the annealing furnace are based on the product of the positive square root of the annealing time (A 1 / 2 ), and the value obtained by taking the natural logarithm of the reciprocal of the moisture concentration (ln(1 / B))). Provided is a method for manufacturing an ultra-high strength galvanized steel sheet with excellent weldability, characterized in that the method is controlled according to the above criteria.

Description

[Technical Field]

[0001] This invention relates to steel sheets and methods for manufacturing the same, and more particularly to ultra-high-strength galvanized steel sheets with excellent weldability and methods for manufacturing the same. [Background technology]

[0002] The automotive industry is focusing on improving fuel efficiency and reducing weight to meet the demands of the times, such as resource depletion, the rapid progression of global warming, and high oil prices. Furthermore, as regulations on passenger safety become increasingly stringent, there is a growing demand for ultra-high-strength steel. There is also a demand for various improvements to galvanized steel sheets, which have excellent sacrificial corrosion protection and, when exposed to corrosive environments, prevent steel corrosion by allowing the lower-potential zinc to dissolve first. For example, there is a growing demand to improve the problem of liquid metal embrittlement (LME), which occurs during spot welding on automobile assembly lines when the galvanized layer melts and molten metallic zinc penetrates to the interface with retained austenite on the surface of the base iron, inducing brittleness.

[0003] A relevant prior art document is Korean Patent Publication No. 10-2020-0075949. [Overview of the project] [Problems that the invention aims to solve]

[0004] The problem that this invention aims to solve is to provide an ultra-high-strength galvanized steel sheet with excellent weldability and a method for manufacturing the same.

[0005] However, these challenges are illustrative, and the technical concept of the present invention is not limited to them. [Means for solving the problem]

[0006] A method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability according to one aspect of the present invention includes the step of performing an annealing heat treatment on a cold-rolled steel sheet in an annealing furnace, wherein the annealing time (A) for the annealing heat treatment and the moisture concentration (B) in the annealing furnace are the positive square root (A) of the annealing time. 1 / 2 The control is based on the product of (1 / B) and the value obtained by taking the natural logarithm of the reciprocal of the moisture concentration (ln(1 / B)).

[0007] In the method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability, the annealing heat treatment can be controlled such that the shorter the annealing time (A) of the annealing heat treatment, the higher the moisture concentration (B) in the annealing furnace.

[0008] In the method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability, the annealing heat treatment can be controlled such that the annealing time (A) of the annealing heat treatment becomes longer as the moisture concentration (B) in the annealing furnace decreases.

[0009] In the method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability, the cold-rolled steel sheet contains, by weight percent, carbon (C): 0.1-0.5%, silicon (Si): 1.0-3.0%, manganese (Mn): 1.5-3.5%, phosphorus (P): more than 0% and 0.02% or less, sulfur (S): more than 0% and 0.01% or less, aluminum (Al): more than 0% and 0.1% or less, nitrogen (N): more than 0% and 0.01% or less, and may contain the remaining iron (Fe) and other unavoidable impurities.

[0010] In the method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability, the annealing heat treatment step may include a step in which the temperature is within the two-phase region of austenite and ferrite, thereby inducing a decarburization reaction on the surface of the steel sheet and transforming the austenite present on the surface layer of the steel sheet into ferrite.

[0011] In the method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability, the annealing heat treatment step can be carried out under the condition of an annealing temperature of 830 to 900°C.

[0012] In the method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability, the annealing heat treatment step may be performed under conditions where the annealing time (A) and the moisture concentration in the annealing furnace (B) satisfy the following formula 1. However, the unit of the annealing time (A) is seconds (s), and the unit of the moisture concentration in the annealing furnace (B) is ppm.

[0013] [Formula 1]

[0014]

number

[0015] The method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability may further include, after the annealing heat treatment step, a step of primary cooling the steel sheet to a temperature of 600°C or higher and less than 800°C at an average cooling rate of 1 to 20°C / s; a step of secondary cooling the steel sheet to 200°C or higher and less than 300°C at an average cooling rate of 20°C / s or higher; a step of reheating the steel sheet to 350°C to 490°C and maintaining it for 100 seconds or less; and a step of performing a galvanizing treatment on the steel sheet.

[0016] In the method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability, after the galvanizing treatment, the steel sheet has a ferrite single-phase microstructure on its surface, and the thickness of the decarburized layer formed on the surface of the steel sheet may be 18 μm or more.

[0017] In the method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability, the galvanized steel sheet may have an applicable welding current range of 6.0kA to 7.5kA.

[0018] The ultra-high strength galvanized steel sheet excellent in weldability according to another aspect of the present invention contains, in terms of weight %, carbon (C): 0.1 to 0.5%, silicon (Si): 1.0 to 3.0%, manganese (Mn): 1.5 to 3.5%, phosphorus (P): more than 0% and 0.02% or less, sulfur (S): more than 0% and 0.01% or less, aluminum (Al): more than 0% and 0.1% or less, nitrogen (N): more than 0% and 0.01% or less, and a base iron containing the remaining iron (Fe) and other inevitable impurities; and a galvanized layer formed on the base iron. The surface layer of the base iron in contact with the galvanized layer has a fine structure of a ferrite single phase, the thickness of the decarburized layer formed on the surface layer of the base iron is 18 μm or more, and the fine structure of the base iron consists of 0 to 40% ferrite, 10 to 30% retained austenite, and the balance martensite.

[0019] In the ultra-high strength galvanized steel sheet excellent in weldability, the range of the applicable welding current of the galvanized steel sheet is 6.0 kA to 7.5 kA, and when welding the galvanized steel sheet, the possibility of the occurrence of liquid metal embrittlement (LME) in which the galvanized layer melts and penetrates the surface of the base iron to induce brittleness can be 0%.

Effect of the Invention

[0020] According to the present invention, it is possible to embody an ultra-high strength galvanized steel sheet excellent in weldability and a method for manufacturing the same.

[0021] The effects of the present invention described above are described exemplarily, and the scope of the present invention is not limited by such effects.

Brief Description of the Drawings

[0022] [Figure 1] It is a flowchart illustrating a method for manufacturing an ultra-high strength galvanized steel sheet according to an embodiment of the present invention. [Figure 2] It is a graph illustrating subsequent heat treatment (annealing, cooling, reheating) steps applied to a cold-rolled steel sheet in a method for manufacturing an ultra-high strength galvanized steel sheet excellent in weldability according to an embodiment of the present invention. [Figure 3]This is a comparative example of the present invention, a photograph showing the occurrence of liquid metal embrittlement cracking in a galvanized steel sheet. [Figure 4] This figure illustrates the outline of the decarburization reaction in a method for manufacturing an ultra-high-strength galvanized steel sheet according to one embodiment of the present invention. [Figure 5] This is a cross-sectional photograph of a steel material in the first experimental example of the present invention, under the conditions of a Gleeble test temperature of 800°C and a general dew point (DP < -45°C). [Figure 6] This is a photograph showing the evaluation results of the elongation test in the first experimental example of the present invention, under the conditions of a Gleeble test temperature of 800°C and a typical dew point (DP < -45°C). [Figure 7] This is a photograph taken in the first experimental example of the present invention, showing whether or not liquid metal embrittlement (LME) cracking occurred under the conditions of a Gleeble test temperature of 800°C and a typical dew point (DP < -45°C). [Figure 8] This is a cross-sectional photograph of a steel material in the first experimental example of the present invention, under the conditions of a Gleeble test temperature of 800°C and a high dew point (DP0°C). [Figure 9] This is a photograph showing the evaluation results of the elongation test in the first experimental example of the present invention, under the conditions of a Gleeble test temperature of 800°C and a high dew point (DP0°C). [Figure 10] This is a photograph taken in the first experimental example of the present invention, showing whether or not liquid metal embrittlement (LME) cracking occurred under the conditions of a Gleeble test temperature of 800°C and a high dew point (DP0°C). [Figure 11] This figure shows the product of the positive square root of the annealing time (A) (A1 / 2) and the value obtained by taking the natural logarithm of the reciprocal of the moisture content in the annealing furnace (B) (ln(1 / B)) in the second experimental example of the present invention. [Figure 12] This figure shows the thickness of the decarburized layer formed on the surface of the base iron in a second experimental example of the present invention, depending on the annealing time (A) and the moisture content in the annealing furnace (B). [Figure 13] This figure shows the incidence rate of liquid metal embrittlement (LME) in a second experimental example of the present invention, depending on the annealing time (A) and the moisture concentration in the annealing furnace (B). [Figure 14] This figure shows the results of applying the spot welding process from the third experimental example. [Modes for carrying out the invention]

[0023] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. These embodiments are provided to further fully illustrate the technical idea of ​​the invention to those ordinary skill in the art, and the embodiments described below can be modified into various other forms; the scope of the technical idea of ​​the invention is not limited to these embodiments. Rather, these embodiments are provided to further enrich and complete this disclosure and to fully communicate the technical idea of ​​the invention to those skilled in the art. Throughout this specification, the same reference numerals mean the same elements. Furthermore, various elements and areas in the drawings are shown schematically. Therefore, the technical idea of ​​the invention is not limited by the relative sizes or spacing shown in the accompanying drawings.

[0024] Figure 1 is a flowchart illustrating a method for manufacturing an ultra-high-strength galvanized steel sheet according to one embodiment of the present invention.

[0025] Referring to Figure 1, a method for manufacturing an ultra-high-strength galvanized steel sheet according to one embodiment of the present invention includes the steps of: providing a steel material (S10); hot rolling the steel material to form a hot-rolled steel sheet (S20); cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet (S30); annealing the cold-rolled steel sheet (S40); primary and secondary cooling of the annealed steel sheet (S50, S60); reheating the cooled steel sheet (S70); and galvanizing the steel sheet (S80).

[0026] The present invention provides a method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability, comprising the step (S40) of performing an annealing heat treatment on a cold-rolled steel sheet in an annealing furnace, wherein the annealing time (A) and the moisture content (B) in the annealing furnace are the positive square root of the annealing time (A 1 / 2The process is controlled based on the product of (A) and the value obtained by taking the natural logarithm of the reciprocal of the moisture concentration (ln(1 / B)). For example, the annealing heat treatment step (S40) may be performed under conditions where the annealing time (A) and the moisture concentration (B) in the annealing furnace satisfy the following formula 1.

[0027] [Formula 1]

[0028]

number

[0029] As an example of a method for manufacturing the aforementioned ultra-high-strength galvanized steel sheet with excellent weldability, the annealing heat treatment can be controlled such that the shorter the annealing time (A) of the annealing heat treatment, the higher the moisture concentration (B) in the annealing furnace.

[0030] As another example of a method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability, the annealing heat treatment can be controlled such that the annealing time (A) of the annealing heat treatment becomes longer as the moisture concentration (B) in the annealing furnace decreases.

[0031] With this configuration, the possibility of liquid metal embrittlement (LME), in which the zinc plating layer melts and penetrates the surface of the base iron during welding to the galvanized steel sheet, thereby inducing brittleness, can be suppressed, and the thickness of the decarburized layer formed on the surface of the steel sheet can be 18 μm or more.

[0032] This invention is not limited to the superficial understanding that lowering the hardness of the surface layer through surface decarburization alleviates liquid metal embrittlement (LME), but recognizes the problem that optimizing the decarburized layer is difficult based solely on the dew point in the annealing furnace. Therefore, in order to secure optimal decarburization conditions, the invention discloses the aforementioned technical concept based on an integrated consideration of annealing time and dew point with respect to annealing temperature.

[0033] The following describes in detail a method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability according to one embodiment of the present invention.

[0034] Steel material supply step (S10)

[0035] Recently, the automotive industry has shown increasing interest in lightweight vehicle bodies using ultra-high-strength steel to simultaneously meet collision stability and fuel efficiency regulations. In response to these demands from automotive manufacturers, the steel industry is actively conducting research on the development of ultra-high-strength steel materials. The Q&P (Quenching and Partitioning) heat treatment technology, developed to simultaneously ensure high strength and high ductility in automotive steel materials, suppresses the formation of carbon carbide precipitates released from the martensitic structure during quenching and allows carbon to diffuse into the retained austenite structure through partitioning. Through carbon re-diffusion, the retained austenite structure is stabilized even at room temperature, ultimately enabling the securing of high ductility from the retained austenite structure and high strength from the martensitic structure.

[0036] Such Q&P steel sheets contain more substitutional solid solution alloy elements such as silicon (Si) and aluminum (Al) than ordinary steel to suppress the formation of carbide precipitates within the microstructure, thereby hindering the movement of iron (Fe) atoms. They also contain large amounts of austenite-stabilizing alloy elements such as carbon (C) and manganese (Mn) to increase the volume fraction of the stabilized retained austenite microstructure and improve the TRIP (Transformation-Induced Plasticity) behavior.

[0037] On the other hand, while it is obvious that the technical concept of the present invention can be applied to the aforementioned Q&P steel sheet, it is clear that the technical concept of the present invention can be broadly applied to various steel sheets and is not limited to being applied only to the aforementioned Q&P steel sheet.

[0038] The roles and contents of exemplary components contained in the ultra-high-strength cold-rolled steel sheet according to one embodiment of the present invention are as follows. In this case, all component element content is expressed in weight percent.

[0039] Carbon (C): 0.1~0.5%

[0040] Carbon is the most important alloying element in steelmaking, primarily serving the roles of fundamental strengthening and austenite stabilization. A high carbon concentration in austenite improves austenite stability, making it easier to secure appropriate austenite for improved material properties. If the carbon content is less than 0.1%, it is difficult to secure the desired yield strength and elongation. If the carbon content exceeds 0.5%, it may lead to a decrease in weldability due to the increase in carbon equivalent. Therefore, the carbon content is preferably 0.1% to 0.5% of the total weight of the steel sheet.

[0041] Silicon (Si): 1.0-3.0%

[0042] Silicon is an element that inhibits the formation of carbides (e.g., Fe3C) in ferrite and increases the diffusion rate of austenite by increasing the activity of carbon. Silicon is also well known as a ferrite-stabilizing element and is known to increase ferrite fraction and ductility during cooling. If the silicon content is less than 1.0%, the effect of silicon addition is insufficient. 3.0% If the silicon content exceeds this amount, oxides (SiO2) may form on the surface of the steel sheet during the process, leading to a decrease in plating quality due to poor wettability in that area. Therefore, the silicon content is preferably 1.0 to 3.0% of the total weight of the steel sheet.

[0043] Manganese (Mn): 1.5-3.5%

[0044] Manganese is an austenite-stabilizing element. The addition of manganese gradually lowers the martensitic transformation initiation temperature (Ms), which increases the residual austenite fraction during the continuous annealing heat treatment process. If the manganese content is less than 1.5%, the effect of manganese addition is insufficient. 3.5If the manganese content exceeds 1%, it can significantly reduce weldability by increasing the carbon equivalent, and oxides (MnO) may form on the surface of the steel sheet during the process, leading to a decrease in plating quality due to poor wettability in that area. Therefore, the manganese content is preferably 1.5 to 3.5% of the total weight of the steel sheet.

[0045] Phosphorus (P): Over 0% ~ 0.02%

[0046] Phosphorus can perform a role similar to silicon in steel. However, if phosphorus is present in the total weight of the steel plate... 0.02 If added in amounts exceeding %, it can reduce the weldability of the steel sheet, increase its brittleness, and cause a decrease in material quality. Therefore, it is preferable to limit the phosphorus content to more than 0% to 0.02% of the total weight of the steel sheet.

[0047] Sulfur (S): More than 0% ~ 0.01% or less

[0048] Sulfur is an element that is inevitably present during the manufacture of steel. It inhibits the toughness and weldability of steel, and by combining with manganese (Mn) to form MnS, it reduces the corrosion resistance and impact properties of steel. Therefore, it is preferable to limit the sulfur content to more than 0% to 0.01% or less of the total weight of the steel plate.

[0049] Aluminum (Al): Over 0% and under 0.1%

[0050] Aluminum (Al) acts similarly to silicon (Si), primarily playing a role in solid solution strengthening and inhibiting carbide formation. It is mainly added for deoxidation purposes, effectively suppressing carbide formation and promoting the formation of a retained austenite phase, thus improving the strength-elongation balance. If the aluminum (Al) content exceeds 0.1%, the workability of the steel sheet may deteriorate due to an increase in inclusions such as alumina. Therefore, it is preferable to add aluminum (Al) at a content of 0.1% or less of the total weight of the steel sheet.

[0051] Nitrogen (N): Greater than 0% and less than or equal to 0.01%

[0052] Nitrogen (N) is an element that degrades aging resistance, and when the nitrogen content exceeds 0.01%, the deterioration of aging resistance becomes significant. Furthermore, nitrogen (N) consumes boron (B) by combining with it to form BN. Therefore, nitrogen (N) reduces the hardenability due to solid solution boron (B), making it difficult to secure a tempered martensite phase with a predetermined area ratio. In addition, nitrogen (N) exists as an impurity element in ferrite and reduces ductility due to strain aging. For this reason, a low nitrogen (N) content is preferable. Therefore, it is preferable to limit the nitrogen (N) content to more than 0% to 0.01% or less of the total weight of the steel sheet.

[0053] On the other hand, in addition to the above-mentioned composition, the ultra-high-strength cold-rolled steel sheet according to one embodiment of the present invention may further selectively contain any combination of the following components.

[0054] At least one of the following: titanium (Ti), niobium (Nb), and vanadium (V): greater than 0% and less than or equal to 0.1%

[0055] Titanium (Ti), niobium (Nb), and vanadium (V) are major elements that precipitate in steel in the form of carbides. In this invention, the purpose is to ensure the stability and improve the strength of retained austenite through the refinement of initial austenite grains by the formation of precipitates, and to refine ferrite grains and achieve precipitation hardening through the presence of precipitates in ferrite. In other words, these are elements that precipitate in the form of NbC, NbN, TiC, TiN, VC, or VN by bonding with carbon (C) or nitrogen (N), or that improve the strength of steel sheets through solid solution strengthening in iron (Fe). For example, titanium (Ti) is effective in improving strength by forming carbonitrides and sulfides, and by bonding with nitrogen to precipitate as titanium nitride (TiN), the formation of boron nitride (BN) can be suppressed, thus effectively enabling hardenability due to boron (B). Niobium (Nb), when added in small amounts, forms NbC precipitates through bonding with carbon (C), contributing to improved strength of the base material through a precipitation strengthening effect. Vanadium (V), like niobium (Nb), is added as a strengthening element.

[0056] If the sum of the aforementioned elements added exceeds 0.1% of the total steel sheet, not only will manufacturing costs increase, but excessive precipitates may be formed in the ferrite phase, leading to excessive precipitation strengthening and a decrease in the elongation of the steel sheet. This can also result in a decrease in low-temperature toughness and weldability due to the large amount of precipitates. Therefore, the amount should be limited to 0.1% or less.

[0057] Chromium (Cr): Over 0% and under 0.8%

[0058] Chromium (Cr) is an element with high hardening potential and is added to increase strength through transformation strengthening. However, if the amount of chromium (Cr) exceeds 0.8% by weight, the toughness decreases due to overall non-uniformity while a structure such as upper bainite is formed. Therefore, it is preferable to control its content to 0.8% by weight or less.

[0059] Molybdenum (Mo): Over 0% and under 0.5%

[0060] Molybdenum (Mo) is an element with greater hardening ability than chromium (Cr) and is added to increase strength through transformation strengthening. If the carbon (C) content exceeds 0.5% by weight within the range of the carbon (C) component of the present invention, toughness decreases due to the formation of a large amount of light secondary phases such as martensite / austenite (MA) phases; therefore, it is preferable to control its content to 0.5% by weight.

[0061] The remaining component of the aforementioned ultra-high-strength cold-rolled steel sheet is iron (Fe). However, in the normal steelmaking process, unintended impurities from the raw materials and surrounding environment inevitably become mixed in, and it is not possible to eliminate them. Since these impurities are known to any engineer in a normal manufacturing process, their details will not be specifically mentioned in this specification.

[0062] In the manufacturing method according to the present invention, the semi-finished product subject to the hot rolling and cold rolling processes may, for example, be a slab. A slab in a semi-finished state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0063] Hot rolling step (S20)

[0064] Step (S20) is performed to form a hot-rolled steel sheet by applying a hot-rolling process to the aforementioned steel material.

[0065] Since the aforementioned steel material is a high-alloy steel, it is necessary to minimize edge breakage and rolling load in order to ensure mass production, so the rolling finish temperature and coiling temperature can be set to a high-temperature range.

[0066] The steel material is reheated at a temperature of Ac3 or higher, for example, at a reheating temperature (Slab Reheating Temperature, SRT) in the range of 1150°C to 1250°C. Through such reheating, redissolution of segregated components and precipitates during casting can occur. If the reheating temperature is below 1150°C, a problem may arise in which the hot rolling load increases rapidly. If the reheating temperature exceeds 1250°C, warping of the slab may make charging and ejection in the heating furnace difficult, and coarsening of the initial austenite grains may make it difficult to ensure the strength of the final produced steel sheet. The reheating temperature can be varied depending on the steel material.

[0067] Next, the reheated steel material can be hot-rolled, for example, at a finish delivery temperature (FDT) of 850°C to 1000°C. If the finish delivery temperature exceeds 1000°C, the quality of the steel sheet may deteriorate due to the formation of scale on the surface of the steel sheet. Also, if the finish delivery temperature is below 850°C, it may result in an increase in rolling load and a decrease in productivity. The finish delivery temperature can be varied depending on the steel material.

[0068] Next, the hot-rolled steel material is cooled at a cooling rate of 10 to 30°C / s, and then wound up at a coiling temperature (CT) in the range of 500°C to 700°C, for example. The coiling temperature can be varied depending on the steel material. If the coiling temperature exceeds 700°C, an undesirable internal oxide layer may form in the hot-rolled steel sheet or the wound hot-rolled coil. In such a wound hot-rolled coil, the internal oxidation will be uneven, making it difficult to control the thickness of the internal oxide layer uniformly. If the coiling temperature is below 500°C, an undesirable low-temperature structure may form.

[0069] Cold rolling step (S30)

[0070] Step (S30) is performed to form a cold-rolled steel sheet by applying a cold-rolling process to the hot-rolled steel sheet.

[0071] On the other hand, in the method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability according to one embodiment of the present invention, the softening heat treatment step and the pickling step can be performed in order before the cold rolling step.

[0072] In the softening heat treatment step, the hot-rolled steel sheet is subjected to a softening heat treatment to soften the material, thereby mitigating 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 ensure cold rolling efficiency. When the strength of the hot-rolled steel sheet is high, problems such as thickness hunting and shape defects may occur during cold rolling. However, in the case of the softening heat treatment process for ultra-high-strength steel according to the present invention, it is applied to hot-rolled coils with residual scale, rather than cold-rolled coils, so countermeasures are needed to address changes in surface properties due to high-temperature reactions of scale during the softening heat treatment. Generally, in the case of ultra-high-strength steel materials containing large amounts of Si and Mn, it is known that internal oxides are generated along the crystal grains of the iron grain boundaries at high temperatures, along with scale. The oxide layer generated by internal oxidation has poor pickling properties because the main component of the matrix phase is Fe. Therefore, the internal oxide layer cannot be completely removed with the same pickling time as general hot-rolled steel sheets, requiring a longer pickling time, which leads to a problem of reduced productivity. Such internal oxidation occurs when the activity of easily oxidizable elements such as Si and Mn is high and they are present under specific oxygen partial pressure conditions. Therefore, when hot-rolled coils with remaining scale are heat-treated in a high-temperature reducing gas atmosphere, further internal oxidation occurs due to oxygen generated during the scale reduction reaction. Internal oxides, which were not observed in the hot-rolled material, develop non-uniformly throughout the entire length of the coil after softening heat treatment, and the development behavior of scale reduction and internal oxidation may differ depending on the location within the winding coil. In the case of the outer winding of the softened heat-treated coil, the growth of internal oxidation due to the hydrogen reduction reaction of scale was mainly observed, while in the case of the inner winding of the coil, the growth of internal oxidation due to the eutectoid reaction of scale (4FeO → 4Fe + 2O2) was observed. The difference in the growth behavior of this internal oxidation was determined to be due to the difference in the ease of penetration of the reaction gas depending on the position of the winding coil. It was understood that the oxygen generated by the hydrogen reduction and eutectoid reaction of scale during the softening heat treatment diffuses into the base material and acts as an internal oxidation reactant.Since it is preferable that the internal oxide layer be formed as uniformly as possible throughout the steel sheet, it is preferable to suppress the formation of the internal oxide layer during the winding step and to form the internal oxide layer during the softening heat treatment. Taking this into consideration, in the method for manufacturing ultra-high-strength cold-rolled steel sheets of the present invention, the winding temperature is adjusted to 500°C to 700°C and the softening heat treatment temperature to 500°C to 650°C. The softening heat treatment can be performed in a batch annealing furnace (BAF) with the hot-rolled steel sheet wound up, 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 has softened material properties and can be made cold-rollable. Furthermore, since the internal oxide layer formed by the softening heat treatment has a predetermined thickness (for example, a thickness of 10 μm or less), subsequent pickling properties can be simultaneously ensured.

[0073] When softening heat treatment is applied at temperatures below 500°C, the degree of softening of the treated hot-rolled steel sheet is minimal, making it difficult to ensure subsequent cold-rollability. In other words, excessive austenite formation during the softening heat treatment may lead to martensite formation during cooling, potentially preventing the effective reduction in strength from occurring.

[0074] Furthermore, when the softening heat treatment is applied at a temperature exceeding 650°C, the internal oxide layer formed by the softening heat treatment has a thickness exceeding 10 μm, making it difficult to ensure subsequent pickling.

[0075] On the other hand, in the method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability according to one embodiment of the present invention, the application or non-application of the softening heat treatment step can be selectively performed depending on the type of steel or the target strength. For example, if the target tensile strength after cold rolling / annealing is 1180 MPa or higher, the softening heat treatment step can be performed, and if the target tensile strength after cold rolling / annealing is 980 MPa, the softening heat treatment step can be omitted. The duration of the softening heat treatment may be 1 to 12 hours.

[0076] In the pickling step, after the softening heat treatment, the hot-rolled steel sheet can be treated with an acid pickling process. By pickling the hot-rolled steel sheet, at least a portion of the internal oxide layer can be removed. The pickling process may be carried out, for example, at a temperature of 70°C to 90°C, with a hydrochloric acid concentration of, for example, 5% to 15%, for, for example, 20 to 40 seconds. The inhibitor concentration may also be 0.1% to 0.5%.

[0077] In the cold-rolled steel sheet forming step (S30), the pickled hot-rolled steel sheet can be cold-rolled with an average reduction ratio of, for example, 40% to 60% and a reduction force of, for example, 700 tons to 1800 tons, thereby producing a cold-rolled steel sheet. The microstructure of the cold-rolled steel sheet has an elongated shape compared to the microstructure of the hot-rolled steel sheet, and the microstructure of the final steel sheet is determined in the subsequent heat treatment.

[0078] Figure 2 is a graph illustrating the subsequent heat treatment steps (annealing, cooling, and reheating) 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.

[0079] Annealing heat treatment step (S40)

[0080] Referring to Figure 2, the cold-rolled steel sheet is subjected to annealing heat treatment in a continuous annealing furnace with a normal slow cooling section. This annealing heat treatment can be performed at a temperature corresponding to the austenite-ferrite two-phase region. By performing heat treatment in the two-phase region to ensure an appropriate fraction of ferrite, the ideal ferrite, tempered martensite, and retained austenite in the final microstructure can be realized, thereby obtaining the target material of the steel sheet.

[0081] The annealing heat treatment temperature and annealing time affect the size of the austenite crystal grains and, therefore, can have a significant impact on the strength of the cold-rolled steel sheet. The annealing heat treatment is performed by heating at a heating rate of 1°C / s or more, for example, in the range of 1°C / s to 15°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 can result in larger crystal grains. The annealing heat treatment may be performed at a temperature of Ae1 or higher, for example, in the range of 830°C to 900°C.

[0082] On the other hand, increasing the annealing time, which is the duration of the annealing heat treatment, affects the coarsening due to the growth of austenite grains, similar to the annealing heat treatment temperature.

[0083] In this invention, it has been confirmed that by controlling the annealing time (A) and the moisture concentration (B) in the annealing furnace during the annealing heat treatment, it is possible to suppress the occurrence of liquid metal embrittlement (LME), in which the zinc plating layer melts and penetrates the surface of the base iron during welding to a galvanized steel sheet, thereby inducing brittleness, and to control the thickness of the decarburized layer formed on the surface of the steel sheet.

[0084] Specifically, the annealing time (A) for performing the annealing heat treatment and the moisture concentration (B) in the annealing furnace are given by the positive square root (A) of the annealing time. 1 / 2 The process can be controlled based on the product of (A) and the value obtained by taking the natural logarithm of the reciprocal of the moisture concentration (ln(1 / B)). For example, the annealing heat treatment step (S40) may be carried out under conditions where the annealing time (A) and the moisture concentration (B) in the annealing furnace satisfy the following formula 1.

[0085] [Formula 1]

[0086]

number

[0087] As an example of a method for manufacturing the aforementioned ultra-high-strength galvanized steel sheet with excellent weldability, the annealing heat treatment can be controlled such that the shorter the annealing time (A) of the annealing heat treatment, the higher the moisture concentration (B) in the annealing furnace.

[0088] As another example of a method for manufacturing the ultra-high-strength galvanized steel sheet with excellent weldability, the annealing heat treatment can be controlled such that the annealing time (A) of the annealing heat treatment becomes longer as the moisture concentration (B) in the annealing furnace decreases.

[0089] With this configuration, the possibility of liquid metal embrittlement (LME), in which the zinc plating layer melts and penetrates the surface of the base iron during welding to the galvanized steel sheet, thereby inducing brittleness, can be suppressed, and the thickness of the decarburized layer formed on the surface of the steel sheet can be 18 μm or more.

[0090] Primary and secondary cooling steps (S50, S60)

[0091] The cold-rolled steel sheet that has undergone annealing treatment is cooled in multiple stages. Specifically, the annealed steel sheet can be subjected to a primary cooling step (S50) and a secondary cooling step (S60).

[0092] The first step (S50) of cooling the steel plate to a temperature of 600°C or higher but less than 800°C at an average cooling rate of 1 to 20°C / s is a slow cooling step, and by attempting to secure a certain amount of ferrite in the final microstructure during the heat treatment process, the plasticity of the final microstructure can be ensured. If the cooling end temperature of the slow cooling is less than 600°C, ferrite transformation may occur in an undesirable amount, which may reduce the strength.

[0093] Next, the step (S60) of secondary cooling the steel plate to 200°C to less than 300°C at an average cooling rate of 20°C / s or more, preferably 50°C / s or more, is a rapid cooling step, and by controlling the rapid cooling completion temperature, it is possible to transform the austenite in the microstructure after slow cooling into martensite, thereby facilitating the securing of the final material. An average cooling rate of 20°C / s or more is required to suppress the phase transformation that may occur during the rapid cooling process.

[0094] Next, the secondary cooled cold-rolled steel sheet can be maintained at the secondary cooling completion temperature, which is 200°C or higher and less than 300°C, for a time in the range of 1 second to 100 seconds. During this maintenance time after rapid cooling, the temperature of the steel may be homogenized in the initial stages. The secondary cooling completion temperature is the martensitic transformation onset temperature (M s ) and the temperature at which martensitic transformation is completed (M f The temperature can be between )

[0095] Subsequently, while the mixture is maintained at the secondary cooling end temperature, some of the retained austenite may transform into lower bainite or the like.

[0096] Reheating step (S70)

[0097] The multi-stage cooled cold-rolled steel sheet can be reheated, for example, at a heating rate in the range of 1°C / s to 50°C / s, and then subjected to partitioning heat treatment by maintaining a temperature of, for example, 350°C to 490°C for a time of 100 seconds or less. The temperature of the partitioning heat treatment is the martensitic transformation onset temperature (M s The temperature may be higher than the temperature. The purpose of the reheating step is to ensure the strength and elongation through carbon concentration in the retained austenite and tempering of the martensite during the step, and finally, to maintain the final microstructure.

[0098] If the temperature of the partitioning heat treatment is below 350°C, the partitioning effect may be insufficient. If the temperature of the partitioning heat treatment exceeds 490°C, the size of the carbides may become coarser, resulting in a decrease in strength. The duration of the partitioning heat treatment may have a smaller effect compared to the partitioning temperature. If the duration of the partitioning heat treatment exceeds 100 seconds, the heat treatment efficiency decreases, the size of the carbides may increase, and a decrease in strength may occur. The partitioning heat treatment step may be performed immediately after the multi-stage cooling, or after being maintained at room temperature for several minutes or more. In the case of non-plated materials, after the partitioning heat treatment step is completed, the material is cooled to room temperature, for example, in the range of 0°C to 40°C.

[0099] The multi-stage cooling steps (S50, S60) and reheating step (S70) described above correspond to the Q&P (Quenching and Partitioning) heat treatment steps developed to simultaneously ensure high strength and high ductility of steel materials. This technique suppresses the formation of carbon carbide precipitates released from the martensitic structure during quenching and allows carbon to diffuse into the retained austenite structure through partitioning. Through carbon re-diffusion, the retained austenite structure is stabilized even at room temperature, ultimately enabling the securing of high ductility due to the retained austenite structure and high strength due to the martensitic structure.

[0100] Zinc plating step (S80)

[0101] The cold-rolled steel sheet that has undergone annealing heat treatment can be immersed in a hot-dip galvanizing bath to hot-dip galvanize it and form a hot-dip galvanized steel sheet (GI). The cold-rolled steel sheet and the hot-dip galvanized steel sheet on which the hot-dip galvanized layer has been formed can be further subjected to alloying heat treatment to form an alloyed hot-dip galvanized steel sheet (GA).

[0102] In the case of plated materials, after the partitioning heat treatment step is performed, they can be directly entered into the plating bath without being cooled to room temperature.

[0103] The entry temperature into 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% by weight of aluminum and saturated Fe. Alternatively, the plating bath may be a Zn-Mg-Al plating bath. On the other hand, the alloying heat treatment temperature may be 450 to 600°C.

[0104] The ultra-high-strength galvanized steel sheet with excellent weldability according to one embodiment of the present invention, embodied by the steps described above, for example, has the following composition by weight %, carbon (C): 0.1-0.5%, silicon (Si): 1.0-3.0%, manganese (Mn): 1.5-3.5%, phosphorus (P): greater than 0% and 0.02% or less, sulfur (S): greater than 0% and 0.01% or less, aluminum (Al): greater than 0% and 0.1% or less, nitrogen (N): greater than 0% and 0.01% or less, and a small amount of titanium (Ti), niobium (Nb), and vanadium (V) The material comprises: a base iron containing more than 0% and 0.1% or less of any of the following: the remaining iron (Fe) and other unavoidable impurities; and a zinc plating layer formed on the base iron, wherein the surface layer of the base iron in contact with the zinc plating layer has a ferrite single-phase microstructure, the thickness of the decarburized layer formed on the surface layer of the base iron is 18 μm or more, and the microstructure of the base iron consists of 0-40% (including 0%) ferrite, 10-30% retained austenite, and the remainder martensite.

[0105] For example, if the tensile strength of the final production steel sheet is 980 to 1180 MPa, the microstructure of the base iron can consist of 20 to 40% ferrite, 10 to 30% retained austenite, and the remainder martensite.

[0106] For example, if the tensile strength of the final production steel sheet is 1180-1470 MPa, the microstructure of the base iron can consist of 5-25% ferrite, 10-30% retained austenite, and the remainder martensite.

[0107] For example, if the tensile strength of the final production steel sheet is 1470 MPa or higher, the microstructure of the base iron can consist of 0-10% (including 0%) ferrite, 10-30% retained austenite, and the remainder martensite.

[0108] The galvanized steel sheet is characterized in that the applicable welding current range is 6.0kA to 7.5kA, and there is a 0% possibility of liquid metal embrittlement (LME), in which the zinc plating layer melts and penetrates the surface of the base iron during welding to the galvanized steel sheet, thereby inducing brittleness.

[0109] The final steel sheet material will have a yield strength of 600 MPa or higher, a tensile strength of 980 MPa or higher, and a total elongation of 10% or higher.

[0110] In the case of the final material, the product of tensile strength and total elongation is at a level of approximately 26,500, which is generally higher than the 25,000 value typically proposed for high-formability steel sheets at this strength level. Through this, it can be estimated that it may have superior formability compared to existing ultra-high-strength steels of the same strength.

[0111] Figure 3 is a photograph showing the occurrence of liquid metal embrittlement cracking in a galvanized steel sheet, which is a comparative example of the present invention.

[0112] When spot welding is performed on general galvanized steel sheets on an automobile assembly line, the galvanized layer melts, and molten metallic zinc penetrates to the interface side of the retained austenite present on the surface of the base iron of the galvanized steel sheet, which can induce the phenomenon of liquid metal embrittlement (LME). This phenomenon of liquid metal embrittlement (LME) causes LME cracking, which leads to a rapid decrease in weld strength and, consequently, a narrowing of the usable current range for welding. Specifically, when spot welding (resistance spot welding) of galvanized steel sheets made of ultra-high tensile steel, in the case of hot-dip galvanized steel sheets (GI), the melting point of the plating layer is very low at 420°C, and even in the case of alloyed galvanized steel sheets, molten metallic zinc is formed by peritectic reaction at around 780°C. The formed molten metallic zinc penetrates along the grain boundaries of the base material in the region where a load from the welding electrode is generated at high temperature, and the strength of the base material rapidly deteriorates. Referring to Figure 3, it can be confirmed that, around liquid metal embrittlement cracks (LME cracks), austenite undergoes a phase transformation to αFe(Zn) at high temperatures due to zinc diffusion into the base material. Since αFe(Zn) is a very brittle substance, this phase transformation further accelerates the brittleness of the steel sheet. Therefore, the frequency of liquid metal embrittlement (LME) occurrence is more sensitive to the amount of retained austenite, and measures to avoid austenite present on the surface have been confirmed as measures that can improve weldability.

[0113] In a method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability according to one embodiment of the present invention, the phase fraction of the steel sheet is controlled by the developed alloy components and the subsequent heat treatment (annealing / cooling / reheating) process. However, the austenite phase formed in the currently developed component system and annealing step either transforms into martensite / bainite during the cooling and reheating process in the subsequent heat treatment, or remains as austenite to form a composite phase.

[0114] Figure 4 is a diagram illustrating the outline of the decarburization reaction in a method for manufacturing an ultra-high-strength galvanized steel sheet according to one embodiment of the present invention.

[0115] The temperature achieved in the annealing step described above is in the region where the austenite phase is formed. However, if the dew point temperature of the atmosphere inside the annealing furnace is increased to -10°C or higher, carbon, which is an austenite-stabilizing element, oxidizes on the surface of the base iron 10 and volatilizes in the form of carbon monoxide, so a decarburization reaction can be carried out by a sustained reaction. Such a reaction can be represented by the following chemical formula 1.

[0116] [C1]

[0117] [ka]

[0118] As the decarburization reaction continues, carbon is depleted on the surface, and if the austenite on the surface has already transformed into ferrite by the annealing step, no austenite will form on the surface during the subsequent cooling / reheating process, ensuring a single-phase ferrite structure even at room temperature in the final stage. The single-phase ferrite structure formed on the surface avoids austenite, which is sensitive to LME cracking, allowing for an expansion of the weldable current range and an improvement in weld strength.

[0119] Experimental example

[0120] The following are preferred experimental examples to aid in understanding the present invention. However, these experimental examples are merely for the purpose of aiding in understanding the present invention, and the present invention is not limited to these experimental examples.

[0121] Table 1 shows the composition of ultra-high-strength cold-rolled steel sheets according to experimental examples of the present invention, and Table 2 shows the process conditions for ultra-high-strength cold-rolled steel sheets according to experimental examples of the present invention.

[0122] [Table 1]

[0123] [Table 2]

[0124] A steel having the composition (unit: weight %) shown in Table 1 was prepared, and a hot-rolled steel sheet was manufactured through a predetermined hot-rolling process. The remainder consisted of iron (Fe) and other unavoidable impurities. Both the examples and comparative examples had the same alloy composition. Referring to Table 2, the hot-rolling process was performed using the following conditions: reheating temperature: 1200°C, finish rolling temperature: 900°C, and coiling temperature: 600°C. After the cold-rolling process, the following conditions were applied: annealing temperature: 850°C, slow cooling temperature: 700°C, rapid cooling temperature: 250°C, and reheating temperature: 460°C.

[0125] First experimental example

[0126] In the first experimental example of the present invention, a 1200 MPa class Q&P steel material was created by applying the composition shown in Table 1 and the process conditions shown in Table 2, and after hot-dip galvanizing to form a hot-dip galvanized (GI) steel sheet, a Gleeble test (700~900°C) was performed. The test conditions applied were a heating rate of 500°C / s, a holding time of 1 second, and a deformation rate of 30 mm / s.

[0127] Table 3 shows whether or not liquid metal embrittlement (LME) occurred in the ultra-high-strength cold-rolled steel sheet according to the first experimental example of the present invention, based on the dew point. In Table 3, the dew point refers to the dew point due to the moisture concentration in the annealing furnace, but a general dew point refers to a dew point below -45°C (e.g., -50°C), and a high dew point refers to a dew point of 0°C. Also, temperatures of 700°C, 750°C, 800°C, 850°C, and 900°C refer to the Gleeble test temperature, and the "LME" column means that the liquid metal embrittlement (LME) phenomenon occurred, while the "No LME" column means that the liquid metal embrittlement (LME) phenomenon did not occur.

[0128] [Table 3]

[0129] Figures 5 to 7 show cross-sectional photographs including the surface layer of the steel material, the evaluation results of the elongation test, and photographs indicating the presence or absence of liquid metal embrittlement (LME) cracking, respectively, under the conditions of a Gleeble test temperature of 800°C and a general dew point (DP < -45°C) in the first experimental example of the present invention. Figures 8 to 10 also show cross-sectional photographs including the surface layer of the steel material, the evaluation results of the elongation test, and photographs indicating the presence or absence of liquid metal embrittlement (LME) cracking, respectively, under the conditions of a Gleeble test temperature of 800°C and a high dew point (DP 0°C) in the first experimental example of the present invention. Referring to Figures 5 to 7 together with Table 3, it can be confirmed that a ferrite single-phase microstructure does not appear on the surface layer of the base iron in contact with the zinc plating layer, and that liquid metal embrittlement (LME) cracking occurs, resulting in a decrease in elongation. In contrast, referring to Figures 8 to 10 along with Table 3, it can be confirmed that a single-phase ferrite (α) microstructure appears on the surface of the base iron in contact with the zinc plating layer, preventing liquid metal embrittlement cracking (LME crack) and improving the elongation properties.

[0130] Second experimental example

[0131] Table 4 shows the annealing time, the thickness of the decarburized layer due to annealing moisture, and the presence or absence of liquid metal embrittlement (LME) in an alloyed hot-dip galvanized steel sheet, which was realized by applying the composition of Table 1 and the process conditions of Table 2 described above, in a second experimental example of the present invention. In the second experimental example, an alloying temperature of 530°C was applied for the zinc plating process. Cases where the dew point is +15°C or higher were excluded from the experimental example because they can induce problems such as corrosion of equipment in the annealing furnace.

[0132] [Table 4]

[0133] Figure 11 shows the positive square root (A) of the annealing time (A) in the second experimental example of the present invention. 1 / 2Figure 11 is a table showing the product of (A) and the value obtained by taking the natural logarithm of the reciprocal of the moisture concentration (B) in the annealing furnace (ln(1 / B)). Figure 12 is a table showing the thickness of the decarburized layer formed on the surface of the base iron in the second experimental example of the present invention, depending on the annealing time (A) and the moisture concentration (B) in the annealing furnace. Figure 13 is a table showing the occurrence rate of liquid metal embrittlement (LME) in the second experimental example of the present invention, depending on the annealing time (A) and the moisture concentration (B) in the annealing furnace. For reference, in Figure 11, the unit of annealing time (A) is seconds (s) and the unit of moisture concentration (B) is ppm. Referring to Table 4 and Figures 11 to 13, when the annealing heat treatment step is performed under conditions where the annealing time (A) and the moisture concentration in the annealing furnace (B) satisfy the following formula 1 (Examples 1 to 6), it can be confirmed that i) there is a 0% possibility of liquid metal embrittlement (LME), in which the zinc plating layer melts and penetrates the surface of the base iron during welding to the galvanized steel sheet, inducing brittleness, and ii) the thickness of the decarburized layer formed on the surface of the base iron is 18 μm or more.

[0134] [Formula 1]

[0135]

number

[0136] Conversely, when the annealing heat treatment step is performed under conditions where the annealing time (A) and the moisture concentration in the annealing furnace (B) do not satisfy the above formula 1 (Comparative Examples 1 to 7), i) the possibility of liquid metal embrittlement (LME), in which the zinc plating layer melts and penetrates the surface of the base iron during welding to the galvanized steel sheet, inducing brittleness, is shown at a significant level, and ii) the thickness of the decarburized layer formed on the surface of the base iron is less than 18 μm.

[0137] That is, in the method for manufacturing an extra-high strength galvanized steel sheet excellent in weldability, the annealing heat treatment can be controlled such that as the annealing time (A) for performing the annealing heat treatment becomes shorter, the moisture concentration (B) in the annealing furnace becomes higher, and it can be confirmed that the annealing heat treatment can be controlled such that as the moisture concentration (B) in the annealing furnace becomes lower, the annealing time (A) for performing the annealing heat treatment becomes longer.

[0138] According to this, in the method for manufacturing an extra-high strength galvanized steel sheet excellent in weldability according to one aspect of the present invention, when annealing heat treatment of a cold-rolled steel sheet is performed in an annealing furnace, the annealing time (A) for performing the annealing heat treatment and the moisture concentration (B) in the annealing furnace are based on the product of the positive square root of the annealing time (A 1 / 2 ), and the value obtained by taking the natural logarithm of the reciprocal of the moisture concentration (ln(1 / B))). It can be understood that they can be controlled.

[0139] As described above, the first parameter disclosed in the above formula 1, which is expressed as the product of the positive square root of the annealing time (A 1 / 2 ), and the value obtained by taking the natural logarithm of the reciprocal of the moisture concentration (B) (ln(1 / B)), can be said to have technical significance as a factor that can effectively control the thickness of the decarburized layer and the occurrence rate of liquid metal embrittlement (LME) simultaneously in an extra-high strength galvanized steel sheet. For example, in Comparative Example 3, Comparative Example 6, Example 2, and Example 5, the positive square root of the annealing time (A 1 / 2 ) is the same, but the thickness of the decarburized layer varies greatly depending on the value obtained by taking the natural logarithm of the reciprocal of the moisture concentration (B) (ln(1 / B)). In Example 4, Example 2, and Comparative Example 7, the value obtained by taking the natural logarithm of the reciprocal of the moisture concentration (B) (ln(1 / B)) is the same, but the occurrence rate of liquid metal embrittlement (LME) varies greatly depending on the positive square root of the annealing time (A 1 / 2 ). Therefore, it is not possible to effectively control the thickness of the decarburized layer and the occurrence rate of liquid metal embrittlement (LME) simultaneously simply by the annealing time (A) alone or by the moisture concentration (B) alone. From this point of view, the above-described first parameter has a causal relationship with a better effect and is not a known physical property shown only by a different expression method, so it can be said to have technical significance.

[0140] On the other hand, referring to Table 4, it can be confirmed that the higher the dew point and the longer the annealing time, the greater the amount of decarburization and the better the ability to avoid liquid metal embrittlement (LME). To improve liquid metal embrittlement (LME), it may be necessary to ensure a decarburized layer thickness of 18 μm or more.

[0141] Third Experiment Example

[0142] In the third experimental example of the present invention, a 1200 MPa class Q&P steel material was created by applying the composition shown in Table 1 and the process conditions shown in Table 2, and after hot-dip galvanizing to form a hot-dip galvanized (GI) steel sheet, a spot welding process was applied. The welding current was set to 0.5 kA lower than the conditions for spatter (expulsion) generation, and the LME was evaluated.

[0143] Table 5 shows the conditions for the spot welding process according to the third experimental example, and Figure 14 shows the results of applying the spot welding process according to the third experimental example. In Figure 14, the dew point refers to the dew point due to the moisture concentration in the annealing furnace, but a general dew point refers to a dew point below -45°C (for example, -50°C), and a high dew point refers to a dew point of 0°C.

[0144] [Table 5]

[0145] Referring to Table 5 and Figure 14, it can be confirmed that in the comparative example of the present invention, the method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability is performed with annealing at a dew point below -45°C and the applicable welding current range is 6.0kA to 6.5kA, whereas in the embodiment of the present invention, the method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability is performed with annealing at a dew point of 0°C and the applicable welding current range is 6.0kA to 7.5kA. According to this, it can be confirmed that the method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability according to the embodiment of the present invention suppresses the austenite phase of the surface layer, which is the cause of LME cracking in the welded area, thereby ensuring strength even under existing welding conditions, expanding the total weldable range, and suppressing defects in the spot welding process.

[0146] The above description has focused on embodiments of the present invention, but various modifications and variations can be made at the level of those skilled in the art. Such modifications and variations can be said to fall within the scope of the present invention as long as they do not depart from the scope of the present invention. Therefore, the scope of the rights of the present invention must be determined by the appended claims.

Claims

1. The step includes annealing a cold-rolled steel sheet in an annealing furnace, The annealing time (A) for the annealing heat treatment and the moisture concentration (B) in the annealing furnace are given by the positive square root of the annealing time (A). １／２ ) is controlled based on the product of the value obtained by taking the natural logarithm of the reciprocal of the moisture concentration (ln(1 / B)), The unit of the annealing time (A) is seconds (s), and the unit of the moisture content (B) in the annealing furnace is ppm. The aforementioned cold-rolled steel sheet consists, by weight percent, of carbon (C): 0.1-0.5%, silicon (Si): 1.0-3.0%, manganese (Mn): 1.5-3.5%, phosphorus (P): more than 0% and 0.02% or less, sulfur (S): more than 0% and 0.01% or less, aluminum (Al): more than 0% and 0.1% or less, nitrogen (N): more than 0% and 0.01% or less, and the remaining iron (Fe) and other unavoidable impurities. The aforementioned annealing heat treatment step is carried out under the conditions of an annealing temperature of 830 to 900°C and a hydrogen concentration of 5 to 7% in the annealing atmosphere. A method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability, characterized in that the annealing heat treatment step is performed under conditions that satisfy the following formula 1 for the annealing time (A) and the moisture concentration in the annealing furnace (B). [Formula 1] [Math 1]

2. A method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability, as described in claim 1, characterized in that the annealing heat treatment is controlled such that the moisture concentration (B) in the annealing furnace increases as the annealing time (A) of the annealing heat treatment decreases.

3. A method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability, as described in claim 1, characterized in that the annealing heat treatment is controlled such that the annealing time (A) for the annealing heat treatment becomes longer as the moisture concentration (B) in the annealing furnace decreases.

4. The method for producing an ultra-high-strength galvanized steel sheet with excellent weldability according to claim 1, wherein the annealing heat treatment step is performed at a temperature corresponding to the austenite and ferrite two-phase region, and the step of inducing a decarburization reaction on the surface of the steel sheet to transform the austenite present in the surface layer of the steel sheet into ferrite.

5. After the annealing heat treatment step, The steps include: first cooling the steel plate to a temperature of 600°C or higher but less than 800°C at an average cooling rate of 1 to 20°C / s; The steps include: secondarily cooling the steel plate to 200°C or higher and less than 300°C at an average cooling rate of 20°C / s or higher; The steps include reheating the steel plate to 350°C to 490°C and maintaining the temperature for 100 seconds or less, A method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability according to claim 1, further comprising the step of performing a zinc plating treatment on the steel sheet.

6. The method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability according to claim 5, characterized in that, after the zinc plating treatment, the steel sheet has a ferrite single-phase microstructure on its surface, and the thickness of the decarburized layer formed on the surface of the steel sheet is 18 μm or more.

7. The method for manufacturing an ultra-high-strength galvanized steel sheet with excellent weldability according to claim 5, characterized in that the galvanized steel sheet has an applicable welding current range of 6.0 kA to 7.5 kA.

8. The base iron consists of, by weight percent, carbon (C): 0.1-0.5%, silicon (Si): 1.0-3.0%, manganese (Mn): 1.5-3.5%, phosphorus (P): more than 0% and 0.02% or less, sulfur (S): more than 0% and 0.01% or less, aluminum (Al): more than 0% and 0.1% or less, nitrogen (N): more than 0% and 0.01% or less, and the remaining iron (Fe) and other unavoidable impurities. The above includes a zinc plating layer formed on the base iron, The surface layer of the base iron in contact with the zinc plating layer has a ferrite single-phase microstructure, and the thickness of the decarburized layer formed on the surface layer of the base iron is 18 μm or more. The microstructure of the aforementioned base iron consists of 0-40% ferrite, 10-30% retained austenite, and the remainder is martensite, resulting in an ultra-high-strength galvanized steel sheet with excellent weldability.

9. The galvanized steel sheet is characterized in that the applicable welding current range is 6.0 kA to 7.5 kA, as described in claim 8, which is an ultra-high-strength galvanized steel sheet with excellent weldability.

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