Ultra-high strength galvanized steel sheet with excellent weldability and method for producing 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] The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to an ultra-high strength zinc-plated steel sheet having excellent weldability and a method for manufacturing the same.

Background Art

[0002] In order to meet the demands of the times such as resource depletion, rapid progress of global warming, and high oil prices, the automobile industry has focused on improving fuel efficiency and weight reduction. Furthermore, as regulations regarding passenger safety are increasing, ultra-high strength steel materials are required. In addition, various improvements are required for zinc-plated steel sheets that have excellent sacrificial corrosion resistance and can prevent corrosion of steel materials by eluting zinc with a low potential first when exposed to a corrosive environment. For example, during spot welding on an automobile assembly line, there is an increasing demand to improve the problem of liquid metal embrittlement (LME) in which the zinc plating layer melts and the molten metal zinc penetrates to the interface side of retained austenite present on the surface layer of the base iron, inducing brittleness.

[0003] As a related prior document, there is Korean Patent Publication No. 10-2020-0075949.

Summary of the Invention

Problems to be Solved by the Invention

[0004] The problem to be solved by the present invention is to provide an ultra-high strength zinc-plated steel sheet having excellent weldability and a method for manufacturing the same.

[0005] However, such problems are exemplary, and the technical idea of the present invention is not limited thereto.

Means for Solving the Problems

[0006] According to one aspect of the present invention, a method for manufacturing an ultra-high strength galvanized steel sheet excellent in weldability includes a step of annealing and heat-treating 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 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)).

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

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

[0009] In the method for manufacturing an ultra-high strength galvanized steel sheet excellent in weldability, the cold-rolled steel sheet contains, by 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 can contain the remaining iron (Fe) and other inevitable impurities.

[0010] In the method for manufacturing an ultra-high strength galvanized steel sheet excellent in weldability, the step of annealing and heat-treating can include a step of performing the annealing and heat-treating at a temperature corresponding to the two-phase region of austenite and ferrite, inducing a decarburization reaction on the surface of the steel sheet, and transforming the austenite present in the surface layer of the steel sheet into ferrite.

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

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

[0013] [Formula 1]

[0014] [Number]

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

[0016] In the method for manufacturing the ultra-high strength zinc-plated steel sheet excellent in weldability, after the zinc plating treatment, the steel sheet may have a fine structure of a ferrite single phase on the surface layer, and the thickness of the decarburized layer formed on the surface layer of the steel sheet may be 18 μm or more.

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

[0018] The ultra-high strength galvanized steel sheet excellent in weldability according to another aspect of the present invention contains, by 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%.

Advantages of the Invention

[0020] According to the present invention, an ultra-high strength galvanized steel sheet excellent in weldability and a method for manufacturing the same can be realized.

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

Brief Description of the Drawings

[0022]

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Embodiments for Carrying Out the Invention

[0023] Hereinafter, with reference to the accompanying drawings, preferred embodiments of the present invention will be described in detail. The embodiments of the present invention are provided to more fully explain the technical idea of the present invention to those having ordinary knowledge in the relevant technical field. The following embodiments can be modified into various other 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 further enrich and complete the present disclosure and to fully convey the technical idea of the present invention to those skilled in the art. Throughout this specification, the same reference numerals mean the same elements. Further, various elements and regions in the drawings are schematically shown. Therefore, the technical idea of the present invention is not limited by the relative sizes and intervals shown in the accompanying drawings.

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

[0025] 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 heat-treating the cold-rolled steel sheet (S40), a step of first and second cooling the annealed heat-treated steel sheet (S50, S60), a step of reheating the cooled steel sheet (S70), and a step of performing a galvanizing treatment on the steel sheet (S80).

[0026] A method for manufacturing an ultra-high strength galvanized steel sheet excellent in weldability according to the technical idea of the present invention includes a step of annealing heat-treating a cold-rolled steel sheet in an annealing furnace (S40), and the annealing time (A) for performing the annealing heat treatment and the moisture concentration (B) in the annealing furnace are such that the positive square root of the annealing time (A 1 / 2) It is controlled based on the product of the annealing time (A) and 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 the condition that the annealing time (A) and the moisture concentration (B) in the annealing furnace satisfy the following mathematical formula 1.

[0027] [Mathematical formula 1]

[0028] [Number]

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

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

[0031] According to such a configuration, when welding the galvanized steel sheet, it is possible to suppress the possibility of occurrence of liquid metal embrittlement (LME) in which the galvanized layer melts and penetrates the surface of the base iron to induce brittleness, and the thickness of the decarburized layer formed on the surface layer of the steel sheet can be 18 μm or more.

[0032] The present invention is not limited to the surface recognition that when the surface hardness is lowered due to surface decarburization, liquid metal embrittlement (LME) is alleviated, and recognizes the problem that it is difficult to optimize the decarburized layer only by the dew point in the annealing furnace, and discloses the above technical idea from an integrated consideration of the annealing time according to the annealing temperature and the dew point in order to ensure the optimum decarburization conditions.

[0033] Hereinafter, a method for manufacturing a hot-dip galvanized high-strength steel sheet excellent in weldability according to an embodiment of the present invention will be described in detail.

[0034] Steel material supply step (S10)

[0035] Recently, in the automotive industry, there has been a growing interest in lightweight vehicle bodies applying ultra-high strength steel in order to simultaneously meet collision stability and fuel consumption regulations. In the steel industry, in line with the requirements of such customers as automobile manufacturers, research on the development of ultra-high strength steel materials has been actively promoted. The Q&P (Quenching and Partitioning) heat treatment technology developed to simultaneously ensure high strength and high ductility of steel materials for automobiles is a technology that suppresses the formation of carbide precipitates of carbon discharged 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 finally, it is possible to ensure high ductility due to the retained austenite structure and high strength due to the martensite structure.

[0036] In order to suppress the formation of carbide precipitates in the structure, such Q&P steel sheets are added with more substitutional solid solution alloy elements such as silicon (Si) and aluminum (Al) that impede the movement of iron (Fe) atoms compared to general steel. Also, in order to increase the volume fraction of the stabilized retained austenite structure and improve the TRIP (Transformation-Induced Plasticity) behavior, they contain a large amount of austenite stabilizing alloy elements such as carbon (C) and manganese (Mn).

[0037] On the other hand, although it is obvious that the technical idea of the present invention can be applied to the above-described Q&P steel sheets, it is clear that the technical idea of the present invention can be widely applied to various steel sheets and is not limited to and applied only to the Q&P steel sheets.

[0038] Hereinafter, the roles and contents of exemplary components included in the ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention will be described as follows. At this time, the content of the component elements all means weight %.

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

[0040] Carbon is the most important alloying element in steelmaking, mainly aiming at the basic strengthening role and the stabilization of austenite. A high carbon concentration in austenite can improve the stability of austenite, making it easy to ensure appropriate austenite for material improvement. When the carbon content is less than 0.1%, it is difficult to ensure the desired yield strength and elongation. When the carbon content exceeds 0.5%, it may lead to a decrease in weldability due to an increase in the carbon equivalent. Therefore, the carbon content is preferably 0.1% - 0.5% of the total weight of the steel plate.

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

[0042] Silicon is an element that suppresses the formation of carbides (e.g., Fe 3 C) 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 as an element that increases the ductility by increasing the ferrite fraction during cooling. When the silicon content is less than 1.0%, the effect of adding silicon is insufficient. When the silicon content exceeds 3.0% , oxides (SiO 2 ) may be formed on the surface of the steel plate during the process, which may lead to a decrease in plating properties due to inferior wettability of the part. Therefore, the silicon content is preferably 1.0 - 3.0% of the total weight of the steel plate.

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

[0044] Manganese is an austenite stabilizing element. When manganese is added, the Ms, which is the martensite transformation start temperature, gradually decreases, and it can have the effect of increasing the retained austenite fraction during the continuous annealing heat treatment process. When the manganese content is less than 1.5%, the effect of adding manganese is insufficient. When the manganese content exceeds 3.5When it exceeds [[%]], the carbon equivalent is increased, significantly reducing weldability. During the process, oxides (MnO) are formed on the surface of the steel plate, which may lead to a decrease in plating performance due to inferior wettability in that area. Therefore, the manganese content is preferably 1.5 - 3.5% of the total weight of the steel plate.

[0045] Phosphorus (P): More than 0% - 0.02%

[0046] Phosphorus can play a role similar to silicon in steel. However, when phosphorus is added in an amount exceeding [[%]] of the total weight of the steel plate, it may reduce the weldability of the steel plate, increase brittleness, and cause deterioration of the material quality. Therefore, the phosphorus content is preferably limited to more than 0% to 0.02% of the total weight of the steel plate. 0.02 % of the total weight of the steel plate, it may reduce the weldability of the steel plate, increase brittleness, and cause deterioration of the material quality. Therefore, the phosphorus content is preferably limited to more than 0% to 0.02% of the total weight of the steel plate.

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

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

[0049] Aluminum (Al): More than 0% - 0.1% or less

[0050] Aluminum (Al) acts similarly to silicon (Si) and mainly plays a role in solid solution strengthening and suppressing the formation of carbides. It is mainly added for the purpose of deoxidation, is effective in suppressing the formation of carbides and generating a retained austenite phase, and is an element effective in improving the strength-elongation balance. When the aluminum (Al) content exceeds 0.1%, the workability of the steel plate may deteriorate due to an increase in inclusions such as alumina. Therefore, it is preferable to add the aluminum (Al) content at 0.1% or less of the total weight of the steel plate.

[0051] Nitrogen (N): More than 0% - 0.01% or less

[0052] Nitrogen (N) is an element that deteriorates the aging resistance. When the nitrogen content exceeds 0.01%, the deterioration of the aging resistance becomes remarkable. Also, nitrogen (N) combines with boron (B) to form BN, thereby consuming boron (B). Therefore, nitrogen (N) reduces the hardenability due to dissolved boron (B) and makes it difficult to secure a tempered martensite phase with a predetermined area ratio. Further, nitrogen (N) exists as an impurity element in ferrite and reduces ductility due to strain aging. Therefore, it is preferable that the content of nitrogen (N) is low. Therefore, it is preferable to limit the content of nitrogen (N) to more than 0% and 0.01% or less of the total weight of the steel sheet.

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

[0054] At least one or more of Titanium (Ti), Niobium (Nb), and Vanadium (V): More than 0% - 0.1% or less

[0055] Elements such as titanium (Ti), niobium (Nb), and vanadium (V) are main elements that precipitate in the form of carbides in steel. In the present invention, the purpose is to ensure the stability and improve the strength of retained austenite through the refinement of initial austenite grain size by the formation of precipitates, and to refine ferrite grain size and precipitation hardening due to the presence of precipitates in ferrite. That is, it is an element that combines with carbon (C) or nitrogen (N) to precipitate in the form of NbC, NbN, TiC, TiN, VC, VN, or improves the strength of the steel sheet through solid solution strengthening in iron (Fe). For example, titanium (Ti) forms carbonitrides and sulfides and is effective for improving strength. By combining with nitrogen and precipitating as titanium nitride (TiN), the formation of boron nitride (BN) can be suppressed, so it is effective for expressing the hardenability due to boron (B). Niobium (Nb) is an element that contributes to the improvement of the strength of the base material by the precipitation strengthening effect by forming NbC precipitates through combination with carbon (C) by adding a small amount. Vanadium (V) is added as a strengthening element in the same manner as niobium (Nb).

[0056] When the total sum of the above elements is added in an amount exceeding 0.1% in the overall steel sheet, not only the production cost increases, but excessive precipitates are generated in the ferrite phase, and the precipitation strengthening acts excessively, resulting in a decrease in the elongation of the steel sheet. Moreover, it may lead to a decrease in low-temperature toughness and weldability due to a large amount of precipitates. Therefore, it is limited to 0.1% or less.

[0057] Chromium (Cr): More than 0% - 0.8% or less

[0058] Chromium (Cr) is an element with a large hardening ability and is added for increasing the strength through transformation strengthening. However, when the chromium (Cr) exceeds 0.8% by weight, the toughness decreases because the structure becomes overall non-uniform while forming a structure such as upper bainite. Therefore, its content is preferably controlled to 0.8% by weight or less.

[0059] Molybdenum (Mo): More than 0% - 0.5% or less

[0060] Molybdenum (Mo) is an element with a larger hardening ability than the above chromium (Cr) and is added for increasing the strength through transformation strengthening. When it exceeds 0.5% by weight within the range of the carbon (C) component of the present invention, the toughness decreases because a large amount of light secondary phases such as martensite / austenite (MA) phase are formed. Therefore, its content is preferably controlled to 0.5% by weight.

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

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

[0063] Hot rolling step (S20)

[0064] Perform a step (S20) of applying a hot rolling process to the steel material to form a hot rolled steel sheet.

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

[0066] Reheat the steel material at a temperature above Ac3, for example, at a reheating temperature (Slab Reheating Temperature, SRT) in the range of 1150°C to 1250°C. Through such reheating, re-solution of segregated components and re-solution of precipitates during casting can occur. If the reheating temperature is less than 1150°C, a problem may occur where the hot rolling load increases rapidly. If the reheating temperature exceeds 1250°C, charging and discharging in the heating furnace may become difficult due to slab warping, and it may become difficult to ensure the strength of the final produced steel sheet due to coarsening of the initial austenite grain size. The reheating temperature can vary according to the steel material.

[0067] Next, hot roll the reheated steel material, for example, hot rolling can be performed at a finish rolling end temperature (Finish Delivery Temperature, FDT) in the range of 850°C to 1000°C. If the finish rolling end temperature exceeds 1000°C, the quality of the steel sheet may deteriorate due to the generation of scale on the surface of the steel sheet. Also, if the finish rolling end temperature is less than 850°C, it may lead to an increase in rolling load and a decrease in productivity. The finish rolling end temperature can vary according to the steel material.

[0068] Next, after cooling the hot-rolled steel material at a cooling rate of 10 to 30 °C / s, it is wound, for example, at a coiling temperature (CT) in the range of 500 °C to 700 °C. The coiling temperature can vary according to the steel material. When the coiling temperature exceeds 700 °C, an undesirable internal oxide layer may occur in the hot-rolled steel sheet or the wound hot-rolled coil. Since the internal oxidation of the hot-rolled coil wound in this way has deviations, it may become difficult to uniformly control the thickness of the internal oxide layer. When the coiling temperature is less than 500 °C, an undesirable low-temperature structure may be formed.

[0069] Cold rolling step (S30)

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

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

[0072] In the softening heat treatment step, by performing softening heat treatment on the hot-rolled steel sheet to realize softening of the material, problems such as the load of the rolling reduction rate and shape defects during subsequent cold rolling can be alleviated. That is, for the efficiency of the cold rolling operation, the softening heat treatment can soften the hot-rolled steel sheet through the softening heat treatment to ensure cold rollability. 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 of the ultra-high strength steel according to the present invention, since it is applied to the hot-rolled coil with scale remaining instead of the cold-rolled coil, countermeasures against changes in surface characteristics due to the high-temperature reaction of the scale during softening heat treatment are required. Generally, in the case of ultra-high strength steel materials containing a large amount of Si, Mn, etc. in steel, it is known that internal oxides are generated along the crystal grains of the base iron grain boundaries together with the scale at high temperatures. Since the main component of the matrix phase of the oxide layer generated by internal oxidation is Fe, the pickling property is poor. Therefore, the internal oxidation layer cannot be completely removed in the same pickling time as that of a general hot-rolled steel sheet, and a long pickling time is required, resulting in a problem of reduced productivity. Such internal oxidation occurs when the activity of easily oxidizable elements such as Si and Mn is high and under specific oxygen partial pressure conditions. Therefore, when heat-treating a hot-rolled coil with scale remaining in a high-temperature reducing gas atmosphere, the generation of further internal oxidation due to oxygen generated during the reduction reaction of the scale is shown. Internal oxides not observed in the hot-rolled material develop non-uniformly over the entire length of the coil after softening heat treatment, and depending on the position within the wound coil, the reduction of the scale and the development behavior of internal oxidation may differ. In the case of the outer wound part of the softening heat treatment coil, the growth of internal oxidation due to the hydrogen reduction reaction of the scale is mainly observed. In the case of the inner wound part of the coil, the eutectoid reaction of the scale (4FeO → 4Fe + 2O 2Internal oxidation growth was observed by ( ). Such differences in the growth behavior of internal oxidation are judged to be due to differences in the ease of penetration of the reaction gas depending on the position of the coiled coil, and it was understood that oxygen generated by the hydrogen reduction and eutectoid reaction of the scale during the softening heat treatment diffused into the base material and acted as an internal oxidation reactant. Since it is preferable that the internal oxidation layer is formed as uniformly as possible throughout the steel sheet, it is preferable to suppress the internal oxidation layer as much as possible in the coiling step and form the internal oxidation layer by the softening heat treatment. Considering such points, in the method for manufacturing an ultra-high strength cold-rolled steel sheet of the present invention, the coiling temperature is adjusted to 500°C to 700°C, and the softening heat treatment temperature is adjusted 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, and can be performed in a hydrogen atmosphere. The hot-rolled steel sheet subjected to the softening heat treatment under the above process conditions can have its material softened to ensure cold rolling properties. In addition, since the internal oxidation 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 ensured simultaneously.

[0073] When the softening heat treatment is applied at a temperature of 500°C or lower, the hot-rolled steel sheet subjected to the softening heat treatment has a weak degree of softening of the material, so it is difficult to ensure subsequent cold rolling properties. That is, when the softening heat treatment is performed, austenite is excessively formed, and martensite may be formed during cooling, and the reduction in strength may not be effectively exhibited.

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

[0075] On the one hand, in the method for manufacturing an ultra-high strength zinc-plated steel sheet excellent in weldability according to an embodiment of the present invention, the application of the softening heat treatment step can be selectively performed according to the steel type or the target strength. For example, when the target tensile strength after cold rolling / annealing is 1180 MPa or more, the softening heat treatment step can be performed. When the target tensile strength after cold rolling / annealing is 980 MPa, the softening heat treatment step cannot be performed. The time for performing the softening heat treatment may be 1 to 12 hours.

[0076] In the pickling treatment step, after performing the softening heat treatment, pickling treatment for cleaning the hot-rolled steel sheet with an acid can be performed. By pickling the hot-rolled steel sheet, at least a part of the internal oxide layer can be removed. The pickling treatment may be performed, for example, at a temperature of 70°C to 90°C, at a hydrochloric acid concentration of, for example, 5% to 15%, and for, for example, 20 seconds to 40 seconds. Also, it can have an inhibitor concentration of 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 at an average reduction ratio of, for example, 40% to 60% and a rolling force of, for example, 700 ton to 1800 ton, thereby manufacturing a cold-rolled steel sheet. The microstructure of the cold-rolled steel sheet has a shape in which the microstructure of the hot-rolled steel sheet is elongated, and in subsequent heat treatment, the microstructure of the finally produced steel sheet is determined.

[0078] FIG. 2 is a graph illustrating subsequent heat treatment (annealing, cooling, reheating) steps applied to a cold-rolled steel sheet in the method for manufacturing an ultra-high strength zinc-plated steel sheet excellent in weldability according to an embodiment of the present invention.

[0079] Annealing heat treatment step (S40)

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

[0081] The annealing heat treatment temperature and annealing time affect the size of austenite crystal grains, and thus can have a great influence on the strength of the cold-rolled steel sheet. The annealing heat treatment is heated, for example, at a heating rate of 1°C / s or more, for example, in the range of 1°C / s to 15°C / s. When the heating rate is less than 1°C / s, it takes a long time to reach the target annealing heat treatment temperature, so the production efficiency decreases and the size of the crystal grains can become large. The annealing heat treatment may be carried out, for example, at a temperature above Ae1, for example, in the range of 830°C to 900°C.

[0082] On the other hand, the longer the annealing time, which is the time for performing the annealing heat treatment, the more it affects the coarsening due to the growth of austenite crystal grains, similar to the annealing heat treatment temperature.

[0083] In the present invention, by controlling the annealing time (A) for performing the annealing heat treatment and the moisture concentration (B) in the annealing furnace, when welding the galvanized steel sheet, the possibility of 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 suppressed, and it has been confirmed that the thickness of the decarburized layer formed on the surface layer of the steel sheet can be controlled.

[0084] 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 can be controlled based on the product of the natural logarithm of the reciprocal of the moisture concentration (ln(1 / B)), for example, it is disclosed that 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 mathematical formula 1.

[0085] [Mathematical formula 1]

[0086] [Number]

[0087] As an example of the method for manufacturing the ultra-high strength zinc-plated steel sheet with excellent 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.

[0088] As another example of the method for manufacturing the ultra-high strength zinc-plated steel sheet with excellent weldability, 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.

[0089] According to such a configuration, when welding the zinc-plated steel sheet, the possibility of the occurrence of liquid metal embrittlement (LME) in which the zinc plating layer melts and penetrates the surface of the base iron to induce brittleness can be suppressed, and the thickness of the decarburized layer formed on the surface layer of the steel sheet can be 18 μm or more.

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

[0091] The cold-rolled steel sheet after the annealing heat treatment is cooled in multiple stages. Specifically, the step (S50) of performing primary cooling and the step (S60) of performing secondary cooling on the steel sheet after the annealing heat treatment can be carried out.

[0092] The step (S50) of first cooling the steel sheet at an average cooling rate of 1 to 20 °C / s to a temperature of 600 °C or higher and less than 800 °C is a slow cooling step. By attempting to ensure a certain amount of ferrite in the final fine structure during the progress of the heat treatment process, the plasticity of the final fine structure 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 cause a decrease in strength.

[0093] Next, the step (S60) of second cooling the steel sheet at an average cooling rate of 20 °C / s or higher, preferably 50 °C / s or higher, to 200 °C or higher and less than 300 °C is a rapid cooling step. By controlling the rapid cooling end temperature, austenite in the fine structure after the slow cooling can be transformed into martensite, facilitating the securing of the final material. In order to suppress the phase transformation that may occur during the rapid cooling process, an average cooling rate of 20 °C / s or higher is required.

[0094] Next, the cold-rolled steel sheet after the second cooling can be maintained at the second cooling end temperature of 200 °C or higher and less than 300 °C for a time in the range of, for example, 1 second to 100 seconds. During such a holding time after rapid cooling, homogenization of the steel temperature can be initially carried out. The second cooling end temperature can be a temperature between the martensite transformation start temperature (M s ) and the martensite transformation completion temperature (M f ).

[0095] Next, while maintaining isothermal at the second cooling end temperature, a part of the retained austenite can transform into lower bainite or the like.

[0096] Reheating step (S70)

[0097] The cold-rolled steel sheet cooled in multiple stages can be reheated at a heating rate in the range of, for example, 1 °C / s to 50 °C / s, and maintained at a temperature of, for example, 350 °C to 490 °C for a time within 100 seconds to perform partitioning heat treatment. The temperature of the partitioning heat treatment is the martensite transformation start temperature (M sIt may also be higher than . The purpose of the reheat step is to ensure strength and elongation through carbon enrichment in retained austenite and tempering of martensite during the step, and finally to maintain the structure of the final fine structure.

[0098] When the temperature of the partitioning heat treatment is less than 350°C, the effect of partitioning may be insufficient. When the temperature of the partitioning heat treatment exceeds 490°C, the size of carbides may coarsen and a decrease in strength may occur. The holding time of the partitioning heat treatment may have a smaller influence compared to the partitioning temperature. When the holding time of the partitioning heat treatment exceeds 100 seconds, the heat treatment efficiency decreases, the size of carbides increases, and a decrease in strength may occur. The partitioning heat treatment step may be performed immediately after the multi-stage cooling is performed, or may be performed after maintaining at room temperature for several minutes or more. In the case of non-plated materials, after the partitioning heat treatment step is completed, it is cooled to room temperature, for example, to a temperature in the range of 0°C to 40°C.

[0099] The multi-stage cooling steps (S50, S60) and reheat 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, and suppress the formation of carbide precipitates of carbon discharged from the martensite structure during quenching, and diffuse carbon into the retained austenite structure through partitioning. Through the re-diffusion of carbon, the retained austenite structure is stabilized even at room temperature, and finally, it is possible to ensure high ductility due to the retained austenite structure and high strength due to the martensite structure.

[0100] Zinc plating treatment step (S80)

[0101] The step (GI) of immersing the annealed and heat-treated cold-rolled steel sheet in a hot-dip galvanizing bath to perform hot-dip galvanizing to form a hot-dip galvanized steel sheet can be carried out, and the step (GA) of subjecting the cold-rolled steel sheet or the hot-dip galvanized steel sheet having the hot-dip galvanized layer formed thereon to an alloying heat treatment to form an alloyed hot-dip galvanized steel sheet can be further carried out.

[0102] In the case of a plating material, after the partitioning heat treatment step is performed, it can enter the plating bath as it is without being cooled to room temperature.

[0103] The entering temperature of the plating bath may be, for example, 460°C, and the composition of the plating bath may be a galvanizing bath containing 0.11 to 0.22% by weight of aluminum and saturated Fe. Or, 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 hot-dip galvanized steel sheet excellent in weldability according to an embodiment of the present invention implemented by performing the above-described steps contains, for example, 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, at least one or more of titanium (Ti), niobium (Nb), and vanadium (V): more than 0% and 0.1% or less, and a base iron containing the remaining iron (Fe) and other inevitable impurities; and a hot-dip galvanized layer formed on the base iron; and has a fine structure of a ferrite single phase on the surface layer of the base iron in contact with the hot-dip galvanized layer, 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% (including 0%) of ferrite, 10 to 30% of retained austenite, and the balance of martensite.

[0105] For example, when the tensile strength of the material of the final production steel plate 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 balance martensite.

[0106] For example, when the tensile strength of the material of the final production steel plate is 1180 to 1470 MPa, the microstructure of the base iron can consist of 5 to 25% ferrite, 10 to 30% retained austenite, and the balance martensite.

[0107] For example, when the tensile strength of the material of the final production steel plate is 1470 Mpa or more, the microstructure of the base iron can consist of 0 to 10% (including 0%) ferrite, 10 to 30% retained austenite, and the balance martensite.

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

[0109] The material of the final production steel plate comes to have a yield strength of 600 MPa or more, a tensile strength of 980 MPa or more, and a total elongation of 10% or more.

[0110] In the case of the final material, it can be confirmed that the value of the product of the tensile strength and the total elongation is at a level of about 26,500, generally exceeding 25,000 which is the value of the product of the tensile strength and the total elongation proposed as a high formability steel plate at this strength level. Through this, it can be estimated that it may have better formability than existing ultra-high strength steels of the same strength.

[0111] FIG. 3 is a photograph taken of the state in which liquid metal embrittlement cracks occur in a galvanized steel sheet, which is a comparative example of the present invention.

[0112] When performing a spot welding process on a general galvanized steel sheet on an automobile assembly line, the zinc plating layer melts, and the molten metal zinc penetrates to the interface side of the retained austenite present on the surface layer of the base iron of the galvanized steel sheet, and a liquid metal embrittlement (LME) phenomenon can be induced. Due to such a liquid metal embrittlement (LME) phenomenon, a liquid metal embrittlement crack (LME crack) occurs, the welding strength drops sharply, and thus there is a problem that the weldable current range becomes narrow. That is, during the resistance spot welding of a galvanized steel sheet of ultra-high strength steel, in the case of a galvanized steel sheet (GI) with a molten zinc plating layer, the melting point of the plating layer is very low at 420 °C, and even in the case of an alloyed galvanized steel sheet, molten metal zinc is formed by a peritectic reaction near 780 °C. The formed molten metal zinc penetrates along the grain boundaries of the base material in the region where the load by the welding electrode occurs at high temperature, and the strength of the base material rapidly becomes inferior. Referring to FIG. 3, it can be confirmed that due to the zinc diffused to the base material side around the liquid metal embrittlement crack (LME crack), austenite transforms into αFe(Zn) at high temperature. Since the αFe(Zn) is a substance with very high brittleness, the brittleness of the steel sheet is further accelerated by such a phase transformation. Therefore, the occurrence frequency of liquid metal embrittlement (LME) is more sensitive as the amount of retained austenite is larger, and a countermeasure for avoiding austenite present on the surface layer has been confirmed as a measure for improving weldability.

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

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

[0115] The temperature developed 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 in 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 that a decarburization reaction can occur by a continuous reaction. Such a reaction can be represented by Chemical Formula 1 below.

[0116] [Chemical Formula 1]

[0117] [Chemical Formula]

[0118] As the decarburization reaction continues, carbon is depleted in the surface layer. If the austenite in the surface layer has already transformed into ferrite even in the annealing step, austenite will not be formed in the surface layer during the subsequent cooling / reheating process, and a ferrite single-phase structure can be ensured even at room temperature, which is the final stage. The ferrite single-phase structure formed in the surface layer can avoid austenite, which is sensitive to LME cracking, so that the weldable current range can be expanded and the welding strength can be improved.

[0119] Experimental example

[0120] Hereinafter, preferred experimental examples are presented to assist in the understanding of the present invention. However, the following experimental examples are only for assisting in the understanding of the present invention, and the present invention is not limited by the following experimental examples.

[0121] Table 1 is a table showing the composition of the ultra-high strength cold-rolled steel sheet according to the experimental example of the present invention, and Table 2 is a table showing the process conditions of the ultra-high strength cold-rolled steel sheet according to the experimental example of the present invention.

[0122] [Table 1]

[0123] [Table 2]

[0124] Prepare steel having the composition shown in Table 1 (unit: wt%) and prepare a hot-rolled steel sheet manufactured through a predetermined hot-rolling process. The balance is iron (Fe) and other inevitable impurities. Both the examples and comparative examples have the same alloy composition. Referring to Table 2, for the hot-rolling process, the process conditions of reheating temperature: 1200°C, finish rolling temperature: 900°C, and coiling temperature: 600°C were applied. After the cold-rolling process, the process conditions of annealing temperature: 850°C, slow cooling temperature: 700°C, rapid cooling temperature: 250°C, and reheating temperature: 460°C were applied.

[0125] First experimental example

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

[0127] Table 3 is a table showing the presence or absence of liquid metal embrittlement (LME) due to dew point in the ultra-high strength cold-rolled steel sheet according to the first experimental example of the present invention. In Table 3, although the dew point means the dew point due to the moisture concentration in the annealing furnace, the general dew point means a dew point of less than -45°C (for example, -50°C), and the high dew point means a dew point of 0°C. Also, the temperatures of 700°C, 750°C, 800°C, 850°C, and 900°C mean the Gleeble test temperatures, the "LME" item means that the liquid metal embrittlement (LME) phenomenon has occurred, and the "No LME" item means that the liquid metal embrittlement (LME) phenomenon has not occurred.

[0128]

Table 3

[0129] Figures 5 to 7 are, in the first experimental example of the present invention, cross-sectional photographs including the surface layer of the steel material, evaluation results of elongation tests, and photographs taken to check for the occurrence of liquid metal embrittlement (LME) cracks under the conditions of a Gleeble test temperature of 800°C and a general dew point (DP < -45°C). Also, Figures 8 to 10 are, in the first experimental example of the present invention, cross-sectional photographs including the surface layer of the steel material, evaluation results of elongation tests, and photographs taken to check for the occurrence of liquid metal embrittlement (LME) cracks under the conditions of a Gleeble test temperature of 800°C and a high dew point (DP 0°C). Referring to Figures 5 to 7 together with Table 3, it can be confirmed that a fine structure of a ferrite single phase did not appear on the surface layer of the base iron in contact with the zinc plating layer, liquid metal embrittlement cracks (LME cracks) occurred, and the elongation decreased. On the contrary, referring to Figures 8 to 10 together with Table 3, it can be confirmed that a fine structure of a ferrite (α) single phase appeared on the surface layer of the base iron in contact with the zinc plating layer, liquid metal embrittlement cracks (LME cracks) did not occur, and the elongation characteristics were improved.

[0130] Second experimental example

[0131] Table 4 shows the second experimental example of the present invention, and shows the thickness of the decarburized layer due to annealing time and annealing moisture, and the presence or absence of occurrence of liquid metal embrittlement (LME) in an alloyed hot-dip galvanized steel sheet embodied by applying the composition of Table 1 and the process conditions of Table 2 described above. In the second experimental example, an alloying temperature of 530°C was applied for the zinc plating treatment. When the dew point is 15°C or higher, problems such as corrosion of equipment in the annealing furnace may be induced, so it was excluded in the experimental example.

[0132]

Table 4

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

[0134] [Formula 1]

[0135]

Number

[0136] On the contrary, when the step of annealing heat treatment is performed under the condition that the annealing time (A) and the moisture concentration (B) in the annealing furnace do not satisfy the above formula 1 (Comparative Examples 1 to 7), i) When welding the galvanized steel sheet, the possibility of occurrence of liquid metal embrittlement (LME) in which the galvanized layer melts and penetrates the surface of the base iron to induce brittleness is shown at a significant level, and ii) It can be confirmed that the thickness of the decarburized layer formed on the surface layer 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 as the moisture concentration (B) in the annealing furnace becomes lower, the annealing time (A) for performing the annealing heat treatment becomes longer. It can be confirmed that the annealing heat treatment can be controlled in this way.

[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 is performed 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 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)), has 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 (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 (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 with a different expression method, so it can be said to have technical significance.

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

[0141] Third experimental example

[0142] In the third experimental example of the present invention, a 1200 MPa grade Q&P steel material was embodied by applying the composition of Table 1 and the process conditions of Table 2 described above, hot dip galvanized, and after forming a hot dip galvanized (GI) steel sheet, a spot welding process was applied. The welding current was set 0.5 kA lower than the generation condition of spatter for welding, and the evaluation of LME was carried out.

[0143] Table 5 is a table showing the conditions of the spot welding process according to the third experimental example, and FIG. 14 is a diagram showing the results of applying the spot welding process according to the third experimental example. In FIG. 14, although the dew point means the dew point due to the moisture concentration in the annealing furnace, the general dew point means a dew point of less than -45°C (for example, -50°C), and the high dew point means a dew point of 0°C.

[0144]

Table 5

[0145] Referring to Table 5 and FIG. 14, in the method for manufacturing a super high strength galvanized steel sheet excellent in weldability according to the comparative example of the present invention, annealing treatment is performed at a dew point of less than -45°C, and the applicable welding current range is 6.0 kA to 6.5 kA. On the other hand, in the method for manufacturing a super high strength galvanized steel sheet excellent in weldability according to the example of the present invention, annealing treatment is performed at a dew point of 0°C, and it can be confirmed that the applicable welding current range is 6.0 kA to 7.5 kA. According to this, in the method for manufacturing a super high strength galvanized steel sheet excellent in weldability according to the example of the present invention, the austenite phase on the surface layer, which is the cause of LME cracking in the welded part, is suppressed, the strength is ensured even under existing welding conditions, the total weldable range is expanded, and it can be confirmed that defects in the spot welding process can be suppressed.

[0146] As described above, the embodiments of the present invention have been mainly described. However, various changes and modifications can be made at the level of those skilled in the art. As long as such changes and modifications do not depart from the scope of the present invention, it can be said that they belong to the present invention. Therefore, the scope of the rights of the present invention must be determined by the appended claims.

Claims

1. including the step of annealing and heat-treating 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 controlled 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)), and a method for manufacturing a hot-dip galvanized steel sheet having excellent weldability.

2. The method for manufacturing a hot-dip galvanized high-strength steel sheet excellent in weldability according to claim 1, wherein the annealing heat treatment is controlled such that the moisture concentration (B) in the annealing furnace increases as the annealing time (A) for performing the annealing heat treatment becomes shorter.

3. The method for manufacturing a hot-dip galvanized high-strength steel sheet excellent in weldability according to claim 1, wherein the annealing heat treatment is controlled such that the annealing time (A) for performing the annealing heat treatment becomes longer as the moisture concentration (B) in the annealing furnace decreases.

4. The cold-rolled steel sheet contains, by 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 contains the remaining iron (Fe) and other inevitable impurities. The method for manufacturing a hot-dip galvanized high-strength steel sheet excellent in weldability according to claim 1.

5. The step of annealing and heat-treating includes performing at a temperature corresponding to a two-phase region of austenite and ferrite, inducing a decarburization reaction on the surface of the steel sheet, and transforming austenite existing in the surface layer of the steel sheet into ferrite. The method for manufacturing a hot-dip galvanized high-strength steel sheet excellent in weldability according to claim 4.

6. The step of annealing and heat-treating is performed under the condition of an annealing temperature of 830 to 900°C. The method for manufacturing a hot-dip galvanized high-strength steel sheet excellent in weldability according to claim 4.

7. The step of annealing and heat-treating is performed under the condition that the annealing time (A) and the moisture concentration (B) in the annealing furnace satisfy the following formula 1. The method for manufacturing a hot-dip galvanized high-strength steel sheet excellent in weldability according to claim 5. [Formula 1] 【Number 1】

8. After the step of annealing and heat-treating, a step of first cooling the steel sheet at an average cooling rate of 1 to 20°C / s to a temperature of 600°C or higher and lower than 800°C; a step of second cooling the steel sheet at an average cooling rate of 20°C / s or higher to a temperature of 200°C or higher and lower than 300°C; a step of reheating the steel sheet to 350 to 490°C and maintaining it within 100 seconds; and further including a step of performing a hot-dip galvanizing treatment on the steel sheet. The method for manufacturing a hot-dip galvanized high-strength steel sheet excellent in weldability according to claim 7.

9. After the zinc plating treatment, the steel sheet has a fine structure of a ferrite single phase on its surface layer, and the thickness of the decarburized layer formed on the surface layer of the steel sheet is 18 μm or more. A method for manufacturing a super high strength zinc-plated steel sheet excellent in weldability according to claim 8, characterized in that.

10. The zinc-plated steel sheet has a range of applicable welding current of 6.0 kA to 7.5 kA. A method for manufacturing a super high strength zinc-plated steel sheet excellent in weldability according to claim 8, characterized in that.

11. By 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, containing a base iron containing the remaining iron (Fe) and other inevitable impurities, 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 fine structure of a ferrite single phase, and the thickness of the decarburized layer formed on the surface layer of the base iron is 18 μm or more, The fine structure of the base iron consists of 0 to 40% ferrite, 10 to 30% retained austenite, and the balance martensite. A super high strength zinc-plated steel sheet excellent in weldability.

12. The zinc-plated steel sheet has a range of applicable welding current of 6.0 kA to 7.5 kA. A super high strength zinc-plated steel sheet excellent in weldability according to claim 11, characterized in that.

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