Ultra-high strength galvanized steel sheet and manufacturing method thereor

Abstract

The present invention relates to a galvanized steel sheet characterized by comprising: a base steel sheet comprising 0.1-0.5 weight% of carbon (C), 0.01-2.0 weight% of silicon (Si), 0.1-5.0 weight% of manganese (Mn), more than 0 and less than or equal to 0.02 weight% of phosphorus (P), more than 0 and less than or equal to 0.01 weight% of sulfur (S), 0.01-2.0 weight% of aluminum (Al), more than 0 and less than or equal to 3.0 weight% of chromium (Cr), more than 0 and less than or equal to 1.0 weight% of molybdenum (Mo), more than 0 and less than or equal to 0.4 weight% of nickel (Ni), more than 0 and less than or equal to 0.4 weight% of copper (Cu), 0.01-0.2 weight% of titanium (Ti), 0.01-0.1 weight% of niobium (Nb), 0.01-1.0 weight% of vanadium (V), 0.001-0.005 weight% of boron (B), 0.0001-0.04 weight% of tin (Sn), and the remainder being iron (Fe) and other inevitable impurities; and a galvanized layer on the base steel sheet, wherein a final microstructure of the base steel sheet has an area fraction of martensite of 65% or more, an average aspect ratio of precipitated carbides of 5.0 or less, a tensile strength (TS) of 1400 MPa or more, an elongation (El) of 3.0% or more, and a bendability (R / t) of 5.0 or less.

Description

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

[0001] The present invention relates to a plated steel sheet and a method for manufacturing the same, and more particularly, to an ultra-high strength plated steel sheet having excellent part formability and a method for manufacturing the same.

[0002] The automotive industry has consistently demanded greater crashworthiness. While the recent proliferation of electric vehicles has reduced the number of automotive components, the introduction of batteries has increased vehicle weight, further expanding the need for crashworthiness. Martensitic steel, the strongest cold-rolled steel, has seen its application expanded with the increasing use of roll forming techniques. However, it still faces issues with corrosion resistance and delayed fracture due to hydrogen. Typically, martensitic steel is manufactured using a rapid cooling-reheating process. This process involves annealing, then rapidly cooling to room temperature to form a single martensite phase, followed by reheating and tempering to remove brittleness. This process makes it impossible to manufacture hot-dip galvanized or alloyed hot-dip galvanized steel sheets. Therefore, martensitic steel is currently being supplied to mass-produced vehicles using electrogalvanized iron (EGI), which is produced by electrogalvanizing unplated materials manufactured using the aforementioned process. In addition, since the delayed fracture issue of martensitic steel is related to hydrogen penetration and corrosion resistance, there is a need to develop hot-dip galvanized and alloyed hot-dip galvanized steel sheets with superior corrosion resistance compared to electrogalvanized steel.

[0003] Related prior art includes Japanese Patent Publication No. 2005-105367.

[0004] The technical problem to be achieved by the present invention is to provide a plated steel sheet and a manufacturing method thereof that reduces brittleness and improves corrosion resistance so as to enable forming of parts using an ultra-high-strength martensitic steel sheet having a tensile strength of 1400 MPa or more.

[0005] However, these tasks are exemplary and the technical idea of ​​the present invention is not limited thereto.

[0006] According to one embodiment of the present invention for solving the above problem, a plated steel sheet comprises carbon (C): 0.1 to 0.5 wt%, silicon (Si): 0.01 to 2.0 wt%, manganese (Mn): 0.1 to 5.0 wt%, phosphorus (P): more than 0 and 0.02 wt% or less, sulfur (S): more than 0 and 0.01 wt% or less, aluminum (Al): 0.01 to 2.0 wt%, chromium (Cr): more than 0 and 3.0 wt% or less, molybdenum (Mo): more than 0 and 1.0 wt% or less, nickel (Ni): more than 0 and 0.4 wt% or less, copper (Cu): more than 0 and 0.4 wt% or less, titanium (Ti): 0.01 to 0.2 wt%, niobium (Nb): 0.01 to 0.1 wt%, vanadium (V): 0.01 to 1.0 wt%, A base steel sheet comprising boron (B): 0.001 to 0.005 wt%, tin (Sn): 0.0001 to 0.04 wt%, and the remainder iron (Fe) and other unavoidable impurities; and a plated steel sheet having a plating layer on the base steel sheet, wherein the final microstructure of the base steel sheet has an area fraction of martensite of 65% or more, an average aspect ratio of precipitated carbides of 5.0 or less, and the plated steel sheet has a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, and a bending workability (R / t): 5.0 or less.

[0007] In the above-mentioned galvanized steel sheet, the final microstructure of the base steel sheet may have an area fraction of bainite and martensite of 90% or more and an area fraction of ferrite of 10% or less.

[0008] In the above-mentioned galvanized steel sheet, the final microstructure of the base steel sheet may have an area fraction of bainite of 25% or more.

[0009] In the above-mentioned plated steel sheet, the carbide may have an average size of 100 nm or less.

[0010] In the above-mentioned galvanized steel sheet, the plating layer may be a hot-dip galvanized layer or an alloyed hot-dip galvanized layer rather than an electrogalvanized layer.

[0011] According to an embodiment of the present invention for solving the above problem, a method for manufacturing a plated steel sheet comprises: carbon (C): 0.1 to 0.5 wt%, silicon (Si): 0.01 to 2.0 wt%, manganese (Mn): 0.1 to 5.0 wt%, phosphorus (P): more than 0 and 0.02 wt% or less, sulfur (S): more than 0 and 0.01 wt% or less, aluminum (Al): 0.01 to 2.0 wt%, chromium (Cr): more than 0 and 3.0 wt% or less, molybdenum (Mo): more than 0 and 1.0 wt% or less, nickel (Ni): more than 0 and 0.4 wt% or less, copper (Cu): more than 0 and 0.4 wt% or less, titanium (Ti): 0.01 to 0.2 wt%, niobium (Nb): 0.01 to 0.1 wt%, vanadium (V): 0.01 to 1.0 wt%, Boron (B): 0.001 to 0.005 wt%, Tin (Sn): 0.0001 to 0.A step of hot rolling a steel material composed of 0.4 wt% and the remainder iron (Fe) and other unavoidable impurities; a step of cold rolling the hot-rolled steel material; And a step of sequentially performing an annealing process, a first heat treatment process (I), a second heat treatment process (II) and a third heat treatment process (III) on the cold-rolled steel; wherein the second heat treatment process (II) includes a hot-dip galvanizing process or an alloyed hot-dip galvanizing process, and the first heat treatment process (I), the second heat treatment process (II) and the third heat treatment process (III) each include a cooling step, wherein the cooling rate in the first heat treatment process (I) is greater than the cooling rate in the second heat treatment process (II), and the cooling rate in the third heat treatment process (III) is greater than the cooling rate in the second heat treatment process (II), and the first cooling end temperature (T1), which is the cooling end temperature in the first heat treatment process (I), is a temperature between the bainite transformation initiation temperature (Bs) and the martensite transformation initiation temperature (Ms), and In the second heat treatment process (II), the second cooling end temperature (T2), which is the cooling end temperature, is a temperature between the bainite transformation initiation temperature (Bs) and the martensite transformation initiation temperature (Ms), and in the third heat treatment process (III), the third cooling end temperature (T3), which is the cooling end temperature, is characterized in that it is lower than the martensite transformation end temperature (Mf).

[0012] In the above method for manufacturing a plated steel sheet, the cooling rate in the first heat treatment process (I) may be 2 to 60°C / s.

[0013] In the above method for manufacturing a plated steel sheet, the process time for performing the third heat treatment process (III) may be 180 seconds or less.

[0014] In the method for manufacturing the above-mentioned plated steel sheet, a step of cooling the steel material to room temperature after performing the third heat treatment process (III) may be further included, but the total process time for performing the step of performing the third heat treatment process (III) and the step of cooling to room temperature may be at least 10 seconds. In this case, the plated steel sheet finally realized after performing the step of cooling to room temperature may have a yield strength (YP): 850 to 1300 MPa, a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, and a bending workability (R / t): 5.0 or less.

[0015] The method for manufacturing the above-mentioned plated steel sheet may further include a step of cooling the steel material to room temperature after performing the third heat treatment process (III); and a tempering step of heating the steel material from room temperature and maintaining it at a predetermined temperature (T4) for a predetermined time (t4); and may satisfy the following Equation 1. In this case, the plated steel sheet finally realized after performing the tempering step may have a yield strength (YP): 1000 to 1600 MPa, a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, and a bending workability (R / t): 5.0 or less.

[0016] Equation 1: 3800 ≤ [(T4+ 300) × (10 + log(t4))] ≤ 5650

[0017] (Here, the unit of the temperature (T4) is ℃, and the unit of the time (t4) is hour)

[0018] In the above method for manufacturing a plated steel sheet, the hot rolling step includes a hot rolling step under conditions of a reheating temperature of 1150 to 1300°C, a finishing rolling temperature of 800 to 1000°C, and a coiling temperature of 300 to 700°C, and the annealing process may have an annealing temperature (T0) of Ac3 or higher.

[0019] According to an embodiment of the present invention, a plated steel sheet can be provided that is implemented with a manufacturing method that reduces brittleness so that forming of a part of an ultra-high-strength martensitic steel sheet having a tensile strength of 1400 MPa or more is possible.

[0020] Of course, the scope of the present invention is not limited by these effects.

[0021] FIG. 1 is a graph illustrating the relationship between time and temperature in steps after the annealing process in a method for manufacturing a plated steel sheet according to one embodiment of the present invention.

[0022] FIG. 2 is a graph illustrating the relationship between time and temperature in steps after the annealing process in a method for manufacturing a plated steel sheet according to another embodiment of the present invention.

[0023] Figures 3 and 4 are schematic diagrams illustrating a method for experimentally measuring the martensite transformation initiation temperature (Ms) and martensite transformation end temperature (Mf) of a specimen in an experimental example of the present invention.

[0024] Figures 5 and 6 are photographs showing the results of carbide analysis after BAF (Batch Annealing Furnace) heat treatment on a specimen (high yield material) according to an invention example (Experimental Example 27) of the present invention.

[0025] Figures 7 and 8 are graphs showing the results of carbide analysis after BAF (Batch Annealing Furnace) tempering heat treatment for a specimen (high yield material) according to an invention example (Experimental Example 27) of the present invention.

[0026] Figures 9 to 12 are photographs of the microstructure of a specimen according to an experimental example of the present invention.

[0027] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Embodiments of the present invention are provided to more completely explain the technical idea of ​​the present invention to those skilled in the art. The following embodiments may be modified in various different forms, and the scope of the technical idea of ​​the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely convey the technical idea of ​​the present invention to those skilled in the art. Like reference numerals throughout this specification denote like elements. Furthermore, various elements and areas in the drawings are schematically drawn. Therefore, the technical idea of ​​the present invention is not limited by the relative sizes or intervals drawn in the attached drawings.

[0028] Martensitic steel is generally known to possess high strength due to supersaturated carbon and grain refinement. However, martensitic steel formed after cooling exhibits high brittleness, making it difficult to apply to parts due to limitations in formability. To overcome this, a common manufacturing method is to cool to room temperature, then reheat and perform an isothermal tempering process. Tempering at relatively low temperatures, which does not cause a rapid decline in the tensile strength of martensitic steel, allows for the competitive interaction of supersaturated carbon migration, segregation, and carbide formation, resulting in increased toughness and ductility, increased homogeneity of each phase, and changes in yield strength. Furthermore, tempering can be performed not only through isothermal tempering but also through auto-tempering, which occurs during cooling after martensite formation begins. Of course, auto-tempering exhibits the aforementioned effects to a lesser extent than isothermal tempering.

[0029] The primary purpose of this patent is to disclose a manufacturing method for reducing brittleness so as to enable forming of parts using ultra-high-strength martensitic steel sheets with a tensile strength of 1400 MPa or more. In particular, in order to increase corrosion resistance compared to electro-galvanized iron, the present invention specifically proposes a method for reducing brittleness of low-yield steel sheets through auto-tempering after the start of martensite formation in the hot-dip galvanizing and alloyed hot-dip galvanizing processes, and a method for manufacturing high-yield steel sheets through isothermal tempering after the above process.

[0030] According to one embodiment of the present invention, a method for manufacturing a plated steel sheet comprises: carbon (C): 0.1 to 0.5 wt%, silicon (Si): 0.01 to 2.0 wt%, manganese (Mn): 0.1 to 5.0 wt%, phosphorus (P): more than 0 and 0.02 wt% or less, sulfur (S): more than 0 and 0.01 wt% or less, aluminum (Al): 0.01 to 2.0 wt%, chromium (Cr): more than 0 and 3.0 wt% or less, molybdenum (Mo): more than 0 and 1.0 wt% or less, nickel (Ni): more than 0 and 0.4 wt% or less, copper (Cu): more than 0 and 0.4 wt% or less, titanium (Ti): 0.01 to 0.2 wt%, niobium (Nb): 0.01 to 0.1 wt%, vanadium (V): 0.01 to 1.0 wt%, A method for producing a steel product, comprising: a step (S10) of hot rolling a steel product comprising boron (B): 0.001 to 0.005 wt%, tin (Sn): 0.0001 to 0.04 wt%, and the remainder iron (Fe) and other unavoidable impurities; a step (S20) of cold rolling the hot-rolled steel product; and a step of sequentially performing an annealing process (S30), a first heat treatment process (S40), a second heat treatment process (S50), and a third heat treatment process (S60) on the cold-rolled steel product.

[0031] First, the role and content of each component included in the above steel are explained.

[0032] carbon (C)

[0033] Carbon (C) is an element added to secure strength by improving martensite hardness. In other words, carbon is the most effective and important element for increasing the strength of steel. In addition, by adding carbon, it is dissolved in austenite and forms a martensite structure during quenching. Furthermore, it combines with elements such as iron, chromium, and molybdenum to form carbides, thereby improving strength and hardness. Carbon (C) may be added in a content ratio of 0.1 to 0.5 wt% of the total steel weight in the base steel sheet constituting the coated steel sheet according to an embodiment of the present invention. If the carbon content is less than 0.1 wt% of the total weight, strength cannot be secured, so the above-described effect cannot be realized, and the problem of not securing sufficient strength arises. Conversely, if the carbon content exceeds 0.5 wt% of the total weight, problems of reduced weldability and workability arise.

[0034] Silicon (Si)

[0035] Silicon (Si) is an element added to secure bendability and hydrogen embrittlement resistance by suppressing cementite formation. In addition, silicon is an element added to increase strength and suppress carbide formation through the effect of strengthening ferrite solid solution. Silicon can be added to secure hydrogen embrittlement resistance by suppressing cementite formation and growth during high-temperature tempering at approximately 600℃. In addition, silicon is well known as a ferrite stabilizing element, so it can increase ductility by increasing the ferrite fraction during cooling. In addition, it is known as an element that can secure strength by promoting martensite formation through austenite carbon enrichment. Meanwhile, silicon is added as a deoxidizing agent to remove oxygen in steel during the steelmaking process together with aluminum, and can also have a solid solution strengthening effect. The silicon may be added in a content ratio of 0.01 to 2.0 wt% of the total steel weight in the base steel sheet constituting the coated steel sheet according to one embodiment of the present invention. When the silicon content is less than 0.01 wt% of the total steel weight, ductility cannot be secured and the aforementioned silicon addition effect cannot be properly exerted. Conversely, when the silicon content exceeds 2.0 wt% of the total weight and is added in large quantities, ferrite is excessively formed, which reduces strength, oxides are formed on the surface of the steel sheet, which reduces the plating properties of the steel sheet, and red scale is generated during reheating and hot rolling, which can cause problems with surface quality, and there are problems of reduced toughness and plastic workability, and it can reduce the weldability of the steel.

[0036] manganese (Mn)

[0037] Manganese (Mn) is an element that contributes to strength enhancement through solid solution strengthening and increased hardenability. For example, manganese facilitates the formation of low-temperature transformation phases and provides the effect of increasing strength through solid solution strengthening. Some of the manganese is dissolved in steel, and some combines with sulfur contained in the steel to form MnS, a non-metallic inclusion. This MnS is ductile and elongates in the working direction during plastic working. However, the formation of MnS reduces the sulfur content in the steel, weakening the grains and inhibiting the formation of FeS, a low-melting-point compound. Although it reduces the acid and oxidation resistance of the steel, it improves the yield strength by making pearlite fine and solid solution strengthening ferrite. Manganese may be added to the base steel sheet constituting the coated steel sheet according to one embodiment of the present invention in an amount of 0.1 to 5.0 wt% based on the total steel weight. If the manganese content is less than 0.1 wt%, the aforementioned strength-enhancing effect cannot be fully achieved. In addition, when the manganese content exceeds 5.0 wt%, problems such as reduced bendability and hydrogen embrittlement resistance arise due to the formation of manganese bands and MnS. For example, problems such as the formation of internal and external segregation zones in continuous casting slabs and steel plates, which induce the initiation and propagation of cracks, arise and reduce bendability. In other words, slab quality and weldability deteriorate, and center segregation may occur, which may reduce the ductility and workability of the base steel plate.

[0038] Person (P)

[0039] Phosphorus (P) can increase strength through solid solution strengthening and suppress the formation of carbides. The phosphorus can be added in a content ratio of more than 0 and less than 0.02 wt% of the total steel weight in the base steel sheet constituting the coated steel sheet according to one embodiment of the present invention. When the content of phosphorus exceeds 0.02 wt%, problems such as reduced spot weldability, brittleness caused by grain boundary segregation, reduced press formability, and reduced impact resistance may occur.

[0040] Yellow (S)

[0041] Sulfur (S) improves the machinability of steel by combining with manganese, titanium, etc., and can improve workability by forming fine MnS precipitates, but is an element that inhibits ductility and weldability. The sulfur may be added in a content ratio of more than 0 and less than 0.01 wt% of the total steel weight in the base steel sheet constituting the coated steel sheet according to an embodiment of the present invention. When the sulfur content exceeds 0.01 wt%, the number of MnS inclusions increases, resulting in poor bendability and hydrogen embrittlement resistance, and problems such as segregation during continuous casting solidification, which may cause high-temperature cracks to occur.

[0042] Aluminum (Al)

[0043] Aluminum (Al) is an element mainly used as a deoxidizer, and it prevents slab cracking during nitride formation, promotes ferrite formation, improves elongation, suppresses carbide formation, and stabilizes austenite by increasing carbon enrichment in austenite. In addition, aluminum is an element that improves plating properties by acting as a layer between iron and a zinc plating layer, and is an effective element for suppressing the formation of manganese bands in hot-rolled coils. It is preferable that the aluminum (Al) is added in a content ratio of 0.01 to 2.0 wt% of the total steel weight in the base steel sheet constituting the plated steel sheet according to an embodiment of the present invention. When the content of aluminum (Al) is less than 0.01 wt%, the deoxidation effect is insufficient, and the above-described aluminum addition effect cannot be properly exhibited. On the other hand, if the content of aluminum (Al) is excessively added exceeding 2.0 wt%, there is a problem that the strength is reduced through the formation of ferrite, the aluminum inclusions increase, which reduces the playability, the aluminum is concentrated on the surface of the steel plate, which reduces the plating property, and AlN is formed in the slab, which causes hot-rolled cracks.

[0044] chromium (Cr)

[0045] Chromium (Cr) is an element that can improve hardenability and secure high strength, and has the effect of improving hardenability as an austenite stabilizing element. That is, chromium is an element that contributes to the improvement of strength through solid solution strengthening and increased hardenability. In addition, chromium increases elongation by precipitating Cr-based precipitates within grains during annealing heat treatment. It is preferable that the chromium (Cr) be added in a content ratio of more than 0 and less than 3.0 wt% of the total steel weight in the base steel sheet constituting the plated steel sheet according to an embodiment of the present invention. If the content of chromium (Cr) exceeds 3.0 wt% and is excessively added, a saturation effect occurs, laser weldability and ductility deteriorate, there are problems of inhibiting plating properties, and there is a problem of ferrite stabilization.

[0046] molybdenum (Mo)

[0047] Molybdenum (Mo) contributes to the improvement of strength through solid solution strengthening and increased hardenability, and is an element that contributes to the improvement of hydrogen embrittlement resistance by refining Ti-based precipitates. That is, molybdenum is an element added to improve hardenability and secure strength and toughness, and is an element that can improve hydrogen embrittlement resistance due to grain refinement and precipitation effects. It is preferable that the molybdenum (Mo) is added in a content ratio of more than 0 and less than 1.0 wt% of the total weight of the base steel sheet constituting the plated steel sheet according to one embodiment of the present invention. When the content of molybdenum (Mo) exceeds 1.0 wt%, the manufacturing cost increases, and there is a problem of lowering weldability.

[0048] Nickel (Ni)

[0049] Nickel is added to suppress hot embrittlement caused by copper, and can be added in a ratio of up to 1:1 with copper based on the weight ratio. Meanwhile, nickel is a precipitate-forming element that forms carbides or nitrides by combining with carbon (C) and nitrogen (N), and can improve the toughness and strength of steel through grain refinement by suppressing such precipitation and recrystallization and grain growth during rolling. If the nickel content exceeds 0.4 wt%, the rolling load may increase significantly during rolling, which may increase the manufacturing cost of steel. In particular, it is necessary to control the ratio with copper to prevent melting during the reheating process. Therefore, it is preferable to add nickel in an amount exceeding 0 and not exceeding 0.4 wt% of the total weight of the steel.

[0050] copper (Cu)

[0051] Copper is a precipitate-forming element that forms carbides or nitrides by combining with carbon (C) and nitrogen (N). It can improve the toughness and strength of steel through grain refinement by suppressing precipitation and recrystallization and grain growth during rolling. It can also be added to increase hydrogen embrittlement resistance. If copper is not added, delayed fracture may occur, and if the copper content exceeds 0.4 wt%, it is an element that induces red heat embrittlement, which may cause cracks during hot rolling, significantly increase the rolling load during rolling, and increase the manufacturing cost of steel. Therefore, it is preferable to add copper in an amount exceeding 0 and not exceeding 0.4 wt% of the total weight of the steel.

[0052] titanium (Ti)

[0053] Titanium (Ti) is an element added for the purpose of forming TiN in order to maintain the solid solution state of boron (B), which is a grain boundary strengthening element. Titanium contributes to grain refinement and suppression of BN formation. It is preferable that the titanium (Ti) is added in a content ratio of 0.01 to 0.2 wt% of the total weight of the base steel sheet constituting the plated steel sheet according to an embodiment of the present invention. When the content of the titanium (Ti) is less than 0.01 wt%, the ductility of the cast slab is reduced due to excessive precipitation of BN precipitates, which causes a problem of lowering the quality of the slab and lowering the strength. On the other hand, when the content of the titanium (Ti) exceeds 0.2 wt%, the bendability and hydrogen embrittlement resistance are lowered due to coarsening of TiN precipitation, and the recrystallization temperature is excessively increased, which causes a problem of inducing a non-uniform structure.

[0054] niobium (Nb)

[0055] Niobium (Nb) is an element added for the purpose of grain refinement, strength improvement, and hydrogen trapping. Niobium (Nb) element can be added in a content ratio of 0.01 to 0.1 wt% of the total weight of the base steel sheet. If the content of niobium (Nb) is less than 0.01 wt%, there is a problem of lowering the ductility of the cast slab due to excessive precipitation of AIN and BN precipitates, which deteriorates the slab quality, and it is difficult to expect the effects of precipitate refinement, grain refinement, and strength improvement. In addition, if the content of niobium (Nb) is greater than 0.1 wt%, grain refinement is difficult due to the formation of coarse TiN and TiC precipitates, there is no increase in strength and hydrogen trap effect, there is a problem of causing a problem of inhomogeneous structure due to excessively high recrystallization temperature, and there is a problem of causing a process load due to excessive precipitation.

[0056] Vanadium (V)

[0057] Vanadium (V) is an element added for the purpose of improving strength through fine precipitation, grain refinement, and hydrogen trapping. As vanadium is added, fine precipitates (VC or (Ti,V)C) are formed, contributing to improving strength. Vanadium (V) is preferably added in a content ratio of 0.01 to 1.0 wt% of the total weight of the base steel sheet constituting the plated steel sheet according to an embodiment of the present invention. When the content of vanadium (V) is less than 0.01 wt%, there is no grain refinement effect and it does not contribute to improving strength. In contrast, when it is added excessively exceeding 1.0 wt%, there is no increase in strength due to precipitate growth, no hydrogen trap effect, and the manufacturing cost of steel may significantly increase. In addition, due to a large amount of precipitates during rolling, the rolling load may significantly increase and the elongation may decrease.

[0058] boron (B)

[0059] Boron (B) is a grain boundary strengthening element that increases resistance to hydrogen embrittlement when distributed at grain boundaries. In addition, boron is an element added to increase the hardenability of steel by suppressing ferrite formation. In addition, boron is a strong quenching element and plays a role in improving strength by preventing segregation of phosphorus (P). If segregation of phosphorus (P) occurs, secondary processing embrittlement may occur, so boron is added to prevent segregation of phosphorus (P) and increase resistance to processing embrittlement. It is preferable that the boron is added in a content ratio of 0.001 to 0.005 wt% of the total weight of the base steel sheet constituting the plated steel sheet according to one embodiment of the present invention. If the boron content is less than 0.001 wt%, there is no grain boundary strengthening effect and strength cannot be secured due to low hardenability. If the boron content exceeds 0.005 wt% and is excessively added, grain boundary brittleness increases due to BN formation, weldability deteriorates, and the formation of boron oxide can cause problems such as deteriorating the surface quality of the steel.

[0060] Sn

[0061] Tin (Sn) does not form an oxide film on its own at high temperatures, so it precipitates on the surface of steel sheets (here, the steel sheet corresponds to the base steel sheet) during annealing heat treatment performed at relatively high temperatures, thereby improving plating properties by inhibiting oxidation-friendly elements such as Al, Si, and Mn from diffusing to the surface and forming oxides. However, tin is an element vulnerable to red-hot embrittlement due to the complex effect with copper (Cu), so if the tin content exceeds 0.04 wt%, the problem of red-hot embrittlement appears, so it is necessary to strictly control the content within the steel to 0.0001 to 0.04 wt%.

[0062] The remaining component of the above ultra-high-strength cold-rolled steel sheet is iron (Fe). However, during the normal manufacturing process, unintended impurities from raw materials or the surrounding environment can inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full content is not specifically addressed in this specification.

[0063] The above hot rolling step (S10) can be performed on the above-described steel material under the following conditions: reheating temperature: 1150 to 1300°C, finishing rolling temperature: 800 to 1000°C, coiling temperature: 300 to 700°C.

[0064] When the above steel is reheated at the above-mentioned temperature (1150 ~ 1300℃), the segregated components during the continuous casting process may be re-dissolved. In order to improve the strength through precipitation and solution strengthening, the strengthening elements must be sufficiently dissolved in austenite before hot rolling, and for this reason, the steel needs to be heated to 1150℃ or higher. If the reheating temperature is lower than 1150℃, the solid solution of various carbides may not be sufficient, and there may be a problem in that the segregated components may not be evenly distributed during the continuous casting process. However, if the reheating temperature exceeds 1300℃, there may be adverse effects such as coarsening or decarburization of austenite, and the desired strength cannot be obtained. In other words, if the reheating temperature exceeds 1300℃, very coarse austenite grains are formed, making it difficult to secure strength. In addition, if the reheating temperature exceeds 1300℃, heating costs increase and process time is added, which may lead to increased manufacturing costs and decreased productivity.

[0065] Finish rolling temperature (FDT) is a very important factor affecting the final material. Rolling at 800 to 1000℃ is the temperature that can refine austenite. However, if the hot rolling temperature is lower than 800℃, the rolling load increases during rolling and mixed grain structure may occur at the edge. Furthermore, rolling at high temperatures exceeding 1000℃ coarsens grains, making it difficult to obtain the target mechanical properties. Cooling after hot rolling is performed at a cooling rate of 1 to 100℃ / s, and the faster the cooling rate, the more advantageous it is for reducing the average grain size.

[0066] Meanwhile, when the coiling temperature is lower than 300°C, the shape of the hot-rolled coil becomes uneven and the cold-rolling load increases. When the coiling temperature is higher than 700°C, the difference in cooling rates between the center and edge of the steel sheet causes an uneven microstructure, and the problem of oxidation within the grain boundaries may occur.

[0067] Meanwhile, the hot rolling can be performed under conditions where the reduction ratio is 35 to 65%. The microstructure of the steel after hot rolling can include bainite, martensite, and ferrite.

[0068] The above cold rolling step (S20) may include a step of cold rolling at a reduction ratio of 35 to 65% after performing a pickling process. The higher the reduction ratio, the finer the grains can be manufactured, and the effect of increased formability due to the structure refinement effect is enhanced. If the reduction ratio in cold rolling is less than 35%, it is difficult to obtain a uniform microstructure, and if it is designed to exceed 65%, the roll force increases, which increases the process load.

[0069] FIG. 1 is a drawing showing an overview of heat treatment processes that sequentially perform annealing, a first heat treatment, a second heat treatment process, and a third heat treatment process in a method for manufacturing a plated steel sheet according to one embodiment of the present invention.

[0070] As used herein, the term "heat treatment process" may include any process that regulates the temperature of a steel material over time for an intended purpose. That is, the heat treatment process may include any process that involves temperature control corresponding to at least one of heating, holding, and cooling of the steel material.

[0071] Referring to Fig. 1, the cold-rolled steel is heated to a temperature higher than Ac3 at a heating rate of 1 to 100°C / s. A rapid heating rate exceeding 100°C / s inhibits the temperature-related recovery / recrystallization of the cold-rolled material, resulting in increased microstructural heterogeneity. The Ac3 temperature can be calculated using the following formula.

[0072] Ac3(℃) = 910 - 203 [C] 0.5 - 30[Mn] + 44.7[Si] + 31.5[Mo] - 15.2[Ni]

[0073] Here, [C], [Mn], [Si], [Mo] and [Ni] are the weight percent values ​​of carbon, manganese, silicon, molybdenum and nickel in the steel.

[0074] According to a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention, an annealing process is performed at an annealing temperature (T0) of Ac3 or higher, for example, between 750 and 950°C, for 60 to 600 seconds.

[0075] In the present invention, the cooling and plating process (hot-dip galvanizing or alloy hot-dip galvanizing) after the end of annealing is classified into three sections and the temperature and time (cooling rate) for each section are proposed.

[0076] After the annealing process (S30), the first heat treatment process (I; S40), the second heat treatment process (II; S50), and the third heat treatment process (III; S60) are sequentially performed.

[0077] The first heat treatment process (I; S40), the second heat treatment process (II; S50), and the third heat treatment process (III; S60) may each include a cooling step, wherein the cooling rate in the first heat treatment process (I; S40) is greater than the cooling rate in the second heat treatment process (II; S50), and the cooling rate in the third heat treatment process (III; S60) is greater than the cooling rate in the second heat treatment process (II).

[0078] Specifically, in the first heat treatment process (I; S40), the cooling rate may be, for example, 2°C / s or more and 60°C / s or less. The first heat treatment process (I; S40) is a relatively high temperature region after annealing, and ferrite formation must be suppressed to secure the final strength. Therefore, the average cooling rate must be 2°C / s or more, and the average cooling rate must not exceed 60°C / s to secure the flatness of the coil.

[0079] In the first heat treatment process (I; S40), the first cooling end temperature (T1), which is the cooling end temperature, is a temperature between the bainite transformation start temperature (Bs) and the martensite transformation start temperature (Ms), in the second heat treatment process (II; S50), the second cooling end temperature (T2), which is the cooling end temperature, is a temperature between the bainite transformation start temperature (Bs) and the martensite transformation start temperature (Ms), and in the third heat treatment process (III; S60), the third cooling end temperature (T3), which is the cooling end temperature, may be lower than or equal to the martensite transformation end temperature (Mf).

[0080] Meanwhile, the second heat treatment process (II; S50) may include a section in which a hot-dip galvanizing process (II(i)) or an alloy hot-dip galvanizing process (II(ii)) is performed.

[0081] In order to secure strength, ferrite formation must be suppressed as much as possible, and for this purpose, the first cooling end temperature (T1), which is the cooling end temperature in the first heat treatment process (I; S40), and the second cooling end temperature (T2), which is the cooling end temperature in the second heat treatment process (II; S50), must be lower than the bainite transformation initiation temperature (Bs), and preferably lower than [bainite transformation initiation temperature (Bs) - 50°C].

[0082] In the first heat treatment process (I; S40), if the first cooling end temperature (T1), which is the cooling end temperature, does not satisfy the temperature range between the martensite transformation initiation temperature (Ms) and the bainite transformation initiation temperature (Bs) and is lower than the martensite transformation initiation temperature (Ms), a problem occurs in which the condition of tensile strength (TS): 1400 MPa or more is not satisfied and is lower than the condition, and in the second heat treatment process (II; S50), if the second cooling end temperature (T2), which is the cooling end temperature, does not satisfy the temperature range between the martensite transformation initiation temperature (Ms) and the bainite transformation initiation temperature (Bs) and is lower than the martensite transformation initiation temperature (Ms), a problem occurs in which the condition of tensile strength (TS): 1400 MPa or more is not satisfied and is lower than the condition.

[0083] In addition, in order to improve the surface quality after the hot-dip galvanizing process (II(i)) or the alloyed hot-dip galvanizing process (II(ii)), the second cooling end temperature (T2), which is the cooling end temperature in the second heat treatment process (II; S50), must be higher than the martensite transformation initiation temperature (Ms), and preferably higher than [martensite transformation initiation temperature (Ms) + 50°C].

[0084] Meanwhile, in order to secure strength, the third cooling end temperature (T3), which is the cooling end temperature in the third heat treatment process (III; S60), must be lower than the martensite transformation end temperature (Mf) so that all of the remaining undeformed regions are transformed into martensite, and preferably, [the third cooling end temperature (T3), which is the cooling end temperature in the third heat treatment process (III; S60), must be lower than the martensite transformation end temperature (Mf) - 10℃]. If the third cooling end temperature (T3), which is the cooling end temperature in the third heat treatment process (III; S60), does not satisfy the range of the martensite transformation end temperature (Mf) but exceeds it, the condition that the area fraction of martensite in the final microstructure is 65% or more is not satisfied, and the problem occurs that the condition that the tensile strength (TS): is 1400 MPa or more is not satisfied, and the condition is not satisfied, and the problem occurs that the condition is not satisfied, and the problem occurs that the condition is not satisfied, and the problem occurs that the problem occurs that the problem occurs that the problem occurs because ...

[0085] In a method for manufacturing a plated steel sheet according to one embodiment of the present invention, the process time for performing the third heat treatment process (III; S60) may be 180 seconds or less. That is, the cooling time from the second cooling end temperature (T2), which is the cooling end temperature in the second heat treatment process (II; S50), to the third cooling end temperature (T3), which is the cooling end temperature in the third heat treatment process (III; S60) should be controlled so as not to exceed 180 seconds. If a cooling rate that is too slow exceeds 180 seconds is maintained, a decrease in strength due to auto tempering may occur. In the present invention, if the cooling time from the second cooling end temperature (T2), which is the cooling end temperature in the second heat treatment process (II; S50), to the third cooling end temperature (T3), which is the cooling end temperature in the third heat treatment process (III; S60), exceeds 180 seconds, a problem occurs in which the condition that the area fraction of martensite in the final microstructure is 65% or more is not satisfied and falls below, and the condition that the tensile strength (TS): is 1400 MPa or more is not satisfied and falls below.

[0086] A method for manufacturing a plated steel sheet according to one embodiment of the present invention comprises: performing the third heat treatment process (III; S60) and then heating the steel material at room temperature (T r ) may further include a step of cooling to room temperature. In this case, the total process time for performing the step of performing the third heat treatment process (III; S60) and the step of cooling to room temperature may be at least 10 seconds or more. That is, in the second heat treatment process (II; S50), the second cooling end temperature (T2), which is the cooling end temperature, may be at least 10 seconds. r ) should be controlled to not be less than 10 seconds. If there is no auto tempering after the formation of martensite, brittleness may occur, so the second cooling end temperature (T2) should be cooled to room temperature (T r ) must be controlled to a cooling time of 10 seconds or more.

[0087] According to an embodiment of the present invention, a plated steel sheet implemented by performing the above-described steps comprises carbon (C): 0.1 to 0.5 wt%, silicon (Si): 0.01 to 2.0 wt%, manganese (Mn): 0.1 to 5.0 wt%, phosphorus (P): more than 0 and 0.02 wt% or less, sulfur (S): more than 0 and 0.01 wt% or less, aluminum (Al): 0.01 to 2.0 wt%, chromium (Cr): more than 0 and 3.0 wt% or less, molybdenum (Mo): more than 0 and 1.0 wt% or less, nickel (Ni): more than 0 and 0.4 wt% or less, copper (Cu): more than 0 and 0.4 wt% or less, titanium (Ti): 0.01 to 0.2 wt%, niobium (Nb): 0.01 to 0.1 wt%, vanadium (V): 0.01 to A coated steel sheet comprising a base steel sheet composed of 1.0 wt% of boron (B), 0.001 to 0.005 wt% of tin (Sn), and the remainder iron (Fe) and other unavoidable impurities; and a plating layer on the base steel sheet, wherein the base steel sheet has an area fraction of martensite of 65% or more in the final microstructure, an average aspect ratio of precipitated carbides of 5.0 or less, a yield strength (YP): 850 to 1300 MPa, a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, and a bending workability (R / t): 5.0 or less.

[0088] In the final microstructure of the above-mentioned steel plate, the area fraction of bainite and martensite may be 90% or more, and the area fraction of ferrite may be 10% or less.

[0089] The area fraction of bainite in the final microstructure of the above-mentioned steel plate may be 25% or more.

[0090] In the final microstructure of the above-mentioned steel plate, the carbide may have an average size of 100 nm or less.

[0091] FIG. 2 is a graph illustrating the relationship between time and temperature in steps after the annealing process in a method for manufacturing a plated steel sheet according to another embodiment of the present invention.

[0092] Referring to FIG. 2, a method for manufacturing a plated steel sheet according to another embodiment of the present invention is characterized in that, unlike the method for manufacturing a plated steel sheet according to one embodiment of the present invention described above with reference to FIG. 1, it further includes a third heat treatment process (III; S60) and a tempering step (S70) of heating the steel material from room temperature after cooling to room temperature and maintaining it at a predetermined temperature (T4) for a predetermined time (t4). The heating rate for heating the steel material to the tempering temperature (T4) is set to 50°C / s or less in consideration of productivity, power, etc.

[0093] Meanwhile, in the method for manufacturing a plated steel sheet according to another embodiment of the present invention (see FIG. 2), instead of performing the tempering step (S70), the method for manufacturing a plated steel sheet according to one embodiment of the present invention (see FIG. 1) performs the third heat treatment process (III; S60) and the step of cooling to room temperature, and it is not necessary to secure the total process time of at least 10 seconds.

[0094] In addition, the method for manufacturing a plated steel sheet according to another embodiment of the present invention (see FIG. 2) is the same as the method for manufacturing a plated steel sheet according to one embodiment of the present invention (see FIG. 1), and therefore, a description thereof is omitted to avoid redundancy.

[0095] In a method for manufacturing a plated steel sheet according to another embodiment of the present invention, the tempering step (S70) includes a tempering step of heating the steel material from room temperature and maintaining it at a predetermined temperature (T4) for a predetermined time (t4), satisfying the following equation 1.

[0096] Equation 1: 3800 ≤ [(T4+ 300) × (10 + log(t4))] ≤ 5650

[0097] (Here, the unit of the temperature (T4) is ℃, and the unit of the time (t4) is hour)

[0098] In the present invention, if the process conditions of the tempering step (S70) do not satisfy the above equation 1, a problem occurs in which the average aspect ratio of the precipitated carbide does not satisfy the condition of 5.0 or less but rather exceeds it, and the bending workability (R / t): does not satisfy the condition of 5.0 or less but rather exceeds it.

[0099] According to an embodiment of the present invention, a plated steel sheet implemented by performing the above-described steps comprises carbon (C): 0.1 to 0.5 wt%, silicon (Si): 0.01 to 2.0 wt%, manganese (Mn): 0.1 to 5.0 wt%, phosphorus (P): more than 0 and 0.02 wt% or less, sulfur (S): more than 0 and 0.01 wt% or less, aluminum (Al): 0.01 to 2.0 wt%, chromium (Cr): more than 0 and 3.0 wt% or less, molybdenum (Mo): more than 0 and 1.0 wt% or less, nickel (Ni): more than 0 and 0.4 wt% or less, copper (Cu): more than 0 and 0.4 wt% or less, titanium (Ti): 0.01 to 0.2 wt%, niobium (Nb): 0.01 to 0.1 wt%, vanadium (V): 0.01 to A coated steel sheet comprising a base steel sheet composed of 1.0 wt% of boron (B), 0.001 to 0.005 wt% of tin (Sn), and the remainder iron (Fe) and other unavoidable impurities; and a plating layer on the base steel sheet, wherein the base steel sheet has an area fraction of martensite of 65% or more in the final microstructure, an average aspect ratio of precipitated carbides of 5.0 or less, a yield strength (YP): 1000 to 1600 MPa, a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, and a bending workability (R / t): 5.0 or less.

[0100] In the final microstructure of the above-mentioned steel plate, the area fraction of bainite and martensite may be 90% or more, and the area fraction of ferrite may be 10% or less.

[0101] The area fraction of bainite in the final microstructure of the above-mentioned steel plate may be 25% or more.

[0102] In the final microstructure of the above-mentioned steel plate, the carbide may have an average size of 100 nm or less.

[0103] In this patent, the target material for hot-dip galvanized and alloyed hot-dip galvanized steel sheets used in automotive components is a tensile strength of 1.4 GPa and a ductility of 3% or more. Yield strength is classified into low-yield types (850 MPa or more and 1,300 MPa or less) and high-yield types (1,000 MPa or more). In addition, to ensure formability for automotive components, bending characteristics are limited as follows.

[0104] The preparation of the specimen for the bending test can be done by any method such as shearing, milling, or water jetting, and it is performed by 90 degree bending. The bending jig is tested by selecting various curvatures (R) compared to the steel plate thickness (t), and the target bending characteristic is 5.0 or less based on R / t. The bending characteristic R / t value is calculated by dividing the minimum bending radius (R) at which no cracks occur by checking for cracks in the bent part using a microscope after processing the steel plate into a specimen with a width of 100 mm × a length of 30 mm and performing a 90˚ bending test.

[0105] From a microstructure perspective, it should be structured as follows to secure a tensile strength of 1.4 GPa or more.

[0106] ① Ferrite: 10% or less, bainite: 25% or less, martensite: 65% or more

[0107] The above microstructure fraction is based on the results of analysis using a scanning electron microscope (SEM) at a point ¼ of the thickness direction in a direction perpendicular to the rolling direction.

[0108] ② In order to secure the target strength, the sum of bainite and martensite must be maintained at 90% or more.

[0109] Next, if carbides are formed as a result of auto-tempering and isothermal hold tempering, their shape and size should be as follows. In particular, if the size is very large or the length is too long in a certain direction, the formation and propagation of cracks are rapid, so their size and shape must be limited.

[0110] ① Type of carbide: Fe-based carbide (including both cementite and transition carbide, atomic formula: (Fe, substitutional element) 2~3 (C)) (Cementite: Fe3C, ε-carbide: Fe 2.5 C, η-carbides: Fe2C, etc.)

[0111] ② Size of carbide: less than 100 nm

[0112] ③ Aspect ratio of carbide: 5.0 or less

[0113] Experimental example

[0114] To aid in understanding the present invention, preferred experimental examples are presented below. However, the following experimental examples are provided solely to aid in understanding the present invention, and the present invention is not limited to the following experimental examples.

[0115] 1. Composition and characteristic temperature of the sample

[0116] In this experimental example, specimens having the alloy element composition (unit: weight %) shown in Table 1 are provided. In Table 1, the remainder is iron (Fe).

[0117] Steel grade composition wt% (P, S, B, Sn: wt-ppm)CSiMnPSAlCrMoNiCuTiNbVBSnA0.2690.0921.94677150.0310.380.190.0100.0260.0030.0012414B0.2460.0131.4671 1570.0220.5950.2120.050.160.0340.020.0013435C0.2650.1231.01298120.0330.1460.0010.130.120.0270.0010.0012346

[0118] Referring to Table 1, steel grades A, B, and C according to the experimental examples of the present invention contain carbon (C): 0.1 to 0.5 wt%, silicon (Si): 0.01 to 2.0 wt%, manganese (Mn): 0.1 to 5.0 wt%, phosphorus (P): more than 0 and 0.02 wt% or less, sulfur (S): more than 0 and 0.01 wt% or less, aluminum (Al): 0.01 to 2.0 wt%, chromium (Cr): more than 0 and 3.0 wt% or less, molybdenum (Mo): more than 0 and 1.0 wt% or less, nickel (Ni): more than 0 and 0.4 wt% or less, copper (Cu): more than 0 and 0.4 wt% or less, titanium (Ti): 0.01 to 0.2 wt%, niobium (Nb): 0.01 to 0.1 wt%, vanadium (V): 0.01 ~ 1.0 wt%, boron (B): 0.001 to 0.005 wt%, tin (Sn): 0.0001 to 0.04 wt%, and the remainder iron (Fe).

[0119] Table 2 shows the calculated value of the bainite transformation initiation temperature (Bs), the calculated value and measured value of the martensite transformation initiation temperature (Ms), and the measured value of the martensite transformation end temperature (Mf) for steel grades having the compositions in Table 1.

[0120] Steel grade Bs(℃)Ms(℃)Mf(℃)CalculationCalculationActual measurement 1Actual measurement 1Actual measurement 2A544.5365.1366173.7166.4B559.1383.9374.3195.5160C589385.4372.6183.6156.1

[0121] Referring to Table 2, the calculated value of the bainite transformation initiation temperature (Bs; unit: ℃) was calculated using the formula proposed by Kirkardy as follows.

[0122] Bs = 656-57.7[C]-35[Mn]-75[Si]-34[Cr]-41.2[Mo]-15.3[Ni]

[0123] Here, [C], [Mn], [Si], [Cr], [Mo] and [Ni] are the weight percent values ​​of carbon, manganese, silicon, chromium, molybdenum and nickel.

[0124] The calculated value of the martensite transformation onset temperature (Ms; unit ℃) is the lower temperature among the transformation onset temperatures determined by the lever rule during cooling using the following formula proposed by Andrew and the length change test. During the experiment, it should be confirmed that the final microstructure is free of ferrite and bainite, and the cooling rate should preferably be 30 ℃ / s or higher.

[0125] Ms = 539-423[C]-30.4[Mn]-12.1[Cr]-7.5[Mo]-17.7[Ni]

[0126] Here, [C], [Mn], [Cr], [Mo] and [Ni] are the weight percent values ​​of carbon, manganese, chromium, molybdenum and nickel.

[0127] The martensite transformation end temperature (Mf) is determined as the lower temperature among the end temperatures determined through the lever rule and the first-order differential curve analysis of the length change during the length change test.

[0128] Figures 3 and 4 are schematic diagrams illustrating a method for experimentally measuring the martensite transformation initiation temperature (Ms) and martensite transformation end temperature (Mf) of a specimen in an experimental example of the present invention.

[0129] First, referring to Fig. 3, a method for measuring the martensite transformation onset temperature (Ms) and martensite transformation end temperature (Mf) by the lever rule is disclosed. Since steel linearly expands with temperature in a section where there is no phase transformation, the decrease in length change with temperature decreases linearly as shown in Fig. 3. By utilizing the straight line extended by linear regression of this section, the fraction (X, unit: %) of the region where transformation is in progress can be calculated from the following equation 2.

[0130] Equation 2: X=(BC) / (AC)

[0131] (Note that A, B, and C correspond to the coordinates of the points shown in Figure 3)

[0132] The temperature at which the calculation result of Equation 2 becomes "0" is determined as the martensite transformation initiation temperature (Ms), and the temperature at which it becomes "100" is determined as the martensite transformation end temperature (Mf). In other words, the temperature at which the temperature begins to deviate from the straight line (G in Fig. 3) obtained by extrapolating data from a linearly decreasing section where no phase transformation exists can be determined as the martensite transformation initiation temperature (Ms), and the temperature at which the temperature begins to change back to a linear section can be determined as the martensite transformation end temperature (Mf).

[0133] Next, referring to Fig. 4, a method for measuring the martensite transformation end temperature (Mf) by analyzing the first-order differential curve is disclosed. Since the length change curve in the section where there is no phase transformation decreases linearly with temperature, a constant value is obtained when this is first differentiated with respect to temperature. Using this, from the temperature (Ti) where the first-order differential curve of the length change shows the lowest value and becomes a positive number (≥ 0.0 ㎛ / ℃), the Ti at which the standard deviation of the first-order differential value of the length change with respect to Ti-30 ℃ is 90% or more is determined as the martensite transformation end temperature (Mf).

[0134] 2. Process conditions and properties / structure of the specimen

[0135] Tables 3 and 4 show the process conditions applied to the experimental examples of the present invention. The steel grades and the characteristic temperatures thereof are described in Tables 1 and 2. The items shown in Tables 3 and 4 correspond to the contents described with reference to FIGS. 1 and 2, and specifically, SS represents the annealing temperature (T0), T1 represents the first cooling end temperature (T1) which is the cooling end temperature in the first heat treatment process (I; S40), T2 represents the second cooling end temperature (T2) which is the cooling end temperature in the second heat treatment process (II; S50), and T3 represents the third cooling end temperature (T3) which is the cooling end temperature in the third heat treatment process (III; S60). Cooling rate represents the cooling speed (unit: ℃ / s). In most of the experimental examples, the second heat treatment process (II; S50) can be understood as a cooling process overall because the second cooling end temperature (T2) is lower than the first cooling end temperature (T1). However, in experimental examples 12, 13, 31, and 32, the second cooling end temperature (T2) is higher than the first cooling end temperature (T1), so it can be classified as a temperature increasing process. In the ‘Tempering Formula A Calculated Value’ item of Table 4, × indicates a case where the tempering process is not performed. In Table 4, Formula A is the relationship between the tempering temperature (T4) and the holding time (t4) of the tempering step (S70), as follows.

[0136] Equation A: [(T4+ 300) × (10 + log(t4))]

[0137] (The unit of temperature (T4) is ℃, and the unit of time (t4) is hour)

[0138] In Experimental Examples 1 to 38, the remaining process conditions were commonly applied to the same values ​​within the range satisfying the process conditions described in the manufacturing method according to the technical idea of ​​the present invention described above.

[0139] ClassificationSteel gradeAnnealingFirst heat treatment(Annealing~T1)Second heat treatment(T1~T2)SS(℃)T1(℃)Cooling speed(℃ / s)T1(℃)T2(℃)Cooling speed(℃ / s)Experimental example 1Comparative exampleA8204502.54504300.2Experimental example 2Comparative exampleA8204502.54504300.2Experimental example 3Comparative exampleA8404502.54504300.2Experimental example 4Comparative exampleA8404502.54504300.2Experimental example 5Invention exampleA8204606.74604500.1Experimental example 6Invention exampleA8204606.74604500.1Experimental example 7Invention exampleA8404606.74604500.1 Experimental Example 8 Invention Example A8404606.74604500.1 Experimental Example 9 Comparative Example A8404606.74604500.1 Experimental Example 10 Comparative Example A84046058.34604500.1 Experimental Example 11 Invention Example A84046058.34604500.1 Experimental Example 12 Comparative Example A8402506.72504502.2 (Temperature Increase) Experimental Example 13 Comparative Example A8402506.72504502.2 (Temperature Increase) Experimental Example 14 Comparative Example A8404606.74502502.2 Experimental Example 15 Comparative Example A8404606.74502502.2 Experimental Example 16 Comparative Example A8404506.74504 500Experimental Example 17Comparative Example A8404506.74504500Experimental Example 18Comparative Example A8404506.74504500Experimental Example 19Comparative Example A8404506.74504500Experimental Example 20Comparative Example B8204502.54504300.2Experimental Example 21Comparative Example B8204502.54504300.2Experimental Example 22Comparative Example B8404502.54504300.2Experimental Example 23Comparative Example B8404502.54504300.2Experimental Example 24Invention Example B8204606.74604500.1Experimental Example 25Invention Example B8204606.74604500.1Experimental Example 26Ball Honor B8404606.74604500.1 Experimental Example 27 Invention Example B8404606.74604500.1 Experimental Example 28 Comparative Example B8404606.74604500.1 Experimental Example 29 Comparative Example B84046058.34604500.1 Experimental Example 30 Invention Example B84046058.34604500.1 Experimental Example 31 Comparative Example B8402506.72504502.2 (Temperature Increase) Experimental Example 32 Comparative Example B8402506.72504502.2 (Temperature Increase) Experimental Example 33 Comparative Example B8404606.74502502.2 Experimental Example 34 Comparative Example B8404606.74502502.2Experimental Example 35 Invention Example C8404606.74604500.1Experimental Example 36 Invention Example C8404606.74604500.1Experimental Example 37 Comparative Example C84046058.34604500.1Experimental Example 38 Invention Example C84046058.34604500.1.

[0140] Classification of steel grade 3 heat treatment (T2~T3) tempering method A Calculated value T3 (℃) T2 ~ T3 (s) T2 ~ Room temperature (s) Process time (s) Cooling speed (℃ / s) Process time (s) Cooling speed (℃ / s) Experimental example 1 Comparative example A 150 2411.22731.5 × Experimental example 2 Comparative example A 150 2411.22731.5 485 0.17 Experimental example 3 Comparative example A 150 2411.22731.5 × Experimental example 4 Comparative example A 150 2411.22731.5 485 0.17 Experimental example 5 Invention example A 140 664.7755.7 × Experimental example 6 Invention example A 140 664.7755.7 4985.6 Experimental example 7 Invention example A 140 664.7755.7 × Experimental example 8 Invention example A 140 664.7755.74985.6 Experimental Example 9 Comparative Example A140664.7755.75928 Experimental Example 10 Comparative Example A150742.9853.1× Experimental Example 11 Invention Example A150742.9853.14422.6 Experimental Example 12 Comparative Example A150664.5755.7× Experimental Example 13 Comparative Example A150664.5755.74985.6 Experimental Example 14 Comparative Example A140661.7753× Experimental Example 15 Comparative Example A140661.77534985.6 Experimental Example 16 Comparative Example A200663.8755.7× Experimental Example 17 Comparative Example A200663.8755.74850.17 Real Experimental Example 18 Comparative Example A 250663755.7 × Experimental Example 19 Comparative Example A 250663755.74850.17 Experimental Example 20 Comparative Example B 1502411.22731.5 × Experimental Example 21 Comparative Example B 1502411.22731.54850.17 Experimental Example 22 Comparative Example B 1502411.22731.5 × Experimental Example 23 Comparative Example B 1502411.22731.54850.17 Experimental Example 24 Invention Example B 140664.7755.7 × Experimental Example 25 Invention Example B 140664.7755.74985.6 Experimental Example 26 Invention Example B 140664.7755.7 × Experimental Example 27 Invention Example B1 40664.7755.74985.6 Experimental Example 28 Comparative Example B 140664.7755.75928 Experimental Example 29 Comparative Example B 150742.9853.1× Experimental Example 30 Invention Example B 150742.9853.14422.6 Experimental Example 31 Comparative Example B 150664.5755.7× Experimental Example 32 Comparative Example B 150664.5755.74985.6 Experimental Example 33 Comparative Example B 140661.7753× Experimental Example 34 Comparative Example B 140661.77534985.6 Experimental Example 35 Invention Example C 140664.7755.7× Experimental Example 36 Invention Example C 140664.7755.74850.17 Experimental Example 37 Comparative Example C150742.9853.1 × Experimental Example 38 Invention Example C150742.9853.14850.17.

[0141] Table 5 shows the mechanical properties and microstructures realized as a result of applying the composition and process conditions according to the experimental example of the present invention. In Table 5, YP represents yield strength, TS represents tensile strength, EL represents elongation, and R / t represents bending workability. Yield strength (YP), tensile strength (TS), and elongation (EL) were evaluated through a tensile test, and the yield strength (YP), tensile strength (TS), and elongation (EL) were measured by evaluating test specimens collected in the 90° direction to the rolling direction of the rolled plate according to the JIS 5 standard. A 90° bend was performed on the specimens for the bending test. The bending jig selects various curvatures (R) compared to the steel plate thickness (t) and tests them, and the target bending characteristics were set to 5.0 or less based on R / t. The bending characteristic R / t value was calculated by dividing the minimum bending radius (R) at which no cracks occur by the thickness of the specimen (t, mm) after processing the steel plate into a specimen with a width of 100 mm × length of 30 mm and performing a 90˚ bending test at a test speed of 100 mm / min and checking the cracks in the bent part using a microscope. The volume fraction of the microstructure can be measured using an optical microscope, but in this experimental example, it was measured using an X-ray diffraction analyzer. The aspect ratio of the carbide in Table 5 was expressed as the average value of the ratio (b / a) after first measuring the major axis length (b) and minor axis length (a) of the carbide and calculating the ratio (b / a) of the major axis length (b) and minor axis length (a) of the carbide. In the microstructure in Table 5, the '-' items correspond to unmeasured data.

[0142] Mechanical properties Microstructure YP (MPa) TS (MPa) EL (%) R / t Martensite (vol.%) Carbide Aspect ratioExperimental example 1Comparative example 840132283.860.3-Experimental example 2Comparative example 91212747.63.857.2-Experimental example 3Comparative example 88213947.43.662.5-Experimental example 4Comparative example 954137183.260.4-Experimental example 5Invention example 99715797.23.470.1-Experimental example 6Invention example 105214246.63.268.8-Experimental example 7Invention example 107216518.23.480.5-Experimental example 8Invention example 1262159073.280.52.5Experimental example 9Comparative example 137314105.55.480.46.2Experimental example 10Comparison Example 113817838.85.8--Experimental Example 11Invention Example 142717018.42.8--Experimental Example 12Comparative Example 113512316.32.4--Experimental Example 13Comparative Example 1133123462.8--Experimental Example 14Comparative Example 111512818.82.8--Experimental Example 15Comparative Example 118712375.82.8--Experimental Example 16Comparative Example 95813427.32.847-Experimental Example 17Comparative Example 106513027.62.655-Experimental Example 18Comparative Example 81112998.3349.2-Experimental Example 19Comparative Example 923128782.458.9-Experimental Example 20B Example 86213605.23.252.51.2 Experimental Example 21 Comparative Example 95513296.62.849.53.6 Experimental Example 22 Comparative Example 89913234.8362.52.1 Experimental Example 23 Comparative Example 101913755.62.862.53.4 Experimental Example 24 Invention Example 90614486.4388.4-Experimental Example 25 Invention Example 106514136.92.888.43.3 Experimental Example 26 Invention Example 1114167982.895-Experimental Example 27 Invention Example 128816408.42.894.92.4 Experimental Example 28 Comparative Example 143215226.7690.55.5 Experimental Example 29 Comparative Example 111616988.36.4--Experimental Example 30 Invention Example 130816757.22.8--Experimental Example 31 Comparative Example 113412018.92.2--Experimental Example 32 Comparative Example 1203111962.6--Experimental Example 33 Comparative Example 98911658.82.4--Experimental Example 34 Comparative Example 103211345.82.6--Experimental Example 35 Invention Example 95114905.8385--Experimental Example 36 Invention Example 102014685.43832.2 Experimental Example 37 Comparative Example 132618764.75.398--Experimental Example 38 Invention Example 152318044.63.697.63.

[0143] Referring to Tables 3 to 5, Experimental Examples 5, 6, 7, 8, 11, 24, 25, 26, 27, 30, 35, 36, and 38 are invention examples that satisfy all of the characteristics of alloy composition, process conditions, mechanical properties, and microstructure according to the technical idea of ​​the present invention described above.

[0144] Specifically, Experimental Examples 5, 7, 24, 26, and 35 correspond to a method for manufacturing a resistance-welding plated steel sheet according to an embodiment of the present invention described with reference to FIG. 1, and satisfy yield strength (YP): 850 to 1300 MPa, tensile strength (TS): 1400 MPa or more, elongation (El): 3.0% or more, and bending workability (R / t): 5.0 or less, and the measured data confirm that the area fraction of martensite in the final microstructure is 65% or more, and the average aspect ratio of precipitated carbide is 5.0 or less.

[0145] Experimental Examples 6, 8, 11, 25, 27, 30, 36, and 38 correspond to a method for manufacturing a high-yield plated steel sheet according to another embodiment of the present invention described with reference to FIG. 2, and satisfy the following: yield strength (YP): 1000 to 1600 MPa, tensile strength (TS): 1400 MPa or more, elongation (El): 3.0% or more, bending workability (R / t): 5.0 or less, and the measured data confirm that the area fraction of martensite in the final microstructure is 65% or more, and the average aspect ratio of precipitated carbide is 5.0 or less.

[0146] In particular, referring to Experimental Example 11, Experimental Example 30, and Experimental Example 38, even if the total process time for performing the step of performing the third heat treatment process (III; S60) and the step of cooling to room temperature is not necessarily secured to be at least 10 seconds, that is, in the second heat treatment process (II; S50), the second cooling end temperature (T2), which is the cooling end temperature, is lowered to room temperature (T r ) is not necessarily secured for at least 10 seconds, when the tempering step (S70) is performed under the condition that Equation 1 (3800 ≤ [(T4 + 300) × (10 + log(t4))] ≤ 5650) is satisfied, the yield strength (YP): 1000 to 1600 MPa, the tensile strength (TS): 1400 MPa or more, the elongation (El): 3.0% or more, and the bending workability (R / t): 5.0 or less are satisfied, and the measured data confirm that the area fraction of martensite in the final microstructure is 65% or more, and the average aspect ratio of the precipitated carbide is 5.0 or less.

[0147] Meanwhile, Experimental Examples 1, 2, 3, and 4 and Experimental Examples 20, 21, 22, and 23 are comparative examples of the present invention, and it can be confirmed that if the cooling time from the second cooling end temperature (T2), which is the cooling end temperature in the second heat treatment process (II; S50), to the third cooling end temperature (T3), which is the cooling end temperature in the third heat treatment process (III; S60), exceeds 180 seconds, the condition that the area fraction of martensite in the final microstructure is 65% or more is not satisfied and falls below, and the condition that the tensile strength (TS): is 1400 MPa or more is not satisfied and falls below.

[0148] Experimental Example 9 and Experimental Example 28 are comparative examples of the present invention, and when the process conditions of the tempering step (S70) do not satisfy Equation 1 (3800 ≤ [(T4+ 300) × (10 + log(t4))] ≤ 5650), it can be confirmed that the condition that the average aspect ratio of the precipitated carbide is 5.0 or less is not satisfied but is exceeded, and the condition that the bending workability (R / t): is 5.0 or less is not satisfied but is exceeded.

[0149] Experimental examples 10, 29, and 37 are comparative examples of the present invention, and are obtained by cooling the temperature of the second cooling end temperature (T2) to room temperature (T) in the second heat treatment process (II; S50). r ) is not satisfied and falls below the condition that the cooling time is 10 seconds or more, and at the same time, if the tempering step (S70) that satisfies Equation 1 (3800 ≤ [(T4+ 300) × (10 + log(t4))] ≤ 5650) is not performed, it can be confirmed that the condition that the bending workability (R / t): is 5.0 or less is not satisfied and is exceeded.

[0150] Experimental Examples 12, 13, 31, and 32 are comparative examples of the present invention, and it can be confirmed that when the first cooling end temperature (T1), which is the cooling end temperature in the first heat treatment process (I; S40), does not satisfy the temperature range between the martensite transformation start temperature (Ms) and the bainite transformation start temperature (Bs) and is lower than the martensite transformation start temperature (Ms), the condition of tensile strength (TS): 1400 MPa or more is not satisfied and is lower than it.

[0151] Experimental Examples 14, 15, 33, and 34 are comparative examples of the present invention, and it can be confirmed that when the second cooling end temperature (T2), which is the cooling end temperature in the second heat treatment process (II; S50), does not satisfy the temperature range between the martensite transformation initiation temperature (Ms) and the bainite transformation initiation temperature (Bs) and is lower than the martensite transformation initiation temperature (Ms), the condition of tensile strength (TS): 1400 MPa or more is not satisfied and is lower than it.

[0152] Experimental Examples 16, 17, 18, and 19 are comparative examples of the present invention, and it can be confirmed that when the third cooling end temperature (T3), which is the cooling end temperature in the third heat treatment process (III; S60), does not satisfy the range of the martensite transformation end temperature (Mf) and exceeds it, the condition that the area fraction of martensite in the final microstructure is 65% or more is not satisfied and falls below it, and the condition that the tensile strength (TS): is 1400 MPa or more is not satisfied and falls below it.

[0153] FIG. 5 and FIG. 6 are photographs showing the results of carbide analysis after BAF (Batch Annealing Furnace) heat treatment on a specimen (high yield material) according to an invention example (Experimental Example 27) of the present invention. FIG. 5 shows the microstructure observed in a BF (Bright Field) image, and FIG. 6 shows the results of a transmission electron microscope (TEM) diffraction pattern analysis.

[0154] Meanwhile, FIGS. 7 and 8 are graphs showing the results of carbide analysis after BAF (Batch Annealing Furnace) tempering heat treatment for a specimen (high yield material) according to an invention example (Experimental Example 27) of the present invention. FIG. 7 is a graph showing the size (radius) distribution of carbide, and FIG. 8 is a graph showing the aspect ratio distribution of carbide.

[0155] Referring to FIGS. 5 to 8, it can be confirmed that the average aspect ratio of carbides precipitated in the final microstructure of the specimens according to the embodiments of the present invention is 5.0 or less, and the carbides have an average size of 100 nm or less. The carbides may include transition carbides, and for example, may include ε-carbides in which the atomic ratio of carbon to a substitutional element selected from the group consisting of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo) is 2.5:1, or η-carbides in which the atomic ratio is 2:1. The average size is an average size including the major axis and minor axis of the carbides in an elliptical or needle-shaped shape, and specifically, an average size including the minor axis size (a) and the major axis size (b). In addition, the average aspect ratio refers to an average of the ratio of the major axis to the minor axis length (b / a).

[0156] Figures 9 to 12 are photographs of the microstructure of a specimen according to an experimental example of the present invention. The area marked B in the photographs represents the bainite area.

[0157] Fig. 9 is a photograph of the microstructure before and after BAF (Batch Annealing Furnace) tempering heat treatment for a specimen of Experimental Example 6 as an invention example among experimental examples of the present invention, and Fig. 10 is a photograph of the microstructure before and after BAF (Batch Annealing Furnace) tempering heat treatment for a specimen of Experimental Example 8 as an invention example among experimental examples of the present invention. Fig. 11 is a photograph of the microstructure after tempering heat treatment for specimens of Experimental Examples 21 and 23 as comparative examples among experimental examples of the present invention, and Fig. 12 is a photograph of the microstructure after tempering heat treatment for specimens of Experimental Examples 25 and 27 as invention examples among experimental examples of the present invention.

[0158] Referring to FIGS. 9, 10, and 12, it can be confirmed that the final microstructure of the steel according to the invention example of the present invention has an area fraction of martensite of 65% or more, an area fraction of bainite of 25% or more, an area fraction of bainite and martensite of 90% or more, and an area fraction of ferrite of 10% or less.

[0159] In contrast, referring to FIG. 11, it can be confirmed that the final microstructure of the steel according to the comparative example of the present invention does not satisfy the condition that the area fraction of martensite is 65% or more and falls below it.

[0160] While the above description focuses on specific embodiments of the present invention, those skilled in the art will appreciate that various modifications and variations can be made. As long as such modifications and variations do not depart from the scope of the present invention, they are considered to be within the scope of the present invention. Therefore, the scope of the present invention should be determined by the claims set forth below.

Claims

1. Carbon (C): 0.1 to 0.5 wt%, Silicon (Si): 0.01 to 2.0 wt%, Manganese (Mn): 0.1 to 5.0 wt%, Phosphorus (P): 0 to 0.02 wt% or less, Sulfur (S): 0 to 0.01 wt% or less, Aluminum (Al): 0.01 to 2.0 wt%, Chromium (Cr): 0 to 3.0 wt% or less, Molybdenum (Mo): 0 to 1.0 wt% or less, Nickel (Ni): 0 to 0.4 wt% or less, Copper (Cu): 0 to 0.4 wt% or less, Titanium (Ti): 0.01 to 0.2 wt%, Niobium (Nb): 0.01 to 0.1 wt%, Vanadium (V): 0.01 to 1.0 wt%, Boron (B): 0.001 to A base steel sheet comprising 0.005 wt% tin (Sn): 0.0001 to 0.04 wt% and the remainder iron (Fe) and other unavoidable impurities; and a plated steel sheet having a plating layer on the base steel sheet. The final microstructure of the above steel sheet has an area fraction of martensite of 65% or more, and an average aspect ratio of precipitated carbides of 5.0 or less. Characterized by tensile strength (TS): 1400 MPa or more, elongation (El): 3.0% or more, and bending workability (R / t): 5.0 or less. Galvanized steel sheet.

2. In paragraph 1, The final microstructure of the above steel plate is characterized in that the area fraction of bainite and martensite is 90% or more and the area fraction of ferrite is 10% or less. Galvanized steel sheet.

3. In paragraph 2, The final microstructure of the above steel plate is characterized in that the area fraction of bainite is 25% or more. Galvanized steel sheet.

4. In paragraph 1, The above carbide is characterized by an average size of 100 nm or less. Galvanized steel sheet.

5. In paragraph 1, The above plating layer is not an electrogalvanized layer, but a hot-dip galvanized layer or an alloyed hot-dip galvanized layer. Galvanized steel sheet.

6. Carbon (C): 0.1 to 0.5 wt%, Silicon (Si): 0.01 to 2.0 wt%, Manganese (Mn): 0.1 to 5.0 wt%, Phosphorus (P): 0 to 0.02 wt% or less, Sulfur (S): 0 to 0.01 wt% or less, Aluminum (Al): 0.01 to 2.0 wt%, Chromium (Cr): 0 to 3.0 wt% or less, Molybdenum (Mo): 0 to 1.0 wt% or less, Nickel (Ni): 0 to 0.4 wt% or less, Copper (Cu): 0 to 0.4 wt% or less, Titanium (Ti): 0.01 to 0.2 wt%, Niobium (Nb): 0.01 to 0.1 wt%, Vanadium (V): 0.01 to 1.0 wt%, Boron (B): 0.001 to A step of hot rolling a steel material composed of 0.005 wt% tin (Sn): 0.0001 to 0.04 wt% and the remainder iron (Fe) and other unavoidable impurities; A step of cold rolling the above hot-rolled steel; and A step of sequentially performing an annealing process, a first heat treatment process (I), a second heat treatment process (II), and a third heat treatment process (III) on the above cold-rolled steel; including, The above second heat treatment process (II) includes a hot-dip galvanizing process or an alloy hot-dip galvanizing process, The first heat treatment process (I), the second heat treatment process (II) and the third heat treatment process (III) each include a cooling step, wherein the cooling rate in the first heat treatment process (I) is greater than the cooling rate in the second heat treatment process (II), the cooling rate in the third heat treatment process (III) is greater than the cooling rate in the second heat treatment process (II), the first cooling end temperature (T1), which is the cooling end temperature in the first heat treatment process (I), is a temperature between the bainite transformation start temperature (Bs) and the martensite transformation start temperature (Ms), the second cooling end temperature (T2), which is the cooling end temperature in the second heat treatment process (II), is a temperature between the bainite transformation start temperature (Bs) and the martensite transformation start temperature (Ms), and the third cooling end temperature (T3), which is the cooling end temperature in the third heat treatment process (III), is Characterized by a temperature below the martensite transformation end temperature (Mf). Method for manufacturing galvanized steel sheet.

7. In paragraph 6, In the above first heat treatment process (I), the cooling rate is characterized by being 2 to 60°C / s. Method for manufacturing galvanized steel sheet.

8. In paragraph 6, The process time for performing the third heat treatment process (III) is characterized in that it is 180 seconds or less. Method for manufacturing galvanized steel sheet.

9. In paragraph 8, After performing the third heat treatment process (III), a step of cooling the steel to room temperature is further included. The total process time for performing the step of performing the third heat treatment process (III) and the step of cooling to room temperature is characterized in that it is at least 10 seconds. Method for manufacturing galvanized steel sheet.

10. In paragraph 9, The final plated steel sheet, which is implemented after performing the cooling step to room temperature, is characterized by having a yield strength (YP): 850 to 1300 MPa, a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, and a bending workability (R / t): 5.0 or less. Method for manufacturing galvanized steel sheet.

11. In paragraph 8, After performing the third heat treatment process (III), a step of cooling the steel to room temperature; and a tempering step of heating the steel from room temperature and maintaining it at a predetermined temperature (T4) for a predetermined time (t4) are further included. Characterized in that it satisfies the following equation 1, Method for manufacturing galvanized steel sheet. Equation 1: 3800 ≤ [(T4+ 300) × (10 + log(t4))] ≤ 5650 (Here, the unit of the temperature (T4) is ℃, and the unit of the time (t4) is hour) 12. In paragraph 11, The final plated steel sheet implemented after performing the above tempering step is characterized by a yield strength (YP): 1000 to 1600 MPa, a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, and a bending workability (R / t): 5.0 or less. Method for manufacturing galvanized steel sheet.

13. In paragraph 6, The above hot rolling step includes a hot rolling step under the conditions of reheating temperature: 1150 to 1300°C, finishing rolling temperature: 800 to 1000°C, and coiling temperature: 300 to 700°C. The above annealing process is characterized in that the annealing temperature (T0) is Ac3 or higher. Method for manufacturing galvanized steel sheet.

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