Galvanized steel sheet and manufacturing method therefor

Abstract

The present application relates to a galvanized steel sheet and a manufacturing method therefor. According to the present application, it is possible to provide a galvanized steel sheet having high yield strength and excellent resistance to hydrogen embrittlement, and a manufacturing method therefor.

Description

Galvanized steel sheet and method of manufacturing the same

[0001] The present application relates to a galvanized steel sheet and a method for manufacturing the same.

[0002] In the automotive industry, the demand for crash safety in vehicle bodies has been continuously increasing. Recently, as the adoption of electric vehicles has expanded, the number of vehicle parts has decreased; however, the increase in vehicle weight due to the introduction of batteries has further amplified the requirements for crash safety. Accordingly, the ultra-high strength of crash components that contribute to crash safety, such as front bumper beams, side sills, and door impact beams, is also continuously being pursued. In particular, the application of martensitic steel, which possesses the highest strength among cold-rolled steel sheets, is expanding as the use of roll forming techniques increases.

[0003] Basically, cold-rolled martensitic steel for automobiles must secure high yield, high toughness, and high bendability through immersion water cooling and tempering processes.

[0004] Patent Document 1 (Korean Published Patent No. 10-2024-0099374) describes a method for manufacturing general martensitic steel, wherein an immersion water cooling and tempering process is performed on the martensitic steel.

[0005] However, martensitic steel that has undergone immersion water cooling and tempering processes is suitable for satisfying the basic material requirements of martensitic steel, but there is a problem in that the surface oxidizes because it comes into contact with water at high temperatures, making it difficult to perform galvanizing on the surface, such as galvanized iron (GI) or galvanized alloy (GA). To solve this, it is common practice to perform surface pickling after immersion water cooling and tempering on such steel plates, and then perform electrogalvanizing (EG).

[0006] However, in the case of electro-galvanizing, there are disadvantages such as severe mold damage due to a plating peeling phenomenon called galling during the processing of materials into parts, low yield for parts manufacturers due to short maintenance periods, and increased production costs resulting from the need to pass through an additional plating line after the heat treatment line.

[0007] Therefore, in order to compensate for these problems and disadvantages, a galvanized steel sheet having high yield capacity and excellent hydrogen embrittlement even when galvanizing is performed, and a method for manufacturing the same, are required.

[0008] The objective of the present application is to provide a galvanized steel sheet having high yield strength and excellent hydrogen embrittlement, and a method for manufacturing the same.

[0009] To solve the above problem, the galvanized steel sheet of the present application is a galvanized steel sheet comprising a steel sheet and a galvanized layer formed on the steel sheet, wherein the steel sheet comprises, in weight%, C: 0.23% or more and 0.30% or less, Mn: greater than 1.5% and less than 3.0%, Si: 0.03% or more and less than 0.3%, Cr: greater than 0% and less than 0.8%, and Mo: greater than 0% and less than 0.3%, and the remainder is Fe and other unavoidable impurities, and the region corresponding to 25% of the thickness from the surface comprises a tempered martensite structure with an area fraction of 65% or more and less than 100%, and is composed of one or more selected from the remaining martensite structure, ferrite structure, bainite structure and retained austenite structure, and has a yield strength (YP) of 1180 MPa or more.

[0010] The above steel plate may further include, in weight%, one or more selected from Sol. Al: greater than 0% and less than or equal to 0.1%, P: greater than 0% and less than or equal to 0.05%, S: greater than 0% and less than or equal to 0.01%, N: greater than 0% and less than or equal to 0.01%, B: greater than 0% and less than or equal to 0.004%, and the sum of Nb, V and Ti: greater than 0% and less than or equal to 0.1%.

[0011] In addition, the above galvanized steel sheet may have a tensile strength (TS) of 1470 MPa or more and 1800 MPa or less.

[0012] In addition, the method for manufacturing a galvanized steel sheet according to the present application comprises the steps of: reheating and hot-rolling a slab comprising, in weight percent, C: 0.23% or more and 0.30% or less, Mn: greater than 1.5% and less than or equal to 3.0%, Si: 0.03% or more and less than or equal to 0.3%, Cr: greater than 0% and less than or equal to 0.8%, and Mo: greater than 0% and less than or equal to 0.3%, and the remainder being Fe and other unavoidable impurities; coiling the hot-rolled steel sheet; cold-rolling the coiled hot-rolled steel sheet while unwinding it; annealing the cold-rolled steel sheet; cooling the annealed steel sheet; over-aging the cooled steel sheet; galvanizing the over-aged steel sheet; and temper-rolling the galvanized steel sheet. A method for manufacturing a galvanized steel sheet comprising the step of tempering a galvanized steel sheet that has been rolled for tempering, wherein the tempered galvanized steel sheet comprises a region corresponding to 25% of the thickness from the surface that contains a tempered martensite structure with an area fraction of 65% or more and less than 100%, and is composed of one or more selected from the remaining martensite structure, ferrite structure, bainite structure and retained austenite structure, and has a yield strength (YP) of 1180 MPa or more.

[0013] In addition, the method for manufacturing the above-mentioned galvanized steel sheet can satisfy the following general formula 1.

[0014] [General Formula 1]

[0015] 1470 ≤ T A +137×(10C+Mn+Cr+Mo)

[0016] In the above general formula 1, T A is the annealing temperature (°C) at the annealing stage, C is the carbon content (weight%) in the slab, Mn is the manganese content (weight%) in the slab, Cr is the chromium content (weight%) in the slab, and Mo is the molybdenum content (weight%) in the slab.

[0017] In addition, the annealing step can be performed at an annealing temperature of 800°C or higher and 900°C or lower.

[0018] In addition, the method for manufacturing the above-mentioned galvanized steel sheet can satisfy the following general formula 2.

[0019] [General Formula 2]

[0020] 8500 ≤ (T T +273)(20+Log(t T ))

[0021] In the above general formula 2, T T is the tempering temperature (°C) at the tempering stage, and t T is the tempering time (minutes) during the tempering stage.

[0022] Additionally, the cooling step may include a first cooling step of cooling to a cooling temperature of 650°C or higher and 750°C or lower; and a second cooling step of cooling to a cooling temperature of 440°C or higher and 500°C or lower.

[0023] In addition, the overaging treatment step described above may be performed by reheating to an overaging treatment temperature of 440°C or higher and 500°C or lower, and then maintaining it for 10 seconds or more and 90 seconds or less.

[0024] In addition, the zinc plating step may include a hot-dip zinc plating step of immersing an over-aged steel plate in a hot-dip zinc plating pot containing a zinc-based plating composition to form a hot-dip zinc plating layer on the over-aged steel plate.

[0025] In addition, the zinc plating step may further include a molten zinc alloying step for alloying the molten zinc plating layer formed on the over-aged steel plate.

[0026] According to the present application, a galvanized steel sheet having high yield strength and excellent hydrogen embrittlement and a method for manufacturing the same can be provided.

[0027] In the description of numerical ranges in this specification, the notation “X~Y” indicates X or greater and Y or less, unless otherwise specifically stated. Additionally, “greater than or equal to” may be replaced with “greater than,” and “less than or equal to” may be replaced with “less than.”

[0028] In addition, regarding the numerical ranges described stepwise in this specification, any upper or lower limit value described in any numerical range may be substituted with an upper or lower limit value of another numerical range described stepwise, or may be substituted with a value shown in the examples.

[0029] The present application relates to a galvanized steel sheet. The galvanized steel sheet comprises a steel sheet and a galvanized layer formed on the steel sheet, wherein the steel sheet comprises, in weight percent, C: 0.23% or more and 0.30% or less, Mn: greater than 1.5% and less than 3.0%, Si: 0.03% or more and less than 0.3%, Cr: greater than 0% and less than 0.8%, and Mo: greater than 0% and less than 0.3%, and the remainder is Fe and other unavoidable impurities, and the region corresponding to 25% of the thickness from the surface comprises a tempered martensite structure with an area fraction of 65% or more and less than 100%, and is composed of one or more selected from the remainder martensite structure, ferrite structure, bainite structure and retained austenite structure, and has a yield strength (YP) of 1180 MPa or more. The galvanized steel sheet of the present application is composed of the aforementioned structure in which the steel sheet satisfies the aforementioned component ratio and the aforementioned region satisfies the aforementioned area fraction range, thereby enabling high yield capacity and excellent hydrogen embrittlement even when a galvanized layer is formed.

[0030] Specifically, the area fraction of the tempered martensite structure formed in the region corresponding to 25% of the thickness from the surface may be 69% or more and 90% or less. Additionally, the yield strength of the galvanized steel sheet may be 1185 MPa or more. At this time, the upper limit of the yield strength of the galvanized steel sheet may be 1500 MPa or less or 1400 MPa or less. If the yield strength of the galvanized steel sheet exceeds the aforementioned range, it may be difficult to form it into a part. The yield strength was measured through a tensile test described later.

[0031] In one example, the steel plate may contain carbides including cementite, transition carbides, and fine precipitates in an area corresponding to 25% of the thickness from the surface.

[0032] The above cementite (Fe3C) is a carbide with an atomic ratio of iron (Fe) to carbon (C) of 3:1. It is formed when the tempering temperature is high or the tempering time is long, and because it is formed coarsely in a needle-like shape, it can reduce bendability and hydrogen embrittlement. Therefore, the area fraction of the above cementite may be 0% or more and 5% or less. At this time, the above cementite may contain some nitrogen.

[0033] In addition, the above-mentioned transition carbide is a carbide comprising an ε-carbide having an atomic ratio of carbon to a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), or molybdenum (Mo), of 2.5:1, or an η-carbide having an atomic ratio of 2:1, which can increase the Yield Ratio (YR), which is the ratio of yield strength to tensile strength, while simultaneously improving bendability and hydrogen embrittlement. Accordingly, the area fraction of the above-mentioned transition carbide may be greater than 0% and less than or equal to 5%. At this time, the above-mentioned transition carbide may contain some nitrogen.

[0034] In addition, the above-mentioned fine precipitates are carbides in which the atomic ratio of carbon (C) or nitrogen (N) to an alloying element, which is one of titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo), is 1:1, and are formed during hot rolling or during coiling after hot rolling, and contain fine alloying elements (Ti, Nb, V, Mo, etc.), and unlike the above-mentioned cementite and the above-mentioned transition carbides, iron (Fe) is not present as a constituent element. The area fraction of the above-mentioned fine precipitates may be greater than 0% and less than or equal to 5%. By having the aforementioned area fraction, the above-mentioned fine precipitates may have a precipitation strengthening effect and a grain refinement effect. Conversely, if the area fraction of the above-mentioned fine precipitates exceeds the aforementioned range, the size of the precipitates (Ti, Nb, V and / or N) becomes coarse during the continuous casting process, and the precipitation strengthening effect and grain refinement effect may be reduced. At this time, the above-mentioned fine precipitates may contain some nitrogen.

[0035] The area fraction of the above cementite, transition carbides, and microprecipitates can be measured through transmission electron microscopy (TEM) analysis using FIB sampling.

[0036] The alloy composition of the above steel plate is described below.

[0037] C: 0.23 wt% or more, 0.30 wt% or less

[0038] Carbon (C) is an element that is dissolved in the martensite structure to secure strength through improved hardness. If the carbon is included in the steel plate in an amount less than the lower limit of the aforementioned range, it may reduce strength. Additionally, if the carbon is included in the steel plate in an amount exceeding the upper limit of the aforementioned range, weldability and workability may be reduced. Accordingly, the carbon may be included in the steel plate in an amount of 0.23 wt% or more and 0.30 wt% or less, and specifically, in an amount of 0.24 wt% or more and 0.26 wt% or less.

[0039] Mn: Exceeding 1.5 wt% and up to 3.0 wt%

[0040] Manganese (Mn) is an element that contributes to strength improvement through solid solution strengthening and increased hardenability. If the manganese is included in the steel plate in an amount less than the lower limit of the aforementioned range, it may reduce strength. Additionally, if the manganese is included in the steel plate in an amount exceeding the upper limit of the aforementioned range, the resistance to hydrogen embrittlement may be reduced due to the formation of manganese bands, MnS, etc., which may reduce bendability. Therefore, the manganese may be included in the steel plate in an amount greater than 1.5 wt% and less than or equal to 3.0 wt%, and specifically, in an amount greater than or equal to 1.6 wt% and less than or equal to 1.8 wt%.

[0041] Si: 0.03 wt% or more and less than 0.3 wt%

[0042] Silicon (Si) is an element used to ensure hydrogen embrittlement resistance by suppressing the formation of cementite. If the silicon is included in the steel plate in an amount less than the lower limit of the aforementioned range, ductility may be reduced. Additionally, if the silicon is included in the steel plate in an amount exceeding the upper limit of the aforementioned range, strength may be reduced by forming a ferrite structure. Accordingly, the silicon may be included in the steel plate in an amount of 0.03 wt% or more and less than 0.3 wt%, and specifically, in an amount of 0.05 wt% or more and 0.2 wt% or less, or in an amount of 0.08 wt% or more and 0.1 wt% or less.

[0043] Cr: Greater than 0 wt% and less than or equal to 0.8 wt%

[0044] Chromium (Cr) is an element that contributes to strength improvement through solid solution strengthening and increased hardenability. If the chromium is included in the steel plate in an amount exceeding the upper limit of the aforementioned range, it may reduce laser weldability and ductility. Therefore, the chromium may be included in the steel plate in an amount greater than 0 weight% and less than or equal to 0.8 weight%.

[0045] Mo: Greater than 0 wt% and less than or equal to 0.3 wt%

[0046] Molybdenum (Mo) is an element that contributes to strength improvement through solid solution strengthening and increased hardenability, and can refine Ti-based precipitates. If the above molybdenum is included in the steel plate in an amount exceeding the upper limit of the aforementioned range, the cost may increase. Therefore, the above molybdenum may be included in the steel plate in an amount greater than 0 weight% and less than or equal to 0.3 weight%.

[0047] Remaining Fe and other unavoidable impurities

[0048] The aforementioned unavoidable impurities are impurities introduced during the steelmaking and steel sheet manufacturing processes, and since this is widely known in the art, a detailed description is omitted. In one embodiment of the present application, the addition of elements other than the components of the steel sheet described above is not excluded, and various elements may be included within a scope that does not impair the technical concept of the present application. If additional elements are included, they may be included to replace the remainder, iron (Fe).

[0049] In one example, the steel plate may further include, in weight percent, one or more selected from Sol. Al: greater than 0% and less than or equal to 0.1%, P: greater than 0% and less than or equal to 0.05%, S: greater than 0% and less than or equal to 0.01%, N: greater than 0% and less than or equal to 0.01%, B: greater than 0% and less than or equal to 0.004%, and the sum of Nb, V and Ti: greater than 0% and less than or equal to 0.1%.

[0050] Sol. Al: Greater than 0 wt% and less than or equal to 0.1 wt%

[0051] Soluble aluminum (Sol. Al) is aluminum that exists in a solid solution state within the iron of a steel plate and is an element used as a deoxidizer by combining with oxygen within the steel plate. If the soluble aluminum is included in the steel plate in an amount less than the lower limit of the aforementioned range, the deoxidation effect may be reduced. Furthermore, if the soluble aluminum is included in the steel plate in an amount exceeding the upper limit of the aforementioned range, it may form AlN and cause cracking of the slab. Accordingly, the soluble aluminum may be included in the steel plate in an amount greater than 0 wt% and less than or equal to 0.1 wt%, and specifically, in an amount greater than or equal to 0.01 wt% and less than or equal to 0.08 wt% or greater than or equal to 0.02 wt% and less than or equal to 0.05 wt%.

[0052] P: Greater than 0 wt% and less than or equal to 0.05 wt%

[0053] Phosphorus (P) is an element that improves the strength of steel plates but increases brittleness through grain boundary segregation and reduces spot weldability. Therefore, the above phosphorus may be included in the steel plates in an amount greater than 0 weight% and less than or equal to 0.05 weight%.

[0054] S: Greater than 0 wt% and less than or equal to 0.01 wt%

[0055] Sulfur (S) is an element that reduces hydrogen embrittlement resistance by forming non-metallic inclusions of MnS. Therefore, the sulfur may be included in the steel plate in an amount greater than 0 weight% and less than or equal to 0.01 weight%.

[0056] N: Greater than 0 wt% and less than or equal to 0.01 wt%

[0057] Nitrogen (N) is an element that is inevitably added. Therefore, the nitrogen may be included in the steel plate in an amount greater than 0 weight% and less than or equal to 0.01 weight%.

[0058] B: Greater than 0 wt% and less than or equal to 0.004 wt%

[0059] Boron (B) is a strong hardenable element and contributes significantly to improving the hardenability of the steel sheet. If the boron is included in the steel sheet in an amount exceeding the upper limit of the aforementioned range, grain boundary brittleness due to BN formation may increase. Therefore, the boron may be included in the steel sheet in an amount greater than 0 weight% and less than or equal to 0.004 weight%.

[0060] Total of Nb, V, and Ti: Greater than 0 wt% and less than or equal to 0.1 wt%

[0061] The above niobium (Nb), the above vanadium (V), and the above titanium (Ti) are elements that help refine the grain size by forming fine precipitates (NbC, VC, TiC, etc.) during the hot rolling and cold rolling stages. If the total amount of the above niobium, the above vanadium, and the above titanium included in the steel plate exceeds the upper limit of the aforementioned range, a problem may occur in which coarse nitrides (TiN, NbN, VN, etc.) are formed during the steelmaking and continuous casting stages and are not uniformly dissolved. Therefore, the above niobium, the above vanadium, and the above titanium may be included in the steel plate in a total amount of more than 0 weight% and less than or equal to 0.1 weight%.

[0062] In addition, the above-mentioned galvanized steel sheet may have a tensile strength (TS) of 1470 MPa or more and 1800 MPa or less. Specifically, the above-mentioned galvanized steel sheet may have a tensile strength of 1480 MPa or more and 1600 MPa or less. The above-mentioned galvanized steel sheet may have excellent hydrogen embrittlement by satisfying the aforementioned range of tensile strength. Conversely, if the tensile strength of the above-mentioned galvanized steel sheet exceeds the upper limit of the aforementioned range, the hydrogen embrittlement may be reduced. At this time, the above-mentioned tensile strength was measured through a tensile test described later.

[0063] In addition, the above-mentioned galvanized steel sheet may have an elongation (EL) of 5% or more. Specifically, the above-mentioned galvanized steel sheet may have an elongation of 6% or more or 7% or more. In addition, the upper limit of the elongation of the above-mentioned galvanized steel sheet may be less than 10%. By satisfying the aforementioned range of elongation, the steel sheet may have excellent formability as a part. At this time, the elongation was measured through a tensile test described later.

[0064] The above zinc plating layer is a plating layer formed to improve the corrosion resistance of the steel sheet. Specifically, the above zinc plating layer may be a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed by a method described below. In this case, the alloyed hot-dip galvanized layer may be a plating layer in which the steel sheet and zinc are alloyed by thermal diffusion.

[0065] For example, the zinc plating layer may have a thickness of 3 μm or more and 30 μm or less from the surface of the steel plate. If the thickness of the zinc plating layer is less than the lower limit of the aforementioned range, corrosion resistance may be reduced. In addition, if the thickness of the zinc plating layer exceeds the upper limit of the aforementioned range, quality such as spangles on the plating surface may be reduced. Therefore, the zinc plating layer may have a thickness within the aforementioned range.

[0066] This application also relates to a method for manufacturing a galvanized steel sheet. The method for manufacturing the galvanized steel sheet relates to a method for manufacturing the galvanized steel sheet described above. Since specific details regarding the galvanized steel sheet described below can be applied in the same way as those described above regarding the galvanized steel sheet, they will be omitted.

[0067] The method for manufacturing a galvanized steel sheet according to the present application comprises the steps of hot rolling, coiling, cold rolling, annealing, cooling, overaging treatment, galvanizing, temper rolling, and tempering. Furthermore, the galvanized steel sheet tempered through the tempering step comprises a region corresponding to 25% of the thickness from the surface that includes a tempered martensite structure with an area fraction of 65% or more and less than 100%, and is composed of one or more selected from the remaining martensite structure, ferrite structure, bainite structure, and retained austenite structure, and has a yield strength (YP) of 1180 MPa or more. Since the specific description of each structure formed in the aforementioned region and the yield strength is the same as that described in the galvanized steel sheet, it will be omitted. According to the method for manufacturing a galvanized steel sheet according to the present application, a galvanized steel sheet having high yield strength and excellent hydrogen embrittlement can be manufactured.

[0068] The above hot rolling step is a step for manufacturing a slab into a hot-rolled steel sheet, and is performed by reheating and then hot rolling a slab comprising, in weight percent, C: 0.23% or more and 0.30% or less, Mn: 1.5% or more and 3.0% or less, Si: 0.03% or more and 0.3% or less, Cr: 0% or more and 0.8% or less, and Mo: 0% or more and 0.3% or less, with the remainder being Fe and other unavoidable impurities. At this time, since the composition of the slab is the same as the composition of the steel sheet, a specific description of the composition of the slab is the same as that described in the above galvanized steel sheet, so it will be omitted.

[0069] In addition, the above slab may further include, in weight percent, one or more selected from Sol. Al: greater than 0% and less than or equal to 0.1%, P: greater than 0% and less than or equal to 0.05%, S: greater than 0% and less than or equal to 0.01%, N: greater than 0% and less than or equal to 0.01%, B: greater than 0% and less than or equal to 0.004%, and the sum of Nb, V, and Ti: greater than 0% and less than or equal to 0.1%. Since the additional composition of the above slab is the same as the additional composition of the above steel plate, a specific description of the additional composition of the above slab is the same as that described in the above galvanized steel plate, so it will be omitted.

[0070] Reheating the slab is performed before hot rolling the slab to bring the slab to a temperature suitable for rolling. In one example, the reheating temperature of the slab is not specifically limited, but may be, for example, between 1150°C and 1300°C. By satisfying the aforementioned range, the slab can be reheated to a temperature of Ac3 or higher to re-dissolve the components during casting. Conversely, if the reheating temperature of the slab is below the lower limit of the aforementioned range, the added components are not uniformly re-dissolved, and the rolling load may increase during the hot rolling stage; if it exceeds the upper limit of the aforementioned range, the finishing temperature of the rough rolling and finishing rolling may rise, leading to grain coarsening and a deterioration in the material quality of the final product. Therefore, the reheating temperature of the slab may satisfy the aforementioned range.

[0071] As described above, the hot rolling is performed to manufacture the reheated slab into a hot-rolled steel sheet. For example, the finishing temperature during the hot rolling may be 800°C or higher and 1000°C or lower. If the finishing temperature during the hot rolling is below the lower limit of the aforementioned range, the rolling load increases, which may lead to a decrease in productivity. Furthermore, if the finishing temperature during the hot rolling exceeds the upper limit of the aforementioned range, it may cause grain coarsening, which may lead to a decrease in strength. Therefore, the finishing temperature during the hot rolling may satisfy the aforementioned range. At this time, the reduction rate during the hot rolling may be 80% or higher and 95% or lower. Subsequently, the hot-rolled steel sheet may be cooled at a cooling rate of 1°C / s or higher and 100°C / s or lower.

[0072] The above-mentioned winding step is a step of winding the hot-rolled steel sheet obtained by the above-mentioned hot rolling.

[0073] In one example, the coiling step may be performed at a temperature of 400°C or higher and 740°C or lower. By performing the coiling step within the aforementioned temperature range, the shape control and cold rolling performance of the hot-rolled coil after coiling are excellent, and it may be advantageous for realizing the material of the final product. On the other hand, if the coiling step is performed below the lower limit of the aforementioned temperature range, the load during cold rolling may increase. Furthermore, if the coiling step is performed above the upper limit of the aforementioned temperature range, the crystal grains of the final product may coarsen, and the material may decrease.

[0074] In one example, the method for manufacturing the galvanized steel sheet may further include a pickling step. The pickling step is a step for removing the surface scale layer of the coiled hot-rolled steel sheet and can be performed by pickling the coiled hot-rolled steel sheet. Since all conditions known in the art can be applied as the pickling conditions, this is omitted.

[0075] The above cold rolling step is a step for manufacturing the above hot-rolled steel sheet into a cold-rolled steel sheet, and is performed by cold rolling while unwinding the coiled hot-rolled steel sheet.

[0076] In one example, the reduction rate during cold rolling may be 35% or more and less than 70%. If the reduction rate during cold rolling is below the lower limit of the aforementioned range, it may cause grain coarsening of the final structure, thereby causing a decrease in strength. Additionally, if the reduction rate during cold rolling exceeds the upper limit of the aforementioned range, the rolling load may increase, leading to a decrease in productivity. Therefore, the reduction rate during cold rolling may satisfy the aforementioned range.

[0077] The above annealing step is a step of heat-treating a cold-rolled steel sheet.

[0078] In one example, the annealing step may be performed by annealing the cold-rolled steel sheet at an annealing temperature of 800°C or higher and 900°C or lower. Specifically, the annealing step may be performed by annealing the cold-rolled steel sheet at an annealing temperature of 800°C or higher and 900°C or lower for 60 seconds or more and 180 seconds or less. Performing the annealing step within the aforementioned annealing temperature range may be advantageous for securing an austenite single-phase region. On the other hand, if the annealing step is performed below the lower limit of the aforementioned annealing temperature range, it is disadvantageous for securing a single-phase region, which may result in a decrease in the material properties of the final product. Furthermore, if the annealing step is performed above the upper limit of the aforementioned annealing temperature range, the crystal grains of the final product may coarsen, which may result in a decrease in material properties.

[0079] A method for manufacturing galvanized steel sheets can satisfy the following general formula 1.

[0080] [General Formula 1]

[0081] 1470 ≤ T A +137×(10C+Mn+Cr+Mo)

[0082] In the above general formula 1, T Ais the annealing temperature (°C) at the annealing stage, C is the carbon content (weight%) in the slab, Mn is the manganese content (weight%) in the slab, Cr is the chromium content (weight%) in the slab, and Mo is the molybdenum content (weight%) in the slab.

[0083] Specifically, the value calculated by the above general formula 1 may be 1480 or higher. In addition, the upper limit of the value calculated by the above general formula 1 may be 1700 or lower or 1650 or lower. By satisfying the above general formula 1, the method for manufacturing the above galvanized steel sheet can manufacture a galvanized steel sheet that satisfies the tensile strength and yield strength ranges of the aforementioned ranges.

[0084] The above cooling step is a step of cooling the annealed steel plate. At this time, the cooling may be performed using a gas cooling method known in the art to ensure the flatness of the steel plate.

[0085] In one example, the cooling step may include a first cooling step and a second cooling step.

[0086] The above first cooling step is a step of cooling the annealed steel sheet in the first step. For example, the above first cooling step may be performed by cooling to a cooling temperature of 650°C or higher and 750°C or lower. Specifically, the above first cooling step may be performed by cooling to a cooling temperature of 650°C or higher and 750°C or lower at a cooling rate of 1°C / s or higher and 40°C / s or lower for 5 seconds or more and 50 seconds or less. If the above first cooling step is performed below the lower limit of the aforementioned cooling temperature range, a ferrite structure may be formed within the steel sheet, which may reduce tensile strength. In addition, if the above first cooling step is performed above the upper limit of the aforementioned cooling temperature range, rapid cooling may cause defects in the sheet shape. Therefore, the above first cooling step may be performed by cooling to the aforementioned cooling temperature.

[0087] In addition, the above-mentioned second cooling step is a step of cooling the steel plate cooled in the first step a second time. For example, the above-mentioned second cooling step may be performed by cooling to a cooling temperature of 440°C or higher and 500°C or lower. Specifically, the above-mentioned second cooling step may be performed by cooling to a cooling temperature of 440°C or higher and 500°C or lower for 10 seconds or more and 60 seconds or less at a cooling rate of 5°C / s or more and 50°C / s or less. If the above-mentioned second cooling step is performed below the lower limit of the aforementioned cooling temperature range, productivity may be reduced as it is implemented by gas cooling. In addition, if the above-mentioned second cooling step is performed above the upper limit of the aforementioned cooling temperature range, it may be disadvantageous in terms of securing material. Therefore, the above-mentioned second cooling step may be performed by cooling to the aforementioned cooling temperature.

[0088] The above over-aging treatment step is a step of over-aging a cooled steel plate.

[0089] In one example, the overaging treatment step may be performed by reheating the cooled steel plate to an overaging treatment temperature of 440°C or higher and 500°C or lower, and then maintaining it for 10 seconds or more and 90 seconds or less. By performing the overaging treatment step by reheating the cooled steel plate to the aforementioned overaging treatment temperature and then maintaining it for the aforementioned time, the toughness of the martensite structure formed in the aforementioned region of the steel plate cooled through the cooling step can be imparted. On the other hand, in the overaging treatment step, if the overaging treatment temperature is below the lower limit of the aforementioned range, when the overaging-treated steel plate is introduced into the plating pot during the zinc plating step, the temperature of the plating pot is lowered, and dross is leached out, which may cause the surface of the zinc plating layer to deteriorate. In addition, in the overaging treatment step, if the overaging treatment temperature exceeds the upper limit of the aforementioned range, too much transformation into a bathite structure occurs in the area corresponding to 25% of the thickness from the surface of the steel plate, and the strength may decrease.

[0090] The above-mentioned zinc plating step is a step of forming a zinc plating layer on an over-aged steel plate, and is performed by zinc plating the over-aged steel plate.

[0091] In one example, the zinc plating step may include a hot-dip galvanizing step. Specifically, the hot-dip galvanizing step may be performed by immersing an over-aged steel plate in a hot-dip galvanizing pot containing a zinc-based plating composition to form a hot-dip galvanizing layer on the over-aged steel plate, specifically on each of the two sides of the over-aged steel plate. At this time, the temperature of the hot-dip galvanizing pot may be, for example, 420°C or higher and 550°C or lower. If the temperature of the hot-dip galvanizing pot is below the lower limit of the aforementioned range, it reaches near the melting point of zinc, causing a decrease in fluidity and making plating difficult; furthermore, excessive dross may be ejected, making it difficult to secure surface quality. Additionally, if the temperature of the hot-dip galvanizing pot exceeds the upper limit of the aforementioned range, a large amount of austenite structure within the steel plate may transform into bainite structure, making it difficult to secure strength. Therefore, the temperature of the hot-dip galvanizing pot may satisfy the aforementioned range.

[0092] In another example, the zinc plating step may further include a molten zinc alloying step. Specifically, the molten zinc alloying step is a step of alloying the molten zinc plating layer, and can be performed by heat-treating a steel plate on which a molten zinc plating layer is formed through the molten zinc plating step, thereby alloying the steel plate and zinc by thermal diffusion. For example, the molten zinc alloying step may be performed at a temperature of 500°C or higher and 600°C or lower. Subsequently, the formed zinc plating layer is an alloyed layer of iron and zinc, and can be formed with a thickness of 3 μm or more and 30 μm or less.

[0093] The above temper rolling step is a step of temper rolling a galvanized steel sheet.

[0094] In one example, the temper rolling step may be performed by temper rolling a galvanized steel sheet with a reduction rate of 0.05% or more and 1.0% or less. By performing the temper rolling step by temper rolling the galvanized steel sheet with the aforementioned reduction rate, the plate shape of the galvanized steel sheet can be properly corrected through light rolling, and deformation exceeding the elastic range is applied to the material, thereby forming dislocations. The formed dislocations act as nucleation sites for transition carbides during tempering, which is described later, thereby increasing the mobile dislocations of the material to a desired value. On the other hand, if the reduction rate in the temper rolling step is below the lower limit of the aforementioned range, the amount of mobile dislocations does not increase significantly, and the precipitation of carbides during tempering decreases, making it difficult to secure the target yield strength. In addition, if the reduction rate exceeds the upper limit of the aforementioned range during the temper rolling step, the material properties increase, but the equipment load becomes severe, making it difficult to apply to a general production process.

[0095] The above tempering step is a step of tempering a tempered galvanized steel sheet.

[0096] In one example, the tempering step may be performed at a tempering temperature of 150°C or higher and 300°C or lower for a tempering time of 3 hours or more and 10 hours or less. In the tempering step, if the tempering temperature is below the lower limit of the aforementioned range, the temperature may be too low to facilitate the precipitation of transition carbides. Additionally, in the tempering step, if the tempering temperature exceeds the upper limit of the aforementioned range, the martensite structure within the galvanized steel sheet may be over-tempered, resulting in a decrease in tensile strength, and acicular cementite may grow, thereby reducing bendability. Therefore, the tempering step may be performed within the aforementioned range of tempering temperature and tempering time.

[0097] The galvanized steel sheet manufactured according to the above method for manufacturing galvanized steel sheets is a galvanized steel sheet made of a martensitic steel grade, and its material properties vary significantly depending on the degree of tempering. In this case, if tempering is insufficient, the yield strength does not increase, and if tempering is excessive, both the yield strength and tensile strength may decrease simultaneously. Furthermore, the galvanized steel sheet manufactured according to the above method for manufacturing galvanized steel sheets may have a different tempering effect depending on the reduction ratio applied during temper rolling before tempering is applied. To resolve this, the above method for manufacturing galvanized steel sheets may satisfy the following general formula 2.

[0098] [General Formula 2]

[0099] 8500 ≤ (T T +273)(20+Log(t T ))

[0100] In the above general formula 2, T T is the tempering temperature (°C) at the tempering stage, and t T is the tempering time (minutes) during the tempering stage.

[0101] Specifically, the value calculated by the above general formula 2 may be 8600 or higher or 8700 or higher. In addition, the upper limit of the value calculated by the above general formula 2 may be 12000 or lower or 10000 or lower. By satisfying the above general formula 2, the method for manufacturing the above galvanized steel sheet can manufacture a galvanized steel sheet that satisfies the tensile strength and yield strength ranges of the aforementioned ranges.

[0102]

[0103] The present application will be described in more detail below through embodiments according to the present application and comparative examples not according to the present application, but the scope of the present application is not limited by the embodiments presented below.

[0104]

[0105] Example 1

[0106] Manufacture of galvanized steel sheets

[0107] The components shown in Table 1 below, the remainder of Fe and other unavoidable impurities were steeled and continuously cast to produce a slab, and then the slab was reheated at a temperature of 1200°C and hot-rolled at a finishing temperature of 900°C and a reduction rate of 90% to produce a hot-rolled steel sheet.

[0108] Afterwards, the hot-rolled steel sheet was cooled at a cooling rate of 50 ℃ / s and then coiled at a coiling temperature of 600℃.

[0109] Afterwards, the above hot-rolled steel sheet was pickled to remove the surface scale layer, and then cold-rolled at a reduction rate of 50% to produce a cold-rolled steel sheet.

[0110] Afterwards, the above cold-rolled steel sheet was annealed at an annealing temperature of 840°C for 108 seconds, as shown in Table 2 below.

[0111] Afterwards, the annealed steel plate was first cooled to 680℃ by cooling at a cooling rate of 7.6 ℃ / s for 21 seconds as shown in Table 2 below, and secondly cooled to 460℃ by cooling at a cooling rate of 6.9 ℃ / s for 32 seconds.

[0112] Afterwards, the cooled steel plate was reheated to an over-aging treatment temperature of 460°C as shown in Table 2 below, and then over-aged by maintaining it for 60 seconds.

[0113] Subsequently, the over-aged steel plate was immersed in a hot-dip galvanizing pot at a temperature of 460°C to form a hot-dip galvanizing layer, and the hot-dip galvanizing layer was heated to 520°C to alloy, thereby producing a galvanized steel plate with a zinc plating layer, specifically an alloyed hot-dip galvanizing layer, formed on the steel plate.

[0114] Afterwards, the galvanized steel sheet was temper-rolled with a reduction rate of 0.1% as shown in Table 2 below.

[0115] Subsequently, a galvanized steel sheet was manufactured by tempering the temper-rolled galvanized steel sheet at a tempering temperature of 150°C for a tempering time of 6 hours, as shown in Table 2 below.

[0116] Slab Alloy Composition (Wet%) CM nSi Sol. Al P S 1.7 0.1 0.0 3 0.0 1 2 0.00 1 0.00 6 0.6 0.0 2 0.0 3 0.2 0.00 3

[0117]

[0118] Examples 2 to 6 and Comparative Examples 1 to 6

[0119] Manufacture of galvanized steel sheets

[0120] A galvanized steel sheet was manufactured in the same manner as in Example 1, except that the annealing temperature, first cooling temperature, second cooling temperature, over-aging treatment temperature, tempering rolling reduction ratio, tempering temperature, and time were changed to the conditions shown in Table 2 below.

[0121] Annealing Temperature (°C) 1st Cooling Temperature (°C) 2nd Cooling Temperature (°C) Over-aging Treatment Temperature (°C) Tempering Rolling Reduction Rate (%) Tempering Temperature (°C) Time (hours) Example 18 40 680 450 4600.11506 Example 28 20 680 450 4600.11506 Example 38 00 680 450 4600.151506 Example 48 40 680 490 4600.11806 Example 58 40 680 450 4600.12006 Example 68 40 680 450 4600.115018 Comparative Example 17 80 680 450 4600.21506 Comparative Example 27 60 680 430 4600.31506 Comparative Example 38406804504600.11206 Comparative Example 48406804504600.11006 Comparative Example 58406804504600.11501 Comparative Example 68406804504600.11500.5

[0122]

[0123] Evaluation Example 1. Evaluation of tissue area fraction

[0124] In each of the examples and comparative examples, the area fraction was evaluated by measuring a region corresponding to 25% of the surface of the steel sheet using a scanning electron microscope (SEM) in a direction perpendicular to the rolling direction of the steel sheet, and the results are shown in Table 3 below.

[0125]

[0126] Evaluation Example 2. Evaluation of satisfaction of Equations 1 and 2

[0127] In each of the examples and comparative examples, when manufacturing a galvanized steel sheet, it was evaluated whether the following general formulas 1 and 2 were satisfied, and the results are shown in Table 3 below.

[0128] [General Formula 1]

[0129] 1470 ≤ T A +137×(10C+Mn+Cr+Mo)

[0130] In the above general formula 1, T A is the annealing temperature (°C) at the annealing stage, C is the carbon content (weight%) in the slab, Mn is the manganese content (weight%) in the slab, Cr is the chromium content (weight%) in the slab, and Mo is the molybdenum content (weight%) in the slab.

[0131] [General Formula 2]

[0132] 8500 ≤ (T T +273)(20+Log(t T ))

[0133] In the above general formula 2, T T is the tempering temperature (°C) at the tempering stage, and t T is the tempering time (minutes) during the tempering stage.

[0134]

[0135] Evaluation Example 3. Tensile Test Evaluation

[0136] Galvanized steel sheets produced in each of the examples and comparative examples were taken according to JIS No. 5 standards and prepared as test specimens. Tensile tests were performed on the test specimens in a direction perpendicular to the rolling direction of the steel sheet to measure yield strength (YP), tensile strength (TS), and elongation (EL), and the results are shown in Table 3 below.

[0137]

[0138] Evaluation Example 4. Evaluation of Hydrogen Embrittlement

[0139] In each of the examples and comparative examples, the galvanized steel sheets were immersed in a 0.1 N hydrochloric acid (HCl) solution for 300 hours under a load of 100% of the yield point (YP), and then it was evaluated whether cracks occurred or fractures occurred due to hydrogen embrittlement. The results are shown in Table 3 below. At this time, if no cracks occurred in the galvanized steel sheets, it was evaluated as OK, and if any cracks occurred in the galvanized steel sheets, it was evaluated as NG.

[0140] Microstructure (%) Calculated value of General Formula 1 Calculated value of General Formula 2 Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) Hydrogen Embrittlement TM FB RA Example 1 Bal.0 201 15258 789.158 12521549 7.8OK Example 2 Bal.5 201 15058 789.158 1235 1510 8.2OK Example 3 Bal.1 201 14858 789.158 1189 14858.4OK Example 4 Bal.0 101 15259 412.50 31285157 18.5OK Example 5 Bal.0 201 152598 28.06 61301 15528.6OK Example 6Bal.020115258990.98120915718.4OK Comparative Example 1Bal.1520114658789.158114814289.8NG Comparative Example 2Bal.2030114458789.1581123140210.5NG Comparative Example 3Bal.020115258165.813113715718.4NG Comparative Example 4Bal.020115257750.25108215718.6NG Comparative Example 5Bal.020115258460117115788.3NG Comparative Example 6Bal.020115258332.664115215828.2NGTM: Tempered Martensitic Structure F: Ferritic Structure B: Bainite Structure RA: Retained Austenite Structure Bal.: Remainder

[0141] As shown in Table 3 above, unlike the galvanized steel sheets produced in Comparative Examples 1 to 6, the galvanized steel sheets produced in each of Examples 1 to 6 satisfied both General Formulas 1 and 2 during production, thereby confirming that a tempered martensite structure was included in a specific area fraction in a specific region. Consequently, it was confirmed that the yield strength, tensile strength, and elongation each satisfied their respective specific ranges, and that hydrogen embrittlement was suppressed. Therefore, it was confirmed that the galvanized steel sheets produced in each of Examples 1 to 6 possessed high yield and excellent hydrogen embrittlement.

Claims

A galvanized steel plate comprising a steel plate and a galvanized layer formed on the steel plate, The above steel sheet comprises, in weight percent, C: 0.23% or more and 0.30% or less, Mn: greater than 1.5% and less than 3.0%, Si: 0.03% or more and less than 0.3%, Cr: greater than 0% and less than 0.8%, and Mo: greater than 0% and less than 0.3%, with the remainder consisting of Fe and other unavoidable impurities, and the region corresponding to 25% of the thickness from the surface comprises a tempered martensite structure with an area fraction of 65% or more and less than 100%, and is composed of one or more selected from the remainder being a martensite structure, a ferrite structure, a bainite structure, and a retained austenite structure. Galvanized steel sheet with a yield strength (YP) of 1180 MPa or higher. In Article 1, The above steel plate is a galvanized steel plate further comprising, in weight%, one or more selected from Sol. Al: greater than 0% and less than or equal to 0.1%, P: greater than 0% and less than or equal to 0.05%, S: greater than 0% and less than or equal to 0.01%, N: greater than 0% and less than or equal to 0.01%, B: greater than 0% and less than or equal to 0.004%, and the sum of Nb, V, and Ti: greater than 0% and less than or equal to 0.1%. In Article 1, Galvanized steel sheet having a tensile strength (TS) of 1470 MPa or more and 1800 MPa or less. A step of reheating and then hot-rolling a slab comprising, in weight%, C: 0.23% or more and 0.30% or less, Mn: greater than 1.5% and less than or equal to 3.0%, Si: 0.03% or more and less than or equal to 0.3%, Cr: greater than 0% and less than or equal to 0.8%, and Mo: greater than 0% and less than or equal to 0.3%, and the remainder being Fe and other unavoidable impurities; Step of winding a hot-rolled steel sheet; A step of cold rolling while unwinding the coiled hot-rolled steel sheet; Step of annealing cold-rolled steel sheets; Step of cooling the annealed steel plate; Step of over-aging the cooled steel plate; Step of galvanizing an over-aged steel plate; A step of temper rolling a galvanized steel sheet; and A method for manufacturing a galvanized steel sheet comprising the step of tempering a temper-rolled galvanized steel sheet, wherein The tempered galvanized steel sheet comprises a region corresponding to 25% of the thickness from the surface that includes a tempered martensite structure with an area fraction of 65% or more and less than 100%, and consists of one or more selected from the remaining martensite structure, ferrite structure, bainite structure, and retained austenite structure. A method for manufacturing a galvanized steel sheet having a yield strength (YP) of 1180 MPa or higher. In Article 4, Method for manufacturing a galvanized steel sheet satisfying the following general formula 1: [General Formula 1] 1470 ≤ T A +137×(10C+Mn+Cr+Mo) In the above general formula 1, T A is the annealing temperature (°C) at the annealing stage, C is the carbon content (weight%) in the slab, Mn is the manganese content (weight%) in the slab, Cr is the chromium content (weight%) in the slab, and Mo is the molybdenum content (weight%) in the slab. In Article 5, A method for manufacturing a galvanized steel sheet in which the above annealing step is performed at an annealing temperature of 800°C or higher and 900°C or lower. In Article 4, Method for manufacturing a galvanized steel sheet satisfying the following general formula 2: [General Formula 2] 8500 ≤ (T T +273)(20+Log(t T )) In the above general formula 2, T T is the tempering temperature (°C) at the tempering stage, and t T is the tempering time (minutes) during the tempering stage. In Article 4, The cooling step described above comprises: a first cooling step of cooling to a cooling temperature of 650°C or higher and 750°C or lower; and A method for manufacturing a galvanized steel sheet comprising a secondary cooling step of cooling to a cooling temperature of 440℃ or higher and 500℃ or lower. In Article 4, A method for manufacturing a galvanized steel sheet, wherein the above-mentioned overaging treatment step is performed by reheating to an overaging treatment temperature of 440°C or higher and 500°C or lower, and then maintaining it for 10 seconds or more and 90 seconds or less. In Article 4, A method for manufacturing a galvanized steel sheet, comprising a galvanizing step in which the galvanizing step comprises immersing an over-aged steel sheet in a hot-dip galvanizing pot containing a zinc-based plating composition to form a hot-dip galvanizing layer on the over-aged steel sheet. In Article 10, A method for manufacturing a galvanized steel sheet, wherein the above-mentioned galvanizing step further comprises a molten zinc alloying step for alloying a molten zinc plating layer formed on an over-aged steel sheet.