Alloyed hot-dip galvanized steel sheet and method for manufacturing same

Abstract

The present application relates to an alloyed hot-dip galvanized steel sheet and a method for manufacturing same. According to the present application, it is possible to provide an alloyed hot-dip galvanized steel sheet having excellent tensile properties and hole expansion ratio as well as excellent productivity and economic feasibility, and a method for manufacturing same.

Description

Alloyed hot-dip galvanized steel sheet and method for manufacturing the same

[0001] The present application relates to an alloyed hot-dip galvanized steel sheet and a method for manufacturing the same.

[0002] Steel materials used in vehicle bodies are required to be ultra-high strength due to the tightening of automotive crash regulations and the need to enhance passenger safety. However, since there is generally a trade-off between strength and formability in steel, there is a problem where formability, such as elongation and hole expansion, decreases as steel strength increases. To address this issue, there is a need to develop steel grades that increase formability while maintaining high strength.

[0003] Dual Phase steel (DP steel), one of the first generation of ultra-high strength steels, consists of a ferrite structure with low strength and high ductility and a martensite structure with high strength and low ductility. Due to these characteristics, DP steel offers an excellent combination of strength and ductility and is widely used as steel for automotive bodies. However, when processes such as hole expansion and bending, which require high local elongation, are applied to DP steel, stress is concentrated at the interphase interface between the ferrite and martensite structures, where there is a large difference in hardness between the phases, causing cracks.

[0004] Conventionally, as a method to reduce the difference in hardness between the ferrite and martensite phases, a tempering process was additionally performed after the production of DP steel through a general process to lower the strength of the martensite phase and reduce the difference in hardness between the phases. However, the disadvantage arose that the addition of a tempering process after the production of DP steel required the construction of equipment for the subsequent process.

[0005] Patent Document 1 (Korean Published Patent No. 10-2024-0098907) developed a technology that reduces the difference in hardness between phases by forming tempered martensite in the final structure through a secondary cooling temperature before immersion in the plating bath, without a subsequent tempering process after the production of DP steel. However, a problem arose in which productivity was reduced because the production speed was decreased due to the need for reheating at the secondary cooling temperature before plating the DP steel to the plating bath temperature.

[0006] Therefore, to compensate for these problems and disadvantages, there is a need for an alloyed hot-dip galvanized steel sheet having excellent tensile properties and hole expansion rate with excellent productivity and economic efficiency, and a method for manufacturing the same.

[0007] The objective of the present application is to provide an alloyed hot-dip galvanized steel sheet having excellent tensile properties and hole expansion rate with excellent productivity and economic efficiency, and a method for manufacturing the same.

[0008] To solve the above problem, the alloyed hot-dip galvanized steel sheet and the method for manufacturing the same according to the present application comprises, in weight percent, C: 0.04% or more and 0.15% or less, Mn: 1.0% or more and 2.5% or less, Cr: 0.5% or more and 1.5% or less, and B: 0.0010% or more and 0.0030% or less, and the remainder being Fe and other unavoidable impurities, a slab comprising these elements is reheated and then subjected to hot rolling, cold rolling, annealing, multi-stage cooling, alloyed hot-dip galvanizing, and final cooling, comprising a steel sheet and an alloyed hot-dip galvanized layer formed on the steel sheet, wherein the steel sheet comprises a fresh martensite structure and a ferrite structure at a depth of 25% of the thickness from the surface and satisfies the following general formula 1.

[0009] [General Formula 1]

[0010] 0.6 ≤ (a×b) / (c×100)

[0011] In the above general formula 1, a is the reduction ratio during cold rolling, b is the volume fraction of the fresh martensite structure at a depth of 25% of the thickness from the surface of the steel sheet, and c is the difference in hardness between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel sheet.

[0012] The above steel plate can be manufactured by cold rolling with a reduction rate of 30% or more and 80% or less.

[0013] In addition, the above steel plate may have a volume fraction of fresh martensite structure at a depth of 25% of the thickness from the surface of 30% or more, and a volume fraction of ferrite structure of 70% or less.

[0014] In addition, the steel plate may have an interphase hardness difference of 3.3 GPa or less between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface.

[0015] In addition, the above slab may further include, in weight%, one or more selected from Si: greater than 0% and less than or equal to 1.0%, P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0050%, Al: greater than 0% and less than or equal to 0.5%, Mo: greater than 0% and less than or equal to 0.20%, Ti: greater than 0% and less than or equal to 0.08%, and N: greater than 0% and less than or equal to 0.0100%.

[0016] In addition, the alloyed hot-dip galvanized steel sheet may have a yield strength of 430 MPa or more and 600 MPa or less, a tensile strength of 780 MPa or more and 900 MPa or less, an elongation of 18.2% or more, and a hole expansion rate of 50% or more.

[0017] In addition, the method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present application comprises the steps of: reheating and hot-rolling a slab comprising, in weight percent, C: 0.04% or more and 0.15% or less, Mn: 1.0% or more and 2.5% or less, Cr: 0.5% or more and 1.5% or less, and B: 0.0010% or more and 0.0030% or less, and the remainder being Fe and other unavoidable impurities; cold-rolling the hot-rolled steel sheet; annealing the cold-rolled steel sheet; multi-stage cooling the annealed steel sheet; and alloying hot-dip galvanizing the multi-stage cooled steel sheet. A method for manufacturing an alloyed hot-dip galvanized steel sheet comprising the step of finally cooling the alloyed hot-dip galvanized steel sheet, wherein the finally cooled alloyed hot-dip galvanized steel sheet comprises a steel sheet and an alloyed hot-dip galvanized layer formed on the steel sheet, and wherein the steel sheet comprises a fresh martensite structure and a ferrite structure at a depth of 25% of the thickness from the surface, and satisfies the following general formula 1.

[0018] [General Formula 1]

[0019] 0.6 ≤ (a×b) / (c×100)

[0020] In the above general formula 1, a is the reduction ratio during cold rolling, b is the volume fraction of the fresh martensite structure at a depth of 25% of the thickness from the surface of the steel sheet, and c is the difference in hardness between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel sheet.

[0021] In addition, the above cold rolling step can be performed with a reduction rate of 30% or more and 80% or less.

[0022] In addition, the above steel plate may have a volume fraction of fresh martensite structure at a depth of 25% of the thickness from the surface of 30% or more, and a volume fraction of ferrite structure of 70% or less.

[0023] In addition, the steel plate may have an interphase hardness difference of 3.3 GPa or less between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface.

[0024] In addition, the above slab may further include, in weight%, one or more selected from Si: greater than 0% and less than or equal to 1.0%, P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0050%, Al: greater than 0% and less than or equal to 0.5%, Mo: greater than 0% and less than or equal to 0.20%, Ti: greater than 0% and less than or equal to 0.08%, and N: greater than 0% and less than or equal to 0.0100%.

[0025] In addition, the annealing step can be performed at a temperature of 760°C or higher and 860°C or lower.

[0026] Additionally, the multi-stage cooling step includes a step of first cooling the annealed steel plate; and a step of second cooling the steel plate that has been first cooled, and the second cooling step may have a cooling end temperature of 460°C or higher and 540°C or lower.

[0027] Additionally, the alloying hot-dip galvanizing step comprises a hot-dip galvanizing step of immersing a multi-stage cooled steel plate in a hot-dip galvanizing bath; and a hot-dip galvanizing step of heat-treating a steel plate in which a hot-dip galvanizing layer is formed through the hot-dip galvanizing step, wherein the temperature of the hot-dip galvanizing bath may be lower than the cooling end temperature in the secondary cooling step.

[0028] In addition, the final cooling step may have a cooling end temperature of 15°C or higher and 25°C or lower.

[0029] According to the present application, an alloyed hot-dip galvanized steel sheet having excellent tensile properties and hole expansion rate with excellent productivity and economic efficiency, and a method for manufacturing the same can be provided.

[0030] 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.”

[0031] 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.

[0032] The present application relates to an alloyed hot-dip galvanized steel sheet. The alloyed hot-dip galvanized steel sheet is an alloyed hot-dip galvanized steel sheet manufactured by reheating a slab comprising, in weight percent, C: 0.04% or more and 0.15% or less, Mn: 1.0% or more and 2.5% or less, Cr: 0.5% or more and 1.5% or less, and B: 0.0010% or more and 0.0030% or less, with the remainder being Fe and other unavoidable impurities, followed by hot rolling, cold rolling, annealing, multi-stage cooling, alloyed hot-dip galvanizing, and final cooling. Furthermore, the alloyed hot-dip galvanized steel sheet comprises a steel sheet and an alloyed hot-dip galvanizing layer formed on the steel sheet. Additionally, the steel sheet comprises a fresh martensite structure and a ferrite structure at a depth of 25% of its thickness from the surface, and satisfies the following general formula 1.

[0033] [General Formula 1]

[0034] 0.6 ≤ (a×b) / (c×100)

[0035] In the above general formula 1, a is the reduction ratio during cold rolling, b is the volume fraction of the fresh martensite structure at a depth of 25% of the thickness from the surface of the steel sheet, and c is the difference in hardness between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel sheet.

[0036] The alloyed hot-dip galvanized steel sheet satisfying the above general formula 1 may be manufactured by including a composition within a specific content range in the present application, and by controlling the reduction rate during cold rolling, the annealing temperature, the cooling end temperature during multi-stage cooling, the hot-dip galvanizing temperature during alloyed hot-dip galvanizing, and the cooling end temperature during final cooling within a specific range in the present application. In this specification, a depth of 25% of the thickness from the surface refers to the area measured with respect to the TD (Transverse direction) plane of the alloyed hot-dip galvanized steel sheet, and refers to a total area of ​​50%, consisting of 25% of the upper surface and 25% of the lower surface of the TD plane. At this time, the area measured on the TD plane may be measured by forming a cross-section through polishing and etching on the TD plane.

[0037] Specifically, the value calculated by the above general formula 1 may be 0.62 or higher or 0.64 or higher. Additionally, the upper limit of the value calculated by the above general formula 1 may be 3.0 or lower, 2.5 or lower, or 2.0 or lower. The alloyed hot-dip galvanized steel sheet of the present application may have excellent tensile properties and hole expansion rate by the steel sheet comprising the aforementioned components and satisfying the above general formula 1. The above tensile properties may be the yield strength (YP), tensile strength (TS), and elongation (EL) described below.

[0038] For example, the above-mentioned alloyed hot-dip galvanized steel sheet may have a yield strength (YP) of 430 MPa or more and 600 MPa or less. Specifically, the lower limit of the yield strength of the above-mentioned alloyed hot-dip galvanized steel sheet may be 432 MPa or more or 434 MPa or more, and the upper limit may be 560 MPa or less or 520 MPa or less. Since the yield strength of the above-mentioned alloyed hot-dip galvanized steel sheet satisfies the aforementioned range, it may be applied to a vehicle body for purposes such as strengthening automobile collision regulations and improving passenger safety. At this time, the above-mentioned yield strength was measured through a tensile test described later.

[0039] In addition, the alloyed hot-dip galvanized steel sheet may have a tensile strength (TS) of 780 MPa or more and 900 MPa or less. Specifically, the tensile strength of the alloyed hot-dip galvanized steel sheet may have a lower limit of 783 MPa or more or 786 MPa or more, and an upper limit of 895 MPa or less or 890 MPa or less. Since the tensile strength of the alloyed hot-dip galvanized steel sheet satisfies the aforementioned range, it may be applied to a vehicle body for purposes such as strengthening automobile collision regulations and improving passenger safety. At this time, the tensile strength was measured through a tensile test described below.

[0040] In addition, the alloyed hot-dip galvanized steel sheet may have an elongation (EL) of 18.2% or more. Specifically, the alloyed hot-dip galvanized steel sheet may have an elongation of 18.3% or more. Furthermore, the upper limit of the elongation of the alloyed hot-dip galvanized steel sheet may be 25.0% or less, and specifically, may be 24.0%, 23.0%, or 22.0% or less. By satisfying the aforementioned range of elongation, the formability of the alloyed hot-dip galvanized steel sheet may be improved. At this time, the elongation was measured through a tensile test described later.

[0041] In one example, the steel sheet may be manufactured by cold rolling with a reduction rate of 30% or more and 80% or less. By manufacturing the steel sheet by cold rolling with a reduction rate within the aforementioned range, a uniform structure is formed after annealing, thereby improving the hole expansion rate. Conversely, if the steel sheet is manufactured by cold rolling with a reduction rate below the lower limit of the aforementioned range, the uniformity of the internal microstructure may be reduced during the annealing process after cold rolling. Additionally, if the steel sheet is manufactured by cold rolling with a reduction rate exceeding the upper limit of the aforementioned range, the load may increase during cold rolling or fracture may occur.

[0042] In addition, the steel plate may have a volume fraction of fresh martensite structure at a depth of 25% of the thickness from the surface of 30% or more and a volume fraction of ferrite structure at 70% or less. At this time, the steel plate may have an upper limit of the volume fraction of fresh martensite structure at a depth of 25% of the thickness from the surface of 60% or less and a lower limit of the volume fraction of ferrite structure at 40% or more. By satisfying the volume fraction ranges of the fresh martensite structure and ferrite structure in the aforementioned region, the steel plate can secure the desired strength and simultaneously improve ductility by lowering the volume fraction of the martensite structure further included in the aforementioned region, and the hole expansion rate can be improved through the configuration of a network structure surrounding the ferrite structure.

[0043] In addition, the interphase hardness difference between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel plate may be 3.3 GPa or less. Specifically, the interphase hardness difference between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel plate may be 3.29 GPa or less. Furthermore, although the lower limit of the interphase hardness difference between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel plate is not specifically limited, it may be, for example, 0 GPa or more, and specifically, 0.5 GPa or more or 1.0 GPa or more. By satisfying the aforementioned range for the interphase hardness difference between the fresh martensite structure and the ferrite structure in the aforementioned region, the formation of cracks at the interphase interface between the fresh martensite structure and the ferrite structure is suppressed, thereby improving the hole expansion rate. On the other hand, if the difference in interphase hardness between the fresh martensite structure and the ferrite structure in the aforementioned region of the steel plate exceeds the upper limit of the aforementioned range, cracks may occur at the interface between the fresh martensite structure and the ferrite structure.

[0044] The hardness of each of the phases of the steel plate is not particularly limited if the difference in interphase hardness between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface satisfies the aforementioned range. For example, the hardness of the fresh martensite structure at a depth of 25% of the thickness from the surface of the steel plate may be 5.00 GPa or more and 7.00 GPa or less, and the hardness of the ferrite structure in the aforementioned region may be 1.70 GPa or more and 5.00 GPa or less.

[0045] The alloy composition of the above slab is described below.

[0046] C: 0.04 wt% or more, 0.15 wt% or less

[0047] Carbon (C) is an element effective in increasing strength, as it dissolves into austenite during annealing and causes an increase in the strength of martensite after final cooling. If the carbon is contained in the slab in an amount less than the lower limit of the aforementioned range, it may reduce the strength of the steel plate contained in the hot-dip galvanized steel plate. Furthermore, if the carbon is contained in the slab in an amount exceeding the upper limit of the aforementioned range, it may reduce the weldability of the steel plate, exceed the desired strength, and increase the difference in hardness between the fresh martensite structure and the ferrite structure, thereby hindering the hole expansion rate. Accordingly, the carbon may be contained in the slab in an amount of 0.04 wt% or more and 0.15 wt% or less, and specifically, in an amount of 0.05 wt% or more and 0.14 wt% or less, or in an amount of 0.06 wt% or more and 0.13 wt% or less.

[0048] Mn: 1.0 wt% or more and 2.5 wt% or less

[0049] Manganese (Mn) is an element that stabilizes the austenite structure within the steel sheet. It can suppress transformation into ferrite, pearlite, and bainite structures during multi-stage cooling and increase the martensite fraction, thereby improving the strength of the steel sheet. If the manganese is included in the slab below the lower limit of the aforementioned range, a large amount of ferrite, pearlite, and bainite structures may be formed during multi-stage cooling, which may reduce the strength of the steel sheet. Additionally, if the manganese is included in the slab above the upper limit of the aforementioned range, a manganese (Mn) enrichment layer may be formed on the surface of the steel sheet, which may reduce the hole expansion rate. Accordingly, the manganese may be included in the slab in an amount of 1.0 wt% or more and 2.5 wt% or less, and specifically, in an amount of 1.2 wt% or more and 2.4 wt% or less, or in an amount of 1.4 wt% or more and 2.3 wt% or less.

[0050] Cr: 0.5 wt% or more and 1.5 wt% or less

[0051] Chromium (Cr) can increase the hardenability of the steel sheet, suppress transformation into ferrite, pearlite, and bainite structures during multi-stage cooling, and increase the volume fraction of the martensite structure, thereby improving the strength of the steel sheet. If the chromium is included in the slab in an amount less than the lower limit of the aforementioned range, a large amount of ferrite, pearlite, and bainite structures may be formed during multi-stage cooling, which may reduce the strength of the steel sheet. Additionally, if the chromium is included in the slab in an amount exceeding the upper limit of the aforementioned range, it may reduce the elongation of the steel sheet. Accordingly, the chromium may be included in the slab in an amount of 0.5 wt% or more and 1.5 wt% or less, and specifically, in an amount of 0.53 wt% or more and 1.42 wt% or less, or in an amount of 0.55 wt% or more and 1.34 wt% or less.

[0052] B: 0.0010 wt% or more, 0.0030 wt% or less

[0053] Boron (B) is a hardenable element and can suppress transformation into ferrite, pearlite, and bainite structures during multi-stage cooling. If the boron (B) is included in the slab in an amount less than the lower limit of the aforementioned range, the aforementioned boron addition effect may be inferior. Additionally, if the boron is included in the slab in an amount exceeding the upper limit of the aforementioned range, it may degrade the impact properties of the steel plate. Accordingly, the boron may be included in the slab in an amount of 0.0010 wt% or more and 0.0030 wt% or less, and specifically, in an amount of 0.0011 wt% or more and 0.0029 wt% or less, 0.0012 wt% or more and 0.0028 wt% or less, or 0.0013 wt% or more and 0.0027 wt% or less.

[0054] Remaining Fe and other unavoidable impurities

[0055] The above-mentioned unavoidable impurities are impurities introduced during the manufacturing process of steelmaking and alloyed hot-dip galvanized steel sheets, 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 slab 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).

[0056] In one example, the slab may further include, in weight percent, one or more selected from Si: greater than 0% and less than or equal to 1.0%, P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0050%, Al: greater than 0% and less than or equal to 0.5%, Mo: greater than 0% and less than or equal to 0.20%, Ti: greater than 0% and less than or equal to 0.08%, and N: greater than 0% and less than or equal to 0.0100%.

[0057] Si: Greater than 0 wt% and less than or equal to 1.0 wt%

[0058] Silicon (Si) is an element that stabilizes the ferrite structure within the steel sheet and can increase the strength of the ferrite structure by being dissolved within it. Additionally, the silicon can improve the elongation of the steel sheet by causing the ferrite structure to be cleaned. If the silicon is included in the slab in an amount less than the lower limit of the aforementioned range, it may reduce the strength and elongation of the steel sheet. Furthermore, if the silicon is included in the slab in an amount exceeding the upper limit of the aforementioned range, silicon (Si)-based oxides are formed on the surface of the steel sheet, causing the surface roughness or plating amount to become uneven due to the silicon (Si)-based oxides. Additionally, during alloying after plating, the temperature rise required for alloying increases, which may increase the load. Therefore, the silicon may be included in the slab in an amount greater than 0 wt% and less than or equal to 1.0 wt%, and specifically, in an amount greater than or equal to 0.10 wt% and less than or equal to 0.9 wt%, or greater than or equal to 0.20 wt% and less than or equal to 0.85 wt%.

[0059] P: Greater than 0 wt% and less than or equal to 0.0200 wt%

[0060] Phosphorus (P) is an element that segregates during the manufacturing process of steel plates, causing a decrease in toughness and delayed fracture. Accordingly, the phosphorus may be included in the slab in an amount greater than 0 wt% and less than or equal to 0.0200 wt%, specifically in an amount greater than 0 wt% and less than or equal to the above. On the other hand, if the phosphorus is included in the slab in an amount exceeding the upper limit of the aforementioned range, it may cause the aforementioned problems.

[0061] S: Greater than 0 wt% and less than or equal to 0.0050 wt%

[0062] Sulfur (S) is an element that reduces the toughness and weldability of steel plates. Therefore, the sulfur may be included in the slab in an amount greater than 0 wt% and less than or equal to 0.0050 wt%, specifically, in an amount greater than 0 wt% and less than or equal to 0.0035 wt% or greater than 0 wt% and less than or equal to 0.0020 wt%. On the other hand, if the sulfur is included in the slab in an amount exceeding the upper limit of the aforementioned range, it may cause the aforementioned problems.

[0063] Al: Greater than 0 wt% and less than or equal to 0.5 wt%

[0064] Aluminum (Al) is an element that stabilizes the ferrite structure within the steel sheet, and by causing the ferrite structure to be cleaned, it can improve the elongation of the steel sheet. If the aluminum is included in the slab in an amount exceeding the upper limit of the aforementioned range, it can form coarse AlN nitrides, thereby reducing the elongation of the steel sheet. Accordingly, the aluminum may be included in the slab in an amount greater than 0 weight% and less than or equal to 0.5 weight%, and specifically, in an amount greater than 0 weight% and less than or equal to 0.47 weight%.

[0065] Mo: Greater than 0 wt% and less than or equal to 0.20 wt%

[0066] Molybdenum (Mo) increases the hardenability of the steel sheet and has the effect of refining the ferrite structure within the steel sheet and improving the strength of the steel sheet. If the molybdenum is included in the slab in an amount exceeding the upper limit of the aforementioned range, it may reduce the elongation of the steel sheet. Accordingly, the molybdenum may be included in the slab in an amount greater than 0 weight% and less than or equal to 0.20 weight%, and specifically, may be included in an amount greater than 0 weight% and less than or equal to 0.15 weight%.

[0067] Ti: Greater than 0 wt% and less than or equal to 0.08 wt%

[0068] Titanium (Ti) is an element that improves slab quality by suppressing AlN formation through the formation of TiN nitrides by combining with nitrogen (N) in the steel sheet, and maximizes the quenching effect of solid solution boron (B) by suppressing BN formation. In addition, the titanium can improve the hole expansion rate by forming fine precipitates such as TiC, thereby refining and homogenizing the crystal grains. Accordingly, the titanium may be included in the slab in an amount greater than 0 wt% and less than or equal to 0.08 wt%, specifically in an amount greater than 0 wt% and less than or equal to 0.06 wt%. On the other hand, if the titanium is included in the slab in an amount exceeding the upper limit of the aforementioned range, the elongation rate may decrease due to excessive precipitation hardening.

[0069] N: Greater than 0 wt% and less than or equal to 0.0100 wt%

[0070] Nitrogen (N) is an element that forms AlN with aluminum and causes cracks to occur during continuous casting. Therefore, the nitrogen may be included in the slab in an amount greater than 0 wt% and less than or equal to 0.0100 wt%, and specifically, may be included in an amount greater than 0 wt% and less than or equal to

[0071] The above-mentioned alloyed hot-dip galvanized layer is a layer in which the hot-dip galvanizing is alloyed. Specifically, the above-mentioned alloyed hot-dip galvanized layer may be a plating layer in which the steel sheet and zinc are alloyed by thermal diffusion.

[0072] For example, the alloyed hot-dip galvanized 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 alloyed hot-dip galvanized layer is less than the lower limit of the aforementioned range, surface deterioration may occur due to insufficient plating amount. In addition, if the thickness of the alloyed hot-dip galvanized layer exceeds the upper limit of the aforementioned range, material degradation may occur. Therefore, the alloyed hot-dip galvanized layer may have a thickness within the aforementioned range.

[0073] This application also relates to a method for manufacturing an alloyed hot-dip galvanized steel sheet. The method for manufacturing the alloyed hot-dip galvanized steel sheet relates to a method for manufacturing the aforementioned alloyed hot-dip galvanized steel sheet. Since specific details regarding the alloyed hot-dip galvanized steel sheet described below can be applied in the same way as those described above, they will be omitted.

[0074] The method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present application comprises the steps of hot rolling, cold rolling, annealing, multi-stage cooling, alloying hot-dip galvanizing, and final cooling. Furthermore, the alloyed hot-dip galvanized steel sheet that has been finally cooled through the final cooling step comprises a steel sheet and an alloyed hot-dip galvanized layer formed on the steel sheet. Additionally, the steel sheet comprises a fresh martensite structure and a ferrite structure at a depth of 25% of its thickness from the surface, and satisfies the following general formula 1.

[0075] [General Formula 1]

[0076] 0.6 ≤ (a×b) / (c×100)

[0077] In the above general formula 1, a is the reduction ratio during cold rolling, b is the volume fraction of the fresh martensite structure at a depth of 25% of the thickness from the surface of the steel sheet, and c is the difference in hardness between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel sheet.

[0078] A detailed explanation of the above general formula 1 is omitted as it is identical to that described in the above alloyed hot-dip galvanized steel sheet. According to the method for manufacturing the alloyed hot-dip galvanized steel sheet of the present application, the alloyed hot-dip galvanized steel sheet manufactured by the above-described manufacturing method satisfies the above general formula 1, thereby having excellent tensile properties and a hole expansion rate.

[0079] 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: 0.04% or more and 0.15% or less, Mn: 1.0% or more and 2.5% or less, Cr: 0.5% or more and 1.5% or less, and B: 0.0010% or more and 0.0030% or less, with the remainder being Fe and other unavoidable impurities. Specifically, reheating the slab brings the slab to a temperature at which it can be rolled, and is performed before hot rolling the slab. The reheating temperature of the slab is not particularly limited, but, for example, may be 1150°C or more and 1300°C or less. By performing reheating of the slab within the aforementioned temperature range, the casting structure is destroyed, and alloy elements and precipitates segregated during casting can be re-dissolved. In contrast, if the reheating temperature of the above slab is below the lower limit of the aforementioned range, it is not easy to perform hot rolling, and if it exceeds the upper limit of the aforementioned range, the oxidation of the surface of the slab becomes severe, which may cause rolling defects. At this time, since the specific description of the composition of the above slab is the same as that described in the above non-oriented electrical steel sheet, it will be omitted.

[0080] 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 950°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 reduce 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.

[0081] In one example, the method for manufacturing the alloyed hot-dip galvanized steel sheet may further include a coiling step. The coiling step may be performed by coiling the hot-rolled steel sheet obtained by hot rolling at a temperature of 400°C or higher and 700°C or lower. If the coiling temperature of the hot-rolled steel sheet is below the lower limit of the aforementioned range, it may cause an increase in the strength of the hot-rolled steel sheet, which increases the rolling load during cold rolling and may reduce productivity. Furthermore, if the coiling temperature of the hot-rolled steel sheet exceeds the upper limit of the aforementioned range, the strength of the final product may be reduced. Therefore, the coiling temperature of the hot-rolled steel sheet may satisfy the aforementioned range.

[0082] In another example, the method for manufacturing the alloyed hot-dip 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.

[0083] 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 through cold rolling.

[0084] In one example, the reduction rate during cold rolling may be 30% or more and 80% or less. If the reduction rate during cold rolling is below the lower limit of the aforementioned range, the uniformity of the final structure may decrease. In addition, if the reduction rate during cold rolling exceeds the upper limit of the aforementioned range, it significantly increases the load on the cold rolling process and poses a risk of plate breakage during the production process. Therefore, the reduction rate during cold rolling may satisfy the aforementioned range.

[0085] The above annealing step is a step of heat-treating a cold-rolled steel sheet.

[0086] In one example, the annealing step may be performed by annealing the cold-rolled steel sheet at a temperature of 780°C or higher and 860°C or lower for 10 seconds or more and 300 seconds or less. Specifically, the annealing may be performed by heating the cold-rolled steel sheet to a temperature of 800°C or higher and 850°C or lower at a heating rate of 1°C / s or higher and 10°C / s or lower, and maintaining it in this temperature range for 10 seconds or more and 300 seconds or less. If the heat treatment temperature of the cold-rolled steel sheet during annealing is below the lower limit of the aforementioned range, there is a risk that unrecrystallized grains may be formed, which may reduce the ductility of the cold-rolled steel sheet. In addition, if the heat treatment temperature of the above cold-rolled steel sheet exceeds the upper limit of the aforementioned range during annealing, the volume fraction of the austenite structure increases, and hardenability alloying elements that improve hardenability are evenly distributed within the austenite structure, and as the content of hardenability alloying elements decreases, transformation into ferrite, pearlite, and bainite structures occurs during multi-stage cooling, which may cause a decrease in strength.

[0087] The above multi-stage cooling step is a step for cooling annealed steel plates.

[0088] In one example, the multi-stage cooling step may include a first cooling step and a second cooling step.

[0089] 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 end temperature of 600°C or higher and 750°C or lower at a cooling rate of 0.5°C / s or higher and 40°C / s or lower. By performing the above first cooling step at the aforementioned temperature, the transformation of the structure within the steel sheet into a bainite structure is suppressed, and through the subsequent second cooling step and alloying hot-dip galvanizing step, it may be composed of a ferrite structure and an austenite structure.

[0090] In addition, the second cooling step is a step of cooling the steel plate cooled in the first step a second time. For example, the cooling end temperature of the second cooling step may be 460°C or higher and 540°C or lower. Specifically, the second cooling step may be performed by cooling to a cooling end temperature of 460°C or higher and 540°C or lower at a cooling rate of 1°C / s or higher and 50°C / s or lower. At this time, the cooling end temperature in the second cooling step may be higher than the temperature of the molten zinc plating bath in the molten zinc plating step described later. In the second cooling step, since the cooling end temperature satisfies the aforementioned temperature range and is higher than the temperature of the molten zinc plating bath in the molten zinc plating step described later, the multi-stage cooled steel plate may undergo the molten zinc plating step and the molten zinc alloying step in the alloying molten zinc plating step described later, and its interior may be composed of a ferrite structure and an austenite structure. On the other hand, if the cooling end temperature in the above-mentioned second cooling step is lower than the molten zinc plating bath temperature in the molten zinc plating step described later, additional equipment for heating is required to increase the temperature of the steel sheet before plating, and the process load due to heating increases, thereby reducing the production speed of the steel sheet, so there is a risk of reduced productivity.

[0091] The above-mentioned alloying hot-dip galvanizing step is a step of forming an alloying hot-dip galvanizing layer on a multi-stage cooled steel plate, and is performed by alloying hot-dip galvanizing the multi-stage cooled steel plate.

[0092] In one example, the alloying hot-dip galvanizing step may include a hot-dip galvanizing step and a hot-dip galvanizing step.

[0093] Specifically, the hot-dip galvanizing step can be performed by immersing a multi-stage cooled steel plate in a hot-dip galvanizing bath containing a zinc-based plating composition to form a hot-dip galvanizing layer on the multi-stage cooled steel plate, specifically on each of the two sides of the multi-stage cooled steel plate, thereby manufacturing a hot-dip galvanized steel plate. At this time, the temperature of the hot-dip galvanizing bath may be lower than the cooling end temperature of the second cooling step. For example, the temperature of the hot-dip galvanizing bath may be 450°C or higher and 470°C or lower, and may be lower than the cooling end temperature of the second cooling step. Specifically, the hot-dip galvanizing step can be performed by immersing the multi-stage cooled steel plate in the aforementioned hot-dip galvanizing bath at a temperature lower than the cooling end temperature of the second cooling step, for example, 450°C or higher and 470°C or lower, to form a hot-dip galvanizing layer on the multi-stage cooled steel plate. Since the temperature of the molten zinc plating bath is lower than the cooling end temperature in the second cooling step, the steel sheet with the molten zinc plating layer formed thereon can undergo the molten zinc alloying step described later, and its interior can be composed of a ferrite structure and an austenite structure. The zinc-based plating composition contained in the molten zinc plating bath can be any composition known in the art without limitation, so it is not particularly limited.

[0094] The above 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 above molten zinc plating step, thereby alloying the steel plate and zinc by thermal diffusion. For example, the above molten zinc alloying step can be performed at a temperature of 500°C or higher. Subsequently, the formed alloyed molten 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.

[0095] The above final cooling step is a step of cooling the alloyed hot-dip galvanized steel sheet.

[0096] In one example, the final cooling step may have a cooling end temperature of room temperature, specifically between 15°C and 25°C. Specifically, the final cooling step may be performed by finally cooling the alloyed hot-dip galvanized steel sheet that has undergone the molten zinc alloying step to a cooling end temperature of room temperature, specifically between 15°C and 25°C. By performing the final cooling step by finally cooling the alloyed hot-dip galvanized steel sheet to the aforementioned cooling end temperature, the austenite structure within the finally cooled alloyed hot-dip galvanized steel sheet transforms into a fresh martensite structure, so that the finally cooled alloyed hot-dip galvanized steel sheet may contain a ferrite structure, a fresh martensite structure, and a remainder of a pearlite structure and a bainite structure. As a result, the alloyed hot-dip galvanized steel sheet may have excellent tensile properties and a hole expansion rate.

[0097] In one example, the steel sheet included in the alloyed hot-dip galvanized steel sheet may have a volume fraction of fresh martensite structure at a depth of 25% of the thickness from the surface of 30% or more, and a volume fraction of ferrite structure of 70% or less. Since the specific description of the volume fraction of each structure included in the aforementioned region of the steel sheet is the same as that described in the alloyed hot-dip galvanized steel sheet, it will be omitted.

[0098] In addition, the steel sheet included in the above-mentioned alloyed hot-dip galvanized steel sheet may have an interphase hardness difference of 3.3 GPa or less between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface. Since the specific description of the interphase hardness difference between the fresh martensite structure and the ferrite structure included in the aforementioned region of the steel sheet is the same as that described in the above-mentioned alloyed hot-dip galvanized steel sheet, it will be omitted.

[0099]

[0100] 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.

[0101]

[0102] Preparation Examples 1 to 8

[0103] Manufacture of slabs

[0104] Each slab was manufactured by steelmaking and continuous casting with the components shown in Table 1 below, the remainder being Fe and other unavoidable impurities.

[0105] Slab Alloy Composition (Weight%) CSI Mn PS Al Cr Mo Ti BN Preparation Example 1 0.06 0.8 1 1.4 3 0.007 9 0.0009 0.47 0.57 0.09 0.031 0.0019 0.0053 Preparation Example 20.07 0.53 1.87 0.009 10.0015 0.32 1.33 0.001 0.042 0.0027 0.0057 Preparation Example 30.08 0.35 2.17 0.0121 0.0017 0.25 1.09 0.12 0.058 0.0013 0.0043 40.10.282.270.01000.00140.280.950.050.030.00170.0058 Preparation Example 50.120.492.280.01050.00100.150.550.010.0230.00130.0033 Preparation Example 60.160.372.050.01350.00110.150.230.0020.0150.00110.0042 Preparation Example 70.120.682.470.00830.00190.250.40.0010.0250.00100.0065 Preparation Example 80.121.152.050.01270.00250.070.630.10.020.00070.0031

[0106]

[0107] Example 1

[0108] Manufacture of alloyed hot-dip galvanized steel sheets

[0109] The slab of Preparation Example 1 was reheated at a temperature of 1230°C and then hot-rolled at a finishing temperature of 880°C to produce a hot-rolled steel sheet.

[0110] Afterwards, the above hot-rolled steel sheet was coiled at a coiling temperature of 550°C.

[0111] Afterwards, the above hot-rolled steel sheet was pickled to remove the surface scale layer, and then cold-rolled to produce a cold-rolled steel sheet at the reduction rate shown in Table 2 below.

[0112] Afterwards, the above cold-rolled steel sheet was heated at a heating rate of 5 ℃ / s to the temperature shown in Table 2 below and maintained at this temperature for 150 seconds, then cooled first to a cooling end temperature of 680℃ at a cooling rate of 20 ℃ / s, and then cooled second to a cooling end temperature shown in Table 2 below at a cooling rate of 25 ℃ / s.

[0113] Afterwards, the cooled steel plate was immersed in a molten zinc plating bath at the temperature shown in Table 2 below to form a molten zinc plating layer, and the molten zinc plating layer was heated to 500°C to alloy it, and then finally cooled to a cooling end temperature of 20°C to produce an alloyed molten zinc plating steel plate with an alloyed molten zinc plating layer formed on the steel plate.

[0114]

[0115] Examples 2 to 10 and Comparative Examples 1 to 11

[0116] Manufacture of alloyed hot-dip galvanized steel sheets

[0117] Alloyed hot-dip galvanized steel sheets were manufactured using the same method as in Example 1, except that the type of slab, the reduction ratio during cold rolling, the annealing temperature during annealing, the cooling end temperature during secondary cooling, and the molten zinc plating bath temperature conditions were changed to the conditions shown in Table 2 below. In this case, Comparative Example 10 showed a decrease in the uniformity of the final microstructure at a depth of 25% of the thickness from the surface of the steel sheet included in the alloyed hot-dip galvanized steel sheet. In addition, Comparative Example 11 could not manufacture the alloyed hot-dip galvanized steel sheet because the reduction ratio during cold rolling was very large during the manufacturing process, which greatly increased the load of the cold rolling process and caused sheet breakage during the cold rolling process.

[0118] Reduction rate (%) during cold rolling of slab Annealing temperature (°C) Cooling end temperature during secondary cooling (°C) Molten zinc plating bath temperature (°C) Example 1 Preparation Example 1 52840480460 Example 2 Preparation Example 252820480460 Example 3 Preparation Example 252840480460 Example 4 Preparation Example 252850480460 Example 5 Preparation Example 352840480460 Example 6 Preparation Example 452820520460 Example 7 Preparation Example 452820480460 Example 8 Preparation Example 552820480460 Example 9 Preparation Example 277840520460 Example 10 Preparation Example 377840520460 Comparative Example 1 Preparation Example 15 28 20 48 0 460 Comparative Example 2 Preparation Example 22 9 8 20 48 0 460 Comparative Example 3 Preparation Example 35 28 10 5 20 460 Comparative Example 4 Preparation Example 35 28 10 48 0 460 Comparative Example 5 Preparation Example 45 27 8 0 48 0 460 Comparative Example 6 Preparation Example 55 27 8 0 48 0 460 Comparative Example 7 Preparation Example 65 28 20 48 0 460 Comparative Example 8 Preparation Example 75 28 20 48 0 460 Comparative Example 9 Preparation Example 83 88 20 48 0 460 Comparative Example 10 Preparation Example 12 88 40 48 0 460 Comparative Example 11 Preparation Example 183---

[0119]

[0120] Evaluation Example 1. Evaluation of tissue volume fraction

[0121] With respect to the TD direction of the alloyed hot-dip galvanized steel sheets prepared in each of the examples and comparative examples, a cross-section was formed through polishing and etching, and then the volume fraction of the microstructure formed in the region corresponding to a thickness of 25% from the surface was evaluated using a scanning electron microscope (SEM), and the results are shown in Table 3 below. At this time, the remainder excluding the ferrite microstructure and the fresh martensite microstructure consists of the pearlite microstructure and the bainite microstructure.

[0122]

[0123] Evaluation Example 2. Evaluation of interphase hardness difference between pre-martensite structure and ferrite structure

[0124] After forming a cross-section through polishing and etching in the TD direction of the alloyed hot-dip galvanized steel sheets prepared in each of the examples and comparative examples, the hardness of the fresh martensite structure and the ferrite structure of the steel sheets included in each of the examples and comparative examples was measured using a nanoindenter, the difference in hardness between the phases was calculated, and the results are shown in Table 3 below.

[0125]

[0126] Evaluation Example 3. Evaluation of satisfaction of General Formula 1

[0127] In each of the examples and comparative examples, the reduction rate during cold rolling of the alloyed hot-dip galvanized steel sheet, the volume fraction of the fresh martensite structure at a depth of 25% of the thickness from the surface of the steel sheet, and the difference in hardness between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel sheet were used to evaluate whether the following general formula 1 is satisfied, and the results are shown in Table 3 below.

[0128] [General Formula 1]

[0129] 0.6 ≤ (a×b) / (c×100)

[0130] In the above general formula 1, a is the reduction ratio during cold rolling, b is the volume fraction of the fresh martensite structure at a depth of 25% of the thickness from the surface of the steel sheet, and c is the difference in hardness between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel sheet.

[0131]

[0132] Evaluation Example 4. Tensile Test Evaluation

[0133] Tensile tests were performed according to ASTM-E8 on alloyed hot-dip galvanized steel sheets prepared in each of the examples and comparative examples to measure yield strength (YP), tensile strength (TS), and elongation (EL), and the results are shown in Table 3 below.

[0134]

[0135] Evaluation Example 5. Evaluation of Hall Expansion Rate

[0136] For the steel plates included in the alloyed hot-dip galvanized steel plates manufactured in each of the examples and comparative examples, experiments were conducted according to ISO / TS 16630, and the hole expansion rate was calculated, and the results are shown in Table 3 below. Specifically, a specimen of a certain length was taken from each alloyed hot-dip galvanized steel plate with full width, and a hole was formed in the specimen by punching. Then, the burr of the specimen with the hole formed was positioned upward, and the hole was expanded using a conical punch. The hole diameter was measured at the point where a crack occurred, and the hole expansion rate was evaluated by calculating the ratio of the increase to the initial hole diameter.

[0137] Volume fraction of structure (vol%) Hardness (GPa) Calculated value of General Formula 1 Tensile properties Hole expansion rate (%) FF MF FM FM - F Interphase hardness difference Yield strength (MPa) Tensile strength (MPa) Elongation (%) Example 1 5 3 4 38.8 15.3 11.5 0 1.4 9 0 5 0 28 46 20.3 5 9.3 Example 2 5 6 39 4.0 46.5 9 2.5 40.7 8 9 435 8 ​​342 25 3.5 Example 3 5 0 46 3.8 9 5.7 11.8 21.3 17 434 8 6 42 26 5.0 Example 4 4 ​​35 34.0 15.6 0 1.5 9 1.7 334 58 88 718.3 6 7.4 Example 554423.756.392.640.82950686819.156.2 Example 655413.716.572.860.75249487118.451.5 Example 758383.446.533.090.64249585320.851.2 Example 846462.726.013.290.73045278620.355.7 Example 958393.855.942.091.43755187718.858.7 Example 1061343.936.632.700.97054687419.357.3 Comparative Example 170253.195.852.660.48647180618.347.3 Comparative Example 252433.786.382.600.47740777923.243.2 Comparative Example 364333.927.133.210.52946883118.147.5 Comparative Example 466314.037.593.570.44645383419.647.6 Comparative Example 564333.757.253.500.48945481819.143.5 Comparative Example 662342.826.413.590.49145180618.348.3 Comparative Example 746453.419.486.070.38848882721.231.4 Comparative Example 859363.708.544.840.38254085220.523.4 Comparative Example 964333.467.804.340.292705101113.028.6 F: Ferrite structure FM: Presimartensite structure FM-F Interphase hardness difference: Interphase hardness difference between presimartensite structure and ferrite structure

[0138] As shown in Table 3 above, unlike the alloyed hot-dip galvanized steel sheets produced in each of Examples 1 to 10, the alloyed hot-dip galvanized steel sheets produced in each of Comparative Examples 1 to 9 satisfy General Formula 1, thereby confirming that the tensile properties and hole expansion rate simultaneously satisfy their respective specific ranges.

Claims

As an alloyed hot-dip galvanized steel sheet manufactured by reheating a slab comprising, in weight%, C: 0.04% or more and 0.15% or less, Mn: 1.0% or more and 2.5% or less, Cr: 0.5% or more and 1.5% or less, and B: 0.0010% or more and 0.0030% or less, and the remainder being Fe and other unavoidable impurities, followed by hot rolling, cold rolling, annealing, multi-stage cooling, alloyed hot-dip galvanizing, and final cooling, It comprises a steel plate and an alloyed hot-dip galvanized layer formed on the steel plate, The above steel sheet is an alloyed hot-dip galvanized steel sheet comprising a fresh martensite structure and a ferrite structure at a depth of 25% of the thickness from the surface, satisfying the following general formula 1: [General Formula 1] 0.6 ≤ (a×b) / (c×100) In the above general formula 1, a is the reduction ratio during cold rolling, b is the volume fraction of the fresh martensite structure at a depth of 25% of the thickness from the surface of the steel sheet, and c is the difference in hardness between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel sheet. In Article 1, The above steel sheet is an alloyed hot-dip galvanized steel sheet manufactured by cold rolling with a reduction rate of 30% or more and 80% or less. In Article 1, The above steel sheet is an alloyed hot-dip galvanized steel sheet having a volume fraction of fresh martensite structure of 30% or more and a volume fraction of ferrite structure of 70% or less at a depth of 25% of the thickness from the surface. In Article 1, The above steel plate is an alloyed hot-dip galvanized steel plate having an interphase hardness difference of 3.3 GPa or less between a fresh martensite structure and a ferrite structure at a depth of 25% of the thickness from the surface. In Article 1, The above slab is an alloyed hot-dip galvanized steel sheet further comprising, in weight%, one or more selected from Si: greater than 0% and less than or equal to 1.0%, P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0050%, Al: greater than 0% and less than or equal to 0.5%, Mo: greater than 0% and less than or equal to 0.20%, Ti: greater than 0% and less than or equal to 0.08%, and N: greater than 0% and less than or equal to 0.0100%. In Article 1, Alloyed hot-dip galvanized steel sheet having a yield strength of 430 MPa or more and 600 MPa or less, a tensile strength of 780 MPa or more and 900 MPa or less, an elongation of 18.2% or more, and a hole expansion rate of 50% or more. A step of reheating and then hot-rolling a slab comprising, in weight percent, C: 0.04% or more and 0.15% or less, Mn: 1.0% or more and 2.5% or less, Cr: 0.5% or more and 1.5% or less, and B: 0.0010% or more and 0.0030% or less, and the remainder being Fe and other unavoidable impurities; Step of cold rolling a hot-rolled steel sheet; Step of annealing cold-rolled steel sheets; Step of multi-stage cooling of annealed steel plates; Step of alloying hot-dip galvanizing multi-stage cooled steel plates; and A method for manufacturing an alloyed hot-dip galvanized steel sheet comprising the step of finally cooling the alloyed hot-dip galvanized steel sheet, The finally cooled alloyed hot-dip galvanized steel sheet comprises a steel sheet and an alloyed hot-dip galvanized layer formed on the steel sheet, and A method for manufacturing an alloyed hot-dip galvanized steel sheet, wherein the steel sheet comprises a fresh martensite structure and a ferrite structure at a depth of 25% of the thickness from the surface, and satisfies the following general formula 1: [General Formula 1] 0.6 ≤ (a×b) / (c×100) In the above general formula 1, a is the reduction ratio during cold rolling, b is the volume fraction of the fresh martensite structure at a depth of 25% of the thickness from the surface of the steel sheet, and c is the difference in hardness between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface of the steel sheet. In Article 7, A method for manufacturing an alloyed hot-dip galvanized steel sheet, wherein the above cold rolling step is performed at a reduction rate of 30% or more and 80% or less. In Article 7, A method for manufacturing an alloyed hot-dip galvanized steel sheet, wherein the above steel sheet has a volume fraction of fresh martensite structure of 30% or more and a volume fraction of ferrite structure of 70% or less at a depth of 25% of the thickness from the surface. In Article 7, A method for manufacturing an alloyed hot-dip galvanized steel sheet in which the difference in interphase hardness between the fresh martensite structure and the ferrite structure at a depth of 25% of the thickness from the surface is 3.3 GPa or less. In Article 7, A method for manufacturing an alloyed hot-dip galvanized steel sheet, wherein the above slab further comprises, in weight%, one or more selected from Si: greater than 0% and less than or equal to 1.0%, P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0050%, Al: greater than 0% and less than or equal to 0.5%, Mo: greater than 0% and less than or equal to 0.20%, Ti: greater than 0% and less than or equal to 0.08%, and N: greater than 0% and less than or equal to 0.0100%. In Article 7, A method for manufacturing an alloyed hot-dip galvanized steel sheet in which the above annealing step is performed at a temperature of 760°C or higher and 860°C or lower. In Article 7, The above multi-stage cooling step comprises: a step of first cooling the annealed steel plate; and It includes a step of secondarily cooling the firstly cooled steel plate, and The above second cooling step is a method for manufacturing an alloyed hot-dip galvanized steel sheet in which the cooling end temperature is 460°C or higher and 540°C or lower. In Article 13, The above-mentioned alloying hot-dip galvanizing step comprises: a hot-dip galvanizing step of immersing a multi-stage cooled steel plate in a hot-dip galvanizing bath; and It includes a molten zinc alloying step for heat-treating a steel plate in which a molten zinc plating layer is formed through the above molten zinc plating step, and A method for manufacturing an alloyed hot-dip galvanized steel sheet in which the temperature of the molten zinc plating bath is lower than the cooling end temperature in the second cooling step. In Article 7, The above final cooling step is a method for manufacturing an alloyed hot-dip galvanized steel sheet in which the cooling end temperature is 15°C or higher and 25°C or lower.