Plated steel sheet and manufacturing method thereof

Abstract

One aspect of the present invention provides a plated steel sheet. The plated steel sheet comprises: a base steel sheet; and a plating layer formed on at least one surface of the base steel sheet, wherein the base steel sheet contains, in wt%, 0.05-0.25% of carbon (C), 0.001-0.6% of silicon (Si), 1.2-3.0% of manganese (Mn), 0.001-1.0% of aluminum (Al), 0.0001-0.05% of phosphorus (P), 0.0001-0.05% of sulfur (S), and 0.0001-0.02% of nitrogen (N), with the remainder comprising Fe and inevitable impurities, the microstructure of the base steel sheet includes 65-90 area% of a first phase containing bainitic ferrite, the microstructure of the base steel sheet includes 10-35 area% of a second phase containing a tempered martensite-austenite composite phase (TMA phase) and at least one among tempered low-temperature bainite and tempered lath martensite, and the area fraction of the tempered martensite-austenite composite phase relative to the total area fraction of the microstructure may be less than 3 area%.

Description

Galvanized steel sheet and its manufacturing method

[0001] The present invention relates to a plated steel sheet and a method for manufacturing the same. More specifically, the present invention relates to a plated steel sheet that can be used for chassis structural members of automobiles.

[0002] The automobile market has recently been transitioning from internal combustion engine vehicles to eco-friendly vehicles, including electric vehicles, to mitigate global warming. This shift has led to changes in the types of components that make up a vehicle, as well as changes in vehicle weight. Specifically, it is known that the weight of electric vehicles increases by approximately the same amount as the battery weight compared to internal combustion engine vehicles.

[0003] Among the components that make up an automobile, the chassis supports the body and absorbs road vibrations and shocks, ensuring a comfortable ride and driving safety. As vehicle weight increases, the fatigue loads applied to chassis components also increase. Therefore, steel materials used in electric vehicle chassis components require superior fatigue strength.

[0004] Generally, the fatigue strength of steel increases proportionally to component thickness. Therefore, when component thickness decreases due to corrosion, fatigue strength declines, compromising driving safety. In particular, components manufactured from high-strength steel experience a relatively significant decrease in fatigue strength with thickness reduction. Therefore, the high-strength steel used in chassis components must be made of steel plates with excellent corrosion resistance.

[0005] Furthermore, the fatigue strength of steel is known to be proportional to its tensile strength and / or yield strength. Therefore, for steel sheets used in automobile chassis components, methods to enhance tensile strength and / or yield strength are necessary to improve fatigue strength.

[0006] Meanwhile, since chassis components are manufactured through press forming, there is a growing demand for improved shape fixation to ensure dimensional accuracy. Specifically, springback is the phenomenon of elastic recovery after stress is removed following plastic deformation. Excessive yield strength of a material can make it difficult to achieve the desired shape due to springback. Therefore, it is necessary to increase yield strength to secure fatigue strength, while simultaneously controlling the upper limit of yield strength to reduce springback.

[0007] Typically, hot-rolled galvanized steel sheets are manufactured by pickling hot-rolled steel sheets to remove the scale layer on the surface, heating the steel sheet to a temperature higher than the melting temperature of zinc to ensure wettability, and then immersing it in molten zinc. Typically, the heating of the steel sheet is performed in a temperature range of 400°C to 700°C. Therefore, in the case of high-strength steels containing low-temperature transformation structures, the yield strength tends to increase rapidly due to heat treatment, resulting in poor shape freezing properties during press forming.

[0008] To address these issues, various technologies have been proposed to manufacture high-strength hot-rolled steel sheets. For example, Patent Document 1 proposes a method for manufacturing hot-rolled steel sheets with excellent tensile strength, with a microstructure composed of 10-55% ferrite and 45-90% bainite and martensite. However, although Patent Document 1 demonstrates that the tensile strength of the hot-rolled steel sheet is 950 MPa or higher, it does not consider the changes in yield strength that occur during heat treatment prior to plating and the resulting changes in press formability.

[0009] Therefore, in order to ensure the driving safety of chassis parts for eco-friendly vehicles such as electric vehicles, it is necessary to develop steel that has high tensile strength and yield strength, thus providing excellent fatigue life, has excellent corrosion resistance by forming a plating layer on the surface, and can control the upper limit of the yield strength to facilitate press forming.

[0010] (Patent Document 1) Korean Patent Publication No. 10-2020-0011475

[0011] One aspect of the present invention is to provide a plated steel sheet having excellent fatigue performance due to high tensile strength and yield strength, and excellent shape freezing properties by controlling the upper limit of the yield strength, and a method for manufacturing the same.

[0012] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0013] One aspect of the present invention is to provide a plated steel sheet. The plated steel sheet comprises: a base steel sheet; And a plating layer formed on at least one surface of the base steel sheet, wherein the base steel sheet contains, in wt%, carbon (C): 0.05 to 0.25%, silicon (Si): 0.001 to 0.6%, manganese (Mn): 1.2 to 3.0%, aluminum (Al): 0.001 to 1.0%, phosphorus (P): 0.0001 to 0.05%, sulfur (S): 0.0001 to 0.05%, nitrogen (N): 0.0001 to 0.02%, the remainder Fe and other inevitable impurities, and the microstructure of the base steel sheet includes 65 to 90 area% of a first phase including bainitic ferrite, and the microstructure of the base steel sheet includes at least one of tempered low-temperature bainite and tempered lath martensite and a tempered martensite-austenite composite phase (TMA phase). The second phase comprises 10 to 35 area%, and the area fraction of the tempered martensite-austenite composite phase among the total area fraction of the microstructure may be less than 3 area%.

[0014] Additionally, in the aforementioned plated steel sheet, the average circle diameter of the second phase may be 2 to 20 ㎛.

[0015] Additionally, in one of the aforementioned plated steel sheets, the microstructure of the base steel sheet may further include carbides of less than 5 area% of the total area fraction of the microstructure.

[0016] In addition, in one of the aforementioned plated steel sheets, the base steel sheet may further include, in weight %, at least one of chromium (Cr): 0.01 to 2.0%, molybdenum (Mo): 0.01 to 2.0%, titanium (Ti): 0.001 to 0.2%, and niobium (Nb): 0.001 to 0.2%.

[0017] Additionally, in one of the aforementioned plated steel sheets, the plated steel sheet may have a yield strength of 700 to 950 MPa.

[0018] In addition, in one of the aforementioned plated steel sheets, the plated steel sheet may have a tensile strength of 980 MPa or more and an elongation of 9% or more.

[0019] Another aspect of the present invention provides a method for manufacturing a plated steel sheet. The method comprises the steps of: reheating a slab containing carbon (C): 0.05 to 0.25%, silicon (Si): 0.001 to 0.6%, manganese (Mn): 1.2 to 3.0%, aluminum (Al): 0.001 to 1.0%, phosphorus (P): 0.0001 to 0.05%, sulfur (S): 0.0001 to 0.05%, nitrogen (N): 0.0001 to 0.02%, the remainder iron (Fe) and other unavoidable impurities at 1100 to 1350°C; hot-rolling the heated slab to obtain a hot-rolled steel sheet; first cooling the hot-rolled steel sheet to a temperature of Bs or lower at a cooling rate of 30°C / s or higher; The method may include a step of secondarily cooling the first-cooled steel plate to a temperature of (Bs+Ms) / 2 or higher at a cooling rate of 20°C / s or lower; a step of thirdly cooling the second-cooled steel plate to a temperature of BS~Ms-40°C at a cooling rate of 30°C / s or higher; a step of coiling the third-cooled steel plate; a step of cooling the coiled steel plate to room temperature; a step of pickling the cooled steel plate and heat-treating it at 400 to 700°C; and a step of plating the heat-treated steel plate.

[0020] In addition, in the method described above, in the step of obtaining the hot-rolled steel sheet, the heated slab can be hot-rolled so that the value of the following relational expression 1 satisfies 5 to 15 in a rolling finish temperature (FDT) range of 800 to 1150°C.

[0021] [Relationship 1]

[0022] Du = [(FDT + 7.4×[C] - 24.7×[Si] - 4.7×[Mn] - 3.9×[Cr] - 5.2[Mo] - 480×[Ti] - 1200×[Nb])×0.049] - 34.2

[0023] (Here, Du is an indicator of the effective grain size of austenite immediately before the first cooling after hot rolling, FDT means the rolling end temperature (℃), and each element represents the content (weight%).)

[0024] Additionally, in one of the aforementioned methods, in the step of obtaining the hot-rolled steel sheet, the total reduction amount of the final two passes may be 10 to 40%.

[0025] Additionally, in one of the above-described methods, the secondary cooling step may be performed for a time (ts) that satisfies the following relational expression 2.

[0026] [Relationship 2]

[0027] 0.65 ≤ 1-exp[-k(T) × (ts) 2 ] ≤ 0.9

[0028] (Here, k(T) represents a value defined by the following relation 3.)

[0029] [Relationship 3]

[0030]

[0031] (Here, T1 is the primary cooling end temperature, T2 is the secondary cooling end temperature, and each element represents the content (weight%).)

[0032] Additionally, in one of the aforementioned methods, the heat treatment step may be performed for 2 to 200 seconds.

[0033] In addition, in one of the above-described methods, in the plating step, the heat-treated steel sheet can be zinc-plated by immersing it in a Zn-based plating bath.

[0034] According to the present invention, the tensile strength and yield strength of the plated steel sheet are high, so that fatigue performance is excellent, and the shape freezing property during press forming can be improved by controlling the upper limit of the yield strength.

[0035] In addition, according to the present invention, since the plated steel sheet includes a plated layer, it can provide excellent corrosion resistance.

[0036] The various advantageous and beneficial advantages and effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0037] Figure 1 is a photograph of the appearance of the plated steel sheets of Invention Example 1 and Comparative Example 1.

[0038] Figure 2 is a photograph of the microstructure of Invention Example 1 and Comparative Example 3 observed using a scanning electron microscope.

[0039] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified in various other forms, and the terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include plural forms, unless the relevant definition clearly indicates a contrary meaning.

[0040] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0041] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0042] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0043] Conventional steel sheets used for chassis components utilized a large amount of low-temperature transformed structures (e.g., bainite, martensite, and martensite-austenite composite (MA) phases) to secure high yield and tensile strengths. Furthermore, with increasing demand for improved corrosion resistance, hot-rolled steel sheets heated to 400 to 700°C and then plated were manufactured and used as the steel material for chassis components.

[0044] However, in the case of the steel plate as described above, the yield strength of the steel plate increases significantly, and the dimensional accuracy of the part is reduced due to springback after forming, making assembly difficult and reducing the productivity of the part.

[0045] Accordingly, the inventors of the present invention sought to develop a steel plate capable of controlling the upper limit while securing a high level of yield strength.

[0046] In general, the phenomenon of increased yield strength after heat treatment before plating is known to be caused by the phenomenon of brittle hardening in which the dissolved carbon, which has become easily diffused by heating during the annealing process, is fixed to the dislocations existing in the matrix structure and hinders the movement of the dislocations.

[0047] That is, the yield strength after heat treatment can be determined by the dislocation density and the content of dissolved carbon present in the matrix structure. The inventors of the present invention have confirmed that the yield strength of a hot-rolled steel sheet after heat treatment can be controlled by controlling the dislocation density and dissolved carbon within the hot-rolled steel sheet before heat treatment, and have conducted research on this matter.

[0048] Specifically, carbon present in a supersaturated state within the second phase diffuses and migrates into the matrix structure as the temperature rises during heat treatment, thereby becoming fixed at the potential present within the matrix structure. In other words, the content of dissolved carbon present within the matrix structure may be affected by the type, fraction, and / or size of the second phase.

[0049] The inventors of the present invention have experimentally confirmed that a high level of yield strength can be secured while controlling its upper limit by optimizing the fraction and type of the matrix structure and the second phase of the microstructure by appropriately controlling the alloy composition of the steel plate and the hot rolling, cooling and heat treatment conditions, and have completed the present invention based on the results.

[0050] In one embodiment of the present invention, the microstructure of the base steel sheet that is the base of the plated steel sheet may include bainitic ferrite as a first phase and tempered low-temperature bainite, tempered lath martensite, and / or tempered martensite-austenite composite phase (TMA phase) as a second phase.

[0051] The present invention will be described in detail below.

[0052] A plated steel sheet according to an embodiment of the present invention may include a base steel sheet; and a plated layer provided on at least one surface of the base steel sheet.

[0053] First, the alloy composition of the steel plate is explained. The contents of the alloy composition below are in weight percent unless otherwise specified.

[0054] Carbon (C): 0.05 to 0.25%

[0055] Carbon (C) is the most economical and effective element for strengthening steel. Furthermore, C diffuses into austenite during bainitic-ferrite transformation, stabilizing the austenite. This allows the austenite to transform into low-temperature bainite and lath martensite, the secondary phases, during subsequent cooling, thereby enhancing the tensile strength and / or yield strength of the steel sheet.

[0056] When the content of C is less than 0.05%, the fraction of the second phase decreases, making it difficult to sufficiently secure tensile strength and / or yield strength. In addition, when the content of C exceeds 0.25%, the fraction of the second phase increases excessively, making it difficult to secure elongation, and weldability may deteriorate. That is, the content of C may be 0.05 to 0.25%, more specifically 0.06 to 0.24%, and even more specifically 0.07 to 0.22%.

[0057] Silicon (Si): 0.001 to 0.6%

[0058] Silicon (Si) is an element that enhances the hardenability of steel, thereby enhancing strength through its solid solution strengthening effect. Furthermore, Si delays the formation of carbides, preventing the formation of pearlite, thereby promoting transformation into secondary phases, low-temperature bainite and lath martensite, thereby enhancing yield strength and / or tensile strength.

[0059] When the content of Si is less than 0.001%, it may be difficult to secure the effect of strength improvement by solid solution strengthening. In addition, when the content of Si exceeds 0.6%, oxides may be formed on the surface during plating, which may reduce the wettability of molten zinc and deteriorate the plating quality. That is, the content of Si may be 0.001 to 0.6%, specifically 0.001 to 0.600%, more specifically 0.01 to 0.55%, and even more specifically 0.02 to 0.5%.

[0060] Manganese (Mn): 1.2 to 3.0%

[0061] Manganese (Mn) is an element that improves the hardenability of steel, and can facilitate the formation of low-temperature transformation structures by preventing the formation of granular ferrite during cooling after finish rolling.

[0062] When the content of Mn is less than 1.2%, it may be difficult to secure sufficient strength due to an insufficient fraction of the second phase. In addition, when the content of Mn exceeds 3.0%, the hardenability increases significantly, so that the transformation of bainite does not occur smoothly in the cooling zone, and the time required to secure the fraction of bainitic ferrite, which is the first phase, excessively increases, which may reduce the elongation. That is, the content of Mn may be 1.2 to 3.0%, more specifically, 1.5 to 2.8%, and even more specifically, 1.8 to 2.5%.

[0063] Aluminum (Al): 0.001 to 1.0%

[0064] Aluminum (Al) is an element added for deoxidation, and can improve strength through the solid solution strengthening effect.

[0065] When the content of Al is less than 0.001%, it may be difficult to sufficiently secure the effect of strength improvement by solid solution strengthening. In addition, when the content of Al exceeds 1.0%, it may cause an increase in oxide and / or nitride inclusions in the steel, thereby reducing the formability of the steel sheet. That is, the content of Al may be 0.001 to 1.0%, specifically 0.001 to 1.000%, more specifically 0.005 to 0.8%, and even more specifically 0.01 to 0.5%.

[0066] Phosphorus (P): 0.0001 to 0.05%

[0067] Phosphorus (P) is an impurity that is inevitably contained in steel, and can be a major cause of lowering the workability of steel due to segregation. Therefore, the lower its content, the more effective it can be for the workability of steel plates.

[0068] The lower limit of the above P content may be 0%, but considering limitations in the manufacturing process or excessive increase in manufacturing cost, the P content may be 0.0001% or more, and more specifically, 0.0002% or more. In addition, if the P content exceeds 0.05%, the workability of the steel sheet may deteriorate, and therefore the P content may be 0.05% or less, and more specifically, 0.050% or less, and even more specifically, 0.045% or less.

[0069] Sulfur (S): 0.0001 to 0.05%

[0070] Sulfur (S) is an impurity that is inevitably contained in steel, and as it combines with manganese and other elements to form non-metallic inclusions, it can be a major cause of reducing the workability of steel. Therefore, the lower its content, the more effective it can be for the workability of steel plates.

[0071] The lower limit of the above S content may be 0%, but considering limitations in the manufacturing process or excessive increase in manufacturing cost, the above S content may be 0.0001% or more, and more specifically, 0.0002% or more. In addition, when the above S content exceeds 0.05%, the workability of the steel sheet may deteriorate, and therefore the above S content may be 0.05% or less, and more specifically, 0.045% or less.

[0072] Nitrogen (N): 0.0001 to 0.02%

[0073] Nitrogen (N) is an impurity that is inevitably contained in steel, and can reduce the workability of steel by reacting with Al, etc. to precipitate nitrides. Therefore, the lower its content, the more effective it can be for the workability of steel plates.

[0074] The lower limit of the above N content may be 0%, but considering limitations in the manufacturing process or excessive increase in manufacturing cost, the N content may be 0.0001% or more, and more specifically, 0.0002% or more. In addition, when the N content exceeds 0.02%, the workability of the steel sheet may deteriorate, and therefore the N content may be 0.02% or less, and specifically, 0.0200% or less, and more specifically, 0.015% or less, and even more specifically, 0.0150% or less.

[0075] In addition to the alloy composition described above, the steel plate may further include one or more of chromium (Cr), molybdenum (Mo), titanium (Ti), and niobium (Nb).

[0076] Chromium (Cr): 0.01 to 2.0%

[0077] Chromium (Cr) is an element that improves the hardenability of steel, suppresses the formation of ferrite during cooling after finish rolling, and causes the second phase to become a low-temperature transformation phase (e.g., tempered low-temperature bainite and / or tempered lath martensite), thereby improving the yield strength and / or tensile strength.

[0078] When the content of Cr is less than 0.01%, the above-described effect cannot be sufficiently obtained. In addition, when the content of Cr exceeds 2.0%, the hardenability increases excessively, so that the bainite transformation does not occur smoothly in the cooling zone, and therefore the time required to secure the fraction of bainitic ferrite, which is the first phase, increases excessively, which may reduce the elongation. That is, the content of Cr may be 0.01 to 2.0%, specifically 0.01 to 2.00%, more specifically 0.05 to 1.5%, and even more specifically 0.1 to 1.0%.

[0079] Molybdenum (Mo): 0.01 to 2.0%

[0080] Molybdenum (Mo) is an element that significantly enhances the hardenability of steel, thereby enhancing strength through a solid solution strengthening effect. Furthermore, Mo inhibits the formation of ferrite during cooling after finish rolling, and causes the second phase to become a low-temperature transformation phase (e.g., tempered low-temperature bainite and / or tempered lath martensite), thereby enhancing yield strength and / or tensile strength.

[0081] When the content of Mo is less than 0.01%, the above-described effect cannot be sufficiently obtained. In addition, when the content of Mo exceeds 2.0%, the hardenability increases excessively, so that the bainite transformation does not occur smoothly in the cooling zone, and therefore the time required to secure the fraction of bainitic ferrite, which is the first phase, increases excessively, which may reduce the elongation. That is, the content of Mo may be 0.01 to 2.0%, specifically 0.01 to 2.00%, more specifically 0.02 to 1.0%, and even more specifically 0.05 to 0.3%.

[0082] Titanium (Ti): 0.001 to 0.2%

[0083] Titanium (Ti) is an element that forms carbonitrides, which delays the recrystallization behavior of steel during hot rolling, and thus can control the austenite grain size along with the rolling end temperature.

[0084] When the content of Ti is less than 0.001%, the above-described effect cannot be sufficiently obtained. In addition, when the content of Ti exceeds 0.2%, the austenite grain size becomes excessively fine, so that the fraction of the martensite-austenite composite phase (TMA phase) constituting the second phase excessively increases, which may reduce the shape freezing property. That is, the content of Ti may be 0.001 to 2.0%, specifically 0.001 to 2.00%, more specifically 0.01 to 1.5%, and even more specifically 0.03 to 0.08%.

[0085] Niobium (Nb): 0.001 to 0.2%

[0086] Niobium (Nb) can control the austenite grain size by delaying the recrystallization behavior of steel during hot rolling.

[0087] When the content of Nb is less than 0.001%, the above-described effect cannot be sufficiently obtained. In addition, when the content of Nb exceeds 0.2%, the austenite grain size becomes excessively fine, so that the fraction of the martensite-austenite composite phase (TMA phase) constituting the second phase excessively increases, which may reduce the shape freezing property. That is, the content of Nb may be 0.001 to 2.0%, specifically 0.001 to 2.000%, more specifically 0.01 to 1.0%, and even more specifically 0.02 to 0.05%.

[0088] The steel sheet may contain the above components, as well as iron (Fe) as a remaining component. Furthermore, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of manufacturing, their full details are not specifically addressed in this specification.

[0089] The microstructure of the steel sheet may include bainitic ferrite as a first phase and tempered low-temperature bainite, tempered lath martensite, and / or tempered martensite-austenite composite phase (TMA phase) as a second phase.

[0090] Primary phase: 65 to 90 area%

[0091] The first phase may include bainitic ferrite. Here, the first phase may refer to the phase having the largest area fraction within the microstructure, and more specifically, may refer to a phase having an area fraction exceeding 50 area% based on the total fraction of the microstructure.

[0092] After hot rolling, the steel plate is cooled to a temperature below Bs (the onset temperature of bainite transformation) in the first cooling process to avoid equiaxed ferrite transformation, and then slowly cooled in the subsequent second cooling process to allow bainite transformation to proceed. At this time, the bainite transformation occurs in the transformation region of high-temperature bainite, so the formation of bainitic ferrite and the diffusion of carbon into untransformed austenite occur, and carbides may not be formed within the bainitic ferrite.

[0093] Furthermore, although a large number of dislocations exist within the bainitic ferrite generated by shear transformation, the dislocation density can be reduced to an appropriate level through secondary cooling and recovery after coiling. In other words, bainitic ferrite not only improves the elongation of the steel sheet but also reduces the possibility of carbon atoms becoming fixed at dislocations within the matrix after heat treatment, thereby preventing excessive increases in yield strength.

[0094] When the area fraction of the first phase is less than 65%, it may be difficult to sufficiently secure elongation. In addition, when the area fraction of the first phase exceeds 90%, the fraction of low-temperature transformation structures (e.g., tempered low-temperature bainite and / or tempered lath martensite, etc.) that play a role in improving strength is not sufficient, so it may be difficult to secure appropriate yield strength and / or tensile strength (e.g., yield strength of 750 MPa or more and / or tensile strength of 980 MPa or more). That is, the area fraction of the first phase may be 65 to 90%, specifically 65.0 to 90.0%, more specifically 73 to 88%, and even more specifically 70 to 85%.

[0095] Secondary phase: 10-35% by area

[0096] The second phase may include at least one of tempered low-temperature bainite and tempered lath martensite.

[0097] During secondary cooling, carbon diffusion into the untransformed austenite progresses along with the formation of bainitic ferrite, and the untransformed austenite may undergo carbon enrichment. Untransformed austenite may transform into secondary phases, such as low-temperature bainite, lath martensite, and a martensite-austenite composite phase (MA phase), during the additional cooling process after secondary cooling and the cooling process after coiling.

[0098] When bainitic ferrite is formed during secondary cooling, the untransformed austenite distributed within the structure has different sizes depending on the location, and since the carbon content within the austenite varies depending on the austenite size, the type of the second phase may also vary. For example, relatively large untransformed austenite may transform into low-temperature bainite during cooling to the coiling temperature due to its low carbon content. In addition, for example, relatively small austenite may not transform into low-temperature bainite due to its high stability, but may transform into lath martensite at a lower temperature.

[0099] In the process of heating the steel sheet cooled after coiling to 400 to 700℃, the low-temperature bainite and lath martensite are tempered, and the microstructure and physical properties may change. In the steel sheet cooled to room temperature after coiling, the low-temperature bainite and lath martensite commonly contain iron carbides at grain boundaries and within grains within the lath structure, so that the small amount of carbon atoms present within the second phase during tempering is mainly consumed by the additional growth of carbides, and thus may have less effect on the increase in the solid solution carbon concentration within the matrix structure. Therefore, when the second phase of the steel sheet is composed of low-temperature bainite and lath martensite, the increase in yield strength after heat treatment can be suppressed.

[0100] Meanwhile, it is to be noted that the above-mentioned tempered low-temperature bainite and tempered lath martensite commonly contain iron carbides at grain boundaries and within grains within the lath structure, and therefore are managed by combined fraction.

[0101] The second phase may include a tempered martensite-austenite composite phase (TMA phase). Here, the tempered martensite-austenite composite phase may refer to a structure tempered by heat treating a martensite-austenite composite phase (MA phase). As the martensite-austenite composite phase is heat treated, some carbon may diffuse and migrate from the MA phase to the matrix structure or form carbides within the MA phase. In this way, since the amount of superabsorbed solid-solution carbon decreases during tempering, the strength of the tempered martensite-austenite composite phase may decrease compared to before the heat treatment.

[0102] During secondary cooling, small-sized austenite has the highest carbon concentration, so it does not transform into low-temperature bainite or martensite immediately after coiling, but transforms into martensite during the final cooling stage, or fails to transform into martensite during cooling to room temperature and remains as austenite. If the carbon concentration is unevenly distributed within the austenite, it can locally transform into martensite within a single austenite unit structure or exist as retained austenite, thus forming a martensite-austenite composite phase (MA phase).

[0103] At this time, the martensite with high carbon content has a plate type martensite rather than a lath type, and has the characteristic that carbides are not generated inside even during the subsequent slow cooling process due to the low Ms temperature. That is, in the process of heating the steel plate cooled after coiling to 400 to 700℃, the MA phase can be the main cause of a significant increase in the yield strength after heat treatment by releasing a significant amount of carbon atoms into the matrix due to the high level of supersaturated carbon concentration and the absence of internal carbides.

[0104] In addition, during phase transformation during cooling after coiling, a large number of dislocations are generated in the surrounding matrix due to lattice expansion. However, since the MA phase has a low phase transformation temperature, a decrease in dislocation density due to dislocation recovery may not occur. Accordingly, the dislocations near the second phase remaining in the matrix may become the main path for carbon atoms to diffuse from the supersaturated second phase into the matrix during heat treatment.

[0105] In addition, during heat treatment, carbon diffusion from the second phase into the matrix structure can proceed via the interface between the matrix structure and the second phase. That is, since the wider the area of ​​the interface between the matrix structure and the second phase, the easier carbon diffusion is, if the MA phase is finely dispersed, the yield strength after heat treatment can be significantly increased.

[0106] The present invention can prevent excessive increase in yield strength and control the upper limit of yield strength by minimizing excessive formation and microdispersion of MA phase. Specifically, by appropriately controlling the equivalent circle diameter of the second phase, excessive formation and microdispersion of MA phase can be minimized. Here, the equivalent circle diameter of the second phase means the diameter of a circle having the same area as the area of ​​a particle when observing a cross-section perpendicular to the steel plate, and can be measured by analyzing at a magnification of 5,000x using a scanning electron microscope and an image analyzer.

[0107] In the present invention, by controlling the lower limit of the average circle diameter of the second phase, the value of Du, which will be described later, can be derived within an appropriate range. For example, when the average circle diameter of the second phase is less than 2 µm, carbon diffusion into austenite may be facilitated, increasing the probability of remaining in the MA phase. That is, when the average circle diameter of the second phase is excessively small, the area of ​​the interface between the matrix and the second phase becomes large, and carbon diffusion from the MA phase to the matrix becomes facilitated during the heat treatment process, so the yield strength may excessively increase. That is, the average circle diameter of the second phase may be 2 µm or more, specifically 2.0 µm or more, more specifically 2.2 µm or more, and even more specifically 2.5 µm or more.

[0108] Meanwhile, when the second phase is coarse, the probability of it existing as tempered low-temperature bainite and tempered martensite increases, but when the average equivalent circle diameter of the second phase exceeds 20 ㎛, the strength-increasing effect may decrease. That is, the average equivalent circle diameter of the second phase may be 20 ㎛ or less, specifically 20.0 ㎛ or less, more specifically 15 ㎛ or less, and even more specifically 10 ㎛ or less.

[0109] If the area fraction of the second phase is less than 10%, it may be difficult to secure sufficient yield strength and tensile strength. In addition, if the area fraction of the second phase exceeds 35%, the fraction of the matrix structure decreases, making it difficult to secure sufficient elongation. That is, the area fraction of the second phase may be 10 to 35%, specifically 10.0 to 35.0%, more specifically 12 to 30%, and even more specifically 15 to 27%.

[0110] The area fraction of the tempered martensite-austenite composite phase (TMA phase) among the total area fraction of the microstructure may be less than 3%. When the area fraction of the TMA phase is 3% or more, as described above, the yield strength after heat treatment may increase excessively, and the shape freezing property may be inferior during press forming. That is, the area fraction of the TMA phase may be less than 3%, specifically less than 3.0%, more specifically less than 1%, and even more specifically less than 0.5%. In addition, since the lower the area fraction of the TMA phase, the more effective it is in terms of deriving an appropriate level of strength, the lower limit thereof may not be separately limited, but as a non-limiting example, the TMA phase may be 0%, more than 0%, 0.01% or more, 0.05% or more, or 0.1% or more.

[0111] The microstructure of the steel sheet may further include carbides of less than 5 area% of the total area fraction of the microstructure as other structures in addition to the above-described matrix structure and second phase.

[0112] For example, iron carbides can be formed along with carbon diffusion into austenite during bainite transformation. Excessive inclusion of iron carbides can result in a decrease in the fraction of the second phase (low-temperature transformation structure) capable of adequately controlling tensile strength and yield strength, making it difficult to achieve the desired effects of the present invention.

[0113] In addition, for example, when the base steel sheet further contains Ti and / or Nb, the microstructure of the base steel sheet may include alloy carbonitrides. In this case, an additional strengthening effect due to grain refinement can be expected, but coarse carbides in the steel may reduce the toughness of the steel. That is, the area fraction of the carbides (including iron carbides and / or alloy carbonitrides) may be less than 5% of the total area fraction of the microstructure, and more specifically, may be less than 5.0%.

[0114] The plated steel sheet may include a plated layer provided on at least one surface of the base steel sheet. The plated layer may be arranged on at least one surface of the steel sheet to improve corrosion resistance.

[0115] The plating layer may be composed of any one selected from zinc, aluminum, a zinc-based alloy, and an aluminum-based alloy, and specifically may be composed of a zinc-based alloy. For example, when composed of a zinc-based alloy, the plating layer may include at least one selected from aluminum (Al), magnesium (Mg), nickel (Ni), and iron (Fe), with the remainder being zinc (Zn). However, the composition of the plating layer is not limited thereto, and may have a composition that can be commonly used in the relevant technical field.

[0116] The plating layer may be formed by hot-dip plating on at least one surface of the above-mentioned base steel sheet. However, the plating method is not limited thereto, and methods commonly used in the relevant technical field, such as electrolytic plating, vacuum deposition plating, and cladding, may be applied.

[0117] According to one embodiment of the present invention, a coated steel sheet may have a yield strength of 700 to 950 MPa. Conventional steel sheets have problems in that the dimensional accuracy of produced parts is inferior due to springback, assembly is not easy, and part productivity is reduced due to excessive increase in yield strength after heat treatment prior to plating. However, the present invention has the effect of ensuring a high level of yield strength by optimizing the type and size of the matrix microstructure and the fraction of the second phase, while controlling the upper limit thereof to provide excellent shape freezing during press forming.

[0118] A coated steel sheet according to one embodiment of the present invention may have a tensile strength of 980 MPa or more and an elongation of 9% or more. The coated steel sheet may secure excellent fatigue strength and formability by having appropriate yield strength, tensile strength, and / or elongation.

[0119] Hereinafter, a method for manufacturing a plated steel sheet according to one embodiment of the present invention will be described.

[0120] A steel plate can be manufactured by performing a series of processes including reheating, hot rolling, cooling, coiling, heat treatment, and plating a slab having the alloy composition described above.

[0121] Below, each of the above process conditions is described in detail.

[0122] [Slab reheating]

[0123] Before performing hot rolling, a slab satisfying the alloy composition of the above-described base steel sheet may be reheated to undergo homogenization treatment. The heating temperature in the reheating step may be 1100 to 1350°C. If the heating temperature is lower than 1100°C, homogenization of the alloy elements may not be sufficiently performed. In addition, if the heating temperature exceeds 1350°C, oxides may be excessively formed on the surface of the slab, which may deteriorate the surface quality of the plated steel sheet. That is, the heating temperature may be 1100 to 1350°C, more specifically, 1150 to 1300°C, and even more specifically, 1180 to 1250°C.

[0124] [Hot rolling]

[0125] A hot rolled steel sheet can be obtained by hot rolling a heated slab.

[0126] The grain size of austenite after hot rolling is affected by the alloy composition, rolling end temperature, and / or reduction, which may affect the formation behavior of ferrite and bainite in the subsequent cooling process and the composition of the final microstructure. In addition, the fraction of the martensite-austenite composite phase (MA phase), which is one of the factors determining the tensile strength and yield strength, is affected by the grain size of austenite after hot rolling, and the smaller the average equivalent circle diameter of the second phase, the higher the probability that it exists as the MA phase. Although the average equivalent circle diameter of the second phase is affected by the nucleation behavior during bainite transformation, due to the nature of shear transformation, the average equivalent circle diameter of the second phase cannot increase compared to the size of the austenite before transformation. Therefore, by controlling the grain size of austenite after hot rolling, the average equivalent circle diameter of the second phase can be controlled more effectively.

[0127] Accordingly, the inventors of the present invention were able to manufacture a plated steel sheet that appropriately controls the final microstructure and secures the desired tensile strength and yield strength by controlling the rolling end temperature, the content of alloy elements included in the slab, the effective grain size of austenite that can be calculated from the rolling end temperature, and the total rolling reduction amount of the final two passes.

[0128] In the step of obtaining the above hot-rolled steel sheet, the rolling finish temperature (FDT) may be 800 to 1150°C. If the FDT is less than 800°C, the rolling load may increase excessively, which may result in reduced workability or poor anisotropy. In addition, if the FDT exceeds 1150°C, oxides may be excessively formed on the surface of the steel sheet after rolling, which may not be effectively removed even after pickling, thereby deteriorating the surface quality of the steel sheet. That is, the FDT may be 800 to 1150°C, more specifically 820 to 1100°C, and even more specifically 850 to 950°C.

[0129] In the step of obtaining the above hot-rolled steel sheet, the heated slab can be hot-rolled so that the value of the following relationship 1 satisfies 5 to 15 in the above FDT range. When the value of Du below is 5 to 15, the area fraction of the TMA phase is controlled to be less than 3%, so that a yield strength of 950 MPa or less can be obtained. For example, when the rolling end temperature is excessively low and the value of Du below is less than 5 (i.e., when the grain size of the austenite is excessively fine), the TMA phase is excessively formed, which facilitates carbon diffusion into the matrix during the heat treatment process, so that the yield strength can significantly increase. In addition, when the value of Du below exceeds 15, the grain size of the austenite is excessively coarse, which may cause a problem of reduced elongation due to delayed bainite transformation.

[0130] [Relationship 1]

[0131] Du = [(FDT + 7.4×[C] - 24.7×[Si] - 4.7×[Mn] - 3.9×[Cr] - 5.2[Mo] - 480×[Ti] - 1200×[Nb])×0.049] - 34.2

[0132] (Here, Du is an indicator of the effective grain size of austenite immediately before the first cooling after hot rolling, FDT means the temperature (℃) of the hot-rolled sheet immediately after hot rolling, and each element represents the content (weight%).)

[0133] In the step of obtaining the hot-rolled steel sheet, the total reduction in the final two passes may be 10 to 40%. When hot rolling is performed by multi-stage rolling, the rolling load can be reduced and the thickness can be precisely controlled. If the total reduction in the final two passes is less than 10%, the reductions of the previous passes may be formed somewhat high, resulting in shape defects, or the temperature of the steel sheet may drop rapidly, resulting in reduced workability. In addition, if the total reduction in the final two passes exceeds 40%, the rolling load of the final two passes may increase excessively, resulting in a problem of reduced workability. That is, the total reduction in the final two passes may be 10 to 40%.

[0134] [Cooling and winding stage]

[0135] The hot-rolled steel sheet can be cooled in stages. Specifically, the hot-rolled steel sheet can be first cooled at a cooling rate of 30°C / s or more to a temperature lower than Bs, and secondarily cooled at a cooling rate of 20°C / s or less to a temperature higher than (Bs+Ms) / 2. Here, Bs is the temperature (°C) at which bainite formation begins, and can be calculated by the formula {Bs = 830-320×[C]-90×[Mn]-35×[Si]-70×[Cr]-120×[Mo]} (wherein, each element represents the content (wt%)). In addition, Ms is the temperature (℃) at which martensite formation begins by cooling, and can be calculated by the formula {Ms = 550-330×[C]-41×[Mn]-20×[Si]-20×[Cr]-10×[Mo]+30×[Al]} (where each element represents the content (weight %)).

[0136] Through the primary cooling step, the steel plate can be cooled to a temperature below Bs, thereby securing the first phase (e.g., bainitic ferrite) of the steel plate at 65 to 90 area%. Thereafter, through the secondary cooling step, the growth of bainitic ferrite and diffusion into untransformed austenite can occur.

[0137] In the primary cooling stage, the primary cooling rate may be 30°C / s or higher. This prevents phase transformation of the granular ferrite formed during cooling, thereby preventing a decrease in strength. The upper limit of the primary cooling rate is not particularly limited; however, rapid cooling of the steel plate may cause distortion of the plate shape, so the primary cooling rate may be 200°C / s or lower.

[0138] In the secondary cooling step, if the secondary cooling end temperature is lower than (Bs+Ms) / 2, the problem of poor elongation may occur due to the inability to smoothly recover dislocations within the bainitic ferrite during transformation. That is, the secondary cooling end temperature may be higher than (Bs+Ms) / 2, and specifically, higher than 500°C.

[0139] The secondary cooling step can be performed for a time (ts) that satisfies the following relational expression 2. k(T) in the following relational expression 3 is an indicator of the growth rate of bainitic ferrite, which can be affected by the alloying components of the steel, the phase transformation temperature, and the austenite grain size after hot rolling. When the following relational expression 2, which represents the relationship between k(T) and the maintenance time of the secondary cooling, is satisfied, the area fraction of bainitic ferrite can be secured at 65 to 90%.

[0140] [Relationship 2]

[0141] 0.65 ≤ 1-exp[-k(T) × (ts) 2 ] ≤ 0.9

[0142] (Here, k(T) represents a value defined by the following relation 3.)

[0143] [Relationship 3]

[0144]

[0145] (Here, T1 is the primary cooling end temperature, T2 is the secondary cooling end temperature, and each element represents the content (weight%).)

[0146] If the value of the above relational expression 2 is less than 0.65, the first phase (bainitic ferrite) may not be sufficiently formed, making it difficult to secure sufficient elongation. In addition, if the value of the following relational expression 2 exceeds 0.9, the fraction of the first phase (bainitic ferrite) is excessive, resulting in excellent elongation, but it may be difficult to secure sufficient yield strength and tensile strength because a sufficient fraction of the second phase is not secured.

[0147] In the secondary cooling step, the secondary cooling rate may be 20°C / s or less. While performing the secondary cooling, the temperature may increase due to transformation exotherm caused by the bainite phase transformation, but excessive exotherm may cause an excessive decrease in dislocation density. That is, to prevent the temperature of the steel plate from increasing due to transformation exotherm, the secondary cooling may be maintained isothermally or performed at a cooling rate of 20°C / s or less. If the secondary cooling rate exceeds 20°C / s, the phase transformation may proceed actively, causing the shape of the plate to be distorted.

[0148] After the secondary cooling is completed, the third cooling is performed at a cooling rate of 30℃ / s or more to BS~Ms-40℃, and the thirdly cooled steel plate can be coiled.

[0149] In the third cooling step, the third cooling end temperature can be BS to Ms-40℃. As the second cooling progresses, austenite becomes stabilized, and the Ms temperature substantially decreases. That is, in the third cooling step, the lower limit of the third cooling end temperature can be applied lower than the Ms temperature, and when bainitic ferrite as the first phase progresses by 65% ​​or more, a temperature higher than Ms-40℃ can be applied.

[0150] In the third cooling step, the third cooling rate may be 30°C / s or more. During the third cooling, low-temperature bainite transformation progresses, and depending on the carbon content within the austenite, some of it may transform into martensite even after coiling. Since the hardenability of austenite increases during the third cooling, the third cooling rate may be 30°C / s or more, in which case the formation of additional high-temperature bainite can be avoided. There is no particular limitation on the upper limit of the third cooling rate, but in order to prevent distortion of the plate shape, the third cooling rate may be 100°C / s or less.

[0151] The coiling step may be performed at a temperature of 350°C or higher. If the coiling temperature is lower than 350°C, carbide growth within the second phase, low-temperature bainite and lath martensite, is not smooth, and thus the concentration of carbon atoms diffusing into the matrix structure in the subsequent heat treatment step increases, which may result in reduced shape freezing properties. In other words, the lower limit of the coiling temperature may be 350°C.

[0152] [Final cooling]

[0153] After completing the cooling and coiling process as described above, final cooling can be performed to obtain the target hot-rolled steel sheet. At this time, final cooling can be completed by performing air cooling.

[0154] [Acid washing and heat treatment steps]

[0155] The above cooled hot-rolled steel sheet can be pickled and heat-treated.

[0156] Scale formed on the surface of a steel plate can be removed through pickling treatment. Specifically, the pickling treatment can be performed in a hydrochloric acid bath. For example, the hydrochloric acid concentration of the hydrochloric acid bath can be 10 to 30 vol.%. If the hydrochloric acid concentration is less than 10 vol.%, the above-described effects cannot be sufficiently achieved. Furthermore, if the hydrochloric acid concentration exceeds 30 vol.%, the problem of the surface layer of the steel plate being corroded by the hydrochloric acid may occur.

[0157] For example, the pickling treatment can be performed for 10 to 500 seconds. If the pickling time is less than 10 seconds, the scale on the steel plate may not be completely removed. Furthermore, if the pickling time exceeds 500 seconds, the surface layer of the steel plate may be corroded by hydrochloric acid.

[0158] Through the heat treatment step, the second phase (low-temperature bainite, lath martensite, and / or MA phase) generated in the steel sheet can be tempered. As the low-temperature bainite and lath martensite are tempered, the carbides generated after coiling grow, the carbon concentration existing in the solid solution state decreases, and the dislocation density also decreases, thereby reducing the effect of contributing to the strengthening effect. In addition, as the MA phase, which did not have carbides inside, is tempered in the heat treatment step, the carbon atoms existing in the solid solution state inside diffuse and move to the first phase, bainitic ferrite, and are fixed to the dislocations existing in the matrix, which can significantly increase the yield strength of the plated steel sheet. In the present invention, by minimizing the excessive generation and microdispersion of the MA phase, an excessive increase in the yield strength can be prevented and the upper limit of the yield strength can be controlled.

[0159] The heat treatment step may be performed at 400 to 700°C. If the heat treatment temperature is less than 400°C, when the base steel sheet is immersed in a zinc-based plating bath for subsequent plating, the interfacial reaction between the liquid zinc and the base steel sheet may not be sufficient to form an alloying-inhibiting layer (e.g., Fe-Al-based compound, etc.), and the plating wettability may be poor, which may deteriorate the plating quality and plating adhesion. In addition, if the heat treatment temperature exceeds 700°C, the second phase formed in the hot-rolled steel sheet may be excessively tempered, resulting in a significant decrease in strength. That is, the heat treatment temperature may be 400 to 700°C, more specifically, 420 to 650°C, and even more specifically, 450 to 600°C.

[0160] The heat treatment step may be performed for 2 to 200 seconds. If the heating time is less than 2 seconds, temperature uniformity in the thickness and width directions of the steel plate may not be sufficiently secured, which may result in deterioration of surface quality and material deviation. Furthermore, if the heating time exceeds 200 seconds, excessive fuel costs may occur. In other words, the heating time may be 2 to 200 seconds.

[0161] The above heat treatment step can be performed in a reducing atmosphere. Specifically, the reducing atmosphere can be maintained by adding 3 to 20 vol.% hydrogen to the nitrogen gas in the annealing furnace. If the hydrogen concentration in the annealing furnace is less than 3 vol.%, the reducing capacity may be insufficient, resulting in excessive oxide formation on the surface of the steel sheet, and deterioration of surface quality and plating adhesion. In addition, the upper limit of the hydrogen concentration in the annealing furnace is not specifically limited, but considering the increased cost and risk of explosion, the hydrogen concentration may be 20 vol.% or less.

[0162] [Plating step]

[0163] A heat-treated steel sheet can be plated to form a plating layer on one surface of the steel sheet. Specifically, in the plating step, the heat-treated steel sheet can be zinc-plated by immersing it in a zinc-based plating bath.

[0164] In the above plating step, the plating bath inlet temperature of the steel sheet may be 420 to 500°C. If the inlet temperature of the steel sheet is less than 420°C, wettability may not be sufficiently secured at the contact interface between the steel sheet and the liquid zinc. In addition, if the plating bath inlet temperature of the steel sheet exceeds 500°C, the reaction between the steel sheet and the liquid zinc may be excessive, causing a zeta phase, which is an Fe-Zn alloy phase, to be generated at the interface, thereby reducing the adhesion of the plating layer, or causing excessive dissolution of Fe from the steel sheet into the plating bath, which may cause problems such as dross generation in the plating bath. That is, the plating bath inlet temperature of the steel sheet may be 420 to 500°C, more specifically, 425 to 495°C, and even more specifically, 430 to 490°C.

[0165] In the above plating step, the Al concentration in the Zn-based plating bath may be 0.10 to 0.30 wt%. By appropriately controlling the Al concentration, the wettability of the plating layer and the fluidity of the plating bath can be secured. If the Al concentration is less than 0.10 wt%, the above-described effect cannot be sufficiently obtained. In addition, if the Al concentration exceeds 0.30 wt%, a problem may occur in which an alloying inhibition layer is formed excessively thickly at the interface of the steel sheet. That is, the Al concentration may be 0.10 to 0.30 wt%, more specifically 0.11 to 0.29 wt%, and even more specifically 0.12 to 0.28 wt%.

[0166] For example, if the steel sheet to be manufactured is a GA (galvanized alloy) steel sheet, the concentration of Al may be 0.10 to 0.20 wt%. Also, for example, if the steel sheet to be manufactured is a GI (galvanized alloy) steel sheet, the concentration of Al may be 0.20 to 0.30 wt%. By appropriately adjusting the concentration of Al depending on the steel sheet to be manufactured, the dross formation in the plating bath can be maintained at an appropriate level, and performance such as the plating surface quality can be secured.

[0167] For example, a manufacturing method according to an embodiment of the present invention may further include a step of alloying heat treating the plated steel sheet. Specifically, the alloying (GA) temperature may be 440 to 580°C. If the alloying temperature is less than 440°C, the diffusion amount of Fe is small, so the degree of alloying is insufficient, and thus the plating properties may not be good. In addition, if the alloying temperature exceeds 580°C, a powdering problem may occur due to excessive alloying. That is, the alloying temperature may be 440 to 580°C, more specifically 450 to 570°C, and even more specifically 460 to 560°C.

[0168] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0169] (Example)

[0170] A slab having the alloy composition shown in Table 1 below (wherein the remaining components of the alloy composition are Fe and unavoidable impurities) was prepared, and the slab was manufactured into a hot-rolled steel sheet according to the manufacturing conditions shown in Table 2. At this time, the reheating temperature of the slab was 1220℃, the thickness of the steel sheet after hot rolling was 3.2 mm, and the total reduction amount of the final 2 passes during the finish rolling was applied equally at 25%. In addition, the first cooling rate was applied equally at 70℃ / s, the second cooling rate was applied at 3 to 7℃ / s, and the third cooling rate was applied at 45℃ / s. In addition, the cooled steel sheet was pickled in a 20% hydrochloric acid solution for 100 seconds to remove scale, and then annealed in a reducing atmosphere containing 10% hydrogen and plated by immersion in a molten zinc plating solution.

[0171] Steel grade CSiMnPSAlCrMoTiNbN Steel grade 10.110.32.10.0120.0020.0250.20000.004 Steel grade 20.070.22.10.0120.0030.02300000.004 Steel grade 30.180.031.80.0120.0030.01800.1000.0 03 Steel grade 40.090.52.50.0110.0030.02500000.004 Steel grade 50.110.22.10.0120.0020.022000.0500.004 Steel grade 60.150.031.80.0120.0020.0280.2000.030.003 Steel grade 70.220.031 .80.0120.0030.0250.50000.004Steel grade 80.070.51.80.0120.0020.0250.80000.004Steel grade 90.080.032.20.0120.0020.300000.004Steel grade 100.110.82.10.0120.0020.02500 000.004 Steel Grade 110.0700.80.0120.0020.02500000.004 Steel Grade 120.110.32.10.00110.0030.021000.080.030.003 Steel Grade 130.180.031.80.0120.0030.0180.50.100.070.003

[0172] Classification Steel grade Hot rolling and cooling heat treatment FDT Relationship 1 Bs (℃) Ms (℃) T1 (℃) T2 (℃) Ts (s) Relationship 2 T3 (℃) Temperature (℃) Time (s) Invention example 1 Steel grade 1 9009.158141854052050.7842052025 Invention example 2 Steel grade 2 92010.261243755053040.7942052025 Invention example 3 Steel grade 3 92010.55974165605405.5 0.7543052025 Invention Example 4 Steel Type 49209.755940952050060.8440052025 Invention Example 5 Steel Type 594010.059942454052040.7740052025 Invention Example 6 Steel Type 69409.760542354052040.8439052025 Invention Example 7 Steel Type 793010.956239452050050.723905 2025 Invention Example 8 Steel Type 89 209.75 7 24 28 5 4 0 5 20 6 0.78 4 20 5 20 25 Invention Example 9 Steel Type 99 2010.46 0.5 4 4 25 4 0 5 20 3.5 0.73 4 4 0 5 20 25 Comparative Example 1 Steel Type 109 209.5 5 7 8 4 1 2 5 4 0 5 20 5 0.72 4 20 5 20 25 Comparative Example 2 Steel Type 119 2010.77 36 4 9 5 6 5 0 6 3 0 3.5 0.89 4 30 5 20 25Comparative Example 3 Steel Type 1 28804.559542254052030.8142052025Comparative Example 4 Steel Type 1 38804.356240654052050.8242052025Comparative Example 5 Steel Type 292010.261243755053050.9242052025Comparative Example 6 Steel Type 1 9009.158141854052030.4242052025Comparative Example 7 Steel Type 19009.158141854052050.7842072025- FDT: Rolling end temperature (℃)- Relationship 1: Du = [(FDT + 7.4×[C] - 24.7×[Si] - 4.7×[Mn] - 3.9×[Cr] - 5.2[Mo] - 480×[Ti] - 1200×[Nb])×0.049] - 34.2(Here, Du is an index indicating the effective grain size of austenite immediately before the first cooling after hot rolling, FDT means the rolling end temperature (℃), and each element represents the content (weight %). - BS: The temperature (℃) at which bainite formation starts, which can be calculated by the formula {Bs = 830-320×[C]-90×[Mn]-35×[Si]-70×[Cr]-120×[Mo]} (each element represents the content (weight %).) - MS: The temperature (℃) at which martensite formation starts by cooling, which can be calculated by the formula {Ms = 550-330×[C]-41×[Mn]-20×[Si]-20×[Cr]-10×[Mo]+30×[Al]} (each element represents the content (weight %).) - T1: The temperature at which the first cooling ends - T2: The temperature at which the second cooling ends - Ts: Secondary cooling holding time - Relationship 2: 1-exp[-k(T) × (ts). 2 ](here , T1 is the first cooling end temperature, T2 is the second cooling end temperature, and each element represents the content (weight %) - T3: the third cooling end temperature

[0173] For the above-mentioned manufactured invention examples 1 to 8 and comparative examples 1 to 7, the fraction of microstructure, the average equivalent circle diameter of the second phase, the yield strength (YS), tensile strength (TS) and elongation (El) before and after heat treatment were measured, and the surface quality after heat treatment was evaluated, which is then shown in Table 3 below.

[0174] The fraction of microstructure and the average equivalent circle diameter of the second phase were measured by etching the specimen using the nital etching method and analyzing it using a scanning electron microscope and an image analyzer at a magnification of 5,000 times.

[0175] Yield strength, tensile strength, and elongation before and after heat treatment were measured by taking JIS-5 standard test specimens in a direction perpendicular to the rolling direction. Here, yield strength and elongation represent 0.2% offset yield strength and fracture elongation, respectively.

[0176] Surface quality was evaluated by observing the surface of the plated steel sheet. Specifically, if the area of ​​the unplated area was less than 3% of the total immersion area, the plating quality was judged to be good and was marked with ○. If the area fraction of the unplated area was 3% or more, the plating quality was judged to be poor and was marked with ×.

[0177] Classification Base Organization 2nd phase Before heat treatment Material After heat treatment BFTLB+TLMTMA Average circle equivalent diameter (㎛) YPTSEl YPTSEL Surface quality Invention example 177.821.90.33.6731108112883103212○Invention example 279.520.30.24.1717105913864101312○Invention example 374.924.90.24.2694105911836100812○Invention example 484.315.50.23.97681093139241042 14○Invention Example 577.322.40.24.0738109812889105012○Invention Example 684.515.30.23.9725105714872100714○Invention Example 771.528.00.44.474411379899108410○Invention Example 877.222.50.33.973810871288910361 2○Invention Example 974.025.80.24.1705107011849101912○Comparative Example 172.422.94.73.87731151141001109914×Comparative Example 289.210.70.14.36189071574486715○Comparative Example 381.611.27.21.877211051310381055 13○Comparative Example 481.312.16.51.77551090131001104312○Comparative Example 591.68.20.14.17209981586695514○Comparative Example 641.857.40.83.687212648105912068○Comparative Example 778.421.20.23.271510731252672618○- BF: Bainitic ferrite- TLB: Tempered low-temperature bainite- TLM: Tempered lath martensite- TMA phase: Tempered martensite-austenite composite phase

[0178] As shown in Tables 1 to 3 above, it was confirmed that Invention Examples 1 to 9, which satisfy both the alloy composition and manufacturing conditions proposed in the present invention, have a microstructure that includes bainitic ferrite as a matrix structure with an area fraction of 65 to 90%, and a tempered low-temperature bainite / tempered lath martensite / tempered martensite-austenite composite phase (TMA phase) as a second phase with an area fraction of 10 to 35%. In particular, it can be seen that Invention Examples 1 to 9 can secure the desired strength and shape freezing property because they satisfy an area fraction of the TMA phase of less than 3%.

[0179] That is, it can be seen that invention examples 1 to 9 satisfy all of the alloy composition and manufacturing conditions proposed in the present invention, and thereby secure an appropriate matrix structure and the type and fraction of the second phase, thereby securing a yield strength of 700 to 950 MPa, a tensile strength of 980 MPa or more, and an elongation of 9% or more.

[0180] Meanwhile, Comparative Example 1 contained an extremely excessive amount of Si, so that the unplated area was prominent, resulting in poor surface quality, and it was found that the TMA phase was also formed excessively, resulting in an excessively high yield strength.

[0181] In addition, it was confirmed that Comparative Example 2 contained a very small amount of Mn, and thus the tensile strength was measured to be low at less than 980 MPa.

[0182] In addition, considering that Comparative Examples 3 and 4 were calculated to have a Du value of less than 5, as shown in Equation 1, it was found that the rolling end temperature during hot rolling was somewhat low, so that the TMA phase was excessively formed, and as a result, the yield strength was excessively high. In Comparative Examples 3 and 4, the Du was low, so that the austenite grain size was formed finely, and therefore the average circle diameter of the second phase was also measured to be low.

[0183] In addition, considering that Comparative Example 5 was calculated to have a value exceeding 0.9 in relational expression 2, it was found that the secondary cooling time was excessive, and therefore a sufficient fraction of the second phase was not secured, resulting in an excessively high yield strength.

[0184] In addition, considering that the value of relational expression 2 in Comparative Example 6 was calculated to be less than 0.65, it was found that due to insufficient secondary cooling time, a sufficient fraction of the matrix structure was not secured, resulting in a low elongation.

[0185] In addition, it was found that Comparative Example 7 had a very high heat treatment temperature, so the second phase was excessively tempered, and thus the tensile strength and yield strength were significantly reduced.

[0186] Meanwhile, Fig. 1 is a photograph of the appearance of the plated steel sheets of Invention Example 1 and Comparative Example 1. Specifically, Fig. 1 (a) is a photograph of Invention Example 1, and it can be seen that no unplated area occurred in the entire area immersed in the plating bath. Meanwhile, Fig. 1 (b) is a photograph of Comparative Example 1, and it can be seen that Si was included in an extremely excessive amount, forming numerous unplated areas and reducing plating properties.

[0187] In addition, Fig. 2 shows photographs of the microstructures of Invention Example 1 and Comparative Example 3 observed using a scanning electron microscope. Specifically, Fig. 2 (a) is a photograph of Invention Example 1, and it can be seen that the matrix structure and second phase targeted by the present invention were appropriately formed. On the other hand, Fig. 2 (b) is a photograph of Comparative Example 3, and it can be seen that the TMA phase was excessively formed, and as a result, the yield strength was measured to be very high.

[0188] The above embodiments are merely examples and the present invention is not limited thereto. Anything that has substantially the same configuration and achieves the same operational effects as the technical concepts described in the claims of the present invention is included within the technical scope of the present invention.

Claims

1. Steel plate; and Including a plating layer provided on at least one surface of the above steel plate, The above steel plate contains, in weight %, carbon (C): 0.05 to 0.25%, silicon (Si): 0.001 to 0.6%, manganese (Mn): 1.2 to 3.0%, aluminum (Al): 0.001 to 1.0%, phosphorus (P): 0.0001 to 0.05%, sulfur (S): 0.0001 to 0.05%, nitrogen (N): 0.0001 to 0.02%, the remainder being iron and other unavoidable impurities. The microstructure of the above steel plate includes 65 to 90 area% of the first phase including bainitic ferrite, The microstructure of the above steel plate includes 10 to 35 area% of a second phase including at least one of tempered low-temperature bainite and tempered lath martensite and a tempered martensite-austenite composite phase (TMA phase). A plated steel sheet having an area fraction of the tempered martensite-austenite composite phase of less than 3 area% among the total area fraction of the above microstructure.

2. In paragraph 1, A plated steel sheet having an average equivalent diameter of the second phase of 2 to 20 ㎛.

3. In paragraph 1, A coated steel sheet wherein the microstructure of the above-mentioned steel sheet further includes carbides of less than 5 area% of the total area fraction of the above-mentioned microstructure.

4. In paragraph 1, The above-mentioned steel sheet is a plated steel sheet further containing, in weight %, at least one of chromium (Cr): 0.01 to 2.0%, molybdenum (Mo): 0.01 to 2.0%, titanium (Ti): 0.001 to 0.2%, and niobium (Nb): 0.001 to 0.2%.

5. In paragraph 1, The above-mentioned galvanized steel sheet is a galvanized steel sheet having a yield strength of 700 to 950 MPa.

6. In paragraph 1, The above-mentioned galvanized steel sheet is a galvanized steel sheet having a tensile strength of 980 MPa or more and an elongation of 9% or more.

7. A step of reheating the slab containing carbon (C): 0.05 to 0.25%, silicon (Si): 0.001 to 0.6%, manganese (Mn): 1.2 to 3.0%, aluminum (Al): 0.001 to 1.0%, phosphorus (P): 0.0001 to 0.05%, sulfur (S): 0.0001 to 0.05%, nitrogen (N): 0.0001 to 0.02%, the remainder iron and other unavoidable impurities at 1100 to 1350°C; A step of hot rolling the above reheated slab to obtain a hot-rolled steel sheet; A step of first cooling the hot-rolled steel sheet to a temperature below Bs at a cooling rate of 30°C / s or more; A step of secondarily cooling the above-mentioned first-cooled steel plate to a temperature of (Bs+Ms) / 2 or higher at a cooling rate of 20℃ / s or less; A step of cooling the secondarily cooled steel plate a third time from BS to Ms-40℃ at a cooling rate of 30℃ / s or more; A step of coiling the third cooled steel plate; A step of cooling the above-mentioned rolled steel plate to room temperature; A step of pickling the cooled steel plate and heat treating it at 400 to 700°C; and Step of plating the above heat-treated steel plate A method for manufacturing a plated steel sheet comprising:

8. In paragraph 7, A method for manufacturing a coated steel sheet, wherein in the step of obtaining the hot-rolled steel sheet, the reheated slab is hot-rolled so that the value of the following relational expression 1 satisfies 5 to 15 in a rolling finish temperature (FDT) range of 800 to 1150°C. [Relationship 1] Du = [(FDT + 7.4×[C] - 24.7×[Si] - 4.7×[Mn] - 3.9×[Cr] - 5.2[Mo] - 480×[Ti] - 1200×[Nb])×0.049] - 34.2 (Here, Du is an indicator of the effective grain size of austenite immediately before the first cooling after hot rolling, FDT indicates the rolling end temperature (℃), and each element indicates the content (weight%).) 9. In paragraph 7, A method for manufacturing a plated steel sheet in which, in the step of obtaining the above hot-rolled steel sheet, the total reduction amount of the final two passes is 10 to 40%.

10. In paragraph 7, A method for manufacturing a plated steel sheet, wherein the above secondary cooling step is performed for a time (ts) that satisfies the following relational expression 2. [Relationship 2] 0.65 ≤ 1-exp[-k(T) × (ts) 2 ] ≤ 0.9 (Here, k(T) represents a value defined by the following relation 3.) [Relationship 3] (Here, T1 represents the end temperature of the first cooling, T2 represents the end temperature of the second cooling, and each element represents the content (weight%).) 11. In paragraph 7, A method for manufacturing a plated steel sheet, wherein the heat treatment step is performed for 2 to 200 seconds.

12. In paragraph 7, A method for manufacturing a galvanized steel sheet, wherein in the above-mentioned plating step, the heat-treated steel sheet is immersed in a zinc-based plating bath to perform zinc plating.

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