Steel sheet and manufacturing method therefor

Abstract

The present invention provides a steel sheet suitable for a structural member of a vehicle, which has high strength of 590 MPa or higher in tensile strength, along with excellent work hardening rate and hole expandability, and also provides a method for manufacturing the steel sheet.

Description

Steel plate and method for manufacturing the same

[0001] The present invention relates to a steel plate suitable as a material for automobile structural members, and more specifically, to a high-strength composite structure steel plate having a tensile strength of 590 MPa or higher and a method for manufacturing the same.

[0002] Currently, environmental and safety regulations in the automotive industry are becoming increasingly stringent, and fuel efficiency regulations for automobiles are also becoming increasingly stricter as carbon dioxide emission regulations are intensified.

[0003] According to the Insurance Institute for Highway Safety in the United States, crash safety regulations for occupant protection have been strengthened, and in particular, since 2013, they have required harsh crash performance such as a 25% small overlap.

[0004] Currently, the only solution to environmental and safety issues is to reduce the weight of automobiles. To achieve this, high-strength steel is necessary, and the application of high-strength steel also requires high formability.

[0005] Methods for strengthening steel include solid solution strengthening, precipitation strengthening, grain refinement strengthening, and transformation strengthening. However, among the aforementioned methods, solid solution strengthening and grain refinement strengthening have the disadvantage of making it difficult to produce high-strength steel with a tensile strength of 590 MPa or higher.

[0006] High-strength steel through precipitation strengthening is a technology that secures strength by strengthening the steel through the precipitation of forming elements such as carbides and nitrides such as copper (Cu), niobium (Nb), titanium (Ti), and vanadium (V), or by refining the grains by suppressing grain growth due to fine precipitates. This technology has the advantage of easily achieving high strength at low manufacturing costs. However, because the recrystallization temperature rises rapidly due to fine precipitates, it has the disadvantage of requiring high-temperature annealing to induce sufficient recrystallization to secure the ductility of the steel. In addition, precipitation-strengthened steel precipitates precipitates such as carbides and nitrides in the ferrite matrix, so it has limitations in achieving high strengths exceeding 600 MPa.

[0007] Meanwhile, transformation-strengthened high-strength steels have been developed, including dual-phase steel (DP steel) with ferrite-martensite dual structure formed in a ferrite matrix, transformation-induced plasticity steel (TRIP steel) utilizing the transformation-induced plasticity of retained austenite, and complex-phase steel (CP steel) composed of ferrite and hard bainite or martensite structures.

[0008] Recently, there has been a growing demand for steel plates with higher strength for automobiles to improve fuel efficiency and durability, while high-strength steel plates with a minimum tensile strength of 590 MPa or more are increasingly used as body structures or reinforcing materials to ensure collision safety and protect passengers.

[0009] However, as strength gradually increases, the manufacturing of complex parts is reaching its limit due to the occurrence of processing defects such as cracks or wrinkles during the process of forming automobile parts (e.g., press forming).

[0010] Accordingly, in terms of improving the workability of high-strength steels, if we can improve workability indices such as work hardening rate, elongation, and hole expandability while maintaining the low yield ratio of DP steel, the most widely used transformation-hardened high-strength steel, we can prevent processing defects that occur during the forming process into parts. Furthermore, such high-strength steels can expand their application to complex parts.

[0011] As a related prior art, Patent Document 1 can be cited. This technology discloses a steel sheet composed of a composite structure mainly composed of martensite, and proposes a method of dispersing fine copper precipitate particles with a diameter of 1 to 100 nm within the structure to improve workability. However, in the case of this technology, since Cu is excessively added at 2 to 5% to precipitate fine Cu particles, there is a concern that red-hot embrittlement due to Cu may occur, and there is also a problem that manufacturing costs excessively increase.

[0012] As another technology, Patent Document 2 discloses a steel sheet having a matrix structure of ferrite and a pearlite content of 2 to 10 area%, and in which carbon-nitride forming elements such as Nb, Ti, and V are added for precipitation strengthening, thereby improving strength through precipitation strengthening and grain refinement. However, the steel sheet according to this document has good hole expandability, but has limitations in improving tensile strength, and has high yield strength and low ductility, which may cause cracks to occur during press forming.

[0013] As another technology, Patent Document 3 discloses a technology for achieving high strength and high ductility by utilizing a tempered martensite phase, specifically, a cold-rolled steel sheet with excellent plate shape after continuous annealing. However, this technology has concerns about poor weldability due to the high carbon content of the steel (over 0.2%), and the possibility of in-furnace dent defects due to the high Si content.

[0014] (Patent Document 1) Japanese Patent Publication No. 2005-264176

[0015] (Patent Document 2) Korean Patent Publication No. 10-2015-0073844

[0016] (Patent Document 3) Japanese Patent Publication No. 2010-090432

[0017] One aspect of the present invention is to provide a steel sheet suitable for use as an automobile structural member, which steel sheet has a high tensile strength of 590 MPa or higher, as well as excellent work hardening rate and hole expandability. A method for manufacturing the steel sheet is also provided.

[0018] In addition, the technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0019] According to one aspect of the present invention, a steel sheet is provided that includes, in wt%, carbon (C): 0.100 to 0.150%, silicon (Si): 0.30 to 1.00%, manganese (Mn): 1.20 to 1.90%, phosphorus (P): 0.100% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), aluminum (sol.Al): 0.500% or less (excluding 0%), chromium (Cr): 0.500% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), the remainder being iron and other unavoidable impurities.

[0020] In one embodiment of the present invention, the steel plate can satisfy the following relationship 1.

[0021] [Relationship 1]

[0022] ((3×Si)+Al+(10×C)) / (Si+Mn+(3×Cr)) ≥ 1.00

[0023] (In equation 1, each element represents the weight content, and if not added, 0 is substituted.)

[0024] In one embodiment of the present invention, the steel plate may have a microstructure composed of an area fraction of 60 to 80% ferrite, 10 to 25% bainite, 3 to 8% retained austenite, and a remainder martensite.

[0025] In one embodiment of the present invention, the martensite phase may be included in an amount of less than 11%.

[0026] Steel plates having such alloy compositions and microstructures can have low yield ratios and high ductility, and at the same time, can have excellent work hardening properties and hole expandability.

[0027] A steel plate according to one embodiment of the present invention may have a yield ratio of 0.7 or less, and a relationship among a work hardening rate (n), an elongation (El), a hole expandability (HER), and a yield ratio (YR) in a 4 to 6% strain range may satisfy the following relational expression 2.

[0028] [Relationship 2]

[0029] (n×El×HER) / YR ≥ 400.0

[0030] (In equation 2, the units of each property are not considered.)

[0031] According to another aspect of the present invention, in manufacturing a steel plate, the method comprises the steps of: preparing a steel slab; heating the steel slab to a temperature range of 1100 to 1300°C; finishing hot-rolling the heated steel slab at a temperature higher than the Ar3 transformation point to obtain a hot-rolled steel plate; coiling the hot-rolled steel plate at a temperature range of 400 to 700°C; cold-rolling the coiled hot-rolled steel plate at a total reduction ratio of 40 to 80% to obtain a cold-rolled steel plate; continuously annealing the cold-rolled steel plate in a two-phase temperature range of 760 to 820°C; first cooling the continuously annealed cold-rolled steel plate to a temperature range of 630 to 690°C at a cooling rate of 10°C / s or less (excluding 0°C / s); The method may include a step of secondarily cooling the first-cooled cold-rolled steel sheet to a temperature range of 350 to 450°C at a cooling rate of 10°C / s or more; a step of maintaining the secondarily cooled cold-rolled steel sheet for 100 seconds or more; and a step of finally cooling the maintained cold-rolled steel sheet to a temperature of Ms-100°C or lower at a cooling rate of 3°C / s or more.

[0032] In one embodiment of the present invention, the steel slab has the above-described alloy composition and can satisfy relational expression 1.

[0033] In one embodiment of the present invention, a step of obtaining a galvanized steel sheet by galvanizing the maintained cold-rolled steel sheet before final cooling may be further included, and optionally, alloying heat treatment may be further performed.

[0034] In one embodiment of the present invention, a step of temper rolling at a reduction ratio of less than 1% after final cooling may be further included.

[0035] According to the present invention, it is possible to provide a cold-rolled steel sheet (composite phase steel) having excellent processing characteristics while having a low yield ratio and high ductility, which are characteristics of DP steel.

[0036] The cold-rolled steel sheet according to the present invention has an effect that makes it suitable for use as an automotive material, particularly as a structural member.

[0037] The inventors of the present invention have conducted in-depth research on a method to improve the processing characteristics such as work hardening rate and hole expandability while preserving the low yield ratio and high elongation of DP steel, which has been used as a material for existing automobile structural members.

[0038] Through repeated research, the inventors of the present invention have confirmed that a structural composition advantageous for securing desired properties can be established by optimizing the alloy composition and manufacturing conditions for steel plates with a tensile strength of 590 MPa or higher. As a result, the present invention is provided.

[0039] Hereinafter, the present invention will be described in detail.

[0040] According to one aspect of the present invention, a steel sheet may contain, in wt%, carbon (C): 0.100 to 0.150%, silicon (Si): 0.30 to 1.00%, manganese (Mn): 1.20 to 1.90%, phosphorus (P): 0.100% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), aluminum (sol.Al): 0.500% or less (excluding 0%), chromium (Cr): 0.500% or less (excluding 0%), and nitrogen (N): 0.010% or less (excluding 0%).

[0041] Below, the reasons for limiting the alloy composition of a steel plate according to one embodiment of the present invention are described in detail. Unless otherwise specified, the content of each element is based on weight, and the ratio of the structure is based on area.

[0042] Carbon (C): 0.100~0.150%

[0043] Carbon (C) is a very important element added to strengthen the transformation structure, and is effective in promoting the formation of martensite phase in composite structure steel while also promoting the high strength of steel.

[0044] As the C content increases, the amount of martensite formed in the steel increases. However, in one embodiment of the present invention, when the C content exceeds 0.150%, the strength is improved by the martensite formed in large quantities, but the difference in strength with ferrite, which has a relatively low carbon concentration, increases. In this case, fracture easily occurs at the interface between phases when stress is applied, so the bending characteristics and extension flangeability of the steel are inferior. In addition, as stress is concentrated on the high-strength martensite, the ductility and work hardening rate are reduced, and the weldability deteriorates, causing welding defects to occur when processing parts at the customer company. On the other hand, when the C content is less than 0.100%, it is very difficult to secure the target strength, and it becomes difficult to secure the retained austenite phase, which is advantageous for securing ductility.

[0045] Therefore, in one embodiment of the present invention, C may be included in an amount of 0.100 to 0.150%. In another embodiment of the present invention, C may be 0.105% or more, or 0.110% or more. In yet another embodiment, C may be 0.145% or less, or 0.140% or less.

[0046] Silicon (Si): 0.30~1.00%

[0047] Silicon (Si) is a ferrite-stabilizing element that promotes the formation of ferrite in steel and contributes to the formation of martensite by promoting the enrichment of carbon in untransformed austenite. Furthermore, Si, due to its high solid-solution strengthening capacity, increases the strength of ferrite, making it an effective element for reducing the hardness difference between phases. Furthermore, Si is a useful element for securing strength without compromising the ductility of steel.

[0048] Meanwhile, Si can effectively concentrate carbon in austenite by delaying carbide precipitation during the bainite transformation during the steel manufacturing process, and is therefore an element that is advantageous for the formation of a retained austenite phase. For this reason, Si must be added. However, if the content is less than 0.30%, it is difficult to sufficiently secure a retained austenite phase that is advantageous for securing ductility. In one embodiment of the present invention, if the Si content exceeds 1.00%, scale defects may be induced on the steel surface, which may deteriorate the plating quality. In addition, the chemical treatment property of the steel may deteriorate, making it difficult to secure paintability.

[0049] Therefore, in one embodiment of the present invention, Si may be included in an amount of 0.30 to 1.00%. In another embodiment of the present invention, the Si may be 0.35% or more, or 0.40% or more. In yet another embodiment, the Si may be 0.90% or less, or 0.85% or less.

[0050] Manganese (Mn): 1.20~1.90%

[0051] Manganese (Mn) is an element that strengthens steel by minimizing grain size without compromising ductility and by completely precipitating sulfur (S) in the steel as MnS, thereby preventing hot embrittlement caused by the formation of FeS. Furthermore, in composite phase steels, it lowers the critical cooling rate for martensite formation, contributing to easier martensite formation.

[0052] In one embodiment of the present invention, if the content of Mn is less than 1.20%, the strengthening effect of the steel is insufficient, making it difficult to secure the target level of strength. On the other hand, if the content exceeds 1.90%, there is a high possibility that problems will occur in the weldability and hot-rollability of the steel, and there is a problem that the martensite phase is formed excessively, making the material unstable, and Mn-Band (band of Mn oxide) is formed in the structure, increasing the risk of processing cracks and plate breakage. In addition, there is a problem that Mn oxide is dissolved on the surface of the steel during the annealing process, which significantly inhibits the plating property during subsequent plating.

[0053] Therefore, in one embodiment of the present invention, Mn may be included in an amount of 1.20 to 1.90%. In another embodiment of the present invention, the Mn may be included in an amount of 1.25% or more, or 1.30% or more. In yet another embodiment, the Mn may be included in an amount of 1.85% or less.

[0054] Phosphorus (P): 0.100% or less (excluding 0%)

[0055] Phosphorus (P) is the substitutional element with the greatest strengthening effect, making it an advantageous element for improving in-plane anisotropy. Furthermore, P is the most advantageous element for securing strength without significantly impairing the formability of steel. However, excessive addition of P significantly increases the likelihood of brittle fracture, increasing the likelihood of slab fracture during hot rolling. Furthermore, it also has the problem of acting as an element that impairs the surface properties of galvanized steel sheets.

[0056] In one embodiment of the present invention, since the aforementioned problem may occur if the P content exceeds 0.100%, the P may be included at 0.100% or less. Meanwhile, since the P may be inevitably added during the steel manufacturing process, a content of 0% may be excluded.

[0057] Sulfur (S): 0.010% or less (excluding 0%)

[0058] Sulfur (S) is an unavoidable impurity added to steel. It impairs the ductility and weldability of steel, so it is important to keep its content as low as possible. In particular, sulfur in steel is highly likely to cause red-hot embrittlement.

[0059] In one embodiment of the present invention, since the aforementioned problem may occur when the content of S exceeds 0.010%, the S may be included at 0.010% or less. Meanwhile, considering the level at which the S is inevitably added during the steel manufacturing process, 0% may be excluded from the content.

[0060] Aluminum (sol.Al): 0.500% or less (excluding 0%)

[0061] Aluminum (sol.Al) is an element added to steel to refine grain size and facilitate deoxidation. Similarly to Si, it acts as a ferrite-stabilizing element, distributing carbon within ferrite to austenite, thereby enhancing martensite hardenability. Furthermore, it effectively suppresses carbide precipitation within bainite during maintenance in the bainite region, making it a useful element for enhancing the ductility of steel.

[0062] In one embodiment of the present invention, if the sol.Al content exceeds 0.500%, excessive inclusions are formed during steelmaking operations, which increases the likelihood of surface defects in the galvanized steel sheet. Furthermore, there is a risk that manufacturing costs will increase, thereby reducing economic feasibility.

[0063] Meanwhile, in one embodiment of the present invention, if the content of sol.Al is 0%, aluminum killed steel cannot be manufactured in a stable state, so 0% can be excluded for the content.

[0064] Therefore, in one embodiment of the present invention, sol.Al may be included in an amount of more than 0% to 0.500%. According to another embodiment of the present invention, the sol.Al may be 0.005% or more, or 0.010% or more. According to yet another embodiment, the sol.Al may be 0.490% or less, or 0.450% or less.

[0065] Chromium (Cr): 0.500% or less (excluding 0%)

[0066] Chromium (Cr) can be added to improve the hardenability of steel and secure high strength. Cr is effective in forming martensite and is an advantageous element for manufacturing composite phase steel with high ductility by minimizing the decrease in elongation compared to the increase in strength. In particular, during the hot rolling process, Cr 23 It forms Cr-based carbides such as C6, some of which dissolve during the annealing process, while others remain undissolved, allowing the amount of dissolved C in martensite to be controlled below an appropriate level after subsequent cooling. This suppresses the occurrence of yield point elongation (YP-El), which is advantageous for the production of composite phase steels with low yield ratios.

[0067] In one embodiment of the present invention, if the Cr content exceeds 0.500%, the aforementioned effect becomes saturated, and there is a concern that the hot-rolled strength may increase excessively, thereby reducing the cold-rollability. In addition, since the fraction of Cr-based carbides increases and coarsens, there is a problem that martensite coarsens after annealing, resulting in a decrease in the elongation of the steel. In one embodiment of the present invention, the lower limit of the Cr content is not particularly limited, and since the effect of Cr can be obtained if it is added at a level during the steel manufacturing process, taking this into consideration, it may be limited to exceeding 0%.

[0068] Therefore, in one embodiment of the present invention, Cr may be included in an amount of more than 0% to 0.500%. In another embodiment of the present invention, Cr may be 0.005% or more, or 0.010% or more. In yet another embodiment, Cr may be 0.450% or less, or 0.400% or less.

[0069] Nitrogen (N): 0.010% or less (excluding 0%)

[0070] Nitrogen (N) is an effective element for stabilizing austenite, but if its content is excessive, not only will the refining cost of steel rise sharply, but the formation of AlN will also increase the risk of cracks forming in the slab during the continuous casting process. Considering this, in one embodiment of the present invention, N may be limited to 0.010% or less. However, considering the level that is inevitably added during the steel manufacturing process, 0% may be excluded for the content.

[0071] The remaining component of the present invention is iron (Fe). However, 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 typical manufacturing, their full details are not specifically discussed in this specification.

[0072] A steel plate according to one embodiment of the present invention may be composed of the above-described alloy elements, and the relationship between some of the alloy elements may be limited to specific conditions.

[0073] According to one embodiment of the present invention, the content relationship of C, Si, Mn, Al, and Cr in the alloy composition can be defined by the following relational expression 1.

[0074] [Relationship 1]

[0075] ((3×Si)+Al+(10×C)) / (Si+Mn+(3×Cr)) ≥ 1.00

[0076] (In equation 1, each element represents the weight content, and if not added, 0 is substituted.)

[0077] The inventors of the present invention, through repeated research, have discovered that the formation of an appropriate ferrite phase and a certain amount of retained austenite phase are essential for securing a low yield ratio and high ductility in composite-structure steel. Accordingly, the combination of specific alloying elements that influence the aforementioned structure formation is factored into Equation 1.

[0078] Specifically, in order to secure a certain amount of retained austenite phase as the microstructure of the steel sheet according to one embodiment of the present invention, it is necessary to concentrate C within the austenite. When the steel contains a certain amount of C, Si and Al delay the precipitation of C carbides during bainite transformation and promote the enrichment of C into the austenite. Therefore, along with C, Si and Al are essential elements for securing the retained austenite phase. In addition, Si and Al are ferrite stabilizing elements and are also effective in increasing the ferrite fraction.

[0079] Meanwhile, Mn and Cr, which are hardenable elements added to steel to secure strength, delay the transformation of ferrite during cooling (slow cooling) after annealing, particularly according to one embodiment of the present invention, after two-phase annealing, thereby limiting the formation of ferrite. In this case, the fractions of relatively hard phases, bainite and martensite, increase, and their sizes become coarser, making it impossible to secure the target retained austenite phase. As a result, while the strength is high, the ductility and processing characteristics (work hardening rate, hole expandability, etc.) are inferior.

[0080] Based on this, when the value of relational expression 1 according to one embodiment of the present invention is secured to 1.00 or more, the target ferrite and residual austenite phases can be secured, and as a result, the workability of the steel can be improved. However, when the value is less than 1.00, the ferrite and residual austenite phases cannot be secured to the target level, resulting in poor workability of the steel. According to another embodiment of the present invention, the value of relational expression 1 may be 1.10 or more.

[0081] Meanwhile, in one embodiment of the present invention, the upper limit value of relational expression 1 may be determined within the content setting range of each element constituting relational expression 1, and there is no particular limitation on the upper limit value.

[0082] Hereinafter, the microstructural characteristics of the steel plate according to one embodiment of the present invention will be described in detail.

[0083] The inventors of the present invention have found that, in order to improve the processing characteristics of a steel plate according to one embodiment of the present invention, that is, a composite structure steel, while having high strength and high ductility, control of the fraction range of each phase is effective.

[0084] Accordingly, the steel sheet according to one embodiment of the present invention may include an area fraction of 60 to 80% ferrite, 10 to 25% bainite, 3 to 8% retained austenite, and a remainder martensite phase.

[0085] As mentioned above, in order to improve the ductility of the steel plate according to one embodiment of the present invention, the fraction of the ferrite phase and the retained austenite phase must be precisely controlled.

[0086] In one embodiment of the present invention, the ferrite phase is a soft phase, a structure advantageous for securing ductility. For example, if the ferrite fraction is less than 60%, the target elongation cannot be achieved, while if the fraction exceeds 80%, the target strength cannot be achieved.

[0087] Furthermore, in one embodiment of the present invention, the retained austenite phase can contribute to securing the ductility of the steel by causing transformation-induced plasticity during the processing process when the steel is processed (formed) to manufacture parts, etc. As an example, if the fraction of the retained austenite phase is less than 3%, it is difficult to secure the target elongation, whereas if it exceeds 8%, the risk of liquid metal embrittlement occurring during spot welding of parts for automobile manufacturing increases.

[0088] In this way, while improving the ductility of steel by limiting the ferrite and retained austenite phases to an appropriate fraction, it is advantageous to include an appropriate fraction of the bainite phase in order to improve the hole expandability of the steel. When a certain amount of Si and Al is added to the steel, the precipitation of carbides is delayed and the thermal stability of austenite is improved, thereby securing the retained austenite phase at room temperature by promoting the enrichment of C into the austenite around the bainite during the process of forming the bainite phase. As an example, if the fraction of the bainite phase is less than 10%, the amount of C enriched into the austenite may decrease, which may lower the fraction of the retained austenite phase. On the other hand, if the fraction of the bainite phase exceeds 25%, the strength of the steel may increase, resulting in poor ductility.

[0089] This bainite phase has a hardness between the relatively soft ferrite phase and the relatively hard martensite phase, and has the effect of reducing the hardness difference between the ferrite phase and the martensite phase (the hardness difference between phases). As a result, the hole expandability of the steel can be improved. If the fraction of the aforementioned bainite phase is less than 10%, the effect of reducing the hardness difference between the phases cannot be sufficiently obtained, and thus the hole expandability of the steel cannot be improved. On the other hand, if the fraction exceeds 25%, the effect of reducing the hardness difference between the phases can be significantly obtained, but it becomes difficult to secure the target low yield ratio due to the increase in yield strength.

[0090] A steel sheet according to one embodiment of the present invention may include a martensite phase as a structure other than the aforementioned structures (phases). In one embodiment of the present invention, the martensite phase may be included as a fraction excluding the ferrite, retained austenite, and bainite phases, and the fraction thereof is not particularly limited.

[0091] As a non-limiting example, the martensite phase may be present in an area fraction of less than 11%, and more advantageously, may be present in an area fraction of less than 10.5%.

[0092] In one embodiment of the present invention, the martensite phase mainly refers to a fresh martensite phase, but does not exclude a tempered martensite phase. The tempered martensite phase may be included at an impurity level.

[0093] Meanwhile, the martensite phase is a phase with high hardness, and when external stress is applied during the processing (forming) of steel, the stress locally becomes significantly higher than that of the ferrite and bainite phases. In order to solve this problem, the inventors of the present invention conducted in-depth research and found that by finely and evenly dispersing the martensite phase within the steel, stress and strain can be controlled so that they are not concentrated in one part. Accordingly, according to one embodiment of the present invention, the fraction (Ms) of fine martensite having an average grain size of 2㎛ or less is controlled with respect to the total fraction (Mt) of the martensite phase, and as an example, the occupancy rate (Ms / Mt) of fine martensite having an average grain size of 2㎛ or less can be controlled to 70% or more. Here, the average grain size means the average value of the equivalent circle diameter.

[0094] That is, according to one embodiment of the present invention, by containing a martensite phase and finely dispersing it within the steel, it is possible to improve the work hardening rate and ductility of the steel. If the occupancy rate of the fine martensite (Ms / Mt) is less than 70%, the fine dispersion effect of martensite is insufficient, resulting in poor work hardening rate and ductility of the steel due to concentration of local stress and strain.

[0095] In addition, in one embodiment of the present invention, the martensite phase has a number of fine martensites having an average grain size of 2 ㎛ or less per unit area (1 mm 2 ) per 2.0×10 5 It can be ideal. The number of the above fine martensite is per unit area (1 mm 2 ) per 2.0×10 5 If it is less than this, the work hardening ability of the steel plate cannot be improved due to the presence of a large number of martensite phases of relatively large size.

[0096] According to one embodiment of the present invention, a steel sheet can exhibit characteristics such as a low yield ratio and a high work hardening rate by initiating deformation at a low stress in the initial stage of plastic deformation by controlling the microstructure. In particular, by controlling the microstructure, local stress and strain can be alleviated, thereby exhibiting characteristics such as a high work hardening rate, and further, by delaying the creation, growth, and coalescence of pores, the effect of improving ductility can be obtained.

[0097] In particular, a steel plate according to one embodiment of the present invention may have a yield ratio (yield strength / tensile strength) of 0.7 or less, and a relationship among a work hardening rate (n), an elongation (El), a hole expandability (HER), and a yield ratio (YR) in a 4 to 6% strain range may satisfy the following relational expression 2.

[0098] [Relationship 2]

[0099] (n×El×HER) / YR ≥ 400.0

[0100] (In equation 2, the units of each property are not considered.)

[0101] Additionally, a steel plate according to one embodiment of the present invention may exhibit high strength and high ductility, and as an example, may have a tensile strength of 590 MPa or more and an elongation of 25% or more.

[0102] Meanwhile, the steel sheet according to one embodiment of the present invention may be a cold-rolled steel sheet, a hot-dip galvanized steel sheet including a zinc-based plating layer on at least one surface of the cold-rolled steel sheet, or an alloyed hot-dip galvanized steel sheet obtained by alloying the hot-dip galvanized steel sheet.

[0103] Although not particularly limited, the zinc-based plating layer may be, for example, a zinc-plated layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and / or magnesium in addition to zinc.

[0104] Hereinafter, a method for manufacturing a steel plate according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method is an example for manufacturing a steel plate according to one embodiment of the present invention.

[0105] According to one embodiment of the present invention, a steel plate can be manufactured by going through the process of [heating - hot rolling - cooling - coiling - cold rolling - annealing - cooling] for a prepared steel slab, and each process step is specifically described below.

[0106] [Heating of steel slabs]

[0107] After preparing a steel slab according to one embodiment of the present invention, the steel slab can be heated. The heating process of the steel slab is a process for smoothly performing the hot rolling process described below and sufficiently obtaining the target physical properties of the steel plate. As one example, the steel slab can have the same alloy composition and alloy composition relationship (relationship 1) as the steel plate according to one embodiment of the present invention, and the description of each alloy element and the description of the composition relationship are replaced with the above-mentioned matters.

[0108] In one embodiment of the present invention, the process of heating the steel slab may be performed under normal conditions, and as an example, may be performed at a temperature range of 1100 to 1300°C. If the heating temperature is lower than 1100°C, friction between the steel plate and the rolling mill increases, which causes a problem in that the load applied to the rollers during hot rolling increases rapidly. On the other hand, if the temperature exceeds 1300°C, not only does the energy cost required for the temperature increase, but the amount of surface scale increases, which may lead to material loss.

[0109] [Hot rolling]

[0110] The above heated steel slab can be hot rolled to obtain a hot rolled steel sheet.

[0111] In one embodiment of the present invention, a hot-rolled steel sheet can be manufactured by performing finishing hot rolling at a temperature higher than the Ar3 transformation point during the hot rolling. In one embodiment of the present invention, if the finishing hot rolling process is performed at a temperature lower than the Ar3 transformation point, there is a concern that ferrite and austenite dual-phase rolling may occur, resulting in material non-uniformity.

[0112] As an example, the above finishing hot rolling can be performed in a temperature range of 800 to 1000°C, and if the temperature exceeds 1000°C, there is a concern that material unevenness may occur due to the formation of abnormal coarse grains caused by high-temperature rolling, and this causes a problem of coil distortion occurring during subsequent cooling.

[0113] [Winding]

[0114] The hot-rolled steel sheet manufactured above can be coiled.

[0115] In one embodiment of the present invention, the coiling process may be performed at a temperature range of 400 to 700°C. If the coiling temperature is lower than 400°C, the strength of the hot-rolled steel sheet may become excessively high, which may cause rolling load during subsequent cold rolling. In addition, the cost and time required to cool the hot-rolled steel sheet to the coiling temperature are excessive, which causes an increase in process costs. On the other hand, if the temperature exceeds 700°C, scale may excessively form on the surface of the hot-rolled steel sheet, which is highly likely to cause surface defects and deteriorate the plating properties.

[0116] [Cold rolling]

[0117] The above-mentioned hot-rolled steel sheet can be cold-rolled to produce a cold-rolled steel sheet.

[0118] In one embodiment of the present invention, cold rolling can be performed at a cold reduction ratio (total reduction ratio) of 40 to 80%. If the cold reduction ratio during cold rolling is less than 40%, it becomes difficult to secure the target thickness and also difficult to correct the shape of the steel sheet. On the other hand, if the cold reduction ratio exceeds 80%, there is a high possibility of cracks occurring at the edge of the steel sheet, and there is a problem of generating a load during cold rolling.

[0119] [Continuous annealing]

[0120] The cold-rolled steel sheet manufactured above can be subjected to continuous annealing treatment. According to one embodiment of the present invention, the continuous annealing treatment can be performed in a continuous alloying galvanizing furnace.

[0121] In one embodiment of the present invention, the continuous annealing treatment can be performed in a temperature range of 760 to 820°C. That is, by performing the continuous annealing treatment of a cold-rolled steel sheet in a two-phase temperature range where ferrite and austenite coexist, not only austenite but also ferrite can be formed simultaneously with the recrystallization of the structure, while carbon can be distributed.

[0122] In the continuous annealing treatment according to one embodiment of the present invention, if the temperature is lower than 760℃, not only will recrystallization not be sufficiently achieved, but the austenite phase will not be sufficiently formed either, making it difficult to secure the target microstructure phase composition, for example, the appropriate fraction of bainite and martensite phases, after the continuous annealing treatment. On the other hand, if the temperature exceeds 820℃, the productivity decreases, and due to the excessive formation of the austenite phase, there is a problem in that the fraction of the martensite and bainite phases becomes excessive after the subsequent cooling process. In this case, the yield strength increases and the ductility decreases, making it impossible to secure the intended characteristics of low yield ratio and high ductility. In addition, the surface enrichment of elements such as Si and Mn, which inhibit the wettability of the hot-dip galvanizing among the alloy compositions, may become severe, which may deteriorate the plating surface quality.

[0123] Therefore, in one embodiment of the present invention, the continuous annealing treatment can be performed at a temperature range of 760 to 820°C. In another embodiment of the present invention, the continuous annealing treatment can be performed at 770°C or higher.

[0124] [Cooling and Maintenance]

[0125] The cold rolled steel sheet that has been continuously annealed in the above two-phase temperature range can be cooled.

[0126] In one embodiment of the present invention, the cooling may be performed in steps, and as an example, the continuous annealing-treated cold-rolled steel sheet may be first cooled to a temperature range of 630 to 690°C at a cooling rate of 10°C / s or less (excluding 0°C / s), and then the first-cooled cold-rolled steel sheet may be secondarily cooled to a temperature range of 350 to 450°C at a cooling rate of 10°C / s or more. At this time, the cooling rate during the second cooling may be performed faster than the cooling rate during the first cooling.

[0127] In this way, in the continuous annealing process according to one embodiment of the present invention, the type and fraction of the microstructure formed can be controlled by performing stepwise cooling in a specific temperature range according to the cooling rate when cooling a cold-rolled steel sheet in which a certain fraction of a ferrite phase is formed together with austenite.

[0128] In one embodiment of the present invention, a ferrite phase can be additionally introduced into the annealed cold-rolled steel sheet by performing primary cooling at a cooling rate of 10°C / s or less to a temperature range of 630 to 690°C.

[0129] If the cooling rate exceeds 10°C / s during the above primary cooling, the additionally introduced ferrite phase may become insufficient, which may reduce the ductility of the steel sheet. Meanwhile, there is no particular limitation on the lower limit of the cooling rate. However, in order to control the fraction of the ferrite phase in the final microstructure to 80% or less, it may be performed at 0.5°C / s or higher.

[0130] In addition, if the cooling end temperature during the first cooling is less than 630°C, the ferrite phase cannot be sufficiently formed. On the other hand, if the temperature exceeds 690°C, there is a problem in that the cooling rate must be excessively increased during the subsequent second cooling process, and there is a concern that the fractions of the bainite phase and martensite phase in the final microstructure may not be sufficient.

[0131] According to one embodiment of the present invention, during the first cooling process of a cold-rolled steel sheet subjected to continuous annealing, a ferrite phase is additionally formed, while the austenite phase remaining after forming the ferrite phase is dispersed.

[0132] In one embodiment of the present invention, after the first cooling of the continuously annealed cold-rolled steel sheet, secondary cooling can be performed at a cooling rate of 10°C / s or more to a temperature range of 350 to 450°C, and by maintaining the cold-rolled steel sheet in this cooled state, a bainite phase can be introduced.

[0133] If the cooling rate during the secondary cooling is less than 10°C / s, pearlite may be generated during the cooling process, preventing the bainite phase from being sufficiently formed. Meanwhile, the upper limit of the cooling rate during the secondary cooling is not particularly limited, and a skilled artisan may select it appropriately, taking into account the specifications of the cooling equipment. As an example, the cooling rate may be 100°C / s or less.

[0134] In addition, if the cooling end temperature during the secondary cooling is less than 350°C, the martensite phase is excessively formed, and the martensite phase is tempered during the subsequent holding process, making it impossible to secure the intended yield strength and high ductility. On the other hand, if the temperature exceeds 450°C, the bainite phase is not sufficiently formed, making it impossible to obtain the effects of the appropriate fraction of the bainite phase, such as the effect of reducing the hardness difference between the phases.

[0135] Meanwhile, in one embodiment of the present invention, the holding process of the secondary cooled cold-rolled steel sheet may be performed for 100 seconds or longer. If the holding time is less than 100 seconds, bainite cannot be obtained in a sufficient fraction. The upper limit of the holding time is not particularly limited, and can be determined by a person skilled in the art as the time required for the bainite phase to form in the desired fraction.

[0136] In one embodiment of the present invention, the maintenance process performed after secondary cooling can be performed within the temperature range at which the secondary cooling is completed, and therefore, there are no particular limitations on the temperature range. However, as one example, it can be performed at 400°C or lower.

[0137] According to one embodiment of the present invention, a certain fraction of the bainite phase can be formed during the secondary cooling and holding process of the primary cooled cold-rolled steel sheet, and by forming the bainite phase in this way, the dispersion effect of the remaining austenite phase is further enhanced. Accordingly, when the remaining austenite phase transforms into martensite during the subsequent cooling process, it can be formed in a fine and uniform distribution.

[0138] [Final cooling]

[0139] The cold rolled steel sheet, which has undergone the above-mentioned stepwise (primary and secondary) cooling and maintenance processes, can be finally cooled.

[0140] In one embodiment of the present invention, the final cooling of the cold-rolled steel sheet that has undergone the stepwise cooling and holding process can be performed at a cooling rate of 3°C / s or more to a temperature range of Ms-100°C or lower, and a martensite phase can be introduced during this process. Here, Ms refers to the martensite transformation initiation temperature, and the cooling can be performed to room temperature. The room temperature is not particularly limited and can be approximately 25 to 35°C.

[0141] If the cooling rate during the final cooling is less than 3°C / s or the cooling end temperature exceeds Ms-100°C, the martensite phase cannot be secured at the intended level. In one embodiment of the present invention, there is no particular limitation on the upper limit of the cooling rate during the final cooling, but in terms of forming a martensite phase of a certain fraction, the cooling may be performed at a cooling rate of, for example, 50°C / s or less.

[0142] According to one embodiment of the present invention, the austenite that is transformed into ferrite and bainite phases during the stepwise cooling and holding process and remains can be transformed into a martensite phase during the final cooling process. At this time, the austenite phase that is transformed into a martensite phase during the final cooling process is uniformly and finely distributed within the steel due to the dispersion effect during the previous stepwise cooling and holding process, and thus the martensite phase formed during the final cooling process can also be formed in a fine and uniform distribution. In particular, the fine martensite phase can be formed at a ratio of 70% or more compared to the total fraction of the martensite phase formed during the final cooling process. As one example of the present invention, the fine martensite phase may refer to a martensite phase having an average crystal grain size of 2 μm or less.

[0143] Meanwhile, in the final cooling process, the martensite phase is formed and the remaining austenite phase remains as is, becoming the residual austenite phase of the final microstructure. According to the alloy composition and manufacturing conditions according to one embodiment of the present invention, the residual austenite phase can be formed at an area fraction of 3 to 8%.

[0144] According to one embodiment of the present invention, a steel sheet (cold rolled steel sheet) having a composite structure can be obtained by controlling the alloy composition and manufacturing conditions. Specifically, the composite structure of the cold rolled steel sheet according to one embodiment of the present invention has a low difference in hardness between the soft phase and the hard phase, and further, along with the introduction of an appropriate fraction of ferrite, retained austenite, and bainite phases, the martensite phase can be formed by being finely and uniformly dispersed. As a result, the cold rolled steel sheet can have the effect of improving the work hardening rate due to the alleviation of stress concentration and improving the hole expandability.

[0145] In one embodiment of the present invention, a cold-rolled steel sheet may be plated to obtain a plated steel sheet. As an example, hot-dip galvanizing may be performed on a cold-rolled steel sheet that has undergone a stepwise cooling and maintenance process before final cooling.

[0146] [Hot-dip galvanizing]

[0147] According to one embodiment of the present invention, a cold-rolled steel sheet, that is, a cold-rolled steel sheet that has undergone a stepwise cooling and maintenance process, can be immersed in a molten zinc-based plating bath to produce a molten zinc-based galvanized steel sheet (galvanized steel sheet).

[0148] In one embodiment of the present invention, the hot-dip galvanizing process may be performed under normal conditions, but as an example, may be performed at a temperature range of 430 to 490°C. In addition, the composition of the hot-dip galvanizing bath is not particularly limited, and may be a pure zinc plating bath or a zinc alloy plating bath containing Si, Al, Mg, etc.

[0149] [Alloying heat treatment]

[0150] In addition, if necessary, an alloyed zinc-plated steel sheet can be obtained by subjecting a hot-dip galvanized steel sheet manufactured by hot-dip galvanizing according to one embodiment of the present invention to an alloying heat treatment.

[0151] In one embodiment of the present invention, the alloying heat treatment process conditions are not particularly limited, and any conventional conditions may be used. As an example, the alloying heat treatment process may be performed at a temperature range of 480 to 600°C.

[0152] [Temperature rolling]

[0153] Furthermore, if necessary, a temper rolling process can be further performed, and the temper rolling process can be performed not only on a cold rolled steel sheet that has undergone final cooling, but also on a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet that has undergone final cooling.

[0154] According to one embodiment of the present invention, a large amount of dislocations are formed in steel through a temper rolling process, thereby further improving the bake hardenability. As an example, this can be performed at a reduction ratio of less than 1% (excluding 0%). When the reduction ratio during the temper rolling is 1% or more, it is advantageous in terms of dislocation formation, but may cause side effects such as plate fracture due to limitations in equipment capacity.

[0155] 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.

[0156] (Example)

[0157] Steel slabs having the alloy compositions shown in Table 1 below were heated at a temperature of 1100 to 1300°C, and each heated slab was then final hot-rolled at Ar3+50°C to 950°C to produce hot-rolled steel sheets. Thereafter, each hot-rolled steel sheet was pickled under normal conditions, coiled at a temperature range of 400 to 700°C, and then cold-rolled at a cold reduction ratio of 40 to 80% to produce cold-rolled steel sheets.

[0158] After this, each cold-rolled steel sheet was subjected to continuous annealing treatment under the conditions shown in Table 2 below, followed by step cooling (first - second cooling) and holding processes. Thereafter, the steel sheet was cooled to room temperature at a cooling rate of 5°C / s, and then temper rolled at a reduction ratio of 0.2% to produce the final cold-rolled steel sheet (composite structure steel).

[0159] Steel alloy composition (weight %) relationship 1CSiMnPSsol.AlCrN10.1100.601.500.0110.0020.2100.1000.0051.3020.1200.721.900.0130.0030.0300.1000.0021.1630.1200.731.600.0110.0040.0200 .2000.0041.1640.1500.511.400.0120.0020.4100.3000.0041.2250.1400.731.400.0110.0080.0250.1000.0051.4960.1000.621.900.0110.0020.2100.050 0.0031.1570.1200.512.000.0150.0040.0300.7000.0020.6080.1000.211.900.0140.0070.0250.2000.0050.6190.1600.331.600.0160.0050.0310.3000.00 60.93100.0800.712.200.0140.0020.0250.6000.0040.63110.1000.052.000.0110.0080.71000.0030.91120.1200.691.900.0120.0050.2000.5000.0050.85

[0160] Steel grade Coiling temperature (℃) Annealing temperature (℃) 1st cooling 2nd cooling Maintenance classification End temperature (℃) Cooling speed (℃ / s) End temperature (℃) Cooling speed (℃ / s) Temperature (℃) Time (sec) 1650 800 630 4390 21380 150 Invention steel 125 80 820 680 5400 31390 210 Invention steel 235 60 780 680 8420 23420 250 Invention steel 345 20 820 650 6400 18380 301 Invention steel 45 450 81 8680 5380 27380 112 Invention steel 56 670 810 670 7420 25400 310 Invention steel 67650830650633033320257Comparison 1863080068054003138082Comparison 29650840650742022410123Comparison 310680820650456011450304Comparison 411580770680340030380208Comparison 512550800680454013450322Comparison 6

[0161] For each cold-rolled steel sheet manufactured as described above, the microstructure was measured and the mechanical properties were evaluated, and the results are shown in Table 3.

[0162] First, the microstructure of each cold-rolled steel sheet was measured using FE-SEM and an image analyzer after Nital corrosion for specimens collected at a point 1 / 4t (t: steel sheet thickness (unit: mm)) in the thickness direction. The fractions of ferrite, bainite, and martensite phases were measured, and the fraction of retained austenite phase was measured using XRD.

[0163] At this time, the size of each particle was converted to the equivalent diameter from the martensite area results measured using the image analyzer and measured. Then, 1 mm 2 The fraction and number of martensite having a size of 2㎛ or less within the unit area were calculated.

[0164] Meanwhile, in order to evaluate the tensile properties of each cold-rolled steel sheet, the same specimens as those used for microstructure measurement were tested in the C direction according to the JIS standard.

[0165] In addition, the strain hardening rate (n) was measured in the 4-6% strain range for the same specimen, and the hole expansion ratio (HER) was evaluated according to the JSF T1001-1996 standard.

[0166] ClassificationMicrostructure CharacteristicsMechanical PropertiesFBM(Mt)R-AMs / MtNumber of Micro MsYS(MPa)TS(MPa)El(%)YRnHER(%)Relationship 2Inventive steel 1692083802.28×10 5 380671270.570.23743482.7 Invention Steel 2731683762.30×10 5 370629280.590.22148503.4 Invention Steel 3731674772.43×10 5 357638290.560.23151610.1 Invention Steel 4672373822.08×10 5 384632270.610.22055535.6Invention Steel 57116103872.49×10 5 340689260.490.23345556.3 Invention Steel 6721675752.12×10 5 363605290.600.22643469.7 Comparison River 1814150431.58×10 5 497699220.710.20646293.6 Comparison River 2833140531.32×10 5 379683230.550.20038317.8 Comparison River 3757180611.14×10 5 483722210.670.19843266.9 Comparison River 4796150501.46×10 5 368676240.540.17639305.1 Comparison River 5806140331.76×10 5 362620270.580.18445385.4 Comparison River 6835111411.27×10 5350675210.520.17939281.9 In the microstructure, F represents ferrite, B represents bainite, M represents martensite, and RA represents retained austenite phases, and each phase is expressed as an area fraction (%). The number of micro martensite (M) phases is per unit area of ​​1 mm. 2 This is the result measured based on .

[0167] As shown in Tables 1 to 3, the inventive steels 1 to 6, which satisfy both the alloy composition and manufacturing conditions according to an embodiment of the present invention, formed microstructures as intended. Accordingly, it can be confirmed that they have a tensile strength of 590 MPa or more, a low yield ratio of 0.70 or less, and high ductility. In addition, the steel sheet according to an embodiment of the present invention satisfies Equation 2 by improving hole expandability and work hardening rate.

[0168] In this way, the cold-rolled steel sheet according to one embodiment of the present invention has the characteristics of DP steel, such as high yield strength and high ductility, and has excellent processing characteristics due to a low hardness difference between phases despite being a composite structure steel.

[0169] On the other hand, comparative steels 1 to 6, which did not satisfy at least one of the alloy composition and manufacturing conditions according to one embodiment of the present invention, did not form the intended microstructure. As a result, the desired physical properties, particularly relational equation 2, were not satisfied.

Claims

1. Contains, in wt%, carbon (C): 0.100 to 0.150%, silicon (Si): 0.30 to 1.00%, manganese (Mn): 1.20 to 1.90%, phosphorus (P): 0.100% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), aluminum (sol.Al): 0.500% or less (excluding 0%), chromium (Cr): 0.500% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), the remainder being iron and other unavoidable impurities, and satisfying the following relational expression 1, A steel plate having a microstructure composed of an area fraction of 60 to 80% ferrite, 10 to 25% bainite, 3 to 8% retained austenite, and residual martensite. [Relationship 1] ((3×Si)+Al+(10×C)) / (Si+Mn+(3×Cr)) ≥ 1.00 (In equation 1, each element represents the weight content, and if not added, 0 is substituted.) 2. In paragraph 1, A steel plate comprising the above martensite phase in an area fraction of less than 11%.

3. In paragraph 1, The above martensite has a number of fine martensites with a size of 2 ㎛ or less in diameter per unit area (1 mm). 2 ) 2.0×10 per 5 A plate that is more than a dog.

4. In paragraph 1, A steel plate having a ratio (Ms / Mt) of the total fraction of martensite (Mt) and the fraction (Ms) of fine martensite having an average grain size of 2 ㎛ or less of 70% or more.

5. In paragraph 1, The above steel plate is a steel plate with a yield ratio (yield strength / tensile strength) of 0.7 or less.

6. In paragraph 1, The above steel plate is a steel plate in which the relationship between the work hardening rate (n), elongation (El), hole expandability (HER) and yield ratio (YR) in the 4 to 6% strain range satisfies the following relationship 2. [Relationship 2] (n×El×HER) / YR ≥ 400.0 (In equation 2, the units of each property are not considered.) 7. A step for preparing a steel slab containing, by weight%, carbon (C): 0.100 to 0.150%, silicon (Si): 0.30 to 1.00%, manganese (Mn): 1.20 to 1.90%, phosphorus (P): 0.100% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), aluminum (sol.Al): 0.500% or less (excluding 0%), chromium (Cr): 0.500% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), the remainder iron and other unavoidable impurities, and satisfying the following relationship 1; A step of heating the above steel slab to a temperature range of 1100 to 1300℃; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated steel slab at a temperature higher than the Ar3 transformation point; A step of coiling the above hot-rolled steel plate at a temperature range of 400 to 700°C; A step of cold rolling the above-mentioned hot-rolled steel sheet at a total reduction ratio of 40 to 80% to obtain a cold-rolled steel sheet; A step of continuously annealing the above cold rolled steel sheet in a two-phase temperature range of 760 to 820°C; A step of first cooling the continuously annealed cold rolled steel sheet to a temperature range of 630 to 690°C at a cooling rate of 10°C / s or less (excluding 0°C / s); A step of secondarily cooling the first-cooled cold-rolled steel sheet at a cooling rate of 10°C / s or more to a temperature range of 350 to 450°C; A step of maintaining the secondarily cooled cold rolled steel sheet for 100 seconds or longer; and A method for manufacturing a steel sheet, comprising a step of finally cooling the maintained cold rolled steel sheet to a temperature of Ms-100°C or lower at a cooling rate of 3°C / s or higher. [Relationship 1] ((3×Si)+Al+(10×C)) / (Si+Mn+(3×Cr)) ≥ 1.0 (In equation 1, each element represents the weight content, and if not added, 0 is substituted.) 8. In paragraph 7, A method for manufacturing a steel sheet further comprising the step of obtaining a galvanized steel sheet by galvanizing the cold rolled steel sheet maintained above before final cooling.

9. In paragraph 8, A method for manufacturing a steel sheet further comprising the step of selectively performing alloying heat treatment on the galvanized steel sheet.

10. In paragraph 7, A method for manufacturing a steel sheet further comprising the step of subjecting the above-described finally cooled cold rolled steel sheet to temper rolling at a reduction ratio of less than 1%.

11. In paragraph 7, A method for manufacturing a steel plate, wherein the above secondary cooling is performed at a faster cooling rate than the above primary cooling.