Cold rolled steel sheet and method for manufacturing same

Abstract

The present invention relates to a cold rolled steel sheet and a method for manufacturing same and, more specifically, to a cold rolled steel sheet and a method for manufacturing same, the cold rolled steel sheet being suitable for use as a steel material for automobile reinforcements such as bumper beams, sill side beams, etc. or a steel material for the protection of electric vehicle battery cases, such as side frames, cross members, etc.

Description

Technical Field

[0001] The present disclosure relates to a cold rolled steel sheet and method for manufacturing same, and, more specifically, to a cold rolled steel sheet and a method for manufacturing same, the cold rolled steel sheet being suitable for use as a steel material for automobile reinforcements such as bumper beams, sill side beams, etc. or a steel material for the protection of electric vehicle battery cases, such as side frames, cross members, etc.Background Art

[0002] In the case of steels mainly used for reinforcing components related to the collision safety of automobile passengers, the development of ultra-high strength steels having high processing characteristics, especially excellent bending properties, is required during manufacturing using a cold forming technique. To this end, research on ultra-high strength steel having a tensile strength of 1470 MPa or higher and a manufacturing method using a martensite single phase has been actively conducted. Recently, a hot press forming (HPF) method has been developed that forms a material at high temperatures at which the material may be easily formed and then secures the required strength through water cooling between a die and a material. Since the HPF method is able to secure high strength for the same thickness, the HPF method is widely used during manufacturing of components, but there are problems in application due to excessive facility investment costs and increased process costs, and therefore, the development of materials for cold stamping is required. Accordingly, the development of an ultra-high strength cold rolled steel sheet which is suitable for use as materials for cold stamping, has high strength and high yield ratio to secure crash performance and has excellent bending characteristics is required.

[0003] A representative prior art reference of this method is Patent Document 1. Patent Document 1 relates to a cold rolled steel sheet, including: by wt%, C: 0.25 to 0.4%, Si: 1.0% or less, Mn: 1.5 to 2.5%, P: 0.02% or less, S: 0.003% or less, Al: 0.01 to 0.1%, N: 0.005% or less, B: 0.0005 to 0.005%, and further including Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, and a sum of 0.005 to 0.1%, and relates to manufacturing the cold rolled steel sheet by heating and holding a steel sheet in a temperature range of 900°C or less to an Ae3 transformation point or more using a martensite single-phase structure, then rapidly cooling the heated steel sheet to 200°C or less at an average cooling rate of 300°C / s or more, and then tempering the rapidly cooled steel sheet at 250°C or less. However, in the case of patent document 1, there may be a problem that the shape (flatness) is inferior due to water cooling, resulting in defects during molding.

[0004] Patent Document 2 relates to a thin steel sheet including: by wt%, C: 0.05% or more and 0.35% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.8% or more and 3.0% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.005% or more and 0.10% or less, and N: 0.0060% or less, and including a steel structure having a ferrite area ratio of 0% or more and 90% or less, a bainite area ratio of 5% or less (including 0%), a martensite and tempered martensite area ratio of 10% or more (including 100%), and a retained austenite area ratio of 2.0% or less (including 0%), wherein a standard deviation of yield strength in a width direction is 30 MPa or less, and a maximum bending amount when sheared at a length of 1 m is 10 mm or less. However, even in the case of patent document 2, there may be a problem that shape defects occur due to rapid cooling after annealing.

[0005] Accordingly, in order to solve the above-mentioned problem, it is necessary to develop an ultra-high strength cold rolled steel sheet and a plated steel sheet having a tensile strength of 1470 MPa or more and having excellent hole expandability, bending characteristics, and weldability.[Prior art documents]

[0006] (Patent document 1) Japanese Patent Laid-Open Publication No. 2010-248565 (Patent document 2) Japanese Patent Laid-Open Publication No. 2020-019992 Summary of InventionTechnical Problem

[0007] An aspect of the present disclosure is to provide a cold rolled steel sheet and method for manufacturing same.

[0008] A preferred aspect of the present disclosure is to provide an ultra-high strength cold rolled steel sheet having a tensile strength of 1470 MPa or higher and having excellent hole expandability, bending properties and weldability and a method for manufacturing the same.Solution to Problem

[0009] According to an aspect of the present disclosure provides a cold rolled steel sheet including: by wt%, carbon (C): 0.19 to 0.26%, silicon (Si): 0.03 to 0.50%, manganese (Mn) : 1.4 to 2.0%, chromium (Cr): 0.03 to 0.30%, molybdenum (Mo): 0.03 to 0.30%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.003% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.01 to 0.10%, niobium (Nb): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.05%, and a balance of Fe and other inevitable impurities, wherein the following relational expressions 1 to 3 are satisfied, and including a central portion; and a surface layer formed on an outer side based on a thickness direction of the central portion, wherein a microstructure of the central portion includes, in area%, a sum of at least one of ferrite and bainite: 5% or less (including 0%), and a balance of at least one of martensite and tempered martensite, a microstructure of the surface layer includes, in area%, a sum of at least one of ferrite and bainite: 11% or less (excluding 0%), and a balance of at least one of martensite and tempered martensite, and an average size of carbides is 260 nm or less. 410 ≤ Y / X ≤ 620 (However, in the relational expressions 1 to 3, the content of each alloy element is wt%.)

[0010] The cold rolled steel sheet may have a surface roughness (Rsk) of -0.7 to -0.1.

[0011] The cold rolled steel sheet may have a yield strength of 1150 to 1400 MPa.

[0012] The cold rolled steel sheet may have a tensile strength of 1470 to 1650 MPa.

[0013] The cold rolled steel sheet may have a yield ratio of 0.75 to 0.96.

[0014] The cold rolled steel sheet may have an elongation of 4 to 11%.

[0015] The cold rolled steel sheet may have a bending workability (R / t) of 2 to 4.

[0016] The cold rolled steel sheet may have a hole expandability of 35 to 70%.

[0017] The cold rolled steel sheet may have a (tensile strength × hole expandability) / (bending workability (R / t)) of 15,000 to 35,000 MPa%.

[0018] The cold rolled steel sheet may have a hardness of a spot welded portion of 440 to 570 Hv after resistance spot welding.

[0019] The cold rolled steel sheet has a crack length of 10µm or less (including 0µm) at a minimum nugget diameter (3√t where t: thickness of steel material) in a spot welded portion after resistance spot welding.

[0020] The cold rolled steel sheet may have an electro-galvanized layer formed on at least one surface.

[0021] The surface layer may be a region from a surface of steel to 20µm in a thickness direction.

[0022] Another aspect of the present disclosure provides a method for manufacturing a cold rolled steel sheet, heating a slab satisfying the following relational expressions 1 to 3 at 1100 to 1300°C, the slab including, by wt%, carbon (C): 0.19 to 0.26%, silicon (Si): 0.03 to 0.50%, manganese (Mn): 1.4 to 2.0%, chromium (Cr): 0.03 to 0.30%, molybdenum (Mo): 0.03 to 0.30%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.003% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.01 to 0.10%, niobium (Nb): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.05%, and a balance of Fe and other inevitable impurities; finishing hot rolling the heated slab at Ar3 to Ar3+120°C to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at Ms to 600°C; cold rolling the coiled hot rolled steel sheet at a cold reduction ratio of 35 to 70% to obtain a cold rolled steel sheet; continuously annealing the cold rolled steel sheet at Ac3+20°C to Ac3+80°C; primarily cooling the continuously annealed cold rolled steel sheet to a primary cooling end temperature (T1) of 670 to 750°C at an average cooling rate of 1 to 6°C / s; secondarily cooling the primarily cooled cold rolled steel sheet to a secondary cooling end temperature (T2) of 50 to 200°C at an average cooling rate of 40 to 110°C / s; reheating the secondarily cooled cold rolled steel sheet to an overaging temperature (H) of 100 to 250°C and then subjecting the reheated steel sheet to an overaging treatment for 5 to 12 minutes; temper rolling the overaging cold rolled steel sheet with reduction force of 500 to 1,000 tons; and tension leveling the temper-rolled cold rolled steel sheet with an elongation of 0.05 to 0.65%, wherein during the secondary cooling, the following relational expression 4 and 5 are satisfied, the primary cooling end temperature (T1) - second cooling end temperature (T2) is controlled to be 650°C or lower, and the overaging temperature (H) - second cooling end temperature (T2) is controlled to be 30°C or higher. 410 ≤ Y / X ≤ 620 940 ≤ F × Z ≤ 1200 (However, in the Relational Expressions 1 to 3, the content of each alloy element is wt%, ΔT is Ms - secondary cooling end temperature (T2), and Z is 3500C + 150Mn + 50 (Si+Cr+Mo) + 1000Nb.)

[0023] The continuous annealing may be performed for 30 to 230 seconds.

[0024] The method for manufacturing a cold rolled steel sheet may further include: after the tension leveling, forming an electro-galvanized layer on at least one surface of the cold rolled steel sheet.Advantageous Effects of Invention

[0025] According to an aspect of the present disclosure, a cold rolled steel sheet and a method for manufacturing the same could be provided.

[0026] According to a preferred aspect of the present disclosure, an ultra-high strength cold rolled steel sheet having a tensile strength of 1470 MPa or higher and having excellent hole expandability, bending properties and weldability and a method for manufacturing the same.

[0027] Brief Description of Drawings

[0028] FIG. 1 is a schematic diagram for evaluating the presence or absence of cracks in a minimum nugget diameter (3√t) of a resistance spot welded portion.

[0029] FIG. 2 is a microstructure image of a surface layer of Inventive Example 1 according to an embodiment of the present disclosure observed with an electron microscope (SEM).

[0030] FIG. 3 is a microstructure image of a surface layer of Comparative Example 5 according to an embodiment of the present disclosure observed with an electron microscope (SEM).

[0031] FIG. 4 is a microstructure image obtained by observing a cross section of a welded portion of Inventive Example 4 according to an embodiment of the present disclosure with an optical microscope.

[0032] FIG. 5 is a microstructure image obtained by observing a cross section of a welded portion of Comparative Example 1 according to an embodiment of the present disclosure with an optical microscope.Best Mode for Invention

[0033] The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the invention. The singular forms used herein include the plural forms as well, unless the phrases clearly indicate the contrary. The term "comprises," "comprising," "includes" and / or "including" as used in the specification specifies a particular feature, region, integer, step, operation, element, and / or component, but does not exclude the presence or addition of other particular features, regions, integers, steps, operations, elements, components, and / or groups.

[0034] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which the present disclosure belongs. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed in an ideal or significantly formal sense unless otherwise defined.

[0035] The inventors of the present disclosure have recognized that a microstructure and carbides may be appropriately controlled by controlling an alloy composition and manufacturing conditions, and specifically, by controlling surface roughness, an ultra-high strength cold rolled steel sheet having excellent bending characteristics, shape, and weldability and a tensile strength of 1470 MPa or more may be manufactured, and have completed the present disclosure.

[0036] Hereinafter, a cold rolled steel sheet according to an embodiment of the present disclosure will be described. First, an alloy composition of the present disclosure will be described. The alloy composition described below refers to weight % unless otherwise specified.Carbon (C): 0.19 to 0.26%

[0037] C is an interstitial solid-solution element and is the most effective and important element for improving the strength of steel. Additionally, C is an element that needs to be added to secure the strength of martensite steel. When the content of C is less than 0.19%, it may be difficult to obtain the yield ratio and tensile strength targeted in the present disclosure. When the content of C exceeds 0.26%, martensite is excessively formed during cooling due to a rapid increase in hardenability, and as a result, the strength may rapidly increase and an elongation may be inferior. Additionally, the weldability may be inferior. Accordingly, the content of C may be preferably in the range of 0.19 to 0.26%. In order to further enhance the effects described above and prevent the occurrence of problems, a lower limit of the content of C may be more preferably 0.20%, and an upper limit of the content of C may be more preferably 0.25%.Silicon (Si): 0.03 to 0.50%

[0038] Si suppresses the formation of carbides and controls a size of carbides during reheating and overaging treatment operations performed after continuous annealing and cooling. When the content of Si is less than 0.03%, it may be difficult to sufficiently obtain the above-described effect. When the content of Si exceeds 0.50%, there is a concern that ferrite may be formed after continuous annealing and cooling, weakening the strength of the steel. Additionally, Si is an element that increases resistivity, which may result in poor resistance spot weldability. Accordingly, the content of Si may be preferably in the range of 0.03 to 0.50%. In order to further enhance the effects described above and prevent the occurrence of problems, a lower limit of the Si content may be more preferably 0.05%, and an upper limit of the Si content may be more preferably 0.40%, and even more preferably 0.30%.Manganese (Mn): 1.4 to 2.0%

[0039] Mn is an element added to secure strength. When the content of Mn is less than 1.4%, the hardenability is low, and thus, if the cooling speed is not sufficiently rapid during cooling after continuous annealing, martensite may not be formed, which may make it difficult to secure the strength targeted by the present disclosure. When the content of Mn exceeds 2.0%, an Ms temperature decreases during cooling after continuous annealing, and thus, a final cooling temperature decreases, which may result in a poor shape of a steel sheet. Additionally, it may be difficult to secure an initial martensite structure. Additionally, during steelmaking / continuous casting, a Mn-based segregation zone occurs in a longitudinal direction of a slab, which may deteriorate the bendability thereof. In other words, manganese segregates in a thickness direction to form a manganese band within a slab, which may cause cracks during continuous casting and increase the occurrence of defects during the rolling process. Accordingly, the content of Mn may be preferably in the range of 1.4 to 2.0%. In order to further enhance the effects described above and prevent the occurrence of problems, a lower limit of the Mn content may be more preferably 1.5%, and an upper limit of the Mn content may be more preferably 1.9%.Chromium (Cr): 0.03 to 0.30%

[0040] Cr is an element that facilitates securing a low-temperature transformation structure by suppressing ferrite transformation. Additionally, when utilizing a continuous annealing process with slow cooling as in the present disclosure, there is an advantage of suppressing ferrite formation. When the content of Cr is less than 0.03%, the hardenability is low, and then, when the cooling speed is not sufficiently rapid during cooling after continuous annealing, martensite may not be formed, which make it difficult to secure the strength on a level targeted by the present disclosure. When the content of Cr exceeds 0.30%, delayed fracture resistance may deteriorate, and carbides such as CrC may be formed, which may hinder hole expandability and bending workability, and may increase costs due to excessive alloy input. Accordingly, the content of Cr may be preferably in the range of 0.03 to 0.30%. In order to further enhance the effects described above and prevent the occurrence of problems, a lower limit of the Cr content may be more preferably 0.03%, and an upper limit of the Cr content may be more preferably 0.25%.Molybdenum (Mo): 0.03 to 0.30%

[0041] Mo is an element that has the effect of improving quenching properties of steel, the effect of generating Mo-based fine carbides that become hydrogen trap sites, and the effect of improving the delayed fracture resistance by refining martensite. When the content of Mo is less than 0.03%, it may be difficult to sufficiently obtain the above-mentioned effects. When the content of Mo exceeds 0.30%, the above-described effects do not increase significantly as compared to an increase in costs due to the addition of expensive alloy elements. Accordingly, the content of Mo may preferably have a range of 0.03 to 0.30%. In order to further enhance the effects described above and prevent the occurrence of problems, a lower limit of the Mo content may be more preferably 0.05%, and an upper limit of the Mo content may be more preferably 0.25%.Boron (B): 0.0005 to 0.005%

[0042] B is an element that suppresses the formation of ferrite, and therefore, the present disclosure has the advantage of suppressing the formation of ferrite during cooling after continuous annealing. When the content of B is less than 0.0005%, there is no hardenability effect at all, and thus, the strength targeted by the present disclosure may not be secured, and also, there is a problem that the bending workability is inferior due to excessive formation of ferrite in a surface layer. When the content of B exceeds 0.005%, ductility may be significantly reduced. Accordingly, the content of B may be preferably in the range of 0.0005 to 0.005%. In order to further enhance the effects described above and prevent the occurrence of problems, a lower limit of the content of B may be preferably 0.0007%, and an upper limit of the content of B may be preferably 0.004%.Phosphorus (P): 0.03% or less (excluding 0%)

[0043] P is an impurity element included in steel. When the content of P exceeds 0.03%, weldability deteriorates and there may be a risk of steel brittleness. Meanwhile, the smaller the amount of P added to the steel, the more advantageous it is, but 0% is excluded in consideration of cases in which P is unavoidably included in a manufacturing process. Accordingly, the content of P may be preferably 0.03% or less (excluding 0%). In order to further enhance the effects described above and prevent the occurrence of problems, the content of P may be preferably 0.025% or less.Sulfur (S): 0.003% or less (excluding 0%)

[0044] S is an impurity element included in steel, similarly to P. When the content of S exceeds 0.003%, S may hinder ductility and weldability, and a large amount of MnS precipitates may form, which result in poor bending workability. Meanwhile, the smaller the amount of S added to steel, the more advantageous it is, but considering a case in which S is unavoidably included in the manufacturing process, 0% is excluded. Accordingly, the content of P may be preferably 0.003% (excluding 0%) or less. The content of S may be more preferably 0.0025% or less, and even more preferably 0.0020% or less.Nitrogen (N): 0.01% or less (excluding 0%)

[0045] N is an impurity element. When the content thereof exceeds 0.01%, N significantly increases the risk of cracks occurring during continuous casting due to AlN formation, or the like. The content of N is excluded from 0% considering a case in which N is unavoidably included in the manufacturing process. Accordingly, the content of N may preferably have a range of 0.01% or less (excluding 0%). In order to further enhance the effects described above and prevent the occurrence of problems, the content of N may be more preferably 0.008% or less, and even more preferably 0.006% or less.Aluminum (Al): 0.01 to 0.10%

[0046] Al may be added to remove oxygen in molten steel. When the Al content is less than 0.01%, deoxidation is not sufficiently performed, which may harm the cleanliness of the steel. When the Al content exceeds 0.10%, not only may the castability of the slab deteriorate, but also the temperature required for single-phase heating during continuous annealing may also increase, which may cause production and facility problems. Accordingly, the Al content may preferably have a range of 0.01 to 0.10%. In order to further enhance the effects described above and prevent the occurrence of problems, an upper limit of the Al content may be more preferably 0.075%.Niobium (Nb): 0.01 to 0.05%

[0047] Nb is an element which is segregated in austenite grain boundaries to suppress the coarsening of austenite grains during the continuous annealing process, and contributes to the improvement of strength by forming fine precipitates. When the content of Nb is less than 0.01%, the austenite grain refinement and precipitation strengthening effects may not be sufficiently obtained. When the content of Nb exceeds 0.05%, there is a concern that the precipitation of coarse carbonitrides increases, and the strength and elongation may decrease due to the reduction in the amount of carbon in the steel. Additionally, there is a problem that the workability of a base material deteriorates and the manufacturing costs increase. Accordingly, the content of Nb may be preferably in the range of 0.01 to 0.05%. In order to further enhance the effects described above and prevent the occurrence of problems, a lower limit of the Nb content may be more preferably 0.015%, and an upper limit of the Nb content may be more preferably 0.045%.Titanium (Ti): 0.005 to 0.05%

[0048] Ti is a nitride-forming element that scavenges by precipitating solid-solution N as TiN. When the content of Ti is less than 0.005%, it may be difficult to obtain the effect of increasing strength, and since the effect of scavenging solid-solution N is reduced, cracks may occur during continuous casting as a large amount of AlN is formed. When the content of Ti exceeds 0.05%, the strength of martensite may decrease due to the precipitation of additional carbides in addition to the removal of solid-solution N, and the formation of excessive carbon and nitrides such as TiC and TiN may impede hole expandability and bending workability. Accordingly, the content of Ti may be preferably in the range of 0.005 to 0.05%. In order to further enhance the effects described above and prevent the occurrence of problems, a lower limit of the content of Ti may be more preferably 0.01%, and an upper limit of the content of Ti may be more preferably 0.04%.

[0049] The remaining components are iron (Fe). However, since unintended impurities may inevitably be mixed from raw materials or a surrounding environment during a normal manufacturing process, iron (Fe) may not be excluded. Since such impurities may be known to anyone skilled in the normal manufacturing process, all of the contents are not specifically mentioned in this specification.

[0050] The cold rolled steel sheet of the present disclosure may satisfy an alloy composition described above and the following relational equations 1 to 3.

[0051] The relational equation 1 is a component relational equation that is closely related to the hardness of a welded portion. When a value of X is less than 0.31, it is difficult to sufficiently secure the hardness and strength of the welded portion. When a value of X exceeds 0.40, the hardness of the welded portion becomes excessively high, which may increase the risk of brittle fracture, and thus, the collision stability may be deteriorated. Accordingly, the value of X may have a range of 0.31 to 0.40. In order to further enhance the effect described above and prevent the occurrence of problems, a lower limit of the value X may be more preferably 0.32, and an upper limit of the value X may be more preferably 0.39.

[0052] The relational expression 2 is a component relationship related to hardenability for securing the microstructure and strength targeted by the present disclosure. When a value of Y is less than 125, it may be difficult to secure sufficient strength because the microstructure targeted by the present disclosure is not obtained due to insufficient hardenability. When the value of Y exceeds 190, not only does the manufacturing costs increase, but there is also a problem that the strength is excessively increased and the elongation is reduced. Accordingly, the value of Y may preferably have a range of 125 to 190. In order to further enhance the effect described above and prevent the occurrence of problems, a lower limit of the value Y may be more preferably 130, and an upper limit of the value Y may be more preferably 185. 410 ≤ Y / X ≤ 620

[0053] The relational expression 3 is a component relational expression for securing an appropriate level of the hardness and hardenability of the welded portion. When a value of the Y / X is less than 410, Ceq is satisfied, but it may be difficult to secure strength because a target microstructure is not obtained due to insufficient hardenability. When the value of the Y / X exceeds 620, the hardenability is sufficiently secured, but it may be difficult to secure the strength of the welded portion because the Ceq is low. Accordingly, the value of the Y / X may preferably have a range of 410 to 620. In order to further enhance the effect described above and prevent the occurrence of problems, a lower limit of the Y / X value may be more preferably 420, and an upper limit of the Y / X value may be more preferably 600.

[0054] The cold rolled steel sheet according to the present disclosure may be divided into a central portion; and a surface layer formed on an outer side of the central portion in a thickness direction, in terms of a microstructure.

[0055] A microstructure of the central portion may include, in area%, a sum of at least one of ferrite and bainite: 5% or less (including 0%), and at least one of residual martensite and tempered martensite, and a microstructure of the surface layer includes, in area%, a sum of at least one of ferrite and bainite: 11% or less (excluding 0%), and a balance of at least one of martensite and tempered martensite.

[0056] A main phase of the microstructure of the central portion may include at least one of martensite and tempered martensite. The martensite and tempered martensite are significantly advantageous structures for securing the strength, hole expandability, bending characteristics, and weldability targeted by the present disclosure. However, inevitably, at least one of ferrite and bainite may be formed during the manufacturing process, and when a total fraction of one or more types of ferrite and bainite exceeds 5%, it may be difficult to secure the properties targeted by the present disclosure. The total fraction of one or more types of the ferrite and bainite may be more preferably 3% or less.

[0057] A main phase of the microstructure of the surface layer may include at least one of martensite and tempered martensite. Additionally, a total fraction of at least one of ferrite and bainite may be preferably 11% or less (excluding 0%). Since the ferrite and bainite are softer than the martensite and tempered martensite, the bending characteristics may be further improved by forming an appropriate level of ferrite and bainite in the surface layer. When the total fraction of at least one of the ferrite and bainite exceeds 11%, it may be difficult to secure sufficient strength, and the bending characteristics may be inferior. The total fraction of at least one of the ferrite and bainite may be more preferably 10% or less. In the present disclosure, a lower limit of the total fraction of at least one of the ferrite and bainite is not particularly limited, but may be 1% as an example.

[0058] Meanwhile, a depth of the surface layer may be changed depending on a thickness of the steel, and as an example, the surface layer may be a region up to 20µm in a thickness direction from a surface of the steel.

[0059] The cold rolled steel sheet of the present disclosure may preferably have an average size of carbides of 260 nm or less. When an average size of the carbides exceeds 260 nm, the bending characteristics may be inferior. The average size of the carbides may be more preferably 250 nm or less. In the present disclosure, the smaller the average size of the carbides is, the advantageous it is, and thus, there is no particular limitation on a lower limit thereof. However, the lower limit of the average size of the carbide may be, for example, 10 nm. Meanwhile, the carbides may be, for example, at least one of carbides including Fe and Mn, carbides including Mn and Cr, and carbides including Fe, Mn, Cr and Mo.

[0060] The cold rolled steel sheet of the present disclosure may have a surface roughness (Rsk) of -0.7 to -0.1. The surface roughness (Rsk (Skewness)) is one of several factors of surface roughness related to an asymmetry of a sharp protruding portion. A value of the surface roughness (Rsk) is closer to 0 or a + value, it is more advantageous for securing bending characteristics. As a - value of the surface roughness (Rsk) increases, a valley on a flat surface becomes deeper, and stress is thus concentrated in this area to increase the sensitivity to crack occurrence, which may cause the bending characteristics to be inferior. When the value of the surface roughness (Rsk) is less than -0.7, the bending characteristics may become inferior. When the value of the surface roughness (Rsk) exceeds -0.1, it is advantageous for securing the bending characteristics, but since the surface of the roll should be processed, the manufacturing costs increases significantly. A lower limit of the value of the surface roughness (Rsk) may be more preferably -0.65. An upper limit of the value of the surface roughness (Rsk) may be more preferably -0.15.

[0061] As described above, the cold rolled steel sheet of the present disclosure provided may have a yield strength: 1150 to 1400 MPa, a tensile strength: 1470 to 1650 MPa, a yield ratio: 0.75 to 0.96, elongation: 4 to 11%, bending workability (R / t): 2 to 4, hole expandability: 35 to 70%, (Tensile strength × Hole expandability) / (Bending workability (R / t)) of 15,000 to 35,000 MPa%, hardness of a spot welded portion: 440 to 570 Hv, and a crack length at a minimum nugget diameter (3√t where t: thickness of steel) of a resistance spot welded portion: 10 µm or less (including 0 µm). The crack length of 0 µm denotes that no crack occurs. Meanwhile, the present disclosure does not specifically limit the type of the welded portion, but as an example, as illustrated in FIG. 1, the welded portion may be formed by resistance spot welding under the conditions of a 1.5t gap between two materials of 30 mm x 100 mm in size, force: 3.8 KN, welding time: 20 cycles, holding time: 10 cycles, and a welding current in a range of 5.0 to 6.0 kA.

[0062] The cold rolled steel sheet of the present disclosure may have a thickness of 0.6 to 2.2 mm. A lower limit of the thickness of the cold rolled steel sheet may be more preferably 0.7 mm, and even more preferably 0.8 mm. An upper limit of the thickness of the cold rolled steel sheet may be more preferably 2.1 mm, and even more preferably 2.0 mm.

[0063] The cold rolled steel sheet of the present disclosure may have a plating layer formed on at least one surface thereof. The present disclosure does not specifically limit the type of plating layers, and all types of plating layers commonly used in the relevant technical field may be formed. However, as an example, the plating layer may be an electro-galvanized layer. On the other hand, as another example, the plating layer may be a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.

[0064] Hereinafter, a method for manufacturing a cold rolled steel sheet according to an embodiment of the present disclosure will be described.

[0065] First, a slab satisfying the above-mentioned alloy composition and the relational expressions 1 to 3 is heated at 1100 to 1300°C. The slab heating process is performed to smoothly perform a subsequent hot rolling process and sufficiently obtain target properties of the steel sheet. When the slab heating temperature is less than 1100°C, a problem occurs in which a hot-rolling load increases rapidly. When the slab heating temperature exceeds 1300°C, the amount of surface scale increases and a material yield decreases. A lower limit of the slab heating temperature may be more preferably 1110°C, even more preferably 1120°C, and most preferably 1130°C. An upper limit of the slab heating temperature may be more preferably 1290°C, even more preferably 1280°C, and most preferably 1270°C.

[0066] Then, the heated slab is subjected to a finishing hot rolling at Ar3 to Ar3+120°C to obtain a hot-rolled steel sheet. When the finishing hot-rolling temperature is lower than Ar3, a two-phase phase or ferrite phase rolling of ferrite+austenite occurs, thereby generating a mixed grain structure, and plate fracture may occur due to a change in the hot-rolling load. When the finishing hot-rolling temperature exceeds Ar3+120°C, a large amount of surface scale may occur, thereby deteriorating the surface quality. A lower limit of the finishing hot-rolling temperature is more preferably Ar3+10°C, even more preferably Ar3+20°C, and most preferably Ar3+30°C. An upper limit of the finishing hot-rolling temperature may be more preferably Ar3+110°C, even more preferably Ar3+100°C, and most preferably Ar3+90°C. Meanwhile, the Ar3 means the temperature at which austenite begins to transform into ferrite during cooling, and may be obtained through the following relational expression 1.

[0067] Then, the hot-rolled steel sheet is coiled at Ms to 600°C. When the coiling temperature exceeds 600°C, since internal oxidation occurs on a surface of the steel sheet, a microstructure formed on a surface portion may be uneven, and thus, the bending characteristics may become inferior. Meanwhile, it is preferable to manage the coiling temperature to be low in order to ensure material uniformity of a full length and a full width by forming the microstructure of the hot-rolled steel sheet in a single-phase structure rather than a composite structure as much as possible. However, when the coiling temperature is lower than Ms, the strength of the hot-rolled steel sheet may become excessively high, which may increase the rolling load during cold rolling as a subsequent process, making actual production impossible. A lower limit of the coiling temperature may be more preferably Ms+50°C. An upper limit of the coiling temperature may be more preferably 550°C. The Ms refers to a temperature at which austenite begins to transform into martensite during cooling, and may be obtained through the following relational expression 2.

[0068] Meanwhile, after the coiling, cooling may be performed through water cooling. Additionally, after the cooling, a pickling process may be performed to remove an oxide layer formed on a surface of the hot-rolled steel sheet.

[0069] Then, the coiled hot-rolled steel sheet is cold rolled at a cold reduction ratio of 35 to 70% to obtain a cold rolled steel sheet. When a cold reduction ratio is less than 35%, it may be difficult to secure a thickness desired in the present disclosure, and also, there may be also a concern that austenite may be generated during annealing heat treatment due to the residual crystal grains formed during hot rolling, which may affect final properties. Additionally, the -value of the surface roughness (Rsk) may excessively increase, resulting in poor bending characteristics. When the cold reduction ratio exceeds 70%, a reduction amount in length and width directions may become uneven due to the work hardening occurring during cold rolling, which may cause material deviations in the steel sheet. Additionally, it may be difficult to secure the thickness desired in the present disclosure due to the rolling load. A lower limit of the cold reduction ratio may be more preferably 36%, even more preferably 37%, and most preferably 38%. An upper limit of the cold reduction ratio may be more preferably 68%, even more preferably 66%, and most preferably 64%.

[0070] Then, the cold rolled steel sheet is continuously annealed at Ac3+10°C to Ac3+80°C. When the continuous annealing temperature is lower than Ac3+10°C, since a two-phase annealing occurs over the full length of the steel sheet instead of a single-phase annealing, a mixed grain structure may be formed, and thus, it may be difficult to secure the properties targeted by the present disclosure. When the continuous annealing temperature exceeds Ac3+80°C, facility trouble may occur due to overload of the annealing furnace. A lower limit of the continuous annealing temperature may be more preferably Ac3+11°C, even more preferably Ac3+14°C, and most preferably Ac3+15°C. An upper limit of the continuous annealing temperature may be more preferably Ac3+70°C, even more preferably Ac3+60°C, and most preferably Ac3+50°C. Meanwhile, the Ac3 refers to a temperature at which austenite begins to appear during heating, and may be obtained through the following relational expression 3.

[0071] The continuous annealing may be performed for 30 to 230 seconds. When the continuous annealing time is less than 30 seconds, there is a disadvantage that it may be difficult to secure a single-phase austenite structure. When the continuous annealing time exceeds 230 seconds, the austenite size becomes coarse, making it difficult to secure strength and bending characteristics. A lower limit of the continuous annealing time may be more preferably 40 seconds, even more preferably 50 seconds, and most preferably 60 seconds. An upper limit of the continuous annealing time may be preferably 220 seconds, even more preferably 210 seconds, and most preferably 200 seconds.

[0072] Then, the continuously annealed cold rolled steel sheet is primarily cooled to a primary cooling end temperature (T1) of 670 to 750°C at an average cooling rate of 1 to 6°C / s. When the primary cooling end temperature (T1) is less than 670°C, during the cooling process, a large amount of soft ferrite and bainite other than martensite may be formed, which may result in poor bending properties. When the primary cooling end temperature (T1) exceeds 750°C, since a temperature difference between the primary cooling end temperature (T1) and the secondary cooling end temperature (T2) becomes severe, rapid phase transformation may be caused, which may result in poor product shape. A lower limit of the primary cooling end temperature may be more preferably 680°C. An upper limit of the primary cooling end temperature may be more preferably 740°C. When the primary average cooling rate is less than 1°C / s, since ferrite is formed during cooling, the strength targeted by the present disclosure may not be secured. When a primary average cooling rate exceeds 6°C / s, the average cooling rate during the subsequent secondary cooling decreases, and a fraction of low-temperature transformation phases other than martensite increases, and thus, the strength targeted by the present disclosure may not be secured. A lower limit of the primary average cooling rate may be more preferably 2°C / s. An upper limit of the primary average cooling rate may be more preferably 5°C / s.

[0073] Then, the primarily cooled cold rolled steel sheet is secondarily cooled to a secondary cooling end temperature (T2) of 50 to 200°C at an average cooling rate of 40 to 110°C / s. The second cooling is performed to secure at least one of martensite and tempered martensite, which are the main phases of the present disclosure. When the secondary cooling end temperature (T2) is less than 50°C, shape defects are caused by rapid phase transformation, and continuous production is difficult due to the meandering problem of the strip. When the secondary cooling end temperature (T2) exceeds 200°C, it may be difficult to secure the strength targeted by the present disclosure. A lower limit of the secondary cooling end temperature may be more preferably 55°C, even more preferably 60°C, and most preferably 65°C. An upper limit of the secondary cooling end temperature may be more preferably 195°C, even more preferably 190°C, and most preferably 185°C. When the secondary average cooling rate is less than 40°C / s, soft ferrite transformation may occur during cooling, which may make it difficult to secure the target strength. When the above-mentioned second average cooling rate exceeds 110°C / s, the product shape may become poor due to rapid phase transformation. A lower limit of the secondary average cooling rate may be more preferably 45°C / s, even more preferably 50°C / s, and most preferably 55°C / s. An upper limit of the secondary average cooling rate may be more preferably 105°C, even more preferably 100°C, and most preferably 95°C.

[0074] During the secondary cooling, the following relational expressions 4 and 5 may be preferably satisfied.

[0075] (However, in the relational expression 4, ΔT is Ms - the second cooling end temperature (T2).)

[0076] The relational expression 4 is a relational expression for obtaining the strength targeted by the present disclosure by securing a total fraction of at least one of martensite and tempered martensite desired by the present disclosure. When a value of the F is less than 0.9, it may be difficult to secure the strength targeted by the present disclosure. A value of the F may be more preferably 0.91 or higher. In the present disclosure, since a higher value of the F is advantageous, there is no particular limitation on an upper limit thereof. However, the upper limit of the F value may be 0.99 as an example. 940 ≤ F × Z ≤ 1200 (where ΔT is Ms - secondary cooling end temperature (T2), and Z is 3500C + 150Mn + 50(Si+Cr+Mo) + 1000Nb.)

[0077] The relational expression 5 is a relational expression that examines the relationship between the relational expression 4 and strength-related factors. When the value of the F×Z is less than 940, not only is the microstructure desired by the present disclosure not secured, but the solid solution strengthening effect is insufficient, making it difficult to secure the strength desired by the present disclosure. When the value of the F×Z exceeds 1200, the strength may be excessively high, which may lower the elongation. A lower limit of the F×Z value may be more preferably 960. An upper limit of the F×Z value may be more preferably 1180.

[0078] The primary cooling end temperature (T1) - secondary cooling end temperature (T2) may be preferably controlled to be 650°C or less. When the primary cooling end temperature (T1) - secondary cooling end temperature (T2) exceeds 650°C, a shape defect may occur. The primary cooling end temperature (T1) - secondary cooling end temperature (T2) may be more preferably 600°C or less.

[0079] Then, the secondarily cooled cold rolled steel sheet is reheated to an overaging temperature (H) of 100 to 250°C, and then overaging is performed for 5 to 12 minutes. Through the reheating and overaging, the martensite obtained by the above-described rapid cooling process may be transformed into tempered martensite, thereby increasing the yield strength. When the reheating temperature and overaging temperature are less than 100°C, since tempering is not sufficiently performed, there is a disadvantage that the yield strength is low and sufficient toughness may not be secured. When the reheating temperature and overaging temperature exceed 250°C, there is a disadvantage that the bending workability is deteriorated due to a large amount of carbide precipitation and coarsening. A lower limit of the reheating temperature and overaging temperature may be more preferably 110°C, even more preferably 120°C, and most preferably 130°C. An upper limit of the reheating temperature and overaging temperature may be more preferably 245°C, even more preferably 240, and most preferably 235°C. When the overaging time is less than 5 minutes, since tempering is not sufficiently performed, the yield strength may be low. When the overaging time exceeds 12 minutes, the carbide may become coarser due to excessive tempering, which may result in poor bending properties. A lower limit of the overaging time may be more preferably 5.5 minutes, even more preferably 6.0 minutes, and most preferably 6.5 minutes. An upper limit of the overaging time may be more preferably 11.5 minutes, even more preferably 11 minutes, and most preferably 10.5 minutes.

[0080] The overaging temperature (H) - secondary cooling end temperature (T2) may be preferably control to be 30°C or higher. When the overaging temperature (H) - secondary cooling end temperature (T2) is less than 30°C, tempering is not sufficiently performed, making it difficult to secure the target yield strength. The overaging temperature (H) - secondary cooling end temperature (T2) may be preferably 50°C or higher.

[0081] Then, the cold rolled steel sheet subjected to the overaging process is subjected to temper rolling (SPM (Skin Pass Mill)) having reduction force of 500 to 1,000 tons. The temper rolling enables control of the surface roughness (Rsk). When the reduction force during the temper rolling is less than 500 tons, the load is low, making it difficult to control the surface roughness (Rsk), and when the reduction force exceeds 1,000 tons, the surface may be severely hardened, resulting in poor bending characteristics. A lower limit of the reduction force during the temper rolling may be more preferably 550 tons, and even more preferably 600 tons. An upper limit of the reduction force during the temper rolling may be more preferably 950 tons, and even more preferably 900 tons.

[0082] Then, the temper-rolled cold rolled steel sheet is subjected to tension leveling (T / L) with an elongation of 0.05 to 0.65%. The tension leveling is for correcting a shape of the steel sheet. When the elongation is less than 0.05% during the tension leveling, shape correction may be difficult. When the elongation exceeds 0.65% during the tension leveling, the work hardening may become severe and the bending characteristics may become inferior. A lower limit of the elongation during the tension leveling may be more preferably 0.10%, and even more preferably 0.15%. An upper limit of the elongation during the tension leveling may be more preferably 0.60%, and even more preferably 0.55%.

[0083] Meanwhile, after the tension leveling, an operation of forming an electro-galvanized layer on at least one surface of the cold rolled steel sheet may be additionally included. The present disclosure does not specifically limit the method for forming the electro-galvanized layer, and any method commonly used in the relevant technical field may be used.Mode for Invention

[0084] Hereinafter, the present disclosure will be described in more detail through examples. However, the description of these examples is only for illustrating an embodiment of the present disclosure, and the present disclosure is not limited by the description of these examples. This is because the scope of the rights of the present disclosure is determined by the matters described in the claims and matters reasonably inferred therefrom.(Example)

[0085] A slab having an alloy composition described in Tables 1 and 2 below was heated at 1200°C, and the heated slab was subjected to finishing hot rolling at 900°C to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet was then coiled at 500°C. Then, the hot-rolled steel sheet was cold rolled at a cold reduction ratio described in Table 3 below to obtain a cold rolled steel sheet. Then, continuous annealing, primary cooling, secondary cooling, reheating, overaging treatment, temper rolling, and tension leveling were performed under the conditions described in Tables 3 and 4 below to manufacture a cold rolled steel sheet. Meanwhile, the conditions described in Tables 3 and 4 below were based on the surface temperature of the steel sheet.

[0086] The microstructure, average carbide size, surface roughness (Rsk), and mechanical properties of the cold rolled steel sheet manufactured in this manner were measured, and the results are shown in Tables 5 and 6 below.

[0087] The microstructure was observed using a scanning electron microscope (SEM) and an optical microscope (OM) at the surface layer of the steel sheet (based on the inventive example, in a position of 20µm from a surface in a thickness direction) and in a position of 1 / 4t (t: thickness of a steel material) from a surface of the steel sheet, and then, fractions of each phase were analyzed three times through image analysis and an average value thereof was calculated.

[0088] An average carbide size was captured using a transmission electron microscope (TEM) in a position of 1 / 4t (t: thickness of the steel material) from the surface of the steel sheet, and an average value thereof was calculated.

[0089] Surface roughness (Rsk) was measured five times using a contact 2D roughness meter, and an average value thereof excluding maximum (Max) and minimum (Min) values was calculated.

[0090] Yield strength, tensile strength, yield ratio, and total elongation were measured by processing the cold rolled steel sheet into the JIS standard (gauge length width × length: 25 × 50 mm, total length of the specimen: 200 to 260 mm) specimens, and then performing a tensile test at a test speed of 28 mm / min.

[0091] Bending workability (R / t) was measured by processing the cold rolled steel sheet into 100 mm wide × 30 mm long specimens, and performing a 90° bending test at a test speed of 100 mm / min, and the presence or absence of cracks in the bending area was confirmed using a stereoscopic microscope, and an R / t value was calculated by dividing a minimum bending radius (an R value of the mold) in which no cracks occurred by the thickness of the specimen (mm).

[0092] Hole expandability (HER) was measured according to the ISO 16330 standard, and the holes were sheared with a clearance of 12% using a 10 mm diameter punch.

[0093] In order to obtain a crack length at a minimum nugget diameter (3√t where t: steel thickness (mm)) of the welded portion, as illustrated in FIG. 1, a 1.5t gap is disposed between two materials of 30 mm × 100 mm size, and resistance spot welding was performed under the conditions of force: 3.8 KN, welding time: 20 cycles, holding time: 10 cycles, and welding current in the range of 5.0 to 6.0 kA, and then, a cross-section of the welded portion was observed with an optical microscope, thus measuring the crack length in an area satisfying the minimum nugget diameter (3√t). FIG. 1 is a schematic diagram for evaluating the presence or absence of cracks at the minimum nugget diameter (3√t) of the resistance spot welded portion.

[0094] The hardness of the spot weld portion was measured 10 times with Vickers hardness (load: 500 gf) for the spot weld portion, and the average value was calculated. [Table 1]Steel TypeAlloy Composition (wt%)CSiMnPSNAlInventive Steel 10.220.11.850.010.0010.0040.03Inventive Steel 20.230.051.780.0110.00120.0030.035Inventive Steel 30.210.151.910.0120.00090.0040.03Inventive Steel 40.240.151.70.0120.00150.0030.031Inventive Steel 50.230.21.650.0090.00110.0040.035Comparative Steel 10.270.151.850.0110.00140.0050.039Comparative Steel 20.250.22.50.0120.00120.0040.035Comparative Steel 30.250.61.950.0150.0010.0040.03Comparative Steel 40.180.11.750.0140.00150.0030.031Comparative Steel 50.220.151.10.010.00110.0050.041Comparative Steel 60.210.091.420.0080.0010.0030.035 [Table 2] Steel TypeAlloy Composition (wt%)CrMoNbTiBXYY / XInventive Steel 10.10.050.0350.020.00250.34168496Inventive Steel 20.090.070.030.0250.0020.347155447Inventive Steel 30.150.090.0340.0230.00210.338174513Inventive Steel 40.110.10.0250.0340.00240.36158439Inventive Steel 50.150.110.020.0290.0020.342150440Comparativ e Steel 10.160.070.0310.0210.00250.395175444Comparativ e Steel 20.150.090.0250.0250.00210.41187456Comparativ e Steel 30.10.090.0250.020.0020.402174432Comparativ e Steel 40.150.080.0290.0270.00240.305159521Comparativ e Steel 50.070.040.030.0250.00250.304133438Comparativ e Steel 60.010.020.0210.020.0020.304118390X = C + Mn / 20 + Si / 30 + 2P + 4SY = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 190410 ≤ Y / X ≤ 620 [Table 3] DivisionSteel TypeCold Rolli ng Reduc tion Rate (%)Thickn ess of Cold Rolled Steel Sheet (Mm)Ac3 (°C)Anneal ing Temper ature (°C)Primar y Coolin g End Temper ature (T1) (°C)Prima y Averag e Coolin g Rate (T2) (°C / s)Second ary Coolin g End Temper ature (°C)Second ary Average Cooling Rate (°C / s)T1-T2 (°C)Inventive Example 1Invent ive Steel 1501.6821859715312060595Inventive Example 2Invent ive Steel 2501.6817869720311758603Inventive Example 3Invent ive Steel 3501.6827867721313260589Inventive Example 4Invent ive Steel 4501.6820859718313557583Inventive Example 5Invent ive Steel 5501.6825860725311555610Comparative Example 1Compar ative Steel 1501.6813857700312160579Comparative Example 2Compar ative Steel 2501.6820855720313254588Comparative Example 3Compar ative Steel 3501.6838875715312055595Comparative Example 4Compar ative Steel 4501.6831865719315260567Comparative Example 5Compar ative Steel 5501.6823860711313561576Comparative Example 6Compar ative Steel 6501.6822865705312163584Inventive Example 6Invent ive Steel 4501.6820855695311765578Inventive Example 7Inventive Steel 4501.6820865701312161580Inventive Example 8Invent ive Steel 4501.6820861725312560600Comparative Example 7Invent ive Steel 4501.6820825720411657604Comparative Example 8Invent ive Steel 4501.6820861660312560535Comparative Example 9Invent ive Steel 4501.6820856725325065475Comparative Example 10Invent ive Steel 4501.6820867715311660599Comparative Example 11Invent ive Steel 4501.6820864705313560570Comparative Example 12Invent ive Steel 4501.6820867721312561596Inventive Example 9Invent ive Steel 2561.4817869720312565595Inventive Example 10Invent ive Steel 2571.2817869720312162599Inventive Example 11Invent ive Steel 2581.6817869720311563605Comparative Example 13Invent ive Steel 2301.4817869720311957601Comparative Example 14Invent ive Steel 2301.2817869720312962591Comparative Example 15Invent ive Steel 2301.6817869720314563575Comparative Example 16Invent ive Steel 2501.6817869720313565585Ac3(°C) = 910 - 203√C - 15.2Ni + 44.7Si + 104V + 31.5Mo + 13.1W [Table 4] DivisionSteel TypeMs (°C)FZF×ZReheat ing / Ov eragin g Temper ature (H) (°C)Overag ing Time (min)H-T2 (°C)Temper Rollin g Reduct ion Force (ton)Tension Leveling Elongati on (%)Inventive Example 1Inventive Steel 13880.95109510401809606000.35Inventive Example 2Inventive Steel 23870.95111310571909.5736000.3Inventive Example 3Inventive Steel 33890.94107510111959.5636000.25Inventive Example 4Inventive Steel 43840.94113810701859.5506000.15Inventive Example 5Inventive Steel 53880.951096104120110866000.2Comparative Example 1Comparati ve Steel 13660.93127311841859.5646000.35Comparative Example 2Comparati ve Steel 23540.91129711801749.5426000.35Comparative Example 3Comparati ve Steel 33680.93123211461869.5666000.35Comparative Example 4Comparati ve Steel 44070.9493888219510436000.35Comparative Example 5Comparati ve Steel 54110.9597892920010656000.35Comparative Example 6Comparati ve Steel 64070.969759362019.5806000.35Inventive Example 6Inventive Steel 43840.95113810781859.5686000.35Inventive Example 7Inventive Steel 43840.941138107518010596000.25Inventive Example 8Inventive Steel 43840.941138107219110666000.35Comparative Example 7Inventive Steel 43840.95113810781859.5696000.35Comparative Example 8Inventive Steel 43840.941138107219010656000.35Comparative Example 9Inventive Steel 43840.77113887726010106000.35Comparative Example 10Inventive Steel 43840.9511381078280101646000.35Comparative Example 11Inventive Steel 43840.941138106418710522000.03Comparative Example 12Inventive Steel 43840.941138107219410692000.80Inventive Example 9Inventive Steel 23870.94111310511789.5537000.25Inventive Example 10Inventive Steel 23870.95111310531819.5607000.25Inventive Example 11Inventive Steel 23870.95111310571859.5707000.25Comparative Example 13Inventive Steel 23870.95111310551809.5617000.25Comparative Example 14Inventive Steel 23870.94111310481879.5587000.25Comparative Example 15Inventive Steel 23870.93111310351819.5366000.25Comparative Example 16Inventive Steel 23870.94111310431899.5542500.25Ms(°C) = 539 - 423C - 30.4Mn - 7.5Si + 30Al - 17.7Ni - 12.1Cr - 7.5MoΔT = Ms - Secondary Cooling End Temperature (T2)Z = 3500C + 150Mn + 50(Si+Cr+Mo) + 1000Nb [Table 5] DivisionMicrostructure (area%)Average Carbide Size (nm)Surface Roughness (Rsk)Surface LayerCentral PortionAt least one of F and BAt least one of M and TMAt least one of F and BAt least one of M and TMInventive Example 15.294.819956-0.35Inventive Example 24795.319950-0.31Inventive Example 359519990-0.3Inventive Example 43.996.119968-0.31Inventive Example 54.795.319945-0.32Comparativ e Example 13.596.519950-0.4Comparativ e Example 23.996.119956-0.39Comparativ e Example 39.690.439795-0.31Comparativ e Example 412.387.7257545-0.32Comparativ e Example 514.585.5307098-0.3Comparativ e Example 616.783.33565105-0.34Inventive Example 64.595.519975-0.38Inventive Example 73.996.119966-0.35Inventive Example 85.394.719987-0.41Comparativ e Example 713.586.5208078-0.32Comparativ e Example 816.883.219956-0.38Comparativ e Example 94.595.5199269-0.35Comparativ e Example 10496199360-0.85Comparativ e Example 114.295.819985-0.79Comparativ e Example 123.996.119955-0.41Inventive Example 95.294.819955-0.21Inventive Example 104.795.319961-0.25Inventive Example 114.795.319948-0.15Comparativ e Example 133.596.519962-0.75Comparativ e Example 144.695.419954-0.79Comparativ e Example 153.996.119955-0.81Comparativ e Example 164.595.519952-0.72F: Ferrite, B: Bainite, M: Martensite, TM: Tempered Martensite [Table 6] DivisionYield Strength (YS) (MPa)Tensile Strength (TS) (MPa)YRTotal Elonga tion (%)Bendin g Workab ility (R / t)Hole Expand abilit y (%)(TS×HER) / (R / t) (MPa%)Weld Streng th (Hv)crack Length at 3√t (µm)Inventive Example 1127615560.8273.4450226164910Inventive Example 2130315750.8373.4452238085010Inventive Example 3126715480.8273.4449220504950Inventive Example 4125415490.8173.4459265675150Inventive Example 5128215530.8373.4455248304910Comparative Example 1135016600.8134.383513265592287Comparative Example 2142016900.8434.384517363589251Comparative Example 3124015200.8263.755020267581198Comparative Example 4101113700.74114.0628122564320Comparative Example 5116814300.82104.0629120554210Comparative Example 691012300.74124.062589394200Inventive Example 6132115740.8473.4450228784900Inventive Example 7129715640.8373.4452236424920Inventive Example 8130115790.8273.444922492950Comparative Example 7105414520.73104.3832106084910Comparative Example 8123115360.8074.692891704980Comparative Example 9102014650.7094.693196834920Comparative Example 10129814790.8884.6942132454900Comparative Example 11113015420.7374.0645201725010Comparative Example 12148515340.9734.6943140644950Inventive Example 9128515560.8373.2152252064920Inventive Example 10130115750.8373.3349231764900Inventive Example 11128215530.8373.2253255624950Comparative Example 13129515650.8374.2941149574920Comparative Example 14131115660.8474.2239144734890Comparative Example 15126515340.8284.0639147354920Comparative Example 16129715640.8374.0645173354950

[0095] As may be seen from Tables 1 to 6 above, in the case of Inventive Examples 1 to 11 that satisfy the alloy composition and manufacturing conditions proposed by the present disclosure, it may be seen that the mechanical properties are excellent by securing the microstructure, carbide average size, and surface roughness (Rsk) desired by the present disclosure.

[0096] In the case of Comparative Examples 1 to 6 that do not satisfy the alloy composition proposed by the present disclosure, it may be seen that the mechanical properties are inferior.

[0097] In the case of Comparative Examples 7 to 16 that do not satisfy the manufacturing conditions proposed by the present disclosure, it may be seen that the mechanical properties are inferior by not satisfying the microstructure, carbide average size, or surface roughness (Rsk).

[0098] FIG. 2 is a microstructure image of a surface layer of Inventive Example 1 observed with a scanning electron microscope (SEM), and FIG. 3 is a microstructure image of a surface layer of Comparative Example 5 observed with a scanning electron microscope (SEM). As may be seen from FIGS. 2 and 3, it may be seen that Invention Example 1 mostly has martensite and tempered martensite structures, but it may be seen that Comparative Example 5 includes a considerable amount of ferrite and bainite structures.

[0099] FIG. 4 is a microstructure image of a cross-section of a welded portion of Inventive Example 4 observed with an optical microscope, and FIG. 5 is a microstructure image of a cross-section of a welded portion of Comparative Example 1 observed with an optical microscope. As may be seen from FIGS. 4 and 5, in the case of Invention Example 4, no cracks occurred, but in the case of Comparative Example 1, cracks occurred.

Claims

1. A cold rolled steel sheet, comprising: by wt%, carbon (C): 0.19 to 0.26%, silicon (Si): 0.03 to 0.50%, manganese (Mn): 1.4 to 2.0%, chromium (Cr): 0.03 to 0.30%, molybdenum (Mo): 0.03 to 0.30%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.003% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.01 to 0.10%, niobium (Nb): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.05%, and a balance of Fe and other inevitable impurities, wherein the following relational expressions 1 to 3 are satisfied, wherein the cold rolled steel sheet includes: a central portion; and a surface layer formed on an outer side based on a thickness direction of the central portion, wherein a microstructure of the central portion includes, in area%, a sum of at least one of ferrite and bainite: 5% or less (including 0%), and a balance of at least one of martensite and tempered martensite, a microstructure of the surface layer includes, in area%, a sum of at least one of ferrite and bainite: 11% or less (excluding 0%), and a balance of at least one of martensite and tempered martensite, and an average size of carbides is 260 nm or less, 410 ≤ Y / X ≤ 620 where, in relational expressions 1 to 3, the content of each alloy element is wt%.

2. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a surface roughness (Rsk) of -0.7 to -0.1.

3. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a yield strength of 1150 to 1400 MPa.

4. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a tensile strength of 1470 to 1650 MPa.

5. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a yield ratio of 0.75 to 0.96.

6. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has an elongation of 4 to 11%.

7. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a bending workability (R / t) of 2 to 4.

8. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a hole expandability of 35 to 70%.

9. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a (tensile strength × hole expandability) / (bending workability (R / t)) of 15,000 to 35,000 MPa%.

10. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a hardness of a spot welded portion of 440 to 570 Hv after resistance spot welding.

11. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a crack length of 10µm or less (including 0µm) at a minimum nugget diameter (3√t where t: thickness of steel material) in a spot welded portion after resistance spot welding.

12. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has an electro-galvanized layer formed on at least one surface.

13. The cold rolled steel sheet of claim 1, wherein the surface layer is a region from a surface of steel to 20µm in a thickness direction.

14. A method for manufacturing a cold rolled steel sheet, heating a slab satisfying the following relational expressions 1 to 3 at 1100 to 1300°C, the slab including, by wt%, carbon (C): 0.19 to 0.26%, silicon (Si): 0.03 to 0.50%, manganese (Mn): 1.4 to 2.0%, chromium (Cr): 0.03 to 0.30%, molybdenum (Mo): 0.03 to 0.30%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.003% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.01 to 0.10%, niobium (Nb): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.05%, and a balance of Fe and other inevitable impurities; finishing hot rolling the heated slab at Ar3 to Ar3+120°C to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at Ms to 600°C; cold rolling the coiled hot rolled steel sheet at a cold reduction ratio of 35 to 70% to obtain a cold rolled steel sheet; continuously annealing the cold rolled steel sheet at Ac3+20°C to Ac3+80°C; primarily cooling the continuously annealed cold rolled steel sheet to a primary cooling end temperature (T1) of 670 to 750°C at an average cooling rate of 1 to 6°C / s; secondarily cooling the primarily cooled cold rolled steel sheet to a secondary cooling end temperature (T2) of 50 to 200°C at an average cooling rate of 40 to 110°C / s; reheating the secondarily cooled cold rolled steel sheet to an overaging temperature (H) of 100 to 250°C and then subjecting the reheated steel sheet to an overaging treatment for 5 to 12 minutes; temper rolling the overaging cold rolled steel sheet with reduction force of 500 to 1,000 tons; and tension leveling the temper-rolled cold rolled steel sheet with an elongation of 0.05 to 0.65%, wherein during the secondary cooling, the following relational expression 4 and 5 are satisfied, the primary cooling end temperature (T1) - second cooling end temperature (T2) is controlled to be 650°C or lower, and the overaging temperature (H) - second cooling end temperature (T2) is controlled to be 30°C or higher, 410 ≤ Y / X ≤ 620 940 ≤ F × Z ≤ 1200 where, in Relational Expressions 1 to 3, the content of each alloy element is wt%, ΔT is Ms - secondary cooling end temperature (T2), and Z is 3500C + 150Mn + 50 (Si+Cr+Mo) + 1000Nb.

15. The method for manufacturing a cold rolled steel sheet of claim 14, wherein the continuous annealing is performed for 30 to 230 seconds.

16. The method for manufacturing a cold rolled steel sheet of claim 14, further comprising: after the tension leveling, forming an electro-galvanized layer on at least one surface of the cold rolled steel sheet.

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