COLD ROLLED STEEL SHEET AND METHOD OF MANUFACTURING THEREOF

Abstract

The present invention relates to a cold rolled steel sheet and a manufacturing method therefor and, more specifically, to a cold rolled steel sheet suitable for use as a steel material for automotive reinforcements such as bumper beams, seal side beams, and for the protection of electric vehicle battery cases like side frames, cross members, etc., and to a manufacturing method therefor. A preferable aspect of the present disclosure is to provide an ultra-high strength cold rolled steel sheet with excellent bending properties, shape and weldability and tensile strength of 1470 MPa or more and a manufacturing method thereof.

Description

TECHNICAL FIELD

The present disclosure relates to a cold rolled steel sheet and a manufacturing method thereof and, more specifically, to a cold rolled steel sheet suitable for use as a steel material for automotive reinforcements such as bumper beams, seal side beams, and for the protection of electric vehicle battery cases like side frames, cross members, etc., and to a manufacturing method thereof.

BACKGROUND ART

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 when manufacturing using cold forming techniques. To this end, research on ultra-high strength steels having a tensile strength of 1470 MPa or higher and a manufacturing method thereof using martensite single phases has been actively conducted. Recently, a hot press forming (HPF) method of forming materials at high temperatures that are easy to form and then securing a required strength through water cooling between a die and a material has been developed. Since high strength may be secured for the same thickness, the HPF method is widely used in the 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 is necessary. Accordingly, the development of ultra-high strength cold rolled steel sheets that are suitable for use as materials for cold stamping, have high strength and high yield ratio to secure crash performance and have excellent bending characteristics is required.

A representative prior art of this method is Patent Document 1. Patent document 1 relates to a steel sheet including 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, and B: 0.0005 to 0.005%, and further including Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, and a total of 0.005 to 0.1%, and relates to manufacturing the steel sheet by heating and maintaining the steel sheet in a temperature range of an Ae3 transformation point or more and 900° C. or less using a martensite single-phase structure, then rapidly cooling the steel sheet to 200° C. or less at an average cooling rate of 300° C./s or more, and then tempering the steel sheet at 250° C. or less. However, in the case of patent document 1, there is a problem that the shape (flatness) is inferior due to water cooling, resulting in defects during molding.

Patent document 2 relates to a thin steel sheet including 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 having 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 the width direction is 30 MPa or less, and a maximum bending amount of the thin steel sheet when sheared at a length of 1 m is 10 mm or less. However, even in the case of Patent Document 2, there is a problem that shape defects occur due to rapid cooling after annealing.

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

PRIOR ART DOCUMENTS

• • (Patent Document 1) Japanese Patent Laid-open Publication No. 2010-248565
  • (Patent Document 2) Japanese Patent Laid-open Publication No. 2020-019992

SUMMARY OF INVENTION

Technical Problem

An aspect of the present disclosure is to provide a cold rolled steel sheet and a manufacturing method thereof.

A preferable aspect of the present disclosure is to provide an ultra-high strength cold rolled steel sheet with excellent bending properties, shape and weldability and tensile strength of 1470 MPa or more and a manufacturing method thereof.

Solution to Problem

According to an aspect of the present disclosure, provided is a cold rolled steel sheet comprising: by weight %, carbon (C): 0.23 to 0.31%, silicon (Si): 0.03 to 0.30%, manganese (Mn): 0.60 to 1.30%, chromium (Cr): 0.05 to 0.50%, molybdenum (Mo): 0.05 to 0.50%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.04% or less (excluding 0%), sulfur(S): 0.003% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.015 to 0.10%, titanium (Ti): 0.01 to 0.05%, and a balance of Fe and other inevitable impurities, wherein the following relational expressions 1 to 4 are satisfied, a microstructure includes, in area %, a sum of at least one of ferrite and bainite: 1% or less (including 0%), and a balance of at least one of martensite and tempered martensite, an average size of carbides is 400 nm or less, a surface roughness (Rsk) is −0.7 to −0.1, and a flatness is 3 mm or less (including 0 mm).

$\begin{array}{cc}95\le X=48.8+49\log C+35.1\mathrm{Mn}+25.9\mathrm{Si}+14.5\mathrm{Ni}+9.6\mathrm{Cu}+76.5\mathrm{Cr}+105.9\mathrm{Mo}+1325\mathrm{Nb}+10000B\le 150& \left[\mathrm{Relational}\mathrm{Expression}1\right]\end{array}$ $\begin{array}{cc}19\le Y=13+10C+15\mathrm{Si}+5\left(\mathrm{Mn}+\mathrm{Cr}+\mathrm{Mo}\right)\le 30& \left[\mathrm{Relational}\mathrm{Expression}2\right]\end{array}$ $\begin{array}{cc}Z=C+\mathrm{Mn}/20+\mathrm{Si}/30+2P+4S\le 0.4& \left[\mathrm{Relational}\mathrm{Expression}3\right]\end{array}$ $\begin{array}{cc}0.16\le Y/X\le 0.23& \left[\mathrm{Relational}\mathrm{Expression}4\right]\end{array}$

The cold rolled steel sheet may have a yield strength of 1150 to 1450 MPa.

The cold rolled steel sheet may have a tensile strength of 1470 to 1630 MPa.

The cold rolled steel sheet may have an elongation of 3.5 to 10%.

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

The cold rolled steel sheet may have a 3-point bending maximum angle of 50 to 80°.

The cold rolled steel sheet may have a yield strength/3-point bending maximum angle of 10 to 30 MPa/°.

The cold rolled steel sheet may have a resistance spot welding current range of 1.5 to 2.5 kA.

The cold rolled steel sheet may have a hardness of a spot welded portion of 480 to 620 Hv after resistance spot welding.

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

Another aspect of the present disclosure, provided is a method of manufacturing a cold rolled steel sheet including: heating a slab at 1100 to 1300° C., the slab including, by weight %, carbon (C): 0.23 to 0.31%, silicon (Si): 0.03 to 0.30%, manganese (Mn): 0.60 to 1.30%, chromium (Cr): 0.05 to 0.50%, molybdenum (Mo): 0.05 to 0.50%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.04% or less (excluding 0%), sulfur(S): 0.003% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.015 to 0.10%, titanium (Ti): 0.01 to 0.05%, and a balance of Fe and other inevitable impurities, and satisfying the following relational expressions 1 to 4; 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+10° 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 90° C./s; reheating the secondarily-cooled cold rolled steel sheet to an overaging treatment temperature (H) of 100 to 280° C., and then performing an overaging treatment for 5 to 12 minutes; and subjecting the overaging cold rolled steel sheet to temper rolling with reduction force of 500 to 1,000 tons, wherein the primary cooling end temperature (T1)−secondary cooling end temperature (T2) is controlled to be 750° C. or lower, and the overaging treatment temperature (H)−secondary cooling end temperature (T2) is controlled to be 50° C. or higher.

$\begin{array}{cc}95\le X=48.8+49\log C+35.1\mathrm{Mn}+25.9\mathrm{Si}+14.5\mathrm{Ni}+9.6\mathrm{Cu}+76.5\mathrm{Cr}+105.9\mathrm{Mo}+1325\mathrm{Nb}+10000B\le 150& \left[\mathrm{Relational}\mathrm{Expression}1\right]\end{array}$ $\begin{array}{cc}19\le Y=13+10C+15\mathrm{Si}+5\left(\mathrm{Mn}+\mathrm{Cr}+\mathrm{Mo}\right)\le 30& \left[\mathrm{Relational}\mathrm{Expression}2\right]\end{array}$ $\begin{array}{cc}Z=C+\mathrm{Mn}/20+\mathrm{Si}/30+2P+4S\le 0.4& \left[\mathrm{Relational}\mathrm{Expression}3\right]\end{array}$ $\begin{array}{cc}0.16\le Y/X\le 0.23& \left[\mathrm{Relational}\mathrm{Expression}4\right]\end{array}$

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

The method of manufacturing a cold rolled steel sheet may further include: forming an electro-galvanized layer on at least one surface of the cold rolled steel sheet after the temper rolling.

Advantageous Effects of Invention

According to an aspect of the present disclosure, a cold rolled steel sheet and a manufacturing method thereof may be provided.

According to a preferable aspect of the present disclosure, an ultra-high strength cold rolled steel sheet with excellent bending properties, shape and weldability and tensile strength of 1470 MPa or more and a manufacturing method thereof may be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a microstructure photograph of Inventive Example 1 according to an embodiment of the present disclosure observed with an electron microscope (SEM).

FIG. 2 is a microstructure photograph of Comparative Example 6 according to an embodiment of the present disclosure observed with an electron microscope (SEM).

BEST MODE FOR INVENTION

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.

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.

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.

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): 0.23 to 0.31%

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 need to be added to secure the strength of martensite steel. When the content of C is less than 0.23%, it may be difficult to obtain the yield ratio and tensile strength targeted in the present disclosure. When the content of C exceeds 0.31%, 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 0.23 to 0.31%. A lower limit of the content of C may be more preferably 0.24%. An upper limit of the content of C may be more preferably 0.30%.

Silicon (Si): 0.03 to 0.30%

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 0.30%, 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 0.30%. A lower limit of the Si content may be more preferably 0.04%, and even more preferably 0.05%. An upper limit of the Si content may be more preferably 0.25%, and even more preferably 0.20%.

Manganese (Mn): 0.60 to 1.30%

Mn is an element added to secure strength. When the content of Mn is less than 0.60%, 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 1.30%, 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 0.60 to 1.30%. A lower limit of the Mn content may be more preferably 0.65%. An upper limit of the Mn content may be more preferably 1.20%.

Chromium (Cr): 0.05 to 0.50%

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 0.05%, 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 0.50%, 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 0.05 to 0.50%. A lower limit of the Cr content may be more preferably 0.10%. An upper limit of the Cr content may be more preferably 0.45%.

Molybdenum (Mo): 0.05 to 0.50%

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 0.05%, it may be difficult to sufficiently obtain the above-mentioned effects. When the content of Mo exceeds 0.50%, 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 0.05 to 0.50%. A lower limit of the Mo content may be more preferably 0.07%. An upper limit of the Mo content may be more preferably 0.45%.

Boron (B): 0.0005 to 0.005%

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%. A lower limit of the content of B may be preferably 0.0007%. An upper limit of the content of B may be preferably 0.004%.

Phosphorus (P): 0.04% or Less (Excluding 0%)

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%). The content of P may be preferably 0.025% or less.

Sulfur(S): 0.003% or Less (Excluding 0%)

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%)

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%). The content of N may be more preferably 0.008% or less, and even more preferably 0.006% or less.

Aluminum (Al): 0.015 to 0.10%

Al may be added to remove oxygen in molten steel. When the Al content is less than 0.015%, 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 0.015 to 0.10%. An upper limit of the Al content may be more preferably 0.075%.

Titanium (Ti): 0.01~0.05%

Ti is a nitride-forming element that scavenges by precipitating solid-solution N as TiN. When the content of Ti is less than 0.01%, 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 0.01 to 0.05%. A lower limit of the content of Ti may be more preferably 0.02%. An upper limit of the content of Ti may be more preferably 0.04%.

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.

The cold rolled steel sheet of the present disclosure may satisfy an alloy composition described above and the following relational expressions 1 to 4.

$\begin{array}{cc}95\le X=48.8+49\log C+35.1\mathrm{Mn}+25.9\mathrm{Si}+14.5\mathrm{Ni}+9.6\mathrm{Cu}+76.5\mathrm{Cr}+105.9\mathrm{Mo}+1325\mathrm{Nb}+10000B\le 150& \left[\mathrm{Relational}\mathrm{Expression}1\right]\end{array}$

The relational expression 1 is a component relational expression for securing hardenability. When a value of the X is less than 95, it may be difficult to secure targeted strength because the soft ferrite and bainite structures are transformed during cooling. When the value of the X exceeds 150, the strength becomes excessively high, which may make it difficult to secure a target elongation, and thus processing cracks may occur during forming. Accordingly, the value of the X may preferably have a range of 95 to 150. A lower limit of the X value may be more preferably 100. An upper limit of the X value may be more preferably 145.

$\begin{array}{cc}19\le Y=13+10C+15\mathrm{Si}+5\left(\mathrm{Mn}+\mathrm{Cr}+\mathrm{Mo}\right)\le 30& \left[\mathrm{Relational}\mathrm{Expression}2\right]\end{array}$

The relational expression 2 is a component relational expression related to resistivity that has a major effect on resistance spot weldability. When the value of the Y is less than 19, the resistivity is significantly low, so that excessive welding current may be required, which may increase manufacturing costs. When the value of the Y exceeds 30, the resistivity becomes excessively high, making it difficult to secure a targeted resistance spot weldability current range. Accordingly, the value of the Y may preferably have a range of 19 to 30. A lower limit of the Y value may be more preferably 20. An upper limit of the Y value may be more preferably 28.

$\begin{array}{cc}Z=C+\mathrm{Mn}/20+\mathrm{Si}/30+2P+4S\le 0.4& \left[\mathrm{Relational}\mathrm{Expression}3\right]\end{array}$

The relational expression 3 is a component Relational Expression that may predict the hardness of a welded portion. When a value of the Z exceeds 0.40, the welded portion becomes excessively high, and processing cracks may occur due to insufficient toughness of the welded portion during component forming. Accordingly, the value of the Z may preferably have a range of 0.40 or less. The value of the Z may be more preferably 0.38 or less. Meanwhile, in the present disclosure, as the value of the Z is lower, it may be more advantageous, and thus, there is no particular limitation on a lower limit thereof. However, the lower limit of the value of the Z may be, for example, 0.30.

$\begin{array}{cc}0.16\le Y/X\le 0.23& \left[\mathrm{Relational}\mathrm{Expression}4\right]\end{array}$

The relational expression 4 is a component relationship for securing hardenability for high strength and optimizing resistance spot weldability. When a value of the Y/X is less than 0.16, since a large amount of hardenable elements should be added, there may be a disadvantage that the manufacturing costs increases. When the value of the Y/X exceeds 0.23, there is a disadvantage that it may be difficult to secure strength due to insufficient hardenability. Accordingly, the value of the Y/X may be preferably in the range of 0.16 to 0.23. A lower limit of the value of the Y/X may be more preferably 0.18. An upper limit of the value of the Y/X may be more preferably 0.225.

The cold rolled steel sheet of the present disclosure may have a microstructure including, in area %, a sum of at least one of ferrite and bainite: 1% or less (including 0%), and a balance of at least one of martensite and tempered martensite. The martensite and tempered martensite are significantly advantageous structures for securing the strength, bending properties, weldability, and the like, which are targeted by the present disclosure. At least one of the martensite and tempered martensite may be preferably 100%. However, at least one of the ferrite and bainite may inevitably be formed during the manufacturing process, and when a total fraction of at least one of the ferrite and bainite exceeds 1%, it may be difficult to secure the properties desired by the present disclosure.

The cold rolled steel sheet of the present disclosure may have an average size of carbides of, preferably, 400 nm or less. When an average size of the carbides exceeds 400 nm, bending properties may be inferior. An average size of carbides may be more preferably 200 nm or less. In the present disclosure, as an average size of the carbide, it is more advantageous, 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 10 nm as an example. Meanwhile, the carbide may be at least one of carbide including Fe and Mn, carbide including Mn and Cr, and carbide including Fe, Mn, Cr, and Mo, as an example.

The cold rolled steel sheet of the present disclosure may preferably 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 the 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 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.

The cold rolled steel sheet of the present disclosure may preferably have a flatness of 3 mm or less (including 0 mm). When the flatness exceeds 3 mm, there is a disadvantage in that the dimensional accuracy may become inferior during a component forming process such as roll forming. Meanwhile, the flatness refers to a wave in the steel sheet, not a normal curve of the steel sheet, and refers to a height difference between the highest point and the lowest point. The flatness may be more preferably 2 mm or less.

The cold rolled steel sheet of the present disclosure provided as described above may have a yield strength of 1150 to 1450 MPa, a tensile strength of 1470 to 1630 MPa, an elongation of 3.5 to 10%, a bending workability (R/t): 2 to 4, a maximum angle of 3-point bending: 50 to 80°, a yield strength/maximum angle of 3-point bending: 10 to 30 MPa/°, a resistance spot weldability current range of 1.5 to 2.5 kA, and a hardness of a point welded portion: 480 to 620 Hv. Meanwhile, the present disclosure does not specifically limit a method for forming the welded portion, but as an example, the cold rolled steel sheet of the present disclosure may be obtained by resistance spot weldability by applying the conditions of force: 4.5 KN, welding time: 170 ms, holding time: 250 ms. Additionally, the cold rolled steel sheet of the present disclosure has the advantage of not causing cracks during component forming.

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.

The cold rolled steel sheet of the present disclosure may have an electro-galvanized layer formed on at least one surface. The present disclosure does not specifically limit the type of the electro-galvanized layer, and all types of electro-galvanized layers commonly used in the relevant technical field may be formed.

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

First, a slab satisfying the above-mentioned alloy composition and the relational expressions 1 to 4 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.

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.

$\begin{array}{cc}\mathrm{Ar}3\left(°C.\right)=910-203\surd C+44.7\mathrm{Si}+31.5\mathrm{Mo}& \left[\mathrm{Relational}\mathrm{Expression}1\right]\end{array}$

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 more preferably be Ms+50° C. An upper limit of the coiling temperature may more preferably be 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.

$\begin{array}{cc}\mathrm{Ms}\left(°C.\right)=539-423C-30.4\mathrm{Mn}-7.5\mathrm{Si}+30\mathrm{Al}-17.7\mathrm{Ni}-12.1\mathrm{Cr}-7.5\mathrm{Mo}& \left[\mathrm{Relational}\mathrm{Expression}2\right]\end{array}$

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.

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%.

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.

$\begin{array}{cc}\mathrm{Ac}3\left(°C.\right)=910-203\surd C-15.2\mathrm{Ni}+44.7\mathrm{Si}+104V+31.5\mathrm{Mo}+13.1W& \left[\mathrm{Relational}\mathrm{Expression}1\right]\end{array}$

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.

Then, the continuously annealed cold rolled steel sheet is primarily cooled to a primary cooling end temperature (T1) of 600 to 750° C. at an average cooling rate of 1 to 6° C./s. When the primary cooling end temperature (T1) is less than 600° 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 610° C., and even more preferably 620° C. An upper limit of the primary cooling end temperature may be more preferably 740° C., and even more preferably 730° 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.

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 90° 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 secondary average cooling rate exceeds 90° C./s, a 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 85° C./s, even more preferably 80° C./s, and most preferably 75° C./s.

It is preferable to control the primary cooling end temperature (T1)−secondary cooling end temperature (T2) to be 750° C. or lower. When the primary cooling end temperature (T1)−secondary cooling end temperature (T2) exceeds 750° C., shape defects may occur. It is more preferable that the primary cooling end temperature (T1)−secondary cooling end temperature (T2) be 700° C. or lower.

Then, the secondarily-cooled cold rolled steel sheet is reheated to an overaging temperature (H) of 100 to 280° 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., there is a disadvantage that since tempering is not sufficiently performed, the yield strength is low and sufficient toughness may not be secured. When the reheating temperature and overaging temperature exceed 280° 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 275° C., even more preferably 270° C., and most preferably 265° C. When the overaging time is less than 5 minutes, tempering is not sufficiently performed, which may decrease the yield strength. When the overaging treatment time exceeds 12 minutes, the carbide may coarsen due to excessive tempering, which may result in poor bending properties. A lower limit of the overaging treatment 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 treatment time may be more preferably 11.5 minutes, even more preferably 11 minutes, and most preferably 10.5 minutes.

It is preferable to control the overaging treatment temperature (H)−secondary cooling end temperature (T2) to be 50° C. or higher. When the overaging treatment temperature (H)−secondary cooling end temperature (T2) is less than 50° C., tempering is not sufficiently performed, making it difficult to secure the target yield strength. The overaging treatment temperature (H)−secondary cooling end temperature (T2) may be more preferably 60° C. or higher, and even more preferably 70° C. or higher.

Then, the overaging cold rolled steel sheet is subjected to temper rolling (Skin Pass Mill (SPM)) with 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, a load thereof is low, making it difficult to control the surface roughness (Rsk), and when the reduction force exceeds 1,000 tons, a surface may be severely hardened, which may result 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.

Meanwhile, after the temper rolling, 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 of forming the electro-galvanized layer, and any method commonly used in the relevant technical field may be used.

MODE FOR INVENTION

Hereinafter, the present disclosure will be described in more detail through examples. However, the description of these examples is only for illustrating the implementation 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 patent claims and matters reasonably inferred therefrom.

EXAMPLE

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

A microstructure, an average carbide size, surface roughness (Rsk), flatness, and mechanical properties of the cold rolled steel sheets manufactured in this manner were measured, and the results are shown in Tables 4 and 5 below.

The microstructure was observed using a scanning electron microscope (SEM) and an optical microscope (OM) in a position of ¼t (t: thickness of steel) from the surface of the steel sheet, and fractions of each phase were analyzed three times through image analysis to calculate an average value thereof.

An average carbide size was captured using a transmission electron microscope (TEM) in a position of ¼t (t: thickness of steel) from the surface of the steel sheet, and an average value thereof was calculated.

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

Flatness was calculated by cutting the cold rolled steel sheet into 500 mm lengthwise sections and calculating a height difference between the highest and lowest sections when there was a wave in the full width.

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

Bending workability (R/t) was measured by processing the cold rolled steel sheet into specimens with a width of 100 mm×length of 30 mm, and then performing a 90° bending test at a test speed of 100 mm/min, and a stereoscopic microscope was used to check whether cracks occurred in a bent portion, and an R/t value was obtained by dividing a minimum bending radius (a R value of a mold) in which no cracks occurred by the thickness of the specimen (mm).

For a maximum angle of 3-point bending, the cold rolled steel sheet into specimens with a width of 60 mm×length of 30 mm was processed, and then, a test was performed at a test speed of 20 mm/min and a punching radius of 0.4R, which are the VDA238-100 standards, thereby measuring a maximum bending angle in a maximum load at which cracks occurred.

The hardness of a point welded portion was measured 10 times with Vickers hardness (load: 500 gf) for the spot welded portion after performing resistance spot welding under the conditions of Force: 4.5 KN, Welding time: 170 ms, and Holding time: 250 ms, thereby an average value thereof.

A resistance spot weldability current range was defined as a difference between a lower limit current satisfying 4√t (t: thickness of steel) after performing the resistance spot welding test and an upper limit current obtained by subtracting 0.2 kA from the welding current at which Expulsion occurred.

For the occurrence of cracks during the molding of the component, the cold rolled steel sheet was molded into a shape of a bumper beam to visually observe whether cracks occurred during the molding.

TABLE 1
	Alloy Composition (wt %)
Steel Type	C	Si	Mn	P	S	N	Al
Inventive	0.27	0.10	0.95	0.010	0.0010	0.003	0.030
Steel 1
Inventive	0.27	0.15	0.90	0.015	0.0009	0.004	0.035
Steel 2
Inventive	0.28	0.10	0.85	0.012	0.0011	0.004	0.032
Steel 3
Inventive	0.26	0.10	0.90	0.011	0.0012	0.003	0.031
Steel 4
Inventive	0.28	0.15	1.00	0.015	0.0007	0.004	0.025
Steel 5
Comparative	0.33	0.27	1.50	0.009	0.0009	0.003	0.031
Steel 1
Comparative	0.22	0.10	0.90	0.010	0.0007	0.004	0.038
Steel 2
Comparative	0.25	0.25	1.80	0.015	0.0010	0.004	0.026
Steel 3
Comparative	0.29	0.27	0.65	0.017	0.0015	0.004	0.031
Steel 4
Comparative	0.25	0.25	0.75	0.018	0.0010	0.003	0.036
Steel 5
Comparative	0.27	0.01	0.30	0.015	0.0011	0.004	0.035
Steel 6

TABLE 2
	Alloy Composition (wt %)
Steel Type	Cr	Mo	Ti	B	X	Y	Z	Y/X
Inventive	0.25	0.15	0.025	0.0020	112	24.0	0.34	0.21
Steel 1
Inventive	0.30	0.20	0.025	0.0025	126	25.0	0.35	0.20
Steel 2
Inventive	0.25	0.21	0.025	0.0020	115	23.9	0.35	0.21
Steel 3
Inventive	0.27	0.25	0.025	0.0020	121	24.2	0.34	0.20
Steel 4
Inventive	0.31	0.15	0.025	0.0020	120	25.4	0.37	0.21
Steel 5
Comparative	0.30	0.22	0.025	0.0020	151	30.5	0.44	0.20
Steel 1
Comparative	0.10	0.15	0.025	0.0020	94.3	22.5	0.29	0.24
Steel 2
Comparative	0.30	0.25	0.025	0.0020	158	31.0	0.38	0.20
Steel 3
Comparative	0.03	0.10	0.025	0.0015	80.2	23.9	0.37	0.30
Steel 4
Comparative	0.25	0.01	0.025	0.0020	92.3	24.3	0.34	0.26
Steel 5
Comparative	0.15	0.05	0.030	0.0025	73.5	18.4	0.32	0.25
Steel 6
X = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb
Y = 13 + 10C + 15Si + 5 (Mn + Cr + Mo)
Z = C + Mn/20 + Si/30 + 2P + 4S

TABLE 3
			Thickness			Primary		Secondary
			of Cold			Cooling	Primary	Cooling	Secondary
		Cold	rolled			End	Average	End	average
		Reduction	Steel		Annealing	Temperature	Cooling	temperature	cooling
	Steel	Ratio	Sheet	Ac3	Temperature	(T1)	Rate	(T2)	rate	T1-T2
Division	Type	(%)	(mm)	(° C.)	(° C.)	(° C.)	(° C./s)	(° C.)	(° C./s )	(° C.)
Inventive	Inventive	52	1.4	814	860	710	3	110	65	600
Example 1	Steel 1
Inventive	Inventive	52	1.4	818	860	710	3	115	60	595
Example 2	Steel 2
Inventive	Inventive	52	1.4	814	860	710	3	121	55	589
Example 3	Steel 3
Inventive	Inventive	52	1.4	819	860	710	3	135	60	575
Example 4	Steel 4
Inventive	Inventive	52	1.4	814	860	710	3	102	60	608
Example 5	Steel 5
Comparative	Comparative	52	1.4	812	860	710	3	105	65	605
Example 1	Steel 1
Comparative	Comparative	52	1.4	824	860	710	3	112	60	598
Example 2	Steel 2
Comparative	Comparative	52	1.4	828	860	710	3	121	60	589
Example 3	Steel 3
Comparative	Comparative	52	1.4	816	860	710	3	128	55	582
Example 4	Steel 4
Comparative	Comparative	52	1.4	820	860	710	3	110	60	600
Example 5	Steel 5
Comparative	Comparative	52	1.4	807	860	710	3	101	65	609
Example 6	Steel 6
Inventive	Inventive	52	1.4	814	865	710	3.2	120	60	590
Example 6	Steel 1
Inventive	Inventive	52	1.4	814	855	710	3.2	120	60	590
Example 7	Steel 1
Inventive	Inventive	52	1.4	814	860	690	3	105	65	585
Example 8	Steel 1
Inventive	Inventive	60	1.4	814	865	720	3.1	110	70	610
Example 9	Steel 1
Inventive	Inventive	56	1.4	814	855	690	3	105	67	585
Example 10	Steel 1
Comparative	Inventive	52	1.4	814	855	830	1	40	98	790
Example 7	Steel 1
Comparative	Inventive	52	1.4	814	860	710	3.5	110	60	600
Example 8	Steel 1
Comparative	Inventive	52	1.4	814	860	700	3	125	65	575
Example 9	Steel 1
Comparative	Inventive	30	1.4	814	865	710	3.1	105	67	605
Example 10	Steel 1
Comparative	Inventive	52	1.4	814	865	715	3	110	65	605
Example 11	Steel 1
Ac3 (° C.) = 910 − 203 √C − 15.2Ni + 44.7Si + 104V + 31.5Mo + 13.1W

TABLE 4
						Microstructure
		Reheating/			Temper	(area %)
		Overaging	Over-		rolling	At least	At	Carbide
		Temperature	aging		reduction	one of	least	Average	Surface
	Steel	(H)	Time	H-T2	force	M and	one of	size	Roughness	Flatness
Division	Type	(° C.)	(min)	(° C.)	(ton)	TM	F and B	(nm)	(Rsk)	(mm)
Inventive	Inventive	210	10	100	500	99	1	100	−0.36	1
Example 1	Steel 1
Inventive	Inventive	200	10	85	500	99	1	95	−0.42	1
Example 2	Steel 2
Inventive	Inventive	190	10	69	500	99	1	84	−0.40	1
Example 3	Steel 3
Inventive	Inventive	200	10	65	500	99	1	90	−0.48	1
Example 4	Steel 4
Inventive	Inventive	210	10	108	500	99	1	35	−0.35	1
Example 5	Steel 5
Comparative	Comparative	190	10	85	500	99	1	210	−0.41	1
Example 1	Steel 1
Comparative	Comparative	195	10	83	500	75	25	105	−0.38	1
Example 2	Steel 2
Comparative	Comparative	189	10	68	500	99	1	95	−0.31	1
Example 3	Steel 3
Comparative	Comparative	200	10	72	500	65	35	100	−0.38	1
Example 4	Steel 4
Comparative	Comparative	205	10	95	500	60	40	97	−0.40	1
Example 5	Steel 5
Comparative	Comparative	210	10	109	500	48	52	25	−0.45	1
Example 6	Steel 6
Inventive	Inventive	220	9	100	500	99	1	105	−0.35	1
Example 6	Steel 1
Inventive	Inventive	220	9	100	500	99	1	152	−0.41	1
Example 7	Steel 1
Inventive	Inventive	230	10	125	500	99	1	135	−0.36	1
Example 8	Steel 1
Inventive	Inventive	210	9	100	500	99	1	101	−0.11	1
Example 9	Steel 1
Inventive	Inventive	205	10	100	500	99	1	120	−0.10	1
Example 10	Steel 1
Comparative	Inventive	220	9.5	180	500	99	1	106	−0.31	8
Example 7	Steel 1
Comparative	Inventive	290	9.5	180	500	99	1	450	−0.41	1
Example 8	Steel 1
Comparative	Inventive	220	9.5	95	500	99	1	135	−0.31	7
Example 9	Steel 1
Comparative	Inventive	200	9.5	95	500	99	1	110	−0.79	1
Example 10	Steel 1
Comparative	Inventive	210	9.5	100	150	99	1	120	−0.82	1
Example 11	Steel 1

TABLE 5
						Yield	Resistance
					Maximum	strength/	spot
					angle	Maximum	weld-	Strength	Crack
	Yield	Tensile			of 3-	Angle of	ability	of	occurrence
	Strength	Strength	Total	Bending	point	3 -point	current	Welded	during
	(YS)	(TS)	Elongation	Workability	bending	bending	range	Portion	Component
Division	(MPa)	(MPa)	(%)	(R/t)	(°)	(MPa/°)	(kA)	(Hv)	Forming
Inventive	1319	1588	7.2	3.57	61	22	2.0	524	No
Example 1									occurrence
Inventive	1300	1582	7.0	3.57	59	22	1.9	535	No
Example 2									occurrence
Inventive	1302	1577	7.6	3.57	59	22	2.1	536	No
Example 3									occurrence
Inventive	1315	1588	8.0	3.57	60	22	2.0	511	No
Example 4									occurrence
Inventive	1315	1589	7.5	3.57	59	22	2.0	550	No
Example 5									occurrence
Comparative	1460	1644	3.0	4.29	47	31	1.2	635	Occurrenc
Example 1
Comparative	1143	1431	9.3	3.57	62	18	2.2	452	No
Example 2									occurrence
Comparative	1308	1580	6.2	3.57	60	22	1.1	580	No
Example 3									occurrence
Comparative	920	1288	10.9	3.21	69	15	2.0	561	No
Example 4									occurrence
Comparative	835	1220	11.1	2.86	65	13	2.1	517	No
Example 5									occurrence
Comparative	735	1130	12.2	2.86	67	11	2.5	492	No
Example 6									occurrence
Inventive	1285	1565	8.1	3.57	62	21	2.0	525	No
Example 6									occurrence
Inventive	1301	1555	7.6	3.57	61	21	2.0	521	No
Example 7									occurrence
Inventive	1297	1569	8.0	3.57	60	22	2.0	520	No
Example 8									occurrence
Inventive	1279	1571	7.5	3.21	64	20	2.0	520	No
Example 9									occurrence
Inventive	1308	1580	7.4	3.21	65	20	2.0	520	No
Example 10									occurrence
Comparative	1395	1601	6.0	3.57	57	24	2.0	528	Occurrenc
Example 7
Comparative	1325	1585	7.2	4.64	42	32	2.0	520	No
Example 8									occurrence
Comparative	1085	1574	7.9	3.57	64	17	2.0	523	No
Example 9									occurrence
Comparative	1301	1574	7.5	5.0	40	33	2.0	520	Occurrenc
Example 10
Comparative	1297	1581	7.6	5.0	41	32	2.0	520	Occurrenc
Example 11

As may be seen from Tables 1 to 5 above, in the case of Inventive Examples 1 to 10 satisfying the alloy composition and manufacturing conditions proposed by the present disclosure, it may be seen that the mechanical properties were excellent by securing the microstructure, carbide average size, surface roughness (Rsk), and flatness, which are desired by the present disclosure.

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 were inferior.

In the case of Comparative Examples 7 to 11 that do not satisfy the manufacturing conditions proposed by the present disclosure, it may be seen that the mechanical properties were inferior by not satisfying a microstructure, a carbide average size, surface roughness (Rsk), or flatness.

FIG. 1 is a microstructure image of Inventive Example 1 according to an embodiment of the present disclosure observed with a scanning electron microscope (SEM), and FIG. 2 is a microstructure image of Comparative Example 6 according to an embodiment of the present disclosure observed with a scanning electron microscope (SEM). As may be seen from FIGS. 1 and 2, Inventive Example 1 may be seen to mostly have martensite and tempered martensite structures, but Comparative Example 6 may be seen to include a considerable amount of ferrite and bainite structures.

Claims

1. A cold rolled steel sheet comprising: by weight %, carbon (C): 0.23 to 0.31%, silicon (Si): 0.03 to 0.30%, manganese (Mn): 0.60 to 1.30%, chromium (Cr): 0.05 to 0.50%, molybdenum (Mo): 0.05 to 0.50%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.04% or less (excluding 0%), sulfur(S): 0.003% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.015 to 0.10%, titanium (Ti): 0.01 to 0.05%, and a balance of Fe and other inevitable impurities, 95 ≤ X = 48.8 + 49 ⁢ log ⁢ C + 35.1 Mn + 25.9 Si + 14.5 Ni + 9.6 Cu + 76.5 Cr + 105.9 Mo + 1325 ⁢ Nb + 10000 ⁢ B ≤ 150 [ Relational ⁢ Expression ⁢ 1 ] 19 ≤ Y = 13 + 10 ⁢ C + 15 ⁢ Si + 5 ⁢ ( Mn + Cr + Mo ) ≤ 30 [ Relational ⁢ Expression ⁢ 2 ] Z = C + Mn / 20 + Si / 30 + 2 ⁢ P + 4 ⁢ S ≤ 0.4 [ Relational ⁢ Expression ⁢ 3 ] 0.16 ≤ Y / X ≤ 0.23. [ Relational ⁢ Expression ⁢ 4 ]

wherein the following relational expressions 1 to 4 are satisfied,

a microstructure includes, in area %, a sum of at least one of ferrite and bainite: 1% or less (including 0%), and a balance of at least one of martensite and tempered martensite,

an average size of carbides is 400 nm or less,

a surface roughness (Rsk) is −0.7 to −0.1, and

a flatness is 3 mm or less (including 0 mm),

2. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a yield strength of 1150 to 1450 MPa.

3. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a tensile strength of 1470 to 1630 MPa.

4. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has an elongation of 3.5 to 10%.

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

6. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a 3-point bending maximum angle of 50 to 80°.

7. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a yield strength/3-point bending maximum angle of 10 to 30 MPa/°.

8. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a resistance spot welding current range of 1.5 to 2.5 kA.

9. The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a hardness of a spot welded portion of 480 to 620 Hv after resistance spot welding.

10. 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 thereof.

11. A method of manufacturing a cold rolled steel sheet, comprising: 95 ≤ X = 48.8 + 49 ⁢ log ⁢ C + 35.1 Mn + 25.9 Si + 14.5 Ni + 9.6 Cu + 76.5 Cr + 105.9 Mo + 1325 ⁢ Nb + 10000 ⁢ B ≤ 150 [ Relational ⁢ Expression ⁢ 1 ] 19 ≤ Y = 13 + 10 ⁢ C + 15 ⁢ Si + 5 ⁢ ( Mn + Cr + Mo ) ≤ 30 [ Relational ⁢ Expression ⁢ 2 ] Z = C + Mn / 20 + Si / 30 + 2 ⁢ P + 4 ⁢ S ≤ 0.4 [ Relational ⁢ Expression ⁢ 3 ] 0.16 ≤ Y / X ≤ 0.23 [ Relational ⁢ Expression ⁢ 4 ]

heating a slab at 1100 to 1300° C., the slab including, by weight %, carbon (C): 0.23 to 0.31%, silicon (Si): 0.03 to 0.30%, manganese (Mn): 0.60 to 1.30%, chromium (Cr): 0.05 to 0.50%, molybdenum (Mo): 0.05 to 0.50%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.04% or less (excluding 0%), sulfur(S): 0.003% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.015 to 0.10%, titanium (Ti): 0.01 to 0.05%, and a balance of Fe and other inevitable impurities, and satisfying the following relational expressions 1 to 4;

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+10° 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 90° C./s;

reheating the secondarily-cooled cold rolled steel sheet to an overaging treatment temperature (H) of 100 to 280° C., and then performing an overaging treatment for 5 to 12 minutes; and

subjecting the overaging cold rolled steel sheet to temper rolling with reduction force of 500 to 1,000 tons,

wherein the primary cooling end temperature (T1)−secondary cooling end temperature (T2) is controlled to be 750° C. or lower, and

the overaging treatment temperature (H)−secondary cooling end temperature (T2) is controlled to be 50° C. or higher,

12. The method of manufacturing a cold rolled steel sheet of claim 11, wherein the continuous annealing is performed for 30 to 230 seconds.

13. The method of manufacturing a cold rolled steel sheet of claim 11, further comprising:

forming an electro-galvanized layer on at least one surface of the cold rolled steel sheet after the temper rolling.