EP4640899A1

EP4640899A1 - Cold rolled steel sheet and method for manufacturing same

Info

Publication number: EP4640899A1
Application number: EP23907629.2A
Authority: EP
Inventors: Jong-Pan Kong; Eun-Young Kim; Sang-Ho Han
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-20
Filing date: 2023-12-15
Publication date: 2025-10-29
Also published as: JP2025541884A; EP4640899A4; WO2024136344A1; MX2025007154A; CN120390821A; KR20240098202A

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

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

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.
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.
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.
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]

(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 method for manufacturing same.
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

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. 0.31 ≤ X = C + Mn/20 + Si/30 + 2P + 4S ≤ 0.40 125 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 190 $410 \leq Y / X \leq 620$
(However, in the relational expressions 1 to 3, the content of each alloy element is wt%.)
The cold rolled steel sheet may have a surface roughness (Rsk) of -0.7 to -0.1.
The cold rolled steel sheet may have a yield strength of 1150 to 1400 MPa.
The cold rolled steel sheet may have a tensile strength of 1470 to 1650 MPa.
The cold rolled steel sheet may have a yield ratio of 0.75 to 0.96.
The cold rolled steel sheet may have an elongation of 4 to 11%.
The cold rolled steel sheet may have a bending workability (R/t) of 2 to 4.
The cold rolled steel sheet may have a hole expandability of 35 to 70%.
The cold rolled steel sheet may have a (tensile strength × hole expandability)/(bending workability (R/t)) of 15,000 to 35,000 MPa%.
The cold rolled steel sheet may have a hardness of a spot welded portion of 440 to 570 Hv after resistance spot welding.
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.
The cold rolled steel sheet may have an electro-galvanized layer formed on at least one surface.
The surface layer may be a region from a surface of steel to 20µm in a thickness direction.
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. 0.31 ≤ X = C + Mn/20 + Si/30 + 2P + 4S ≤ 0.40 125 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 190 $410 \leq Y / X \leq 620$ F = 1 - exp{-(1.10 × 10 - 2 × ΔT)} ≥ 0.9 $940 \leq F \times Z \leq 1200$
(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.)
The continuous annealing may be performed for 30 to 230 seconds.
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

According to an aspect of the present disclosure, a cold rolled steel sheet and a method for manufacturing the same could be provided.
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.
Brief Description of Drawings
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.
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).
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).
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.
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

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): 0.19 to 0.26%

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%

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%

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%

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%

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%

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%)

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%)

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%). 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%

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%

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%

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%.
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 equations 1 to 3. 0.31 ≤ X = C + Mn/20 + Si/30 + 2P + 4S ≤ 0.40
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. 125 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 190
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. $410 \leq Y / X \leq 620$
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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 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.
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. Ar3(°C) = 910 - 203√C + 44.7Si + 31.5Mo
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. Ms(°C) = 539 - 423C - 30.4Mn - 7.5Si + 30Al - 17.7Ni - 12.1Cr - 7.5Mo
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. Ac3(°C) = 910 - 203√C - 15.2Ni + 44.7Si + 104V + 31.5Mo + 13.1W
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 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.
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.
During the secondary cooling, the following relational expressions 4 and 5 may be preferably satisfied. F = 1 - exp{-(1.10 × 10^-2 × ΔT)} ≥ 0.9
(However, in the relational expression 4, ΔT is Ms - the second cooling end temperature (T2).)
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. $940 \leq F \times Z \leq 1200$
(where ΔT is Ms - secondary cooling end temperature (T2), and Z is 3500C + 150Mn + 50(Si+Cr+Mo) + 1000Nb.)
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.
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.
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.
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.
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.
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%.
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

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)

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.
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.
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.
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.
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.
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.
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).
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.
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.

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 Type	Alloy Composition (wt%)
Steel Type	C	Si	Mn	P	S	N	Al
Inventive Steel 1	0.22	0.1	1.85	0.01	0.001	0.004	0.03
Inventive Steel 2	0.23	0.05	1.78	0.011	0.0012	0.003	0.035
Inventive Steel 3	0.21	0.15	1.91	0.012	0.0009	0.004	0.03
Inventive Steel 4	0.24	0.15	1.7	0.012	0.0015	0.003	0.031
Inventive Steel 5	0.23	0.2	1.65	0.009	0.0011	0.004	0.035
Comparative Steel 1	0.27	0.15	1.85	0.011	0.0014	0.005	0.039
Comparative Steel 2	0.25	0.2	2.5	0.012	0.0012	0.004	0.035
Comparative Steel 3	0.25	0.6	1.95	0.015	0.001	0.004	0.03
Comparative Steel 4	0.18	0.1	1.75	0.014	0.0015	0.003	0.031
Comparative Steel 5	0.22	0.15	1.1	0.01	0.0011	0.005	0.041
Comparative Steel 6	0.21	0.09	1.42	0.008	0.001	0.003	0.035

[Table 2]

Steel Type	Alloy Composition (wt%)
Steel Type	Cr	Mo	Nb	Ti	B	X	Y	Y/X
Inventive Steel 1	0.1	0.05	0.035	0.02	0.0025	0.34	168	496
Inventive Steel 2	0.09	0.07	0.03	0.025	0.002	0.347	155	447
Inventive Steel 3	0.15	0.09	0.034	0.023	0.0021	0.338	174	513
Inventive Steel 4	0.11	0.1	0.025	0.034	0.0024	0.36	158	439
Inventive Steel 5	0.15	0.11	0.02	0.029	0.002	0.342	150	440
Comparativ e Steel 1	0.16	0.07	0.031	0.021	0.0025	0.395	175	444
Comparativ e Steel 2	0.15	0.09	0.025	0.025	0.0021	0.41	187	456
Comparativ e Steel 3	0.1	0.09	0.025	0.02	0.002	0.402	174	432
Comparativ e Steel 4	0.15	0.08	0.029	0.027	0.0024	0.305	159	521
Comparativ e Steel 5	0.07	0.04	0.03	0.025	0.0025	0.304	133	438
Comparativ e Steel 6	0.01	0.02	0.021	0.02	0.002	0.304	118	390
X = C + Mn/20 + Si/30 + 2P + 4S
Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 190
410 ≤ Y/X ≤ 620

[Table 3]

Division	Steel Type	Cold 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 1	Invent ive Steel 1	50	1.6	821	859	715	3	120	60	595
Inventive Example 2	Invent ive Steel 2	50	1.6	817	869	720	3	117	58	603
Inventive Example 3	Invent ive Steel 3	50	1.6	827	867	721	3	132	60	589
Inventive Example 4	Invent ive Steel 4	50	1.6	820	859	718	3	135	57	583
Inventive Example 5	Invent ive Steel 5	50	1.6	825	860	725	3	115	55	610
Comparative Example 1	Compar ative Steel 1	50	1.6	813	857	700	3	121	60	579
Comparative Example 2	Compar ative Steel 2	50	1.6	820	855	720	3	132	54	588
Comparative Example 3	Compar ative Steel 3	50	1.6	838	875	715	3	120	55	595
Comparative Example 4	Compar ative Steel 4	50	1.6	831	865	719	3	152	60	567
Comparative Example 5	Compar ative Steel 5	50	1.6	823	860	711	3	135	61	576
Comparative Example 6	Compar ative Steel 6	50	1.6	822	865	705	3	121	63	584
Inventive Example 6	Invent ive Steel 4	50	1.6	820	855	695	3	117	65	578
Inventive Example 7	Inventive Steel 4	50	1.6	820	865	701	3	121	61	580
Inventive Example 8	Invent ive Steel 4	50	1.6	820	861	725	3	125	60	600
Comparative Example 7	Invent ive Steel 4	50	1.6	820	825	720	4	116	57	604
Comparative Example 8	Invent ive Steel 4	50	1.6	820	861	660	3	125	60	535
Comparative Example 9	Invent ive Steel 4	50	1.6	820	856	725	3	250	65	475
Comparative Example 10	Invent ive Steel 4	50	1.6	820	867	715	3	116	60	599
Comparative Example 11	Invent ive Steel 4	50	1.6	820	864	705	3	135	60	570
Comparative Example 12	Invent ive Steel 4	50	1.6	820	867	721	3	125	61	596
Inventive Example 9	Invent ive Steel 2	56	1.4	817	869	720	3	125	65	595
Inventive Example 10	Invent ive Steel 2	57	1.2	817	869	720	3	121	62	599
Inventive Example 11	Invent ive Steel 2	58	1.6	817	869	720	3	115	63	605
Comparative Example 13	Invent ive Steel 2	30	1.4	817	869	720	3	119	57	601
Comparative Example 14	Invent ive Steel 2	30	1.2	817	869	720	3	129	62	591
Comparative Example 15	Invent ive Steel 2	30	1.6	817	869	720	3	145	63	575
Comparative Example 16	Invent ive Steel 2	50	1.6	817	869	720	3	135	65	585
Ac3(°C) = 910 - 203√C - 15.2Ni + 44.7Si + 104V + 31.5Mo + 13.1W

[Table 4]

Division	Steel Type	Ms (°C)	F	Z	F×Z	Reheat 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 1	Inventive Steel 1	388	0.95	1095	1040	180	9	60	600	0.35
Inventive Example 2	Inventive Steel 2	387	0.95	1113	1057	190	9.5	73	600	0.3
Inventive Example 3	Inventive Steel 3	389	0.94	1075	1011	195	9.5	63	600	0.25
Inventive Example 4	Inventive Steel 4	384	0.94	1138	1070	185	9.5	50	600	0.15
Inventive Example 5	Inventive Steel 5	388	0.95	1096	1041	201	10	86	600	0.2
Comparative Example 1	Comparati ve Steel 1	366	0.93	1273	1184	185	9.5	64	600	0.35
Comparative Example 2	Comparati ve Steel 2	354	0.91	1297	1180	174	9.5	42	600	0.35
Comparative Example 3	Comparati ve Steel 3	368	0.93	1232	1146	186	9.5	66	600	0.35
Comparative Example 4	Comparati ve Steel 4	407	0.94	938	882	195	10	43	600	0.35
Comparative Example 5	Comparati ve Steel 5	411	0.95	978	929	200	10	65	600	0.35
Comparative Example 6	Comparati ve Steel 6	407	0.96	975	936	201	9.5	80	600	0.35
Inventive Example 6	Inventive Steel 4	384	0.95	1138	1078	185	9.5	68	600	0.35
Inventive Example 7	Inventive Steel 4	384	0.94	1138	1075	180	10	59	600	0.25
Inventive Example 8	Inventive Steel 4	384	0.94	1138	1072	191	10	66	600	0.35
Comparative Example 7	Inventive Steel 4	384	0.95	1138	1078	185	9.5	69	600	0.35
Comparative Example 8	Inventive Steel 4	384	0.94	1138	1072	190	10	65	600	0.35
Comparative Example 9	Inventive Steel 4	384	0.77	1138	877	260	10	10	600	0.35
Comparative Example 10	Inventive Steel 4	384	0.95	1138	1078	280	10	164	600	0.35
Comparative Example 11	Inventive Steel 4	384	0.94	1138	1064	187	10	52	200	0.03
Comparative Example 12	Inventive Steel 4	384	0.94	1138	1072	194	10	69	200	0.80
Inventive Example 9	Inventive Steel 2	387	0.94	1113	1051	178	9.5	53	700	0.25
Inventive Example 10	Inventive Steel 2	387	0.95	1113	1053	181	9.5	60	700	0.25
Inventive Example 11	Inventive Steel 2	387	0.95	1113	1057	185	9.5	70	700	0.25
Comparative Example 13	Inventive Steel 2	387	0.95	1113	1055	180	9.5	61	700	0.25
Comparative Example 14	Inventive Steel 2	387	0.94	1113	1048	187	9.5	58	700	0.25
Comparative Example 15	Inventive Steel 2	387	0.93	1113	1035	181	9.5	36	600	0.25
Comparative Example 16	Inventive Steel 2	387	0.94	1113	1043	189	9.5	54	250	0.25
Ms(°C) = 539 - 423C - 30.4Mn - 7.5Si + 30Al - 17.7Ni - 12.1Cr - 7.5Mo
ΔT = Ms - Secondary Cooling End Temperature (T2)
Z = 3500C + 150Mn + 50(Si+Cr+Mo) + 1000Nb

[Table 5]

Division	Microstructure (area%)				Average Carbide Size (nm)	Surface Roughness (Rsk)
	Surface Layer		Central Portion
	At least one of F and B	At least one of M and TM	At least one of F and B	At least one of M and TM
Inventive Example 1	5.2	94.8	1	99	56	-0.35
Inventive Example 2	47	95.3	1	99	50	-0.31
Inventive Example 3	5	95	1	99	90	-0.3
Inventive Example 4	3.9	96.1	1	99	68	-0.31
Inventive Example 5	4.7	95.3	1	99	45	-0.32
Comparativ e Example 1	3.5	96.5	1	99	50	-0.4
Comparativ e Example 2	3.9	96.1	1	99	56	-0.39
Comparativ e Example 3	9.6	90.4	3	97	95	-0.31
Comparativ e Example 4	12.3	87.7	25	75	45	-0.32
Comparativ e Example 5	14.5	85.5	30	70	98	-0.3
Comparativ e Example 6	16.7	83.3	35	65	105	-0.34
Inventive Example 6	4.5	95.5	1	99	75	-0.38
Inventive Example 7	3.9	96.1	1	99	66	-0.35
Inventive Example 8	5.3	94.7	1	99	87	-0.41
Comparativ e Example 7	13.5	86.5	20	80	78	-0.32
Comparativ e Example 8	16.8	83.2	1	99	56	-0.38
Comparativ e Example 9	4.5	95.5	1	99	269	-0.35
Comparativ e Example 10	4	96	1	99	360	-0.85
Comparativ e Example 11	4.2	95.8	1	99	85	-0.79
Comparativ e Example 12	3.9	96.1	1	99	55	-0.41
Inventive Example 9	5.2	94.8	1	99	55	-0.21
Inventive Example 10	4.7	95.3	1	99	61	-0.25
Inventive Example 11	4.7	95.3	1	99	48	-0.15
Comparativ e Example 13	3.5	96.5	1	99	62	-0.75
Comparativ e Example 14	4.6	95.4	1	99	54	-0.79
Comparativ e Example 15	3.9	96.1	1	99	55	-0.81
Comparativ e Example 16	4.5	95.5	1	99	52	-0.72
F: Ferrite, B: Bainite, M: Martensite, TM: Tempered Martensite

[Table 6]

Division	Yield Strength (YS) (MPa)	Tensile Strength (TS) (MPa)	YR	Total 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 1	1276	1556	0.82	7	3.44	50	22616	491	0
Inventive Example 2	1303	1575	0.83	7	3.44	52	23808	501	0
Inventive Example 3	1267	1548	0.82	7	3.44	49	22050	495	0
Inventive Example 4	1254	1549	0.81	7	3.44	59	26567	515	0
Inventive Example 5	1282	1553	0.83	7	3.44	55	24830	491	0
Comparative Example 1	1350	1660	0.81	3	4.38	35	13265	592	287
Comparative Example 2	1420	1690	0.84	3	4.38	45	17363	589	251
Comparative Example 3	1240	1520	0.82	6	3.75	50	20267	581	198
Comparative Example 4	1011	1370	0.74	11	4.06	28	12256	432	0
Comparative Example 5	1168	1430	0.82	10	4.06	29	12055	421	0
Comparative Example 6	910	1230	0.74	12	4.06	25	8939	420	0
Inventive Example 6	1321	1574	0.84	7	3.44	50	22878	490	0
Inventive Example 7	1297	1564	0.83	7	3.44	52	23642	492	0
Inventive Example 8	1301	1579	0.82	7	3.44	49	22492	95	0
Comparative Example 7	1054	1452	0.73	10	4.38	32	10608	491	0
Comparative Example 8	1231	1536	0.80	7	4.69	28	9170	498	0
Comparative Example 9	1020	1465	0.70	9	4.69	31	9683	492	0
Comparative Example 10	1298	1479	0.88	8	4.69	42	13245	490	0
Comparative Example 11	1130	1542	0.73	7	4.06	45	20172	501	0
Comparative Example 12	1485	1534	0.97	3	4.69	43	14064	495	0
Inventive Example 9	1285	1556	0.83	7	3.21	52	25206	492	0
Inventive Example 10	1301	1575	0.83	7	3.33	49	23176	490	0
Inventive Example 11	1282	1553	0.83	7	3.22	53	25562	495	0
Comparative Example 13	1295	1565	0.83	7	4.29	41	14957	492	0
Comparative Example 14	1311	1566	0.84	7	4.22	39	14473	489	0
Comparative Example 15	1265	1534	0.82	8	4.06	39	14735	492	0
Comparative Example 16	1297	1564	0.83	7	4.06	45	17335	495	0

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.
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.
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).
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.
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

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, 0.31 ≤ X = C + Mn/20 + Si/30 + 2P + 4S ≤ 0.40 125 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 190 $410 \leq Y / X \leq 620$

where, in relational expressions 1 to 3, the content of each alloy element is wt%.
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.
The cold rolled steel sheet of claim 1,
wherein the cold rolled steel sheet has a yield strength of 1150 to 1400 MPa.
The cold rolled steel sheet of claim 1,
wherein the cold rolled steel sheet has a tensile strength of 1470 to 1650 MPa.
The cold rolled steel sheet of claim 1,
wherein the cold rolled steel sheet has a yield ratio of 0.75 to 0.96.
The cold rolled steel sheet of claim 1,
wherein the cold rolled steel sheet has an elongation of 4 to 11%.
The cold rolled steel sheet of claim 1,
wherein the cold rolled steel sheet has a bending workability (R/t) of 2 to 4.
The cold rolled steel sheet of claim 1,
wherein the cold rolled steel sheet has a hole expandability of 35 to 70%.
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%.
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.
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.
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.
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.
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, 0.31 ≤ X = C + Mn/20 + Si/30 + 2P + 4S ≤ 0.40 125 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 190 $410 \leq Y / X \leq 620$ F = 1 - exp{-(1.10 × 10 - 2 × ΔT)} ≥ 0.9 $940 \leq F \times Z \leq 1200$

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