Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/22—Electroplating: Baths therefor from solutions of zinc
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• 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.
• 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.
• Patent Document 1 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.
• the shape flatness
• 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.
• a standard deviation of yield strength in a width direction is 30 MPa or less, and a maximum bending
• 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.
• 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 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 10 ⁇ m or less (including 0 ⁇ m) 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 20 ⁇ m 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
• 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.
• a cold rolled steel sheet and a method for manufacturing the same could be provided.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the content of C may be preferably in the range of 0.19 to 0.26%.
• 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%.
• Si suppresses the formation of carbides and controls a size of carbides during reheating and overaging treatment operations performed after continuous annealing and cooling.
• the content of Si is less than 0.03%, it may be difficult to sufficiently obtain the above-described effect.
• 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.
• 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%.
• 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%.
• Mn is an element added to secure strength.
• 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.
• 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.
• the content of Mn may be preferably in the range of 1.4 to 2.0%.
• 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%.
• 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.
• 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.
• 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.
• the content of Cr may be preferably in the range of 0.03 to 0.30%.
• 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%.
• 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.
• the content of Mo is less than 0.03%, it may be difficult to sufficiently obtain the above-mentioned effects.
• 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%.
• 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%.
• the present disclosure has the advantage of suppressing the formation of ferrite during cooling after continuous annealing.
• 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.
• the content of B exceeds 0.005%, ductility may be significantly reduced.
• 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%, and an upper limit of the content of B may be preferably 0.004%.
• P is an impurity element included in steel.
• the content of P exceeds 0.03%, weldability deteriorates and there may be a risk of steel brittleness.
• S is an impurity element included in steel, similarly to P.
• S may hinder ductility and weldability, and a large amount of MnS precipitates may form, which result in poor bending workability.
• 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.
• N is an impurity element.
• 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.
• Al may be added to remove oxygen in molten steel.
• the Al content is less than 0.01%, deoxidation is not sufficiently performed, which may harm the cleanliness of the steel.
• 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.
• the Al content may preferably have a range of 0.01 to 0.10%.
• an upper limit of the Al content may be more preferably 0.075%.
• 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.
• the content of Nb is less than 0.01%, the austenite grain refinement and precipitation strengthening effects may not be sufficiently obtained.
• 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%.
• Ti is a nitride-forming element that scavenges by precipitating solid-solution N as TiN.
• 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.
• 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.
• the content of Ti may be preferably in the range of 0.005 to 0.05%.
• 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%.
• iron (Fe) The remaining components are iron (Fe).
• iron (Fe) 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.
• a value of X is less than 0.31, it is difficult to sufficiently secure the hardness and strength of the welded portion.
• the 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.
• the value of X may have a range of 0.31 to 0.40.
• 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.
• 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.
• 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.
• 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.
• the relational expression 3 is a component relational expression for securing an appropriate level of the hardness and hardenability of the welded portion.
• 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.
• 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.
• the value of the Y/X may preferably have a range of 410 to 620.
• 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.
• 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.
• 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 20 ⁇ m 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.
• the average size of the carbides may be more preferably 250 nm or less.
• 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.
• the lower limit of the average size of the carbide may be, for example, 10 nm.
• 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.
• 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.
• the value of the surface roughness (Rsk) is less than -0.7, the bending characteristics may become inferior.
• 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 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 ⁇ m or less (including 0 ⁇ m).
• the crack length of 0 ⁇ m denotes that no crack occurs.
• 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.
• the plating layer may be an electro-galvanized layer.
• the plating layer may be a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.
• 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.
• the slab heating temperature is less than 1100°C, a problem occurs in which a hot-rolling load increases rapidly.
• 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.
• the heated slab is subjected to a finishing hot rolling at Ar3 to Ar3+120°C to obtain a hot-rolled steel sheet.
• a 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.
• 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.
• 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
• the hot-rolled steel sheet is coiled at Ms to 600°C.
• 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.
• 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
• 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.
• 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.
• 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.
• 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.
• 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%.
• the cold rolled steel sheet is continuously annealed at Ac3+10°C to Ac3+80°C.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• T1 a primary cooling end temperature
• T2 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• a value of the F may be more preferably 0.91 or higher.
• the upper limit of the F value may be 0.99 as an example.
• the relational expression 5 is a relational expression that examines the relationship between the relational expression 4 and strength-related factors.
• 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.
• 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.
• 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.
• H overaging temperature
• the martensite obtained by the above-described rapid cooling process may be transformed into tempered martensite, thereby increasing the yield strength.
• 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.
• 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.
• the overaging time is less than 5 minutes, since tempering is not sufficiently performed, the yield strength may be low.
• 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.
• 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.
• SPM Standard Pass Mill
• the temper rolling enables control of the surface roughness (Rsk).
• Rsk surface roughness
• 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.
• 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.
• shape correction may be difficult.
• 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%.
• 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.
• 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 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 20 ⁇ m 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.
• SEM scanning electron microscope
• OM optical microscope
• 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.
• TEM transmission electron microscope
• 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 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 was measured according to the ISO 16330 standard, and the holes were sheared with a clearance of 12% using a 10 mm diameter punch.
• 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.
• FIG. 2 is a microstructure image of a surface layer of Inventive Example 1 observed with a scanning electron microscope (SEM)
• FIG. 3 is a microstructure image of a surface layer of Comparative Example 5 observed with a scanning electron microscope (SEM).
• SEM scanning electron microscope
• FIG. 4 is a microstructure image of a cross-section of a welded portion of Inventive Example 4 observed with an optical microscope
• FIG. 5 is a microstructure image of a cross-section of a welded portion of Comparative Example 1 observed with an optical microscope.
• FIGS. 4 and 5 in the case of Invention Example 4, no cracks occurred, but in the case of Comparative Example 1, cracks occurred.

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