Classifications

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  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
  • B21C47/02—Winding-up or coiling
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/20—Isothermal quenching, e.g. bainitic hardening
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • 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
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  • 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
  • C21D8/0247—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
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  • 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
  • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0421—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
  • C21D8/0426—Hot rolling
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  • 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
  • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0421—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
  • C21D8/0436—Cold rolling
• • C—CHEMISTRY; METALLURGY
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  • 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
  • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0447—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the heat treatment
  • C21D8/0463—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 to produce plates or strips for drawing, e.g. for deep-drawing 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
  • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0447—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation annealing
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
• • 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
  • 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
  • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • 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
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  • C22C—ALLOYS
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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  • 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
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  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
  • B21D22/20—Deep-drawing
  • B21D22/208—Deep-drawing by heating the blank or deep-drawing associated with heat treatment
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/62—Quenching devices
  • C21D1/673—Quenching devices for die quenching
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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  • C21D2211/003—Cementite
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  • C21D2211/00—Microstructure comprising significant phases
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  • C21D2211/00—Microstructure comprising significant phases
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite

Definitions

• the present disclosure relates to a cold-rolled steel sheet for hot forming having excellent bendability, a hot-formed member, and a method for manufacturing the same.
• a hot-formed ultra-high-strength member has recently been widely applied to a structural member of automobiles for the purpose of improving fuel efficiency and protecting passenger through the weight reduction of automobiles.
• Patent Document 1 has proposed a representative technology for such hot forming.
• Patent Document 1 proposes a technology for securing ultra-high strength having a tensile strength exceeding 1600 MPa by heating a steel sheet to a temperature of 850°C or higher, and then forming a structure of the member into martensite through hot forming and rapid cooling using a press.
• the technology proposed in Patent Document 1 since the steel sheet is formed at high temperatures, even a complex shape thereof can be easily formed, and a weight reduction effect due to high strength may be expected through the increased strength due to rapid cooling within a mold.
• Bendability is considered to be a representative indicator for evaluating collision resistance properties of an HPF formed member used for the purpose such as passenger protection, or the like.
• the property (bendability) able to withstand bending of a certain distance (angle) without fracturing is required.
• An aspect of the present disclosure is to provide a steel material for hot forming that can have high strength and impart excellent bendability to a member, a hot-formed member, and a method for manufacturing the same.
• An object of the present disclosure is not limited to the above description.
• the object of the present disclosure will be understood from the entire contents of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.
• a cold-rolled steel sheet for hot forming including, by weight%: 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities,
• the cold-rolled steel sheet for hot forming may further include at least one selected from the following a) to f).
• a method for manufacturing a hot-formed member including operations of: heating a slab, including by weight%, 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities, to a temperature within a range of 1000 to 1300°C; hot rolling the heated slab at a finishing rolling temperature of Ar3 to 1000°C to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet at a cooling rate of 400°C/s or more and 750°C/s or less; coiling the hot-rolled steel sheet at a temperature within a range of more than Ms to 750°C or lower; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;
• the steel slab further comprises at least one selected from the following a) to f):
• the cold rolling may be performed at a cumulative reduction ratio of 30 to 80%.
• the continuous annealing may be performed at a temperature within a range of 700 to 900°C for 1 to 1000 seconds.
• a method for manufacturing a hot-formed member including operations of: manufacturing a cold-rolled steel sheet according to the method for manufacturing the cold-rolled steel sheet for hot forming; heating the cold-rolled steel sheet to a temperature of 700°C or higher at a heating rate of 1 to 1000°C/sec; hot forming the heated cold-rolled steel sheet; and cooling the hot-formed steel sheet at a cooling rate of 10 to 1000°C/sec.
• the cooling may be performed by setting a cooling stop temperature to a martensite transformation finish temperature (Mf) or lower.
• the cooling may be performed by setting the cooling stop temperature to a martensite transformation finish temperature (Mf) or higher and a martensite transformation start temperature (Ms) or lower, and in this case, after the cooling, maintaining the temperature at a cooling end temperature or reheating the same to an Ac1 or lower, may be further included.
• a hot-formed member including by weight%: 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities,
• the hot-formed member may further include at least one selected from the following a) to f).
• the hot-formed member may have a tensile strength of 1800 MPa or more and a yield strength of 1200 MPa or more.
• a steel material for hot forming having a high tensile strength of 1800MPa or more, while ensuring high bendability, a hot-formed member using the same, and a method for manufacturing the same may be provided.
• a unit of content of each element is based on weight, and a unit of a ratio of structure is based on an area.
• the inventors of the present disclosure have recognized that in the case of a unplated ultra-high strength cold-rolled steel sheet for hot forming, there is a problem that it is difficult to secure excellent bendability because the bendability is reduced after the hot forming process under normal hot rolling conditions, and have conducted in-depth research to solve the problem.
• a cold-rolled steel sheet for hot forming having excellent surface quality may include by weight %, 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities.
• Carbon (C) is essential element to increase the strength of a heat-treated member and should be added appropriately.
• a more preferable lower limit of the content of C is 0.26%, and an even more preferable lower limit of the content of C is 0.27%.
• the content of C exceeds 0.45%, the strength of a hot-rolled material becomes too high when the hot-rolled material is cold rolled, which greatly reduces the cold-rollability, and also greatly reduces the spot weldability, so it is preferable that the content of C is 0.45% or less.
• a more preferable upper limit of the content of C is 0.42%, and an even more preferable upper limit of the content of C is 0.40%.
• Silicon (Si) plays an important role in forming a Si-based amorphous oxide layer by being concentrated on the surface during annealing of a cold-rolled steel sheet in a continuous annealing line, and also plays a role in securing spot weldability of a member by suppressing the formation of (Fe, Mn, Cr) oxide layers during the hot forming process.
• a lower limit of the content of Si is preferably 0.01%.
• a more preferable lower limit of the content of is 0.1%.
• the content of Si exceeds 3.0%, there is a problem in that the spot weldability is rather reduced due to the formation of an Si-based amorphous oxide layer, which is too thick.
• a more preferable upper limit of the content of Si is 2.8%, and an even more preferable upper limit of the content of Si is 2.5%.
• Chromium (Cr) not only improves hardenability of a steel sheet, but also plays a role in helping to form a stable surface Si-based amorphous oxide layer through an appropriate reaction with Si.
• Manganese (Mn) may not only secure a solid solution strengthening effect, but also needs to be added to lower a critical cooling rate for securing martensite in a hot-formed member.
• a content of Mn is less than 0.01%, the above-described effect is insufficient.
• a more preferable lower limit of the content of Mn is 0.05%, and an even more preferable lower limit of the content of Mn is 0.1%.
• the content of Mn exceeds 4.0%, since the strength of a steel sheet before the hot forming process increases too much, there are disadvantages in that not only may a blanking operation be difficult, but also the cost thereof may be increased due to the excessive addition of alloy iron and the spot weldability may be inferior.
• a more preferable upper limit of the content of Mn is 3.0%, and an even more preferable upper limit of the content of is 2.5%.
• Aluminum (Al), along with Si, may act as a deoxidizer in steelmaking and improve the cleanliness of steel.
• a content of Al is less than 0.001%, the above-described effect is insufficient.
• a more preferable lower limit of the content of Al is 0.002%, and a more preferable lower limit of the content of Al is 0.003%.
• the content of the content of Al exceeds 0.4%, there is a problem that an Ac3 temperature increases excessively and a heating temperature should be increased.
• a more preferable upper limit of the content of Al is 0.3%, and an even more preferable upper limit of the content of Al is 0.2%.
• Phosphorus (P) is an impurity, and to control a content of P be less than 0.001%, a lot of manufacturing costs are required, and when the content of P exceeds 0.05%, the weldability of a hot-formed member may be significantly reduced.
• a more preferable upper limit of the content of P is 0.03%.
• S Sulfur
• S is an impurity, and to control a content of S be less than 0.0001%, a lot of manufacturing costs are required, and when the content of S exceeds 0.02%, the ductility, impact properties, and weldability of a member are deteriorated.
• a more preferable upper limit of the content of S is 0.01%.
• N Nitrogen
• N is an impurity, and to control a content of N be less than 0.001%, a lot of manufacturing costs are required, and when the content of N exceeds 0.02%, it will not only be susceptible to cracking during continuous casting of a slab, but also the impact characteristics may deteriorate.
• a more preferable upper limit of the content of N is 0.01%.
• the remaining component of the present disclosure is iron (Fe).
• Fe iron
• the component since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.
• At least one selected from the following a) to f) may be further included.
• characteristics such as surface quality and hot formability may be further improved.
• Ti, Nb, Zr and V are effective in improving the strength of a heat-treated member by forming fine precipitates, stabilizing retained austenite and improving impact toughness by refining grains.
• the content meaning the total contents of two or more elements when two or more elements are added
• the above-described effect may be insufficient, and a more preferable lower limit of the contents thereof is 0.005%, and an even more preferable lower limit of the contents thereof is 0.008%.
• the content of the contents thereof exceeds 0.4%, not only will the effect be saturated, but it may also cause an increase in costs due to excessive addition of alloy iron.
• a more preferable upper limit of the contents thereof is 0.38%, and an even more preferable upper limit of the contents thereof is 0.35%
• Boron (B) is an element that can improve hardenability even with a small amount of addition, and can suppress brittleness of a hot-formed member due to grain boundary segregation of P and/or S by being segregated in grain boundaries of old austenite.
• Mo and W may be added to improve hardenability, improve strength through precipitation strengthening effect, and refine grains.
• the content meaning the sum of Mo and W when both Mo and W are added
• the above-described effect is insufficient, and a more preferable lower limit of the contents of Mo and W is 0.0015%, and an even more preferable lower limit of the contents of Mo and W is 0.002%.
• the contents of Mo and W exceeds 1.0%, not only is the effect saturated, but there is also the problem of increased costs.
• a more preferable upper limit of the contents of Mo and W is 0.95%, and an even more preferable upper limit the contents of Mo and W is 0.9%.
• Cu may be added as an element which improves strength by forming fine precipitates.
• hot brittleness can be caused when Cu is added alone, so Ni can be added as needed.
• the sum of these components is less than 0.005%, the above-described effect may be insufficient, and a more preferable lower limit of the contents of Cu and Ni is 0.006%, and an even more preferable lower limit of the contents of Cu and Ni is 0.007%. If the sum of the contents of Cu and Ni exceeds 2.0%, a more preferable upper limit thereof is 1.95%, and an even more preferable upper limit thereof is 1.9%.
• Sb and Sn have the effect of suppressing the formation of oxides that may be formed at grain boundaries of a surface layer of a hot-rolled material of a steel material to which Si is added, and may suppress dent defects caused by the detachment of the grain boundaries of the surface layer when a cold-rolled material is annealed.
• a more preferable lower limit of the contents of Sb and Sn is 0.002%, and an even more preferable lower limit of the contents of Sb and Sn is 0.03%.
• An REM element may control the activity of Fe in steel to control a thickness of Fe scales formed in the surface layer during hot forming.
• the addition of REM element of 0.0001% or more is required.
• a more preferable lower limit a content of an REM element is 0.00015%, and an even more preferable lower limit the content of the REM element of is 0.0002%.
• the content of the REM element exceeds 0.02%, the controllability of Fe activity may be lost, and the surface quality may deteriorate. Therefore, it is preferable that the content of the REM element be controlled to 0.02% or less, and more preferable that the content of the REM element be controlled to 0.01% or less.
• the cold-rolled steel sheet for hot forming according to an aspect of the present disclosure may not only satisfy the alloy composition described above, but also may have a value of a structure ratio represented by the following [Relational Expression 1] of 0.2 or more and 1.3 or less.
• Structure ratio V p S + V ⁇ S / V p C + V ⁇ C
• V p S and V ⁇ S represent an area ratio of pearlite and cementite in a surface layer portion, respectively
• V p C and V ⁇ C represent an area ratio of pearlite and cementite in a central portion, respectively.
• the surface layer portion may mean a region within 100 ⁇ m in a thickness direction from the surface, and the central portion in the present disclosure may mean a region of 1/2t ⁇ 50 ⁇ m in the thickness direction from the surface, where t means a thickness of a steel material (mm).
• the structure ratio represented by [Relational Expression 1] exceeds 1.3, a deviation in hardness between the martensite formed on the surface layer after hot forming increases, so that stress due to bending may be concentrated in the stronger martensite, and the stress unevenness in the thickness direction may increase due to a difference in the deviation in hardness in the thickness direction, which may result in poor bendability.
• the structure ratio may be 1.15 or less, and even more preferably, 0.95 or less.
• the structure ratio is less than 0.2 as suggested by the present disclosure, strength after hot forming may not be secured, and the tensile strength may be 1800 MPa or less.
• a more preferable lower limit thereof is 0.25, and an even more preferable lower limit thereof is 0.3.
• the microstructure of the cold-rolled steel sheet according to the present disclosure may include ferrite and cementite.
• the area ratio is not particularly limited, but, for example, the total contents of ferrite and cementite may be 5% or more, by area ratio.
• the member the present disclosure may satisfy a value of a hardness ratio represented by the following [Relational Expression 2] of 0.1 or more and 10 or less.
• Hardness ratio ⁇ H ⁇ S / ⁇ H ⁇ C
• ⁇ H ⁇ S represents a standard deviation of hardness in a surface layer portion
• ⁇ H ⁇ C represents a standard deviation of hardness in a central portion
• the bendability may be deteriorated due to a stress concentration phenomenon and stress imbalance in the thickness direction caused by the deviation in hardness between martensites.
• the hardness ratio is 10 or less, the deviation in hardness is good, so that the bendability may be improved.
• the hardness ratio is less than 0.1, the deviation in hardness in the central portion in the thickness direction may be relatively severe, resulting in poor bendability.
• a composition of a base steel sheet of a member according to the present disclosure is the same as the composition of the cold-rolled steel sheet described above, so it is not described separately.
• a hot-formed member according to an aspect of the present disclosure may include martensite or bainite as a main phase to secure high strength.
• the main phase may mean a phase having the largest area ratio among several phases forming a microstructure.
• An area ratio thereof is not particularly limited, but more preferably, the area ratio may be 50% or more.
• a method for manufacturing a cold-rolled steel sheet for hot forming including operations of: heating a slab, including by weight%, 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities, to a temperature within a range of 1000 to 1300°C; hot rolling the heated slab at a finishing rolling temperature of Ar3 to 1000°C to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet at a cooling rate of 400°C/s or more and 750°C/s or less; coiling the hot-rolled steel sheet at a temperature within a range of more than Ms to 750°C or lower; cold rolling the coiled hot-rolled steel sheet at a cumulative reduction ratio of
• the heating temperature is lower than 1000°C, it is difficult to homogenize a structure of the slab, and when the heating temperature exceeds 1300°C, the formation of excessive oxides and increased manufacturing costs may occur.
• the heated slab is hot rolled at a finishing rolling temperature of Ar3 to 1000°C to obtain a hot-rolled steel sheet.
• finishing rolling temperature is lower than the Ar3 temperature, since rolling in two phase region is likely to occur, a mixed grain structure can be generated on the surface, and it is difficult to control the shape of the hot-rolled steel sheet.
• finishing rolling temperature exceeds 1000°C, the grains of the hot-rolled steel sheet are likely to be coarse.
• the hot-rolled steel sheet having the hot rolling performed is cooled at a cooling rate of 400°C/s or more and 750°C/s or less.
• the hot-rolled steel sheet is coiled at a temperature within a range of more than Ms to 750°C or lower.
• the coiling temperature is a martensite transformation start temperature (Ms) or lower, the strength of the hot-rolled steel sheet becomes too high, which reduces cold-rollability.
• Ms martensite transformation start temperature
• the coiling temperature exceeds 750°C, it causes an increase in a thickness of an oxide layer and oxidation of grain boundaries on a surface layer, which may not only reduce the pickling property but also cause the problem of the grain boundaries on the surface layer being removed during annealing in a continuous annealing furnace.
• the coiled hot-rolled steel sheet is cold rolled to obtain a cold-rolled steel sheet. This is to control the thickness of the steel sheet more precisely, and pickling may be performed before cold rolling.
• a reduction ratio of the cold rolling does not need to be particularly limited, but cold rolling may be performed at a reduction ratio of 30 to 80% to secure a predetermined target thickness.
• the cold-rolled steel sheet is continuously annealed at a temperature within a range of 700 to 900°C.
• an annealing temperature is lower than 700°C, it is difficult for a rolled structure created by cold rolling to recover and recrystallize, and when the annealing temperature exceeds 900°C, it can deteriorate an annealing equipment, which can become a factor in increasing process costs due to frequent replacement of the equipment.
• an annealing time may be 1 to 1000 seconds.
• the annealing time is less than 1 second, it is difficult to obtain the annealing effect, and when the annealing time exceeds 1000 seconds, productivity may decrease.
• a method for manufacturing a hot-formed member includes heating a cold-rolled steel sheet manufactured by the method for manufacturing a cold-rolled steel sheet according to the present disclosure described above to a temperature of 700°C or higher at a heating rate of 1 to 1000°C/sec; hot forming the heated cold-rolled steel sheet; and cooling the hot-formed steel sheet at a cooling rate of 10 to 1000°C/sec.
• the cold-rolled steel sheet manufactured by the method for manufacturing a cold-rolled steel sheet according to the present disclosure described above is heated to a temperature of 700°C or higher at a heating rate of 1 to 1000°C/sec.
• the steel sheet After hot forming the heated cold-rolled steel sheet, the steel sheet is cooled at a cooling rate of 10 to 1000°C/sec.
• a cooling stop temperature in the cooling operation may be lower than M f (martensite transformation end temperature). This is because when cooling is stopped at a temperature exceeding M f and then cooled to room temperature again, it may be difficult to secure the shape fixability of a hot-formed member.
• cooling may be stopped between M f (martensite transformation finish temperature) and M s (martensite transformation start temperature), and then maintained at a cooling end temperature or reheated to Ac1 or lower to temper martensite and stabilize retained austenite.
• the hot-formed member may have martensite or bainite as a main phase in order to secure high strength.
• the main phase means a phase having the largest area ratio among various phases forming the microstructure.
• An area ratio thereof does not need to be specifically limited, but the area ratio may be, for example, 50% or more.
• the hot-formed member may have a tensile strength of 1800 MPa or more.
• a tensile strength of 1800 MPa or more By securing a high strength of 1800 MPa or more, it may be preferably applied to automobile structural members, reinforcing materials, or the like, requiring collision resistance properties.
• a slab having a thickness of 40 mm having the composition illustrated in Table 1 below was vacuum melted, heated in a heating furnace at a temperature of 1200°C for 1 hour, and then hot rolled at a finishing rolling temperature of 930°C to manufacture a hot-rolled steel sheet having a final thickness of 3 mm.
• the hot-rolled steel sheet was cooled at the cooling rate illustrated in Table 2 below and then coiled at a temperature of 640°C. Subsequently, the hot-rolled steel sheet was pickled and then cold rolled at a cold reduction ratio of 50%.
• a cold-rolled steel sheet for hot forming was manufactured by performing continuous annealing at a temperature of 800°C for 80 seconds after the cold rolling.
• the manufactured cold-rolled steel sheet was heated at a heating rate of 20°C/sec, heat treated at 900°C for 6 minutes, and the heated cold-rolled steel sheet was hot formed. Subsequently, the hot-formed steel sheet was cooled to room temperature at a cooling rate of 20°C/sec to manufacture a hot-formed member.
• An area ratio of structures in a surface layer portion and a central portion of the manufactured cold-rolled steel sheet for hot forming, and a structure ratio of [Relational Expression 1] are shown in Table 2.
• OM optical microscope
• the area ratio of the structures in the surface layer portion and central portion were measured three times, respectively, using CLEMEX Vision PE software after the measurement using an optical photograph, and average values thereof were shown in Table 2.
• a hardness ratio which is a ratio of the standard deviation in the hardness in the surface layer portion and central portion of the hot-formed member manufactured after hot forming, was shown in Table 2 based on [Relational Expression 2].
• a tensile strength and maximum bending angle were shown.
• Hardness was measured at a minimum of 10 points at 1 mm intervals using a Vickers hardness tester (Dura Scan 80G5) with a load of 10 kgf, and a value of the tensile strength was measured through a room temperature tensile test using a JIS-5 specimen according to the ISO6892 standard.
• the maximum bending angle is described as a value of an outer bending angle converted from a maximum bending strength specified in the standard according to a bendability evaluation method according to the VDA238-100 standard.
• a change rate of the bending angle represents a deviation ratio of a bending angle between a specimen manufactured under the manufacturing conditions proposed in the present disclosure and a specimen manufactured outside the proposed manufacturing conditions.
• a cooling rate after hot rolling was controlled within the range limited by the present disclosure, so that a structure ratio of the cold-rolled steel sheet was satisfied to have a range of 0.2 or more and 1.3 or less, and at the same time, a hardness ratio thereof was 10 or less, and the hot-formed member thus manufactured exhibited good bendability.

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