WO2021066274A1 - 고강도 및 고성형성을 가지는 강판 및 그 제조방법 - Google Patents

고강도 및 고성형성을 가지는 강판 및 그 제조방법 Download PDF

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WO2021066274A1
WO2021066274A1 PCT/KR2020/006385 KR2020006385W WO2021066274A1 WO 2021066274 A1 WO2021066274 A1 WO 2021066274A1 KR 2020006385 W KR2020006385 W KR 2020006385W WO 2021066274 A1 WO2021066274 A1 WO 2021066274A1
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steel sheet
steel
strength
high strength
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PCT/KR2020/006385
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English (en)
French (fr)
Korean (ko)
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엄호용
구남훈
김민성
오규진
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현대제철 주식회사
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Priority to JP2021564566A priority Critical patent/JP7419401B2/ja
Priority to CN202080035734.7A priority patent/CN113825852B/zh
Priority to DE112020004666.4T priority patent/DE112020004666T5/de
Priority to BR112022001969A priority patent/BR112022001969A2/pt
Priority to MX2022001389A priority patent/MX2022001389A/es
Priority to US17/608,068 priority patent/US20220220576A1/en
Publication of WO2021066274A1 publication Critical patent/WO2021066274A1/ko

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    • 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties 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 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
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C21D2211/001—Austenite
    • 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

Definitions

  • the present invention relates to a steel plate and a method of manufacturing the same, and more particularly, to a steel plate having high strength and high formability, and a method of manufacturing the same.
  • Patent Application No. 10-2016-0077463 title of the invention: ultra-high strength, high ductility steel sheet with excellent yield strength and a manufacturing method thereof.
  • the problem to be solved by the present invention is to provide a steel sheet having high formability and high strength, and a method of manufacturing the same.
  • the steel sheet having high strength and high formability is in weight%, carbon (C): 0.05 to 0.15%, silicon (Si): more than 0 0.4% or less, manganese (Mn): 4.0 to 9.0%, Aluminum (Al): greater than 0 and 0.3% or less, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.006% or less, balance iron (Fe) and other inevitable impurities included do. It has a microstructure made of ferrite and retained austenite. The grain size of the microstructure is 3 ⁇ m or less. Yield strength (YS): 800 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 25% or more, hole expandability (HER): 20% or more.
  • YS Yield strength
  • TS tensile strength
  • EL elongation
  • HER hole expandability
  • the steel sheet includes at least one of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), and the at least one may be greater than 0 and less than 0.02% by weight. .
  • the steel sheet may further include boron (B): greater than 0 and less than or equal to 0.001%.
  • the volume fraction of the retained austenite in the microstructure may be 10 to 30% by volume.
  • the method of manufacturing a steel sheet having high strength and high formability is (a) in weight%, carbon (C): 0.05 to 0.15%, silicon (Si): more than 0 and not more than 0.4%, manganese (Mn) : 4.0 to 9.0%, aluminum (Al): more than 0 and 0.3% or less, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.006% or less, balance iron (Fe) And manufacturing a hot-rolled sheet material using a steel slab containing other inevitable impurities; (b) cold rolling the hot-rolled sheet to manufacture a cold-rolled sheet; (c) subjecting the cold-rolled sheet to a first heat treatment at a temperature of AC3 to AC3 + 15°C; And (d) subjecting the cold-rolled sheet material subjected to the first heat treatment to a second heat treatment at an ideal temperature. After step (d), the cold-rolled sheet has a microstructure made of ferrite
  • the steel slab includes at least one of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), wherein the at least one is greater than 0 and less than 0.02% by weight, respectively. have.
  • the steel slab may further include boron (B): greater than 0 and less than or equal to 0.001%.
  • step (c) may include cooling the heat-treated cold-rolled sheet material to 350 to 450°C at a cooling rate of 4 to 10°C/s.
  • step (d) may include cooling the heat-treated cold-rolled sheet material to 350 to 450°C or less at 4 to 10°C/s.
  • the step (a) comprises the steps of (a1) reheating the steel slab to a temperature of 1150 ⁇ 1250 °C; (a2) hot rolling the reheated steel slab at a finish rolling temperature of 925 to 975°C; And (a3) cooling the hot-rolled steel to 700°C to 800°C at a cooling rate of 10 to 30°C/s and then winding it up.
  • the hot-rolled sheet may further include a step of softening heat treatment at 550°C to 650°C.
  • the cold-rolled sheet has yield strength (YS): 800 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 25% or more, hole expandability (HER): It can have more than 20%.
  • the crystal grain size of the cold-rolled sheet may be 3 ⁇ m or less.
  • a steel sheet having a microstructure made of ultrafine ferrite and retained austenite can be manufactured through component system control and process condition control. Due to the fine grain ferrite, the steel sheet has high strength, and due to the residual austenite present in the microstructure in 10 to 30% by volume, it has high strength and elongation, and by controlling the shape of the microstructure, high It can function to have hole expandability (HER). As a result, it is possible to effectively secure a steel sheet having high formability and high strength.
  • HER hole expandability
  • FIG. 1 is a process flow diagram schematically showing a method of manufacturing a steel sheet having high strength and high formability according to an embodiment of the present invention.
  • FIG. 4 is a photograph showing the microstructure of a high-strength steel sheet according to an embodiment of the present invention.
  • a steel sheet having high strength and high formability may have a fine grain ferrite and residual austenite present in an amount of 10 to 30% by volume as a final microstructure.
  • the steel sheet may have high strength, high elongation, and high hole expandability (HER).
  • the steel sheet is made to sufficiently contain residual austenite at a level of 10 to 30% by volume.
  • the retained austenite can improve the elongation of the steel sheet in substantially the same manner as in the conventional metamorphic organic plastic steel.
  • an austenite stabilizing element may be appropriately added to the steel sheet as described later.
  • the first and second annealing heat treatment may be continuously performed, and the second annealing heat treatment may be performed in an ideal area.
  • the interface between the hard phase and the soft phase, which can function as a crack formation point in the steel sheet is reduced.
  • hard phases such as martensite and bainite
  • the boundary surface between precipitates and crystal grains is reduced.
  • the content of a precipitate generating element such as titanium, niobium, vanadium, and the like and a precipitate growth inhibiting element such as molybdenum may be controlled.
  • the fraction of High Angle Grain Boundaries (HAGBs) in the final structure may be increased.
  • the high-angle grain boundary may mean a grain boundary having an angle of 15° or more between neighboring grains.
  • the shape of the microstructure may be controlled so that the steel sheet has high hole expandability.
  • the annealing heat treatment may be divided into a first and a second heat treatment in a two-stage heat treatment process.
  • the crystal grains of the final microstructure are refined.
  • the grain size of ferrite and retained austenite can be controlled to be 3 ⁇ m or less.
  • the primary annealing temperature can proceed at AC3 ⁇ AC3 +15 °C.
  • the high-strength steel sheet according to an embodiment of the present invention is in weight%, carbon (C): 0.05 to 0.15%, silicon (Si): more than 0 0.4%, manganese (Mn): 4.0 to 9.0%, aluminum (Al) : More than 0 0.3%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, Nitrogen (N): 0.006% or less
  • the balance contains iron (Fe) and other unavoidable impurities.
  • the high-strength steel sheet further includes at least one of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), wherein each of the at least one may exceed 0 and be 0.02% or less.
  • the high-strength steel sheet may further include boron (B): greater than 0 and less than or equal to 0.001% by weight.
  • each component included in the high-strength cold-rolled steel sheet according to an embodiment of the present invention will be described in detail (the content of each component is a weight percent of the total steel sheet, hereinafter expressed as %).
  • Carbon (C) is the most important alloying element in steel making, and in the present invention, the main purpose is to play a basic reinforcing role and to stabilize austenite.
  • the high carbon (C) concentration in austenite improves austenite stability, making it easy to secure appropriate austenite for material improvement.
  • an excessively high carbon (C) content may lead to a decrease in weldability due to an increase in carbon equivalent, and since a large number of cementite precipitated structures such as pearlite may be formed during cooling, carbon (C) is 0.05 to 0.15% of the total weight of the steel sheet. It is preferable to add.
  • the carbon is included in an amount of less than 0.05%, it is difficult to secure the strength of the steel sheet, and when it is included in an amount exceeding 0.15%, toughness and ductility may be deteriorated.
  • Silicon (Si) is an element that suppresses the formation of carbides in ferrite, and increases the activity of carbon (C) to increase the diffusion rate of austenite. Silicon (Si) is also well known as a ferrite stabilizing element and is known as an element that increases the ferrite fraction during cooling to increase ductility. In addition, since the suppression of the formation of carbide is very high, it is a necessary element to secure the TRIP effect by increasing the carbon concentration in the retained austenite during the formation of bainite. However, when silicon (Si) is added in excess of 0.4%, oxide (SiO2) may be formed on the surface of the steel sheet during the process, the rolling load may be increased during hot rolling, and a large amount of red scale may be generated. Therefore, it is preferable to add silicon (Si) in an amount of 0.4% or less of the total weight of the steel sheet.
  • Manganese (Mn) is an austenite stabilizing element, and as manganese (Mn) is added, the starting temperature of martensite, Ms, gradually decreases, thereby increasing the residual austenite fraction after heat treatment.
  • Manganese is contained in 4.0 to 9.0% of the total weight of the steel sheet. When manganese is added in an amount of less than 4.0%, the above-described effect cannot be sufficiently secured. Conversely, when manganese is added in excess of 9.0%, weldability decreases due to an increase in carbon equivalent and oxides (MnO) are formed on the surface of the steel sheet during processing, resulting in a decrease in plating properties due to poor wettability.
  • MnO oxides
  • Al aluminum
  • Al aluminum
  • Niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo) may be selectively included in the steel.
  • niobium (Nb), titanium (Ti), and vanadium (V) are elements precipitated in the form of carbides in steel, and are elements added to secure strength through precipitation of carbides.
  • titanium (Ti) it is possible to suppress the formation of AlN to suppress the formation of cracks during playing.
  • niobium (Nb), titanium (Ti), and vanadium (V) are added in excess of 0.2%, respectively, by forming coarse precipitates, the amount of carbon in the steel is reduced to deteriorate the material, and niobium (Nb) , Titanium (Ti), and vanadium (V)
  • Nb niobium
  • Ti Titanium
  • V vanadium
  • molybdenum may play a role of controlling the size of the carbide by suppressing the growth of the carbide.
  • molybdenum when molybdenum is added in excess of 0.2%, the above effect is saturated and there is a disadvantage of an increase in manufacturing cost.
  • Boron (B) may be selectively added to the steel sheet, and may function as a grain boundary strengthening element. Boron may be added in an amount greater than 0 and 0.001% or less of the total weight of the steel sheet. When boron is added in excess of 0.001%, high-temperature ductility can be reduced by forming nitrides such as BN.
  • Phosphorus (P), sulfur (S) and nitrogen (N) may inevitably be added into the steel during the steelmaking process. That is, ideally, it is preferable not to include it, but it is difficult to completely remove it due to the process technology, so a certain small amount may be included.
  • Phosphorus (P) can play a similar role to silicon in steel. However, when phosphorus is added in excess of 0.02% of the total weight of the steel sheet, it may reduce the weldability of the steel sheet and increase brittleness, resulting in material degradation. Therefore, phosphorus can be controlled to be added to 0.02% or less of the total weight of the steel sheet.
  • sulfur (S) may impair toughness and weldability in the steel, it may be controlled to be contained in 0.005% or less of the total weight of the steel sheet.
  • nitrogen (N) When nitrogen (N) is excessively present in the steel, a large amount of nitride may be precipitated and ductility may be deteriorated. Therefore, nitrogen (N) can be controlled to be contained in 0.006% or less of the total weight of the steel sheet.
  • the high-strength steel sheet of the present invention having the above alloying components has a microstructure made of ferrite and retained austenite.
  • the volume fraction of the retained austenite in the microstructure may be 10 to 30% by volume.
  • the crystal grains of the high-strength steel sheet may be fine grains having a size of 3 ⁇ m or less. Among the crystal grains, a fraction of an elevation grain boundary may be 70% or more.
  • the high-strength steel sheet may have material properties such as yield strength (YS): 800 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 25% or more, and hole expandability (HER): 20% or more. Accordingly, the high-strength steel sheet according to an embodiment of the present invention can be applied to fields requiring high strength and high formability.
  • the high-strength steel sheet according to the exemplary embodiment of the present invention described above may be manufactured by the method of an exemplary embodiment as follows.
  • the present invention intends to present a steel sheet having excellent elongation, hole expandability and strength by performing a two-stage annealing heat treatment after proceeding with an alloy component of an appropriately controlled composition ratio, a hot rolling process and a cold rolling process, and a manufacturing method thereof.
  • FIG. 1 is a process flow diagram schematically showing a method of manufacturing a steel sheet having high strength and high formability according to an embodiment of the present invention.
  • the method of manufacturing the steel sheet includes reheating a steel slab (S110), manufacturing a hot-rolled sheet material by hot rolling the steel slab (S120), and cold-rolling the hot-rolled sheet material (S130). , And annealing and heat treatment of the cold-rolled sheet (S140).
  • the steel slab reheating step (S110) is, in weight%, carbon (C): 0.05 to 0.15%, silicon (Si): more than 0 0.4%, manganese (Mn): 4.0 to 9.0%, aluminum (Al) : More than 0 0.3%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, Nitrogen (N): 0.006% or less Prepare a steel slab containing the balance iron (Fe) and other inevitable impurities And, it is a step of reheating the steel slab to re-dissolve the segregated components during casting and homogenize the components at the time of casting.
  • the steel slab further includes at least one of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), wherein each of the at least one may exceed 0 and be 0.02% or less.
  • the steel slab may further include boron (B): greater than 0 and not more than 0.001% by weight.
  • the steel slab reheating temperature is preferably about 1150 to 1250°C so as to secure a normal hot rolling temperature. If the reheating temperature is less than 1150°C, the hot rolling load may rapidly increase, and if it exceeds 1250°C, it may be difficult to secure the strength of the final produced steel sheet due to coarsening of initial austenite grains.
  • the hot rolling step (S120) is a step of forming a hot-rolled sheet material by performing hot rolling in a conventional method after reheating the slab, and performing finish rolling at a temperature of 925 to 975°C.
  • the finish rolling may be performed at a high temperature of 925 to 975°C.
  • the hot-rolled sheet is cooled to 700 to 800°C at a cooling rate of 10 to 30°C/s, and then wound up.
  • the cooling method may be applied to the non-injection cooling method.
  • the hot-rolled sheet may have a full martensite structure after cooling.
  • softening heat treatment may be performed to reduce a rolling load during cold rolling.
  • the softening heat treatment may be performed at 550 to 650°C.
  • the temperature of the softening heat treatment is less than 550° C., recrystallization does not occur for martensite generated after the hot rolling, and only tempering proceeds, so that supersaturated carbon in the structure may be formed in the form of cementite and spheroidized. In this case, the brittleness of the martensite may be expressed, and thus the plate may be fractured during cold rolling.
  • the softening heat treatment when the temperature of the softening heat treatment exceeds 650° C., austenite is excessively formed, and martensite is formed from the austenite during cooling, so that the effect of the softening heat treatment may not occur effectively.
  • the softening heat treatment in the above temperature range the martensite structure after hot rolling can be converted into a composite structure of ferrite and retained austenite.
  • the cold-rolling step (S130) is a step of cold-rolling the hot-rolled sheet after pickling.
  • the cold rolling may be performed under conditions of a reduction ratio of 40 to 60% of the hot-rolled sheet material.
  • the annealing heat treatment step includes a step of primary heat treatment at a temperature of AC3 ⁇ AC3 + 15 °C for the cold rolled sheet material and a second heat treatment of the cold rolled sheet material subjected to the first heat treatment at an ideal temperature. Can proceed.
  • the temperature of AC3 to AC3 + 15°C in the first heat treatment step may be, for example, a temperature of 735 to 750°C.
  • the ideal temperature in the second heat treatment step may be, for example, a temperature of 640 to 660°C.
  • the first heat treatment may convert a composite structure of ferrite and martensite of a sheet material after cold rolling into a structure of martensite.
  • the cold-rolled sheet is heated to a target temperature of 735 to 750°C at a temperature increase rate of 1 to 3°C/s, and a heat treatment for maintaining 40 to 120 seconds is performed.
  • the temperature increase rate is less than 1°C/s
  • the time to stay at the target temperature of 735 to 750°C exceeds the range of 40 to 120 seconds, so that the austenite grain size at the target temperature may increase excessively.
  • the heating rate exceeds 3°C/s
  • the time to stay at the target temperature of 735 to 750°C is less than the range of 40 to 120 seconds, so that the austenite grains of sufficient size at the target temperature are not secured. It may not be possible.
  • the heat-treated cold-rolled sheet is cooled to 350 to 450°C at a cooling rate of 4 to 10°C/s.
  • the cold-rolled sheet material cooled to the above temperature may be aged for 120 to 330 seconds.
  • the second heat treatment may be continuously performed on the cold-rolled sheet material on which the above-described first heat treatment has been completed.
  • the cold-rolled sheet is heated to a target temperature of 640 to 660°C at a temperature increase rate of 1 to 3°C/s, and heat treatment is performed for 40 to 120 seconds.
  • the secondary heat treatment may be performed at an ideal temperature within the target temperature range, so that the structure of martensite after the first heat treatment may be changed into a structure of ferrite and retained austenite. In this case, the volume fraction of retained austenite may be 10 to 30% by volume.
  • the cold-rolled sheet When the heating rate is less than 1°C/s, the cold-rolled sheet may not be secured by deteriorating material properties by forming or spheroidizing unnecessary cementite before reaching the abnormal temperature.
  • the temperature increase rate exceeds 3°C/s, it may not be maintained in the target temperature range for 40 to 120 seconds, and thus a sufficient fraction of retained austenite may not be secured in the final tissue.
  • the heat-treated cold-rolled sheet is cooled to 350 to 450°C at a cooling rate of 4 to 10°C/s.
  • the cold-rolled sheet material cooled to the above temperature may be aged for 120 to 330 seconds.
  • the steel sheet of the present invention manufactured by the above-described process has yield strength (YS): 800 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 25% or more, and hole expandability (HER): 20% or more.
  • YS yield strength
  • TS tensile strength
  • EL elongation
  • HER hole expandability
  • a predetermined amount of an austenite stabilizing element may be added to the steel slab as described above.
  • the steel sheet may have a composite structure of ferrite of fine grains and 10 to 30% by volume of retained austenite as a final microstructure. Since the steel sheet retains a sufficient fraction of retained austenite, it may have a high elongation of 25% or more due to the metamorphic organic plastic properties.
  • the interface between the hard phase and the soft phase can be reduced.
  • the content of the precipitate forming element such as titanium, niobium, vanadium, and the precipitate growth inhibiting element such as molybdenum by controlling the content of the precipitate forming element such as titanium, niobium, vanadium, and the precipitate growth inhibiting element such as molybdenum, the boundary between the precipitate and the crystal grains can be reduced.
  • the annealing heat treatment is performed as a two-stage heat treatment in which the annealing heat treatment is divided into the first and the second in a predetermined temperature range, thereby increasing the fraction of High Angle Grain Boundaries (HAGBs) in the final structure.
  • HAGBs High Angle Grain Boundaries
  • the steel sheet may have a high hole expandability of 20% or more.
  • crystal grains of the final microstructure may be refined.
  • the grain size of ferrite and retained austenite in the final microstructure can be controlled to be 3 ⁇ m or less.
  • a steel slab having a comparative component system and an actual component system in Table 1 was manufactured.
  • a specimen was prepared from the steel slab, and a high temperature tensile test was performed.
  • the content range of silicon and aluminum exceeded the upper limit of the content range of silicon and aluminum according to an embodiment of the present invention.
  • Figure 2 is a high-temperature tensile test result of the comparative component-based specimen of the present invention
  • Figure 3 is a high-temperature tensile test result of the actual component-based specimen of the present invention.
  • the sample of the comparative component system and the sample of the actual component system are respectively 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, 1000°C, 1050°C, and 1100°C. After heating to a temperature, it is a result of performing a tensile test at the temperature.
  • the high-temperature tensile test is a graph 201 and -20°C/ when the specimen is heated to a temperature exceeding 1100°C and then cooled to each tensile test temperature at a cooling rate of -1°C/s.
  • a graph 202 in the case of cooling at each tensile test temperature at a cooling rate of s is shown together.
  • the area reduction rate is 50% or more at a predetermined temperature, it can be determined that ductility is secured at the predetermined temperature.
  • the area reduction rate is 55% at 1100°C
  • the area reduction rate is 50% at 700 to 800°C
  • the area reduction rate is less than the target value of 50% in the temperature range of 800 to 1050°C. I did.
  • the area reduction rate exceeded the target value of 50%.
  • Table 2 is a chart showing the rolling force per pass calculated by simulating hot rolling according to an embodiment of the present invention for a comparative component specimen and an implementation component specimen.
  • the specimen of the comparative component system should be applied with a large reduction force compared to the specimen of the actual component system. That is, it can be seen that a relatively large load is applied to the rolling mill when the specimen of the comparative component system is hot-rolled.
  • the first and second annealing heat treatment processes were each performed according to Table 3 for the specimens of the implementation component system of Table 1.
  • the secondary annealing temperature was lower than the lower limit of the secondary annealing temperature according to the embodiment of the present invention 640 °C.
  • the secondary annealing temperature was higher than the upper limit of 660 °C of the secondary annealing temperature according to the embodiment of the present invention.
  • the primary annealing temperature was higher than the upper limit of 750°C of the primary annealing temperature according to an embodiment of the present invention.
  • the secondary annealing temperature was higher than the upper limit of 660°C of the secondary annealing temperature according to an embodiment of the present invention.
  • the primary annealing heat treatment was not performed, and only the secondary annealing heat treatment was performed.
  • the secondary annealing temperature was higher than the upper limit of 660°C of the secondary annealing temperature according to an embodiment of the present invention.
  • Table 4 is a chart evaluating the material properties of the specimens of Comparative Examples 1 to 11 and Examples 1 to 6 subjected to annealing heat treatment according to Table 3.
  • Target values of the material properties of the high-strength steel sheet according to an embodiment of the present invention yield strength 800 MPa or more, tensile strength 980 MPa or more, elongation 25% or more, residual austenite volume fraction 10 to 30%, high angle grain boundaries (HAGBs) fraction More than 70% and more than 20% of hole expandability.
  • the specimens of Examples 1 to 6 satisfied all of the above target values. In the case of Comparative Example 1, the elongation and the fraction of high-angle grain boundaries (HAGBs) were less than the target values. In the case of Comparative Example 2, the elongation was less than the target value.
  • tensile strength, elongation, and fraction of high-angle grain boundaries were less than the target values.
  • the elongation, tensile strength x elongation, average grain size, and high-angle grain boundary (HAGBs) fraction were less than the target values.
  • the tensile strength, elongation, average grain size, and high angle grain boundaries (HAGBs) fraction were less than the target values.
  • the yield strength, tensile strength, elongation, average grain size, and high angle grain boundaries (HAGBs) fraction were less than the target values.
  • Figure 4 is a photograph showing the microstructure of a high-strength steel sheet according to an embodiment of the present invention. Specifically, Figure 4 is a microstructure photograph of the specimen of Example 1. Referring to Table 4 and FIG. 4, in the specimen of Example 1, residual austenite and excess ferrite having a volume fraction of 17% were observed.
  • the first and second annealing heat treatment processes were each performed according to Table 5 for the specimens of the implementation component system of Table 1.
  • Table 6 is a chart evaluating the material properties of the specimens of Comparative Examples 12 to 14 and Examples 13 to 16 subjected to annealing heat treatment according to Table 5.
  • the first and second annealing heat treatment processes were each performed according to Table 7 for the specimens of the implementation component system of Table 1.
  • Table 8 is a chart evaluating the material properties of the specimens of Comparative Examples 15 and 16 and Examples 11 to 14 subjected to annealing heat treatment according to Table 7.

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PCT/KR2020/006385 2019-09-30 2020-05-15 고강도 및 고성형성을 가지는 강판 및 그 제조방법 WO2021066274A1 (ko)

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JP2021564566A JP7419401B2 (ja) 2019-09-30 2020-05-15 高強度および高成形性を有する鋼板およびその製造方法
CN202080035734.7A CN113825852B (zh) 2019-09-30 2020-05-15 具有高强度和高成型性的钢板及其制造方法
DE112020004666.4T DE112020004666T5 (de) 2019-09-30 2020-05-15 Stahlblech, welches hohe festigkeit und hohe umformbarkeit hat, und verfahren zum herstellen desselben
BR112022001969A BR112022001969A2 (pt) 2019-09-30 2020-05-15 Lâmina de aço com alta resistibilidade e alta formabilidade e método para fabricar uma lâmina de aço com alta resistibilidade e alta formabilidade
MX2022001389A MX2022001389A (es) 2019-09-30 2020-05-15 Hoja de acero que tiene alta resistencia y alta conformabilidad y metodo para fabricar la misma.
US17/608,068 US20220220576A1 (en) 2019-09-30 2020-05-15 Steel sheet having high strength and high formability and method for manufacturing same

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BR112022001969A2 (pt) 2022-05-10
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