US20220220576A1 - Steel sheet having high strength and high formability and method for manufacturing same - Google Patents

Steel sheet having high strength and high formability and method for manufacturing same Download PDF

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US20220220576A1
US20220220576A1 US17/608,068 US202017608068A US2022220576A1 US 20220220576 A1 US20220220576 A1 US 20220220576A1 US 202017608068 A US202017608068 A US 202017608068A US 2022220576 A1 US2022220576 A1 US 2022220576A1
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steel sheet
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amount
temperature
heat treatment
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Ho Yong Um
Nam Hoon Goo
Min Sung KIM
Gyu Jin OH
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Hyundai Steel Co
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Hyundai Steel Co
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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 sheet and a method for manufacturing the same, and more particularly, to a steel sheet having high strength and high formability and a method for manufacturing the same.
  • a problem to be solved by the present invention is to provide a steel sheet having high formability and high strength and a method for manufacturing the same.
  • a steel sheet having high strength and high formability including, % by weight, an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities.
  • the steel sheet has a microstructure consisting of ferrite and retained austenite. The microstructure has a grain size of 3 ⁇ m or less.
  • the steel sheet has a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.
  • the steel sheet may further include one or more of niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each of which may be included in an amount of greater than 0 and less than or equal to 0.02 wt %.
  • Nb niobium
  • Ti titanium
  • V vanadium
  • Mo molybdenum
  • the steel sheet may further include more than 0 and less than or equal to 0.001 wt % boron (B).
  • the volume fraction of the retained austenite in the microstructure may be 10 to 30 vol %.
  • a method for manufacturing a steel sheet having high strength and high formability including steps of: (a) manufacturing a hot-rolled steel sheet from a steel slab including, % by weight, an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities; (b) manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; (c) subjecting the cold-rolled steel sheet to first heat treatment at a temperature of AC3 to (AC3+15) ° C.; and (d) subjecting the cold-rolled steel sheet, subjected to the first heat treatment, to second heat treatment
  • the steel slab may further include one or more of niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each of which may be included in an amount of greater than 0 and less than or equal to 0.02 wt %.
  • Nb niobium
  • Ti titanium
  • V vanadium
  • Mo molybdenum
  • the steel slab may further include an amount greater than 0 and less than or equal to 0.001 wt % boron (B).
  • step (c) may include a step of cooling the heat-treated cold-rolled steel sheet to a temperature of 350 to 450° C. at a cooling rate of 4 to 10° C./s.
  • step (d) may include a step of cooling the heat-treated cold-rolled steel sheet to a temperature of 350 to 450° C. at 4 to 10° C./s.
  • step (a) may include steps of: (a1) reheating the steel slab to a temperature of 1,150 to 1,250° C.; (a2) hot-rolling the reheated steel slab to a finish delivery temperature of 925 to 975° C.; and (a3) cooling the hot-rolled steel sheet to a temperature of 700° C. to 800° C. at a cooling rate of 10 to 30° C./s, followed by coiling.
  • the method may further include, between steps (a) and (b), a step of subjecting the hot-rolled steel sheet to softening heat treatment at a temperature of 550° C. to 650° C.
  • the cold-rolled steel sheet after step (d) may have a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.
  • Yield strength 800 MPa or greater
  • TS tensile strength
  • EL elongation
  • HER hole expansion ratio
  • the cold-rolled steel sheet after step (d) may have a grain size of 3 ⁇ m or less.
  • the present invention it is possible to manufacture a steel sheet having a microstructure consisting of ultrafine grained ferrite and retained austenite through component system control and process condition control. Due to the fine grained ferrite, the steel sheet may have high strength, and due to the retained austenite present in an amount of 10 to 30 vol % in the microstructure, the steel sheet may have high strength and elongation. In addition, the steel sheet may have a high hole expansion ratio (HER) as a result of controlling the shape of the microstructure. As a result, it is possible to effectively obtain a steel sheet having high formability and high strength.
  • HER hole expansion ratio
  • FIG. 1 is a process flow chart schematically showing a method for manufacturing a steel sheet having high strength and high formability according to an exemplary embodiment of the present invention.
  • FIG. 2 shows the results of a high-temperature tensile test for a comparative component system specimen of the present invention.
  • FIG. 3 shows the results of a high-temperature tensile test for an implementation component system specimen of the present invention.
  • FIG. 4 is a photograph showing the microstructure of a high-strength steel sheet according to an exemplary embodiment of the present invention.
  • a steel sheet having high strength and high formability may have a final microstructure consisting of fine grained ferrite and a retained austenite present in an amount of 10 to 30 vol %.
  • the steel sheet may have a high strength, high elongation and high hole expansion ratio (HER).
  • the steel sheet sufficiently contains retained austenite at a level of 10 to 30 vol %.
  • the retained austenite may enhance the elongation of the steel sheet in substantially the same manner as in conventional transformation-induced plasticity steel.
  • an austenite stabilizing element may be appropriately added into 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 at an intercritical temperature.
  • the phase boundary between a hard phase and a soft phase which can act as a crack formation site in the steel sheet, is reduced.
  • the steel sheet may not contain hard phases such as martensite and bainite in the final microstructure thereof.
  • the interfaces between precipitates and grains are reduced.
  • the contents of precipitate forming elements, such as titanium, niobium and vanadium, and precipitate growth inhibiting elements 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 boundaries may refer to grain boundaries at which the angle between adjacent grains is 15° or greater.
  • the shape of the microstructure may be controlled so that the steel sheet has a high hole expansion ratio.
  • annealing heat treatment may be performed in two steps consisting of first heat treatment and second heat treatment.
  • the grains of the final microstructure are refined.
  • the grains sizes of ferrite and retained austenite may be controlled to 3 ⁇ m or less.
  • the first annealing heat treatment may be performed at a temperature of AC3 to (AC3+15) ° C.
  • a high-strength steel sheet includes, % by weight, an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities.
  • C carbon
  • Si silicon
  • Mn manganese
  • Al aluminum
  • P phosphorus
  • S sulfur
  • N nitrogen
  • Fe iron
  • the high-strength steel sheet further includes one or more of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), each of which may be included in an amount of greater than 0 and less than or equal to 0.02 wt %.
  • the high-strength steel sheet may further include more than 0 and less than or equal to 0.001 wt % boron (B).
  • Carbon (C) is the most important alloying element in steel making, and is used for the main purpose of providing basic strengthening and stabilizing austenite in the present invention.
  • High carbon (C) concentration in austenite improves austenite stability, making it easy to ensure proper austenite for material property improvement.
  • an excessively high carbon (C) content may result in a decrease in weldability due to an increase in carbon equivalent, and a large number of precipitated cementite structures such as pearlite may be formed during cooling.
  • carbon (C) is preferably added in an amount of 0.05 to 0.15% of the total weight of the steel sheet. If the carbon content is less than 0.05%, it may be difficult to secure the strength of the steel sheet, and when the carbon content is more than 0.15%, the toughness and ductility of the steel sheet may deteriorate.
  • Silicon (Si) is an element that suppresses carbide formation in ferrite and increases the diffusion rate of austenite by increasing the activity of carbon (C). Silicon (Si) is also well known as a ferrite stabilizing element, which increases ductility by increasing the ferrite fraction during cooling. In addition, silicon has a very high ability to suppress the formation of carbides, and thus is a necessary element for securing the TRIP effect by increasing the carbon concentration in retained austenite during bainite formation. However, if silicon (Si) is added in an amount greater than 0.4%, it may form silicon oxide (SiO 2 ) on the surface of the steel sheet during the process, increase the rolling load during hot rolling, and generate a large amount of red scale. Thus, silicon (Si) is preferably added in an amount of 0.4% or less of the total weight of the steel sheet.
  • Manganese (Mn) is an austenite stabilizing element. As manganese (Mn) is added, Ms, which is a martensite formation starting temperature, is gradually lowered, thereby exhibiting the effect of increasing the fraction of retained austenite after heat treatment.
  • Manganese is included in an amount of 4.0 to 9.0% of the total weight of the steel sheet. If manganese is added in an amount of less than 4.0%, the above-described effect cannot be sufficiently secured. On the other hand, if manganese is added in an amount greater than 9.0%, weldability may decrease due to an increase in carbon equivalent, and manganese oxide (MnO) may be formed on the surface of the steel sheet during the process, resulting in a decrease in platability due to a decrease in the wettability of the corresponding portion.
  • MnO manganese oxide
  • Aluminum (Al) is known as an element that stabilizes ferrite and inhibits the formation of carbides, like silicon (Si).
  • silicon Si
  • aluminum has the effect of increasing the equilibrium temperature, and thus when aluminum (Al) is added, there is an advantage in that an appropriate heat treatment temperature range is widened.
  • aluminum may be added in an amount of more than 0 and less than or equal to 0.3% of the total weight of the steel sheet.
  • Niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo) may optionally be included in the steel.
  • niobium (Nb), titanium (Ti) and vanadium (V) are elements that are precipitated in the form of carbides in steel, and are added to secure strength through carbide precipitation. Titanium (Ti) may function to suppress the formation of cracks during continuous casting by suppressing the formation of AlN.
  • niobium (Nb), titanium (Ti) and vanadium (V) are each added in an amount greater than 0.2%, they may form coarse precipitates, which causes disadvantages in that the amount of carbon in the steel is reduced and the material properties thereof are degraded, and the manufacturing cost increases due to the addition of niobium (Nb), titanium (Ti) and vanadium (V).
  • titanium is added excessively, it may cause nozzle clogging during continuous casting.
  • each of niobium (Nb), titanium (Ti) and vanadium (V) may be added in an amount greater than 0 and less than or equal to 0.2% of the total weight of the steel sheet.
  • molybdenum may serve to control the size of carbides by suppressing the growth of the carbides.
  • molybdenum is added in an amount greater than 0.2%, there are disadvantages in that the above effect is saturated and the manufacturing cost increases.
  • Boron (B) may optionally be 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 less than or equal to 0.001% of the total weight of the steel sheet. If boron is added in an amount of more than 0.001%, it may lower the high-temperature ductility of the steel sheet by forming a nitride such as BN.
  • Phosphorus (P), sulfur (S) and nitrogen (N) may inevitably be added to the steel during the steelmaking process. That is, it is preferable that these elements are ideally not included, but they may be included in certain amounts because it is difficult to completely remove these elements in terms of process technology.
  • Phosphorus (P) may play a role similar to silicon in the steel. However, if phosphorus is added in an amount greater than 0.02% of the total weight of the steel sheet, it may reduce the weldability of the steel sheet and increase the brittleness thereof, thereby causing material property deterioration. Accordingly, the amount of phosphorus added may be controlled to 0.02% or less of the total weight of the steel sheet.
  • S may inhibit the toughness and weldability of the steel, and hence the content thereof may be controlled to 0.005% or less of the total weight of the steel sheet.
  • nitrogen (N) is excessively present in the steel, a large amount of nitride may be precipitated, resulting in deterioration in the ductility of the steel sheet. Accordingly, the content of nitrogen (N) may be controlled to 0.006% or less of the total weight of the steel sheet.
  • the high-strength steel sheet of the present invention which has the above-described alloying components, has a microstructure consisting of ferrite and retained austenite.
  • the volume fraction of the retained austenite in the microstructure may be 10 to 30 vol %.
  • the grains of the high-strength steel sheet may be fine grains having a size of 3 ⁇ m or less.
  • the fraction of high-angle grain boundaries among the grains may be 70% or greater.
  • the high-strength steel sheet may have material properties, including a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.
  • YS yield strength
  • TS tensile strength
  • EL elongation
  • HER hole expansion ratio
  • the high-strength steel sheet according to the embodiment of the present invention may be applied to fields requiring high strength and high formability.
  • the above-described high-strength steel sheet according to the embodiment of the present invention may be manufactured by a method of an exemplary embodiment as follows.
  • the present invention intends to provide a steel sheet having excellent elongation, hole expansion ratio and strength as a result of using alloying components having appropriately controlled composition ratios and performing two-step annealing heat treatment after performing a hot rolling process and a cold rolling process, and a method for manufacturing the same.
  • FIG. 1 is a process flow chart schematically showing a method for manufacturing a steel sheet having high strength and high formability according to an exemplary embodiment of the present invention.
  • the method for manufacturing a steel sheet includes: steps of: (S 110 ) reheating a steel slab; (S 120 ) manufacturing a hot-rolled steel sheet by hot-rolling the steel slab; (S 130 ) cold-rolling the hot-rolled steel sheet; and (S 140 ) subjecting the cold-rolled steel sheet to annealing heat treatment.
  • step (S 110 ) of reheating a steel slab is a step of preparing a steel slab including, % by weight an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities, and reheating the steel slab to re-dissolve components segregated during casting and homogenize as-cast components.
  • C carbon
  • Si silicon
  • Mn manganese
  • Al aluminum
  • P phosphorus
  • S sulfur
  • N nitrogen
  • Fe iron
  • the steel slab may further include one or more of niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each of which may be included in an amount of more than 0 and less than or equal to 0.02 wt %.
  • the steel slab may further include more than 0 and less than or equal to 0.001 wt % boron (B).
  • the steel slab reheating temperature is preferably about 1,150 to 1,250° C. so that a normal hot delivery temperature may be ensured. If the reheating temperature is lower than 1,150° C., a problem may arise in that the hot rolling load increases rapidly, and if the reheating temperature is greater than 1,250° C., it may be difficult to secure the strength of the final manufactured steel sheet, due to the coarsening of initial austenite grains.
  • hot-rolling step (S 120 ) is performed after the steel slab reheating, and is a step of forming a hot-rolled steel sheet by performing hot rolling by a conventional method 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 steel sheet is cooled to a temperature of 700 to 800° C. at a cooling rate of 10 to 30° C./s and then coiled.
  • the cooling method may be performed using a water-free cooling method.
  • the hot-rolled steel sheet may have a full martensitic structure after cooling.
  • softening heat treatment may be performed to reduce the rolling load during cold rolling.
  • the softening heat treatment may be performed at a temperature of 550 to 650° C. If the temperature of the softening heat treatment is lower than 550° C., recrystallization of the martensite produced after the hot rolling may not occur, and only tempering may proceed, and thus supersaturated carbon in the structure may be formed in the form of cementite and spheroidized. In this case, since the brittleness of the martensite may be expressed, fracture of the steel sheet may occur during cold rolling.
  • the temperature of the softening heat treatment is higher than 650° C.
  • austenite may be excessively formed, and martensite may be formed from the austenite during cooling, so that the effect of the softening heat treatment may not be effective.
  • the softening heat treatment performed in the above temperature range the martensitic structure after the hot rolling may be transformed into a composite structure of ferrite and retained austenite.
  • cold rolling step (S 130 ) is a step of cold-rolling the hot-rolled steel sheet after pickling.
  • the cold rolling may be performed under a condition where the hot-rolled steel sheet is cold-rolled at a reduction ratio of 40 to 60%.
  • the composite structure of ferrite and retained austenite after the softening heat treatment may be transformed into a composite structure of ferrite and martensite.
  • annealing heat treatment step (S 140 ) may include a step of subjecting the cold-rolled steel sheet to first heat treatment at a temperature of AC3 to (AC3+15)° C., and a step of subjecting the cold-rolled steel sheet, subjected to the first heat treatment, to second heat treatment at an intercritical temperature.
  • 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 intercritical temperature in the second heat treatment step may be, for example, a temperature of 640 to 660° C.
  • the first heat treatment may transform a composite structure of ferrite and martensite in the steel sheet after cold rolling into a martensitic structure.
  • heat treatment is performed by heating the cold-rolled steel sheet to a target temperature of 735 to 750° C. at a heating rate of 1 to 3° C./s and holding the cold-rolled steel sheet at the target temperature for 40 to 120 seconds.
  • the heat treatment temperature is lower than 735° C., it is not possible to secure austenite grains having a sufficient size at the target temperature, and a composite structure of martensite and ferrite may be formed after heat treatment, and thus the strength and ductility of the final structure following the annealing heat treatment may decrease.
  • the heat treatment temperature is higher than 750° C., the size of austenite grains at the target temperature may excessively increase, which is disadvantageous in securing the stabilization of austenite in the final structure following the annealing heat treatment, so that the steel sheet may have inferior strength.
  • the retention time at the target temperature of 735 to 750° C. may be longer than the upper limit of the range of 40 to 120 seconds, so that the austenite grain size at the target temperature may excessively increase.
  • the heating rate is greater than 3° C./s, the retention time at the target temperature of 735 to 750° C. may be shorter than the lower limit of the range of 40 to 120 seconds, so that it is impossible to secure austenite grains having a sufficient size at the target temperature.
  • the heat-treated cold-rolled steel sheet is cooled to a temperature of 350 to 450° C. at a cooling rate of 4 to 10° C./s.
  • the cold-rolled steel sheet cooled to the above temperature may be aged for 120 to 330 seconds.
  • the cold-rolled steel sheet that has been subjected to the first heat treatment may be continuously subjected to second heat treatment.
  • heat treatment is performed by heating the cold-rolled steel sheet to a target temperature of 640 to 660° C. at a heating rate of 1 to 3° C./s and holding the cold-rolled steel sheet at the target temperature for 40 to 120 seconds.
  • the martensitic structure after the first heat treatment may be transformed into a structure consisting of ferrite and retained austenite.
  • the volume fraction of the retained austenite may be 10 to 30 vol %.
  • the second heat treatment temperature is lower than 640° C.
  • excessively few austenite structures may be formed at the target temperature and the austenite stability may increase, and for this reason, the austenite in the microstructure after cooling may not exhibit phase transformation during plastic deformation, and thus the strength and ductility of the steel sheet may decrease.
  • the second heat treatment temperature is higher than 660° C.
  • excessively many austenite structures may be formed at the target temperature and the austenite stability may be lowered, and for this reason, martensite may be formed in the microstructure after cooling, resulting in decreases in the ductility and hole expansion ratio of the steel sheet.
  • the heating rate is less than 1° C./s, unnecessary cementite may be formed or spheroidized before the cold-rolled sheet material reaches the above-described intercritical temperature range, resulting in deterioration in the material properties of the steel sheet. If the heating rate is greater than 3° C./s, the steel sheet may not be held for 40 to 120 seconds in the target temperature range, so that it is not possible to secure a sufficient fraction of retained austenite in the final structure.
  • the heat-treated cold-rolled steel sheet is cooled to a temperature of 350 to 450° C. at a cooling rate of 4 to 10° C./s.
  • the cold-rolled steel sheet cooled to the above temperature may be aged for 120 to 330 seconds.
  • a steel sheet having high strength and high formability may be manufactured.
  • the steel sheet of the present invention manufactured by the above-described process, may have a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.
  • Yield strength 800 MPa or greater
  • TS tensile strength
  • EL elongation
  • HER hole expansion ratio
  • austenite stabilizing elements may be added into the steel slab in predetermined amounts as described above.
  • the steel sheet may have a final microstructure as a composite structure consisting of fine grained ferrite and 10 to 30 vol % of retained austenite. Since the steel sheet has a sufficient fraction of retained austenite, it may have a high elongation of 25% or greater due to transformation-induced plasticity properties thereof.
  • phase boundary between a hard phase and a soft phase may be reduced by controlling so that hard phases such as martensite and bainite are not contained in the final microstructure as described above.
  • interfaces between precipitates and grains may be reduced by controlling the contents of precipitate forming elements, such as titanium, niobium and vanadium and precipitate growth inhibiting elements such as molybdenum, in the component system of the steel slab.
  • precipitate forming elements such as titanium, niobium and vanadium
  • precipitate growth inhibiting elements such as molybdenum
  • the martensite formed through the first heat treatment is heat-treated, and thus recrystallization is relatively suppressed before the martensite is reversely transformed into austenite, whereby the fraction of high-angle grain boundaries in the final microstructure may increase to 70% or greater of the grains.
  • the steel sheet may have a high hole expansion ratio of 20% or greater.
  • the grains of the final microstructure may be refined in order for the steel sheet to have high strength.
  • the grain sizes of ferrite and retained austenite in the final microstructure may be controlled to 3 ⁇ m or less through the second heat treatment performed in the intercritical temperature range.
  • FIG. 2 shows the results of a high-temperature tensile test for the comparative component system specimen of the present invention
  • FIG. 3 shows the results of a high-temperature tensile test for the implementation component system specimen of the present invention.
  • the results of the high-temperature tensile tests are the results obtained by heating each of the comparative component system specimen and the implementation component system specimen to temperatures of 700° C., 750° C., 800° C., 850° C., 900° C., 950° C., 1,000° C., and 1,100° C. and then subjecting each specimen to a tensile test at the above temperatures.
  • FIG. 1 shows the results of a high-temperature tensile test for the comparative component system specimen of the present invention
  • FIG. 3 shows the results of a high-temperature tensile test for the implementation component system specimen of the present invention.
  • the results of the high-temperature tensile tests are the results obtained by heating each of the comparative component system specimen and
  • FIG 3 shows a graph 201 obtained by heating the specimen to a temperature higher than 1,100° C. and then cooling the specimen to each of the tensile test temperatures at a cooling rate of ⁇ 1° C./s, together with a graph 202 obtained by cooling the specimen to each of the tensile test temperatures at a cooling rate of ⁇ 20° C./s.
  • the area reduction rate at a predetermined temperature is 50% or greater, it may be determined that the ductility at the predetermined temperature is secured.
  • the area reduction rate at 1,100° C. was 55%
  • the area reduction rate in the temperature range of 700 to 800° C. was 50%
  • the area reduction rate in the temperature range of 800 to 1,050° C. was less than 50%, which is the target value.
  • the area reduction rate in the temperature range of 800 to 1,100° C. exceeded 50%, which is the target value.
  • the high-temperature ductility is not ensured in the high temperature range equal to or higher than 800° C. in which continuous casting is performed according to the embodiment of the present invention, and thus cracks may occur during continuous casting, making it impossible to secure a good slab.
  • Table 2 below shows the rolling force for each pass calculated by simulating the hot rolling according to an exemplary embodiment of the present invention for each of the comparative component system specimen and the implementation component system specimen.
  • the specimen prepared from the implementation component system shown in Table 1 above was subjected to each of first and second annealing heat treatment processes according to Table 3 below.
  • the second annealing temperature was lower than 640° C., which is the lower limit of the second annealing temperature according to the embodiment of the present invention.
  • the second annealing temperature was higher than 660° C., which is the upper limit of the second annealing temperature according to the embodiment of the present invention.
  • the first annealing temperature was higher than 750° C., which is the upper limit of the first annealing temperature according to the embodiment of the present invention.
  • the second annealing temperature was higher than 660° C., which is the upper limit of the second annealing temperature according to the embodiment of the present invention.
  • the first annealing heat treatment was not performed, and only the second annealing heat treatment was performed.
  • the second annealing temperature was higher than 660° C., which is the upper limit of the second annealing temperature according to the embodiment of the present invention.
  • Table 4 shows the results of 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.
  • the target values of the material properties of the high-strength steel sheet according to an exemplary embodiment of the present invention are a yield strength of 800 MPa or greater, a tensile strength of 980 MPa or greater, an elongation of 25% or greater, a retained austenite volume fraction of 10 to 30%, a high-angle grain boundary (HAGB) fraction of 70% or greater, and a hole expansion ratio of 20% or greater.
  • 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 below the target values. In the case of Comparative Example 2, the elongation was below the target value.
  • FIG. 4 is a photograph showing the microstructure of the high-strength steel sheet according to an exemplary embodiment of the present invention. Specifically, FIG. 4 is a microstructure photograph of the specimen of Example 1. Referring to Table 4 and FIG. 4 , retained austenite having a volume fraction of 17% and the remainder ferrite were observed in the specimen of Example 1.
  • the heating rate during the first annealing heat treatment was greater than 3° C./s, which is the upper limit of the heating rate during the first annealing heat treatment according to an exemplary embodiment of the present invention, and the first annealing holding time did not satisfy 40 seconds or more.
  • the heating rate during the first annealing heat treatment was less than 1° C./s, which is the lower limit of the heating rate during the first annealing heat treatment according to an exemplary embodiment of the present invention, and the first annealing holding time exceeded 120 seconds, which is the upper limit.
  • the heating rate during the first annealing heat treatment was less than 1° C./s, which is the lower limit of the heating rate during the first annealing heat treatment according to an exemplary embodiment of the present invention, and the first annealing holding time exceeded 120 seconds, which is the upper limit.
  • the cooling rate was less than 4° C./s, which is the lower limit. Examples 7 to 10 satisfied both the first and second annealing heat treatment conditions according to an exemplary embodiment of the present invention.
  • Table 6 shows the results of evaluating the material properties of the specimens of Comparative Examples 12 to 14 and Examples 7 to 10, subjected to annealing heat treatment according to Table 5 above.
  • the specimen prepared from the implementation component system shown in Table 1 above was subjected to each of first and second annealing heat treatment processes.
  • Example 15 735 6 400 4.0 660 31 13 400
  • Example 11 735 6 400 3.0 660 43 10
  • Example 12 735 6 400 2.0 660 59 7 400
  • Example 13 735 6 400 1.5 660 80 6
  • Example 14 735 6 400 1.0 660 118 4 400
  • Example 16 735 6 400 0.5 660 236 2 400
  • the heating rate during the second annealing heat treatment exceeded 3° C./s, which is the upper limit of the heating rate during the second annealing according to an exemplary embodiment of the present invention, and the second annealing holding time did not satisfy 40 seconds or more.
  • the heating rate during the second annealing was lower than 1° C./s, which is the lower limit of the heating rate during the second annealing according to an exemplary embodiment of the present invention, and the second annealing holding time exceeded 120 seconds, which is the upper limit.
  • Examples 11 to 14 satisfied both the first and second annealing heat treatment conditions according to an exemplary embodiment of the present invention.
  • Table 8 shows the results of 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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