EP3243923B1 - Super high strength plated steel sheet having tensile strength of 1300 mpa or more - Google Patents
Super high strength plated steel sheet having tensile strength of 1300 mpa or more Download PDFInfo
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- EP3243923B1 EP3243923B1 EP15877109.7A EP15877109A EP3243923B1 EP 3243923 B1 EP3243923 B1 EP 3243923B1 EP 15877109 A EP15877109 A EP 15877109A EP 3243923 B1 EP3243923 B1 EP 3243923B1
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- steel sheet
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- 229910000831 Steel Inorganic materials 0.000 title claims description 72
- 239000010959 steel Substances 0.000 title claims description 72
- 229910052739 hydrogen Inorganic materials 0.000 claims description 35
- 239000001257 hydrogen Substances 0.000 claims description 35
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 30
- 229910000859 α-Fe Inorganic materials 0.000 claims description 15
- 239000011572 manganese Substances 0.000 claims description 14
- 229910000734 martensite Inorganic materials 0.000 claims description 14
- 239000010936 titanium Substances 0.000 claims description 12
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 11
- 239000010955 niobium Substances 0.000 claims description 11
- 239000011651 chromium Substances 0.000 claims description 10
- 238000007747 plating Methods 0.000 claims description 10
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 8
- 229910052799 carbon Inorganic materials 0.000 claims description 7
- 239000012535 impurity Substances 0.000 claims description 7
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 6
- 229910052782 aluminium Inorganic materials 0.000 claims description 5
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 5
- 229910001563 bainite Inorganic materials 0.000 claims description 5
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 claims description 4
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims description 4
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims description 4
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 claims description 4
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims description 4
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 claims description 4
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 claims description 4
- 229910052796 boron Inorganic materials 0.000 claims description 4
- 229910052804 chromium Inorganic materials 0.000 claims description 4
- 229910052748 manganese Inorganic materials 0.000 claims description 4
- 229910052758 niobium Inorganic materials 0.000 claims description 4
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 claims description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 229910052698 phosphorus Inorganic materials 0.000 claims description 4
- 239000011574 phosphorus Substances 0.000 claims description 4
- 229910052710 silicon Inorganic materials 0.000 claims description 4
- 239000010703 silicon Substances 0.000 claims description 4
- 229910052717 sulfur Inorganic materials 0.000 claims description 4
- 239000011593 sulfur Substances 0.000 claims description 4
- 229910052719 titanium Inorganic materials 0.000 claims description 4
- 238000010438 heat treatment Methods 0.000 description 34
- 238000000034 method Methods 0.000 description 14
- 230000015572 biosynthetic process Effects 0.000 description 12
- 238000005336 cracking Methods 0.000 description 11
- 230000008569 process Effects 0.000 description 11
- 238000000137 annealing Methods 0.000 description 9
- 230000002829 reductive effect Effects 0.000 description 8
- 238000010583 slow cooling Methods 0.000 description 6
- 150000002431 hydrogen Chemical class 0.000 description 5
- 238000004519 manufacturing process Methods 0.000 description 5
- 239000010960 cold rolled steel Substances 0.000 description 4
- 239000007789 gas Substances 0.000 description 4
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 3
- 229910045601 alloy Inorganic materials 0.000 description 3
- 239000000956 alloy Substances 0.000 description 3
- 238000005275 alloying Methods 0.000 description 3
- 229910001566 austenite Inorganic materials 0.000 description 3
- 230000006866 deterioration Effects 0.000 description 3
- 230000002401 inhibitory effect Effects 0.000 description 3
- 229910052742 iron Inorganic materials 0.000 description 3
- 230000009467 reduction Effects 0.000 description 3
- 230000009466 transformation Effects 0.000 description 3
- 229910001335 Galvanized steel Inorganic materials 0.000 description 2
- 229910000797 Ultra-high-strength steel Inorganic materials 0.000 description 2
- 230000008901 benefit Effects 0.000 description 2
- 238000009749 continuous casting Methods 0.000 description 2
- 230000007797 corrosion Effects 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 239000008397 galvanized steel Substances 0.000 description 2
- 238000005246 galvanizing Methods 0.000 description 2
- 238000005098 hot rolling Methods 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 239000000203 mixture Substances 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 229910052761 rare earth metal Inorganic materials 0.000 description 2
- 150000002910 rare earth metals Chemical class 0.000 description 2
- 238000005496 tempering Methods 0.000 description 2
- -1 AlN nitride Chemical class 0.000 description 1
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 238000001816 cooling Methods 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 238000004817 gas chromatography Methods 0.000 description 1
- 230000000670 limiting effect Effects 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 239000011733 molybdenum Substances 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 239000006104 solid solution Substances 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 230000000087 stabilizing effect Effects 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 230000008719 thickening Effects 0.000 description 1
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 description 1
Images
Classifications
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- 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
- 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
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
- C23C2/06—Zinc or cadmium or alloys based thereon
Definitions
- the present disclosure relates to a plated steel sheet having ultra-high strength and used in vehicles and the like, and more particularly, to a plated steel sheet having ultra-high strength and a tensile strength of 1300 MPa or more, and a method of manufacturing the same.
- KR 2013 0056052 A describes a galvannealed steel sheet with high strength and a corresponding production method.
- US 2014/234657 A12 discloses a high strength hot dip galvanized steel sheet and its production process. Further examples for high strength galvanized steel sheets can be found, for example, in US 2014/377584 A1 , JP 2012 229466 A and US 2011/198002 A1 .
- martensite steel having a tensile strength of 1300 MPa or more has been developed and used as an ultra-high strength plated steel sheet, and plating products having enhanced corrosion resistance have also been being developed.
- steel sheet coils commonly produced in steel mills are slit and coiled to be formed as coils having a relatively narrow width, and then, are formed as components by applying a roll forming method or a simple forming method thereto.
- An aspect of the present disclosure is to provide a plated steel sheet having ultra-high strength, in which the occurrence and propagation of cracking in edges thereof in a width direction may be prevented, even when a slitting and coiling process is performed on a plated steel sheet having ultra-high strength, and a method of manufacturing the same.
- a plated steel sheet having ultra-high strength in which the occurrence of cracking in an edge portion in a width direction, after a slitting and coiling process, may be prevented, may be provided.
- FIG. 1 illustrates a result of observing whether or not cracking occurs after slitting heat-treated and untreated plated sheets having ultra-high strength.
- the present inventors have found that cracking in a width direction edge portion after slitting and coiling a plated steel sheet having ultra-high strength is related to an amount of hydrogen, and thus, a plated steel sheet having ultra-high strength and a reduced amount of hydrogen, and a method for effectively reducing an amount of hydrogen, may be provided.
- a plated steel sheet having ultra-high strength, and having an amount of hydrogen of 0.000015 wt% or less and a tensile strength of 1300 MPa or more, may be provided.
- the plated steel sheet having ultra-high strength may include 0.12 wt% to 0.2 wt% of carbon (C), 0.5 wt% or less of silicon (Si) (excluding 0wt%), 2.6 wt% to 4.0 wt% of manganese (Mn), 0.03 wt% or less of phosphorus (P) (excluding 0 wt%), 0.015 wt% or less of sulfur (S) (excluding 0 wt%), 0.1 wt% or less of aluminum (Al) (excluding 0 w%), 1 wt% or less of chromium (Cr) (excluding 0 wt%), 48/14*[N] to 0.1 wt% of titanium (Ti), 0.1 wt% or less of niobium (Nb) (excluding 0 wt%), 0.005 wt% or less of boron (B) (excluding 0 wt%), 0.01 wt% or less of nitrogen
- Carbon (C) is an element essentially added to secure strength of a steel. In order to obtain the above-mentioned effect, carbon (C) may be added in an amount of 0.12% or more. However, if the content thereof is relatively high, exceeding 0.20%, a problem in which weldability is deteriorated may occur, which may be problematic.
- the content of C may be limited to 0.12% to 0.20%.
- Silicon (Si) is a ferrite stabilizing element and may have a disadvantage in that strength may be reduced by accelerating generation of ferrite at the time of slow cooling after annealing in a continuous annealing type hop-dip heat treatment furnace of the related art in which a slow cooling section is present.
- an upper limit of Si may be determined. In detail, the content of Si may be limited to 0.5% or less.
- Manganese (Mn) is well known as an element inhibiting the formation of ferrite and facilitating the formation of austenite.
- Mn Manganese
- a content of Mn is less than 2.6%, ferrite may be easily formed during slow cooling, while if the content of Mn exceeds 4.0%, band formation due to slabs, and segregation caused in a hot rolling process, may be excessive, and a problem in which a cost of alloy iron is increased due to an excessive amount of alloying input when a convertor is operated.
- the content of Mn may be limited to 2.6% to 4.0%.
- Phosphorus (P) may be an impurity element in steel. If the content thereof exceeds 0.03%, weldability may be decreased, a risk of brittleness of steel may be increased, and a possibility of the occurrence of dent defects may increase. Thus, the content of P may be limited to 0.03% or less.
- S may be an impurity element in steel as well as P, and if the content thereof exceeds 0.015%, a possibility of deterioration of ductility and weldability of steel may increase. Thus, the content of S may be limited to 0.015% or less.
- Aluminum (Al) is an element for expanding a ferrite region.
- a general continuous annealing type hop-dip heat treatment furnace having a slow cooling section there may be a disadvantage in that ferrite formation is promoted, and the possibility of causing deteriorations in high temperature heat hot-rolling characteristics due to the formation of AlN may increase.
- the content of Al may be limited to 0.1% or less.
- Chromium (Cr) is an element suppressing ferrite transformation and facilitating low-temperature transformation.
- a general continuous annealing type hop-dip heat treatment furnace having a slow cooling section there may be an advantage in that ferrite formation is suppressed.
- the content thereof exceeds 1%, a problem in which a cost of alloying iron is increased due to an excessive amount of alloying may occur.
- the content thereof may be limited to 1% or less.
- Titanium (Ti) is an element for forming a nitride, and may serve to precipitate N in steel as TiN to scavenge N therein. To this end, Ti may be required to be added at a chemical equivalent of 48/14 *[N] or more. On the other hand, if Ti is not added, a problem in which cracks may occur during continuous casting by AlN formation may be caused. However, if the content thereof exceeds 0.1%, a problem in which the strength of martensite is reduced due to precipitation of additional carbides in addition to the removal of solid solution N, may be caused.
- Niobium is an element segregated at an austenite grain boundary to suppress coarsening of austenite grains during an annealing heat treatment, and thus, may be added. However, if the content thereof exceeds 0.1%, a problem in which a cost of alloy iron is increased due to an excessive amount of added alloy may occur. Thus, the content of Nb may be limited 0.1% or less.
- B Boron (B) is an element inhibiting ferrite formation.
- B has an advantage of inhibiting the formation of ferrite at the time of cooling after annealing, and thus, may be added.
- the content thereof exceeds 0.005%, since a problem in which ferrite formation is promoted by precipitation of Fe 23 (C,B) 6 may occur, the content thereof may be limited to 0.005% or less.
- Nitrogen (N) is an element reacting with Al to be precipitated into AlN nitride, and the formed AlN may have a problem in that it is a cause of occurrence of cracking during continuous casting.
- the content of Al may be limited to 0.01% or less, and thus, the formation of AlN may be suppressed.
- Fe and unavoidable impurities may be included as a remainder.
- the impurities may include molybdenum (Mo), vanadium (V), nickel (Ni), rare earth metals (REM), and the like.
- a steel sheet used to obtain a plated steel sheet having ultra-high strength may have a microstructure comprised of 90% or more of martensite and 10% or less of ferrite and bainite in a volume fraction, while satisfying the above-mentioned compositional composition.
- a microstructure comprised of 90% or more of martensite and 10% or less of ferrite and bainite in a volume fraction, while satisfying the above-mentioned compositional composition.
- As effective characteristics according to the configuration of the microstructure as martensite of a hard phase has a microstructure, a main phase, securing ultra-high strength may be facilitated.
- the plated steel sheet having ultra-high strength, ultimately obtained by heat-treating the steel sheet as described above, according to an exemplary embodiment may also have the same microstructure as above, and when an additional tempering heat treatment thereto is performed, martensite may be converted into tempered martensite.
- volume fraction may not be easy to actually measure a volume fraction, a three-dimensional concept, and thus, measurement of the volume fraction may be replaced with area fraction measurement through a cross-sectional observation normally used in observation of a microstructure.
- the steel sheet having the component system and the microstructure as described above may be plated and heat-treated, and an amount of hydrogen after heat treatment may be 0.000015 wt% or less, as compared with the case before the heat treatment.
- the target ratio of a yield strength and a tensile strength of the plated steel sheet having ultra-high strength according to an exemplary embodiment may be 0.75 or more.
- a steel sheet including 0.12 wt% to 0.2 wt% of carbon (C), 0.5 wt% or less of silicon (Si) (excluding 0wt%), 2.6 wt% to 4.0 wt% of manganese (Mn), 0.03 wt% or less of phosphorus (P) (excluding 0 wt%), 0.015 wt% or less of sulfur (S) (excluding 0 wt%), 0.1 wt% or less of aluminum (Al) (excluding 0 wt%), 1 wt% or less of chromium (Cr) (excluding 0 wt%), 48/14*[N] to 0.1 wt% of titanium (Ti), 0.1 wt% or less of niobium (Nb) (excluding 0 wt%), 0.005 wt% or less of boron (B) (excluding 0 wt%), 0.01 wt% or less of nitrogen (N) (excluding 0 wt%
- the steel sheet may be plated to produce a plated steel sheet, and the plated steel sheet may be subjected to a heat treatment.
- a plating process is not particularly limited, and for example, a process such as hot-dip galvanizing, hot-dip aluminum plating, electro-galvanizing, or the like may be performed.
- a heat treatment after plating may be performed such that an amount of hydrogen in the plated steel sheet may be 0.000015 wt% or less.
- an amount of hydrogen in the plated steel sheet may be 0.000015 wt% or less.
- the heat treatment temperature and time may be set in consideration of a tensile strength level required by a customer.
- a plated steel sheet having ultra-high strength is generally manufactured as a coil having a constant width through a slitting and coiling process, and the slitting process is a process of adding relatively high stress to an edge portion of a steel sheet.
- the slitting process is a process of adding relatively high stress to an edge portion of a steel sheet.
- a disadvantage in that the quality of a cut surface of an edge portion may be deteriorated due to a plating layer may be present. Hydrogen in steel tends to segregate under a relatively high stress state.
- hydrogen in steel may segregate on a relatively highly stressed portion of an edge portion of the plated steel sheet after the slitting, whereby cracks may start to occur in the edge portion of the plated steel sheet having ultra-high strength and the propagation of cracks may occur in a width direction.
- an amount of hydrogen of the plated steel sheet having ultra-high strength may be reduced to 0.000015 wt% or less, and thus, cracking of an edge portion over time during coiling, after slitting, may be effectively suppressed.
- a plated steel sheet having ultra-high strength, an initial yield strength of 1149 MPa, and an initial tensile strength of 1556 MPa was evaluated for changes in an amount of hydrogen before a heat treatment and after a heat treatment under conditions provided in Table 1 below. Evaluation results are provided as illustrated in Table 1.
- a steel material having a component system consisting of 0.18% of C, 0.1% of Si, 3.6% of Mn, 0.011% of P, 0.11% of Cr, 0.021% of Ti, 0.038% of Nb, 0.0017% of B, 0.003% of S, 0.025% of Al, and 0.004% of N was prepared as a specimen having a size of thickness*12mm*100mm, and was heated to a temperature from 25°C to 250°C at a heating rate of 100°C per hour. An amount of hydrogen was measured using gas chromatography, simultaneously with performing a heat treatment.
- the cold rolled steel sheet had no hydrogen, 0 wt% of hydrogen, while the plated steel sheet had a relatively high content, 0.000022 wt% of hydrogen.
- the amount of hydrogen may be further reduced.
- a plated steel sheet (A) not subjected to heat treatment a plated steel sheet (B) having been subjected to heat treatment at 150°C for 24 hours in a 100% of hydrogen atmosphere, and a plated steel sheet (C) having been subjected to heat treatment at 200°C for 24 hours in a 7% of hydrogen atmosphere were slit, whether or not cracks occurred as time passed was observed, and the results thereof are provided in FIG. 1 .
- a plated steel sheet having ultra-high strength and having a yield strength ratio of 0.75 or more with respect to a tensile strength by tempering heat treatment of an ultra-high strength plated steel sheet having martensite as a main phase, may be provided.
- a decrease in tensile strength may increase.
- setting heat treatment temperature and time according to a tensile strength level required by a customer may be required.
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Description
- The present disclosure relates to a plated steel sheet having ultra-high strength and used in vehicles and the like, and more particularly, to a plated steel sheet having ultra-high strength and a tensile strength of 1300 MPa or more, and a method of manufacturing the same.
- In recent years, in order to increase the stability and to lighten the weight of vehicles, the ultra-high strengthening of steel sheets for vehicles has been continuously increased. In addition, plated steel sheets having been subjected to plating on surfaces of ultra-high strength steel sheets have been mainly used to improve the corrosion resistance of steel sheets. For example,
KR 2013 0056052 A US 2014/234657 A12 discloses a high strength hot dip galvanized steel sheet and its production process. Further examples for high strength galvanized steel sheets can be found, for example, inUS 2014/377584 A1 ,JP 2012 229466 A US 2011/198002 A1 . - At present, martensite steel having a tensile strength of 1300 MPa or more has been developed and used as an ultra-high strength plated steel sheet, and plating products having enhanced corrosion resistance have also been being developed.
- Since such ultra-high strength steel sheets usually have an elongation of 10% or less, steel sheet coils commonly produced in steel mills are slit and coiled to be formed as coils having a relatively narrow width, and then, are formed as components by applying a roll forming method or a simple forming method thereto.
- However, in the case in which ultra-high strength plated steel sheets are slit and then coiled, a problem in which cracking may occur in edge portions of the produced steel plate coils in a width direction, and may propagate to center portions of the steel sheets, may occur.
- Thus, the development of a technology, in which cracking of edge portions of ultra-high strength plated steel sheets which will subsequently be subjected to slitting and coiling processes may be reduced, is required.
- An aspect of the present disclosure is to provide a plated steel sheet having ultra-high strength, in which the occurrence and propagation of cracking in edges thereof in a width direction may be prevented, even when a slitting and coiling process is performed on a plated steel sheet having ultra-high strength, and a method of manufacturing the same.
- In order to achieve the above object, the subject matter of claim 1 is proposed.
- According to an exemplary embodiment in the present disclosure, a plated steel sheet having ultra-high strength, in which the occurrence of cracking in an edge portion in a width direction, after a slitting and coiling process, may be prevented, may be provided.
-
FIG. 1 illustrates a result of observing whether or not cracking occurs after slitting heat-treated and untreated plated sheets having ultra-high strength. - As a result of research to prevent the occurrence and propagation of cracking in edges of a manufactured coil in a width direction in the case of slitting and coiling a plated
steel sheet having ultra-high strength and a tensile strength of 1300 MPa or more, it has been confirmed that the aforementioned problems may be solved by performing a heat treatment before slitting and coiling processes are performed on a plated steel sheet having ultra-high strength to reduce a hydrogen concentration in the plated steel sheet, from which an exemplary embodiment in the present disclosure is provided as below. - In detail, the present inventors have found that cracking in a width direction edge portion after slitting and coiling a plated steel sheet having ultra-high strength is related to an amount of hydrogen, and thus, a plated steel sheet having ultra-high strength and a reduced amount of hydrogen, and a method for effectively reducing an amount of hydrogen, may be provided.
- According to an exemplary embodiment in the present disclosure, a plated steel sheet having ultra-high strength, and having an amount of hydrogen of 0.000015 wt% or less and a tensile strength of 1300 MPa or more, may be provided.
- The plated steel sheet having ultra-high strength according to an exemplary embodiment may include 0.12 wt% to 0.2 wt% of carbon (C), 0.5 wt% or less of silicon (Si) (excluding 0wt%), 2.6 wt% to 4.0 wt% of manganese (Mn), 0.03 wt% or less of phosphorus (P) (excluding 0 wt%), 0.015 wt% or less of sulfur (S) (excluding 0 wt%), 0.1 wt% or less of aluminum (Al) (excluding 0 w%), 1 wt% or less of chromium (Cr) (excluding 0 wt%), 48/14*[N] to 0.1 wt% of titanium (Ti), 0.1 wt% or less of niobium (Nb) (excluding 0 wt%), 0.005 wt% or less of boron (B) (excluding 0 wt%), 0.01 wt% or less of nitrogen (N) (excluding 0wt%), iron (Fe) as a remainder thereof, and other inevitably contained impurities. The plated steel sheet may have a microstructure obtained by plating and heat-treating a steel sheet comprised of 90% or more of tempered martensite and 10% or less of ferrite and bainite in a volume fraction.
- Hereinafter, the reason for limiting the components of the steel sheet will be described in detail. The content unit of each component will refer to weight% unless otherwise specified.
- Carbon (C) is an element essentially added to secure strength of a steel. In order to obtain the above-mentioned effect, carbon (C) may be added in an amount of 0.12% or more. However, if the content thereof is relatively high, exceeding 0.20%, a problem in which weldability is deteriorated may occur, which may be problematic.
- Thus, the content of C may be limited to 0.12% to 0.20%.
- Silicon (Si) is a ferrite stabilizing element and may have a disadvantage in that strength may be reduced by accelerating generation of ferrite at the time of slow cooling after annealing in a continuous annealing type hop-dip heat treatment furnace of the related art in which a slow cooling section is present. In addition, in the case in which a relatively large amount of Mn is added to suppress phase transformation as in the present disclosure, since a risk of deterioration of molten plating characteristics due to formation of a surface oxide by Si during annealing and the occurrence of dent defects due to surface thickening and oxidation by Si may be present, an upper limit of Si may be determined. In detail, the content of Si may be limited to 0.5% or less.
- Manganese (Mn) is well known as an element inhibiting the formation of ferrite and facilitating the formation of austenite. In the case of slow cooling after annealing in a continuous annealing type hop-dip heat treatment furnace, if a content of Mn is less than 2.6%, ferrite may be easily formed during slow cooling, while if the content of Mn exceeds 4.0%, band formation due to slabs, and segregation caused in a hot rolling process, may be excessive, and a problem in which a cost of alloy iron is increased due to an excessive amount of alloying input when a convertor is operated.
- Thus, the content of Mn may be limited to 2.6% to 4.0%.
- Phosphorus (P) may be an impurity element in steel. If the content thereof exceeds 0.03%, weldability may be decreased, a risk of brittleness of steel may be increased, and a possibility of the occurrence of dent defects may increase. Thus, the content of P may be limited to 0.03% or less.
- Sulfur (S) may be an impurity element in steel as well as P, and if the content thereof exceeds 0.015%, a possibility of deterioration of ductility and weldability of steel may increase. Thus, the content of S may be limited to 0.015% or less.
- Aluminum (Al) is an element for expanding a ferrite region. In the case in which a general continuous annealing type hop-dip heat treatment furnace having a slow cooling section is used, there may be a disadvantage in that ferrite formation is promoted, and the possibility of causing deteriorations in high temperature heat hot-rolling characteristics due to the formation of AlN may increase. Thus, the content of Al may be limited to 0.1% or less.
- Chromium (Cr) is an element suppressing ferrite transformation and facilitating low-temperature transformation. In the case in which a general continuous annealing type hop-dip heat treatment furnace having a slow cooling section is used, there may be an advantage in that ferrite formation is suppressed. However, if the content thereof exceeds 1%, a problem in which a cost of alloying iron is increased due to an excessive amount of alloying may occur. Thus, the content thereof may be limited to 1% or less.
- Titanium (Ti) is an element for forming a nitride, and may serve to precipitate N in steel as TiN to scavenge N therein. To this end, Ti may be required to be added at a chemical equivalent of 48/14 *[N] or more. On the other hand, if Ti is not added, a problem in which cracks may occur during continuous casting by AlN formation may be caused. However, if the content thereof exceeds 0.1%, a problem in which the strength of martensite is reduced due to precipitation of additional carbides in addition to the removal of solid solution N, may be caused.
- Niobium (Nb) is an element segregated at an austenite grain boundary to suppress coarsening of austenite grains during an annealing heat treatment, and thus, may be added. However, if the content thereof exceeds 0.1%, a problem in which a cost of alloy iron is increased due to an excessive amount of added alloy may occur. Thus, the content of Nb may be limited 0.1% or less.
- Boron (B) is an element inhibiting ferrite formation. In detail, B has an advantage of inhibiting the formation of ferrite at the time of cooling after annealing, and thus, may be added. However, if the content thereof exceeds 0.005%, since a problem in which ferrite formation is promoted by precipitation of Fe23(C,B)6 may occur, the content thereof may be limited to 0.005% or less.
- Nitrogen (N) is an element reacting with Al to be precipitated into AlN nitride, and the formed AlN may have a problem in that it is a cause of occurrence of cracking during continuous casting. Thus, the content of Al may be limited to 0.01% or less, and thus, the formation of AlN may be suppressed.
- Fe and unavoidable impurities may be included as a remainder. In this case, examples of the impurities may include molybdenum (Mo), vanadium (V), nickel (Ni), rare earth metals (REM), and the like.
- A steel sheet used to obtain a plated steel sheet having ultra-high strength according to an exemplary embodiment may have a microstructure comprised of 90% or more of martensite and 10% or less of ferrite and bainite in a volume fraction, while satisfying the above-mentioned compositional composition. As effective characteristics according to the configuration of the microstructure, as martensite of a hard phase has a microstructure, a main phase, securing ultra-high strength may be facilitated.
- The plated steel sheet having ultra-high strength, ultimately obtained by heat-treating the steel sheet as described above, according to an exemplary embodiment may also have the same microstructure as above, and when an additional tempering heat treatment thereto is performed, martensite may be converted into tempered martensite.
- On the other hand, it may not be easy to actually measure a volume fraction, a three-dimensional concept, and thus, measurement of the volume fraction may be replaced with area fraction measurement through a cross-sectional observation normally used in observation of a microstructure.
- In addition, the steel sheet having the component system and the microstructure as described above may be plated and heat-treated, and an amount of hydrogen after heat treatment may be 0.000015 wt% or less, as compared with the case before the heat treatment. Thus, the target ratio of a yield strength and a tensile strength of the plated steel sheet having ultra-high strength according to an exemplary embodiment may be 0.75 or more.
- In order to produce the plated steel sheet having ultra-high strength and the composition and microstructure as described above, a process as below may be performed.
- First, a steel sheet including 0.12 wt% to 0.2 wt% of carbon (C), 0.5 wt% or less of silicon (Si) (excluding 0wt%), 2.6 wt% to 4.0 wt% of manganese (Mn), 0.03 wt% or less of phosphorus (P) (excluding 0 wt%), 0.015 wt% or less of sulfur (S) (excluding 0 wt%), 0.1 wt% or less of aluminum (Al) (excluding 0 wt%), 1 wt% or less of chromium (Cr) (excluding 0 wt%), 48/14*[N] to 0.1 wt% of titanium (Ti), 0.1 wt% or less of niobium (Nb) (excluding 0 wt%), 0.005 wt% or less of boron (B) (excluding 0 wt%), 0.01 wt% or less of nitrogen (N) (excluding 0 wt%), iron (Fe) as a remainder thereof, and other inevitably contained impurities; and having a microstructure comprised of 90% or more of tempered martensite and 10% or less of ferrite and bainite in a volume fraction, may be prepared.
- Subsequently, the steel sheet may be plated to produce a plated steel sheet, and the plated steel sheet may be subjected to a heat treatment.
- In this case, a plating process is not particularly limited, and for example, a process such as hot-dip galvanizing, hot-dip aluminum plating, electro-galvanizing, or the like may be performed.
- In addition, a heat treatment after plating may be performed such that an amount of hydrogen in the plated steel sheet may be 0.000015 wt% or less. In this case, by performing the heat treatment for a relatively short time at a high temperature or at a relatively low temperature for a relatively long time, the amount of hydrogen may be reduced to a required level. Thus, a heat treatment time and temperature conditions according to an exemplary embodiment are not particularly limited.
- However, as a normal heat treatment temperature increases, a reduction in tensile strength may be increased. Thus, the heat treatment temperature and time may be set in consideration of a tensile strength level required by a customer.
- A plated steel sheet having ultra-high strength is generally manufactured as a coil having a constant width through a slitting and coiling process, and the slitting process is a process of adding relatively high stress to an edge portion of a steel sheet. In the case of a plated steel sheet having ultra-high strength, a disadvantage in that the quality of a cut surface of an edge portion may be deteriorated due to a plating layer may be present. Hydrogen in steel tends to segregate under a relatively high stress state. Thus, for example, when a slitting process is performed on the plated steel sheet having ultra-high strength, hydrogen in steel may segregate on a relatively highly stressed portion of an edge portion of the plated steel sheet after the slitting, whereby cracks may start to occur in the edge portion of the plated steel sheet having ultra-high strength and the propagation of cracks may occur in a width direction.
- Thus, by performing the heat treatment according to an exemplary embodiment, an amount of hydrogen of the plated steel sheet having ultra-high strength may be reduced to 0.000015 wt% or less, and thus, cracking of an edge portion over time during coiling, after slitting, may be effectively suppressed.
- Hereinafter, a plated steel sheet having ultra-high strength according to an exemplary embodiment will be described in detail with reference to Embodiment. It should be noted, however, that the following embodiments are intended to illustrate the present disclosure in more detail and not to limit the scope of the invention. In other words, the scope of the invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.
- A plated steel sheet having ultra-high strength, an initial yield strength of 1149 MPa, and an initial tensile strength of 1556 MPa was evaluated for changes in an amount of hydrogen before a heat treatment and after a heat treatment under conditions provided in Table 1 below. Evaluation results are provided as illustrated in Table 1.
- In this case, a steel material having a component system consisting of 0.18% of C, 0.1% of Si, 3.6% of Mn, 0.011% of P, 0.11% of Cr, 0.021% of Ti, 0.038% of Nb, 0.0017% of B, 0.003% of S, 0.025% of Al, and 0.004% of N was prepared as a specimen having a size of thickness*12mm*100mm, and was heated to a temperature from 25°C to 250°C at a heating rate of 100°C per hour. An amount of hydrogen was measured using gas chromatography, simultaneously with performing a heat treatment.
- First, as a result of measuring amounts of hydrogen of a cold rolled steel sheet, not subjected to plating, and in a plated steel sheet, the cold rolled steel sheet had no hydrogen, 0 wt% of hydrogen, while the plated steel sheet had a relatively high content, 0.000022 wt% of hydrogen.
- These results show that in the case of a relatively small amount of bainite having a BCC structure and martensite having a BCT structure (in the case of martensite having a relatively low carbon content, the martensite has substantially the same crystal structure as that of BCC), since solubility of hydrogen is relatively low and diffusion of hydrogen is relatively fast, hydrogen may have disappeared within a few minutes to several hours after the production of a cold-rolled steel sheet, and thus, the amount of hydrogen was measured as 0 wt% in the cold-rolled steel sheet formed of martensite as a main phase.
[Table 1] Heat Treatment Conditions Temperature 150°C 200°C Hydrogen Atmosphere 100% 7% 0% 7% 0% Time 24h 24h 72h 24h 72h 24h 48h 24h 48h Amount of Hydrogen (Weight%) 0.000017 0.000016 0.000010 0.000010 0.000007 0.000008 0.000006 0.000007 0.000004 - As illustrated in Table 1, when the heat treatment temperature was 150°C, in a case in which an amount of hydrogen in atmospheric gas was 0%, a reduction of hydrogen proceeded relatively fast as compared to the case in which an amount of hydrogen in atmospheric gas was 7%. In addition, it can be seen that when an amount of hydrogen in atmospheric gas was 0%, in the case in which the heat treatment temperature was 200°C, a reduction of hydrogen progressed relatively fast as compared to 150°C.
- For example, as the content of hydrogen in atmospheric gas during a heat treatment is reduced, and as the heat treatment temperature is increased, the amount of hydrogen may be further reduced.
- Further, after a plated steel sheet (A) not subjected to heat treatment, a plated steel sheet (B) having been subjected to heat treatment at 150°C for 24 hours in a 100% of hydrogen atmosphere, and a plated steel sheet (C) having been subjected to heat treatment at 200°C for 24 hours in a 7% of hydrogen atmosphere were slit, whether or not cracks occurred as time passed was observed, and the results thereof are provided in
FIG. 1 . - As illustrated in
FIG. 1 , it can be confirmed that cracking has occurred in the case of the plated steel sheet (A) not subjected to the heat treatment and in the case of the plated steel sheet (B) in which an amount of hydrogen exceeded 0.000015 wt% even after heat treatment. Meanwhile, in the case of the plated steel sheet (C) having been subjected to a heat treatment at a relatively low rate in a hydrogen atmosphere and at a relatively high temperature, no cracking occurred. - These results may indicate that a plated steel sheet having ultra-high strength and having a yield strength ratio of 0.75 or more with respect to a tensile strength, by tempering heat treatment of an ultra-high strength plated steel sheet having martensite as a main phase, may be provided. However, as the heat treatment temperature increases, a decrease in tensile strength may increase. Thus, setting heat treatment temperature and time according to a tensile strength level required by a customer may be required.
Claims (1)
- A plated steel sheet having ultra-high strength of a tensile strength of 1300 MPa or more and a yield strength ratio of 0.75 or more, being characterized in that an amount of hydrogen in the plated steel sheet is 0.000015 wt% or less, wherein the plated steel sheet comprises 0.12 wt% to 0.2 wt% of carbon (C), 0.5 wt% or less of silicon (Si) (excluding 0 wt%), 2.6 wt% to 4.0 wt% of manganese (Mn), 0.03 wt% or less of phosphorus (P) (excluding 0 wt%), 0.015 wt% or less of sulfur (S) (excluding 0 wt%), 0.1 wt% or less of aluminum (Al) (excluding 0 wt%), 1 wt% or less of chromium (Cr) (excluding 0 wt%), 48/14*[N] to 0.1 wt% of titanium (Ti), 0.1 wt% or less of niobium (Nb) (excluding 0 wt%), 0.005 wt% or less of boron (B) (excluding 0 wt%), 0.01 wt% or less of nitrogen (N) (excluding 0wt%), iron (Fe) as a remainder thereof, and other inevitably contained impurities; and has a microstructure obtained by plating and heat-treating a steel sheet comprised of 90% or more of tempered martensite and 10% or less of ferrite and bainite in a volume fraction.
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