EP3889305B1 - High-strength steel plate having excellent low-temperature fracture toughness and elongation ratio, and manufacturing method therefor - Google Patents

High-strength steel plate having excellent low-temperature fracture toughness and elongation ratio, and manufacturing method therefor Download PDF

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Publication number
EP3889305B1
EP3889305B1 EP19889727.4A EP19889727A EP3889305B1 EP 3889305 B1 EP3889305 B1 EP 3889305B1 EP 19889727 A EP19889727 A EP 19889727A EP 3889305 B1 EP3889305 B1 EP 3889305B1
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Prior art keywords
steel plate
recrystallized
region
less
rolling
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German (de)
English (en)
French (fr)
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EP3889305A1 (en
EP3889305A4 (en
EP3889305C0 (en
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Jang-Yong Yoo
Moo-Jong BAE
Yoen-Jung PARK
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Posco Holdings Inc
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Posco Co Ltd
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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
    • C22C38/001—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties 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
    • C21D8/0247—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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a high-strength steel plate and a manufacturing method therefor., and more particularly, to a high-strength steel plate for a pipeline capable of being stably used even in a harsh environment by having high strength characteristics through optimization of a steel composition, a microstructure, and a manufacturing process and having excellent low-temperature fracture toughness and elongation ratio, and a manufacturing method therefor.
  • a steel used in such a pipeline project should necessarily have durability against deformation of the pipeline due to a cryogenic temperature and frost heave (a phenomenon of pushing up the surface of Earth when the ground freezes at the change of seasons) as well as a pressure of a transport gas, and is thus required to have high strength characteristics and excellent drop weight tearing test (DWTT) fracture toughness, and high elongation ratio characteristics.
  • DWTT drop weight tearing test
  • a DWTT percent ductile fracture is a kind of index for determining whether or not a steel for a pipeline has brittle fracture arrestability for being safely used at a low temperature.
  • the pipelines provided in the cold regions are required to have a DWTT percent shear of 85% or more at -20°C in a pipe state.
  • a DWTT percent shear of a steel plate provided for manufacturing a pipe should satisfy 85% or more at -30°C.
  • DWTT property has a deep association with an effective grain size of the steel plate.
  • the effective grain size is defined as a size of grains having a high angle grain boundary, and as the effective grain size is refined, crack arrestability increases. The reason therefor that a propagation path of a crack changes at an effective grain boundary when the crack is initiated and propagated.
  • a method of performing accelerated cooling immediately after rolling is widely used.
  • a mixed structure of acicular ferrite and bainite may be implemented by accelerated cooling immediately after rolling.
  • a microstructure formed through usual accelerated cooling has high hardness because carbon (C) is supersaturated in grains, and accordingly, exhibits inferior ductility such as a uniform elongation ratio less than 9% and a total elongation ratio less than 20%.
  • C carbon
  • a microstructure formed through usual accelerated cooling has high hardness because carbon (C) is supersaturated in grains, and accordingly, exhibits inferior ductility such as a uniform elongation ratio less than 9% and a total elongation ratio less than 20%.
  • a manufacturing method for a steel plate for a pipeline having excellent low-temperature fracture toughness and having excellent ductility by having a uniform elongation ratio of 9% or more and a total elongation ratio of 28% or more by suppressing deterioration of an elongation ratio of the steel plate manufactured by accelerated cooling has been demanded.
  • Patent Literature 1 proposes a method of manufacturing a steel containing a mixed structure of 30 of 60% of equiaxed ferrite and 40 to 70% of bainite in terms of area fraction as a microstructure by non-recrystallized-region-rolling a steel containing nickel (Ni), niobium (Nb), and molybdenum (Mo) at a rolling reduction of 65% or more, primarily cooling the steel to a Bs temperature at a cooling rate of 15 to 30°C/s, and secondarily cooling the steel to a temperature range of 350 to 500°C at a cooling rate of 30 to 60°C/s.
  • Ni nickel
  • Nb niobium
  • Mo molybdenum
  • Patent Document 1 in which low-temperature rolling is performed on a steel plate having a thickness of 20 mm or more, there is a technical difficulty in applying the corresponding process condition to a steel plate having a thickness less than 20 mm.
  • the reason is that the steel plate having the thickness less than 20 mm is subjected to low-temperature rolling and then cooled rapidly, and it is thus difficult to secure desired low-temperature fracture toughness, strength, and elongation ratio in an entire length direction of the steel plate, particularly, at a rear end portion of the steel plate.
  • Patent Document 2 teaches a steel plate for a line pipe having superior uniform elongation ratio and low-temperature fracture toughness and a manufacturing method of the steel plate which comprises by weight%, 0.04-0.10% carbon, 0.05-0.50% silicon, 1.4-2.0% manganese, 0.01-0.05% aluminum, 0.005-0.02% titanium, 0.002-0.01% nitride, 0.02-0.07% niobium, 0.05-0.3% chromium, 0.1-0.4% nickel, 0.05-0.3% molybdenum, identical to or less than 0.015% phosphorus, equal to or less than 0.005% sulfate, 0.0005-0.004% calcium, and a balance of iron and unavoidable impurities, of which a microstructure comprises on area percentage, 50-65% ferrite, 30-40% bainite, 5-10% martensite-austenite constituent, in which the average valid particle size of the ferrite is identical to or less than 10pm, and the average valid particle size of the bainite is identical to or
  • An aspect of the present invention is to provide a high-strength steel plate having excellent low-temperature toughness, and a manufacturing method therefor.
  • a high-strength steel plate having excellent low-temperature fracture toughness and elongation ratio contains: by wt%, 0.05 to 0.1% of carbon (C), 0.05 to 0.5% of silicon (Si), 1.4 to 2.0% of manganese (Mn), 0.01 to 0.05% of aluminum (Al), 0.005 to 0.02% of titanium (Ti), 0.002 to 0.01% of nitrogen (N), 0.04 to 0.07% of niobium (Nb), 0.05 to 0.3% of chromium (Cr), 0.05 to 0.4% of nickel (Ni), 0.02% or less of phosphorus (P), 0.005% or less of sulfur (S), 0.0005 to 0.004% of calcium (Ca), remaining iron (Fe), and inevitable impurities; and 20 to 60 area% of ferrite, 35 to 75 area% of bainite and 5 area% or less of martensite-austenite constituent as a microstructure, wherein a grain size of upper 80% of high angle
  • a manufacturing method for a high-strength steel plate having excellent low-temperature fracture toughness and elongation ratio includes: reheating a slab containing, by wt%, 0.05 to 0.1% of carbon (C), 0.05 to 0.5% of silicon (Si), 1.4 to 2.0% of manganese (Mn), 0.01 to 0.05% of aluminum (Al), 0.005 to 0.02% of titanium (Ti), 0.002 to 0.01% of nitrogen (N), 0.04 to 0.07% of niobium (Nb), 0.05 to 0.3% of chromium (Cr), 0.05 to 0.4% of nickel (Ni), 0.02% or less of phosphorus (P), 0.005% or less of sulfur (S), 0.0005 to 0.004% of calcium (Ca), remaining iron (Fe), and inevitable impurities; maintaining and extracting the slab; recrystallized-region-rolling the maintained and extracted slab in a temperature range of Tnr or higher; non-recrystallized
  • [C], [Si], [Mn], [Al], [Ti], [Nb], [V], [Cr], [Mo], and [Cu] refer to wt% of respective alloy compositions, and when a corresponding alloy composition is not contained, calculation is performed by replacing a value of the corresponding alloy composition with 0.
  • a steel plate particularly suitable as a material for a pipeline by having high strength characteristics and having excellent low-temperature fracture toughness and elongation ratio, and a manufacturing method therefor is provided.
  • the present invention relates to a high-strength steel plate having excellent low-temperature fracture toughness and elongation ratio, and a manufacturing method therefor, and exemplary embodiments in the present invention will hereinafter be described. Exemplary embodiments in the present invention may be modified to have several forms, and it is not to be interpreted that the scope of the present invention is limited to exemplary embodiments described below. The present exemplary embodiments are provided in order to further describe the present invention in detail to those skilled in the art to which the present invention pertains.
  • compositions of a steel according to the present invention will be described in more detail.
  • % indicating a content of each element is based on weight.
  • a high-strength steel plate having excellent low-temperature fracture toughness and elongation ratio contains: by wt%, 0.05 to 0.1% of carbon (C), 0.05 to 0.5% of silicon (Si), 1.4 to 2.0% of manganese (Mn), 0.01 to 0.05% of aluminum (Al), 0.005 to 0.02% of titanium (Ti), 0.002 to 0.01% of nitrogen (N), 0.04 to 0.07% of niobium (Nb), 0.05 to 0.3% of chromium (Cr), 0.05 to 0.4% of nickel (Ni), 0.02% or less of phosphorus (P), 0.005% or less of sulfur (S), 0.0005 to 0.004% of calcium (Ca), remaining iron (Fe), and inevitable impurities.
  • the high-strength steel plate having excellent low-temperature fracture toughness and elongation ratio according to an exemplary embodiment in the present invention may further contain, by wt%, 0.3% or less of molybdenum (Mo).
  • Carbon (C) is an element that is the most effective in improving strength of a steel.
  • expensive alloying elements such as molybdenum (Mo) and nickel (Ni) need to be added in a large amount in order to secure the strength of the steel, which is not preferable in terms of economy.
  • a lower limit of a content of carbon (C) is limited to 0.05% in order to achieve such an effect.
  • an upper limit of the content of carbon (C) is limited to 0.1%. Therefore, the content of carbon (C) of the present invention is in the range of 0.05 to 0.1%, and may be more preferably in the range of 0.05 to 0.095%.
  • Silicon (Si) is an element useful for deoxidation of a molten steel, and is also an element that contributes to improving strength of the steel by solid solution strengthening.
  • a lower limit of a content of silicon (Si) is limited to 0.05% in order to achieve such an effect.
  • a more preferable lower limit of the content of silicon (Si) may be 0.1%.
  • silicon (Si) is an element having strong oxidation properties, it is preferable to limit an upper limit of the content of silicon (Si) to a predetermined range. That is, when silicon (Si) is added in an excessive amount, it causes red scale formation at the time of hot rolling, which is not preferable in terms of surface quality, and has an undesirable influence on toughness of a weld zone.
  • the upper limit of the content of silicon (Si) is limited to 0.5%.
  • a more preferable upper limit of the content of silicon (Si) may be 0.4%.
  • Manganese (Mn) is an element that is effective in solid solution strengthening of the steel.
  • a lower limit of a content of manganese (Mn) is limited to 1.4% in order to secure high strength properties of the steel.
  • a segregation portion may be formed over a wide range in a central portion of a thickness at the time of casting a slab in a steelmaking process, which is not preferable in terms of weldability of a final product.
  • an upper limit of the content of manganese (Mn) is limited to 2.0%.
  • a more preferable upper limit of the content of manganese (Mn) may be 1.8%.
  • Aluminum (Al) is a representative element that is added as a deoxidizing agent along with silicon (Si).
  • aluminum (Al) is an element that contributes to improving strength of the steel by solid solution strengthening.
  • a lower limit of a content of aluminum (Al) is limited to 0.01% in order to achieve such an effect.
  • a more preferable lower limit of the content of aluminum (Al) may be 0.015%.
  • an upper limit of the content of aluminum (Al) is limited to 0.05%.
  • a more preferable upper limit of the content of aluminum (Al) may be 0.04%.
  • Titanium (Ti) is an element that forms TiN precipitates in a solidification process of the steel to suppress growth of austenite grains in a slab heating and hot rolling process, and thus refines a grain size of a final structure.
  • a lower limit of the content of titanium (Ti) is limited to 0.005% in order to achieve a toughness improvement effect of the steel according to the refinement of the final structure.
  • a more preferable content of titanium (Ti) may be 0.008%.
  • an upper limit of the content of titanium (Ti) is limited to 0.02%.
  • a more preferable upper limit of the content of titanium (Ti) may be 0.018%.
  • Nitrogen (N) is solid-dissolved in the steel and then precipitated to serve to increase strength of the steel, and it is known that such a strength improvement effect is much greater than that of carbon (C).
  • TiN is formed through a reaction between titanium (Ti) and nitrogen (N) and it is intended to suppress growth of grains in a reheating process.
  • a lower limit of a content of nitrogen (N) is limited to 0.002%.
  • nitrogen (N) exists in a form of solid solution nitrogen (N) rather than a form of TiN precipitates, so that toughness of the steel may be significantly reduced.
  • an upper limit of the content of nitrogen (N) is limited to 0.01%.
  • a preferable upper limit of the content of nitrogen (N) may be 0.006 %, and a more preferable upper limit of the content of nitrogen (N) may be 0.005%.
  • Niobium (Nb) is an element that is very useful for refining grains, and is an element that significantly contributes to improving strength of the steel by promoting formation of acicular ferrite or bainite, which is a high-strength structure.
  • niobium (Nb) that has the greatest effect on an increase in a non-recrystallization temperature needs to be added in a predetermined amount or more.
  • a lower limit of a content of niobium (Nb) is limited to 0.04%.
  • an upper limit of the content of niobium (Nb) is limited to 0.07%.
  • a preferable upper limit of the content of niobium (Nb) may be 0.06%.
  • Chromium (Cr) is an element that improves hardenability and is an element that is effective in increasing strength of the steel.
  • chromium (Cr) is an element that contributes to improving a uniform elongation ratio by promoting formation of martensite-austenite constituent (MA) at the time of accelerated cooling.
  • a lower limit of a content of chromium (Cr) is limited to 0.05% in order to achieve such an effect.
  • a more preferable lower limit of the content of chromium (Cr) may be 0.08%.
  • chromium (Cr) is excessively added, deterioration of weldability of the steel may be caused.
  • an upper limit of the content of chromium (Cr) is limited to 0.3%.
  • a preferable upper limit of the content of chromium (Cr) may be 0.25%, and a more preferable upper limit of the content of chromium (Cr) may be 0.2%.
  • Nickel (Ni) is an element that effectively contributes to improving toughness and strength of the steel.
  • a lower limit of a content of nickel (Ni) is limited to 0.05% in order to achieve such an effect.
  • nickel (Ni) is an expensive element, and excessive addition of nickel (Ni) is not preferable in terms of economy
  • an upper limit of a content of nickel (Ni) is limited to 0.4%.
  • a preferable upper limit of the content of nickel (Ni) may be 0.3%, and a more preferable upper limit of the content of nickel (Ni) may be 0.25%.
  • Phosphorus (P) 0.02% or less
  • Phosphorus (P) is a representative impurity element that exists in the steel, and is mainly segregated in a central portion of the steel plate to cause a decrease in toughness of the steel, and it is thus preferable to manage phosphorus (P) at a level as low as possible.
  • a content of phosphorus (P) is limited to 0.02% or less.
  • a more preferable content of phosphorus (P) may be 0.015% or less.
  • Sulfur (S) is also a representative impurity element that exists in the steel, and is an element that combines with manganese (Mn) or the like in the steel to form nonmetallic inclusions such as MnS, and accordingly, significantly impairs toughness and strength of the steel.
  • Mn manganese
  • S sulfur
  • a content of sulfur (S) is limited to 0.005% or less.
  • a more preferable content of sulfur (S) may be 0.003% or less.
  • Calcium (Ca) is an element that is effective in suppressing crack formation around nonmetallic inclusions by spheroidizing nonmetallic inclusions such as MnS.
  • a lower limit of a content of calcium (Ca) is limited to 0.0005% in order to achieve such an effect.
  • an upper limit of a content of calcium (Ca) is limited to 0.004%.
  • a preferable upper limit of the content of calcium (Ca) may be 0.002%.
  • Molybdenum (Mo) is an element that is effective in securing both of high strength and high toughness by promoting formation of bainite, which is a low-temperature transformation structure. Therefore, in the present invention, molybdenum (Mo) may be selectively added in order to achieve such an effect.
  • molybdenum (Mo) is an expensive element and it is not preferable in terms of economy to add molybdenum (Mo) in an excessive amount.
  • an upper limit of a content of molybdenum (Mo) may be limited to 0.3%.
  • the remainder contains Fe and inevitable impurities.
  • the inevitable impurities may be unintentionally mixed in a general steelmaking process and may not be completely excluded, and those skilled in a general steelmaking field may easily understand the meaning of the inevitable impurities.
  • the present invention does not entirely exclude addition of a composition other than the steel compositions described above.
  • a microstructure according to the present invention will hereinafter be described in more detail.
  • the steel plate according to an exemplary embodiment in the present invention contains ferrite and bainite as a microstructure, and further contain martensite-austenite constituent. Fractions of the ferrite and the bainite is 20 to 60 area% and 35 to 75 area%, respectively, and a fraction of the martensite-austenite constituent is 5 area% or less.
  • the steel plate according to the present invention contains ferrite having a fine high angle grain boundary in an area of 20% or more, and may thus effectively secure low-temperature drop weight tearing test (DWTT) characteristics.
  • the steel plate according to the present invention contains the ferrite in an area of 60% or less and contains the bainite in an area of 35% or more, and thus secure a yield strength of 485 MPa or more.
  • a fraction of the bainite is limited to 75 area% or less in order to prevent the high angle grain boundary from becoming excessively coarse, and accordingly, low-temperature DWTT characteristics may be effectively secured.
  • the martensite-austenite constituent has an undesirable influence on the low-temperature DWTT characteristics, and it is thus preferable to suppress a fraction of the martensite-austenite constituent as much as possible. Therefore, in the present invention, the fraction of the martensite-austenite constituent is limited to 5 area% or less.
  • the steel plate according to an exemplary embodiment in the present invention has a grain size of 70 um or less in upper 80% of high angle grain sizes based on 15° in a central portion of the steel plate. That is, in the present invention, effective grain sizes may be refined by refining the high angle grain sizes, and accordingly, low-temperature DWTT characteristics may be effectively secured.
  • the central portion of the steel plate may be interpreted as an area including a point of t/2, and may also be interpreted as an area of a point of t/4 to 3*t/4 (t: thickness (mm) of steel plate).
  • the steel plate according to an exemplary embodiment in the present invention has a thickness less than 20 mm, and a preferable thickness of the steel plate may be 16 mm or less.
  • the steel plate according to an exemplary embodiment in the present invention has a yield strength of 485 MPa or more, a total elongation ratio of 28% or more, and a uniform elongation ratio of 9% or more with respect to a rolling orthogonal direction, and has a DWTT percent ductile fracture of 85% or more at -30°C with respect to the rolling orthogonal direction of the steel plate. Therefore, in the present invention, a steel plate particularly suitable as a material for a pipeline by effectively securing strength, low-temperature fracture toughness, and an elongation ratio in spite of having the thickness less than 20 mm may be provided.
  • the high-strength steel plate having excellent low-temperature fracture toughness and elongation ratio is manufactured by reheating a slab containing, by wt%, 0.05 to 0.1% of carbon (C), 0.05 to 0.5% of silicon (Si), 1.4 to 2.0% of manganese (Mn), 0.01 to 0.05% of aluminum (Al), 0.005 to 0.02% of titanium (Ti), 0.002 to 0.01% of nitrogen (N), 0.04 to 0.07% of niobium (Nb), 0.05 to 0.3% of chromium (Cr), 0.05 to 0.4% of nickel (Ni), 0.02% or less of phosphorus (P), 0.005% or less of sulfur (S), 0.0005 to 0.004% of calcium (Ca), remaining iron (Fe), and inevitable impurities, maintaining and extracting the slab, recrystallized-region-rolling the maintained and extracted slab in a temperature range of Tnr or higher, non-recrystallized-region-roll
  • the slab according to the present invention has the same alloy composition as the alloy composition of the steel plate described above, and a description of the alloy composition of the slab according to the present invention is thus replaced by the description of the alloy composition of the steel plate described above.
  • the slab reheating is a process of heating a steel in order to smoothly perform the subsequent rolling processes and secure desired physical properties of the steel plate, a heating process needs to be performed within an appropriate temperature range according to a purpose.
  • a lower limit of a slab reheating temperature needs to be determined in consideration of whether or not it is a temperature at which precipitated elements may be sufficiently dissolved in the steel.
  • the slab according to the present invention essentially contains niobium (Nb) in order to secure high strength properties
  • the lower limit of the slab reheating temperature is limited to 1140°C in consideration of a resoluble temperature of niobium (Nb).
  • an upper limit of the slab reheating temperature is limited to 1200°C.
  • the reheated slab is subjected to a maintaining and extracting process, if necessary, and a maintaining and extracting temperature of the slab is limited to a temperature range of 1140 to 1200°C for reasons similar to those of the slab reheating temperature.
  • the recrystallized-region-rolling is performed in a temperature range of Tnr or more.
  • Tnr refers to a lower limit of a temperature range at which recrystallization of austenite occurs. That is, the recrystallized-region-rolling is performed in a temperature range of an austenite recrystallized region.
  • the recrystallized-region-rolling may be performed in multiple passes, and rolling may be performed at an average reduction ratio of 10% or more per pass. The reason is that when the average reduction ratio per pass is less than 10%, a grain size of recrystallized austenite becomes coarse, which may cause a decrease in toughness of the final steel plate.
  • the recrystallized-region-rolled material may be cooled to a temperature range of Tnr or lower under a cooling condition of air cooling. That is, the recrystallized-region-rolled material is not immediately subjected to non-recrystallized-region-rolling, and may wait for a predetermined time to be cooled to a temperature range of a non-recrystallized region by air cooling. The reason is that when a rolling force is applied in the corresponding section, partial recrystallization may occur, such that a brittle fracture due to a coarse austenite grain size may occur.
  • the non-recrystallized-region-rolling is performed on the recrystallized-region-rolled material.
  • a start temperature of the non-recrystallized-region-rolling is Tnr or lower, and an end temperature of the non-recrystallized-region-rolling is (Ar3+100°C).
  • the non-recrystallized-region-rolling is a process for elongating austenite produced by recrystallized-region-rolling to be elongate and forming a deformed structure in a grain to obtain fine ferrite and bainite, and strength, an elongation ratio and brittle fracture arrestability of the steel plate may be effectively improved by the non-recrystallized-region-rolling.
  • the end temperature of the non-recrystallized-zone-rolling may be limited to (Ar3 + 50°C) or higher.
  • a rolling reduction of the non-recrystallized-region-rolling is an important factor in securing low-temperature toughness of the steel.
  • the rolling reduction of the non-recrystallized-region-rolling is limited to 30% or more in order to secure low-temperature DWTT percent ductile fracture characteristics according to refinement of grain sizes of a final steel. Since it is effective in improving low-temperature toughness that the rolling reduction of the non-recrystallized-region-rolling becomes larger, an upper limit of the rolling reduction of the non-recrystallized-region-rolling may not be limited.
  • the rolling reduction in the non-recrystallized-region-rolling may be limited to 90% or less.
  • the non-recrystallized-region-rolled steel plate is cooled from a cooling start temperature of (Ar3 + 30°C) or higher to a cooling stop temperature of (Bs - 80°C) to Bs.
  • a cooling start temperature of (Ar3 + 30°C) or higher to a cooling stop temperature of (Bs - 80°C) to Bs.
  • the cooling start temperature is excessively low, a large amount of ferrite having low strength is produced, and accordingly, strength of the steel plate may be significantly decreased.
  • the cooling starts in a temperature range of (Ar3 + 30°C) or higher.
  • the steel plate according to the present invention has a final thickness less than 20 mm, it is most preferable in terms of strength and an elongation ratio to stop cooling in a temperature range of (Bs - 80°C) to Bs.
  • the reason is that when the cooling stop temperature is lower than (Bs - 80°C), acicular ferrite and bainite having a high angle grain boundary formed to be coarse and a low angle grain boundary are formed in a large amount, such that an elongation ratio may be decreased, and when the cooling stop temperature exceeds Bs, an amount of bainite produced is small, such that strength of the steel plate may not be secured.
  • the steel plate may be quenched to the cooling stop temperature of (Bs - 80°C) to Bs, then cooled to room temperature by air cooling or radiation cooling.
  • the cooling of the present invention is performed at a cooling rate of 10 to 50°C/s.
  • the reason is that when the cooling rate is less than 10°C/s, a fraction of equiaxed ferrite is significantly increased, such that high strength characteristics of the steel plate may not be effectively secured.
  • an upper limit of the cooling rate is limited to 50°C/s.
  • the steel plate manufactured by the manufacturing method described above may contain ferrite and bainite as a microstructure, and further contain martensite-austenite constituent. Fractions of the ferrite and the bainite is 20 to 60 area% and 35 to 75 area%, respectively, and a fraction of the martensite-austenite constituent is 5 area% or less.
  • the steel plate manufactured by the manufacturing method described above has a grain size of 70 um or less in upper 80% of high angle grain sizes based on 15° in a central portion of the steel plate.
  • the steel plate manufactured by the manufacturing method described above has a thickness less than 20 mm, and has a yield strength of 485 MPa or more, a total elongation ratio of 28% or more, and a uniform elongation ratio of 9% or more with respect to a rolling orthogonal direction, and has a DWTT percent ductile fracture of 85% or more at -30°C with respect to the rolling orthogonal direction of the steel plate. Therefore, according to the manufacturing method according to an exemplary embodiment in the present invention, a steel plate particularly suitable as a material for a pipeline by effectively securing strength, low-temperature fracture toughness, and an elongation ratio in spite of having the thickness less than 20 mm is provided.
  • Slabs having alloy compositions of Table 1 and having a thickness of 250 mm were manufactured, and steel plate specimens having thicknesses of 11 mm, 11.5 mm, and 22 mm, respectively, were manufactured by applying process conditions of Table 3.
  • the slabs were manufactured by applying process conditions used for manufacturing a general slab, and recrystallized-region-rolling was performed by applying a condition of an average rolling reduction per pass of 10% or more in a temperature range of Tnr or higher for all specimens.
  • air cooling to a non-recrystallized region temperature range after the recrystallized-region-rolling was applied to all specimens.
  • Tnr temperature, an Ar3 temperature, and a Bs temperature were calculated on the basis of each alloy composition in Table 1 and shown in Table 2, and Equations used for calculating the Tnr temperature, the Ar3 temperature, and the Bs temperature of Table 2 were separately described below Table 2.
  • a microstructure, a yield strength and a tensile strength, an elongation ratio, and a DWTT percent shear at -30°C were measured and shown in Table 4.
  • the microstructure of each specimen was evaluated using an optical microscope structure photograph and an electron backscatter diffraction (EBSD) grain size distribution chart.
  • the yield strength, the tensile strength, and the elongation ratio were evaluated by performing a room temperature tensile test on each specimen.
  • a yield strength and a tensile strength shown in Table 4 refer to measured values with respect to a rolling orthogonal direction, respectively.
  • tensile properties and a percent ductile fracture were evaluated by performing a DWTT test at -30°C on each specimen.
  • Specimens 1 to 12 satisfying the alloy composition and process conditions of the present invention contain 20 to 60 area% of ferrite and 35 to 75 area% of bainite as a microstructure, contain 5 area% or less of island martensite, and have a grain size of 70 ⁇ m or less in upper 80% of high angle grain sizes based on 15° in a central portion of a steel plate, a yield strength of 485MPa or more, a total elongation ratio of 28% or more, a uniform elongation ratio of 9% or more with respect to a rolling orthogonal direction, and a DWTT percent shear of 85% or more at -30°C with respect to the rolling orthogonal direction, and thus, has physical properties particularly suitable as a material for a pipeline provided in a cryogenic environment.
  • Specimens 13 to 15 and 17 are specimens in a case where the alloy composition of the present invention is satisfied, but cooling is performed in a temperature range lower than the cooling start temperature or the cooling end temperature of the present invention. It may be confirmed that in a case of Specimens 13 to 15 and 17, ferrite less than 20 area% and bainite more than 75 area% were formed, a grain size of upper 80% of high angle grain sizes based on 15° in a central portion of a steel plate exceeded 70 um, and a uniform elongation ratio was thus less than 9%.
  • Specimen 16 is a specimen in a case where the alloy composition of the present invention is satisfied, but non-recrystallized-region-rolling was performed in a temperature range lower than the end temperature of the non-recrystallized-region-rolling of the present invention, and cooling started in a temperature range lower than the cooling start temperature of the present invention, such that cooling ended in a temperature range higher than the cooling stop temperature of the present invention. It may be confirmed that in a case of Specimen 16, ferrite more than 60 area% was formed, such that a yield strength was less than 485 MPa.
  • Specimens 18 to 21 which are specimens that do not satisfy the alloy composition and the process condition of the present invention, do not secure a microstructure and physical properties desired by the present invention.
  • Specimens 22 and 23 satisfy the alloy composition of the present invention, but have a thickness of a steel plate exceeding 20 mm, such that ferrite is excessively formed.
  • FIG. 1 is a photograph of Specimen 2 observed with an optical microscope
  • FIG. 2 is graphs illustrating results obtained by measuring high angle grain boundary grain sizes based on 15° of Specimen 2 using an EBSD. As illustrated in the graphs of FIG. 2 , it may be confirmed that an average grain size of high angle grain boundaries of Specimen 2 is 22.3 um and a grain size of upper 80% of the high angle grain boundaries is 40.5 ⁇ m.
  • FIG. 3 is a photograph of Specimen 18 observed with an optical microscope
  • FIG. 4 is graphs illustrating results obtained by measuring high angle grain boundary grain sizes based on 15° of Specimen 18 using an EBSD. As illustrated in the graphs of FIG. 4 , it may be confirmed that an average grain size of high angle grain boundaries of Specimen 18 is 38 um and a grain size of upper 80% of the high angle grain boundaries is 93 um.
  • the steel plate particularly suitable as the material for a pipeline by having the yield strength of 485MPa or more, the total elongation ratio of 28% or more, the uniform elongation ratio of 9% or more with respect to the rolling orthogonal direction, and the DWTT percent ductile fracture of 85% or more at -30°C with respect to the rolling orthogonal direction of the steel plate in spite of having a thickness less than 20 mm, and the manufacturing method therefor may be provided.

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EP19889727.4A 2018-11-30 2019-11-29 High-strength steel plate having excellent low-temperature fracture toughness and elongation ratio, and manufacturing method therefor Active EP3889305B1 (en)

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KR1020180151565A KR102164107B1 (ko) 2018-11-30 2018-11-30 저온파괴인성 및 연신율이 우수한 고강도 강판 및 그 제조방법
PCT/KR2019/016785 WO2020111891A1 (ko) 2018-11-30 2019-11-29 저온파괴인성 및 연신율이 우수한 고강도 강판 및 그 제조방법

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KR102393906B1 (ko) * 2020-12-18 2022-05-03 주식회사 포스코 Dwtt 천이온도를 예측하는 방법

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EP3889305A1 (en) 2021-10-06
CN113166904A (zh) 2021-07-23
KR20200066394A (ko) 2020-06-10
JP7372325B2 (ja) 2023-10-31
KR102164107B1 (ko) 2020-10-13
CA3120271C (en) 2023-09-12
CN113166904B (zh) 2023-10-27
EP3889305A4 (en) 2021-10-06
EP3889305C0 (en) 2024-04-10
WO2020111891A1 (ko) 2020-06-04
RU2771151C1 (ru) 2022-04-27
JP2022510935A (ja) 2022-01-28
CA3120271A1 (en) 2020-06-04

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