EP3392367B1 - High-strength steel material having excellent low-temperature strain aging impact properties and method for manufacturing same - Google Patents

High-strength steel material having excellent low-temperature strain aging impact properties and method for manufacturing same Download PDF

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EP3392367B1
EP3392367B1 EP16876051.0A EP16876051A EP3392367B1 EP 3392367 B1 EP3392367 B1 EP 3392367B1 EP 16876051 A EP16876051 A EP 16876051A EP 3392367 B1 EP3392367 B1 EP 3392367B1
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steel material
strength
temperature
steel
strain aging
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English (en)
French (fr)
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EP3392367A1 (en
EP3392367A4 (en
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Kyung-Keun Um
Woo-Gyeom KIM
Hong-Ju Lee
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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
    • C22C38/18—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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties 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
    • 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
    • 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
    • C22C38/001—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C21D2211/009—Pearlite

Definitions

  • the present disclosure relates to a steel material used as a material for pressure vessels, offshore structures and the like, and more particularly, to a high-strength steel material having excellent low-temperature strain aging impac8 t properties, and a method for manufacturing the same.
  • the steel material is required to significantly avoid a decrease in toughness due to strain aging by cold deformation.
  • the mechanism of decreased toughness due to strain aging is as follows: The toughness of a steel material measured by a Charpy impact test is explained by a correlation between yield strength and fracture strength at the test temperature; and when the yield strength of a steel material at the test temperature is higher than the fracture strength, the steel material undergoes brittle fracture without ductile fracture, so that an impact energy value is lowered, but when the yield strength is lower than the fracture strength, the steel material is deformed to be ductile, thereby absorbing impact energy during work hardening, and being changed to undergo brittle fracture when the yield strength reaches fracture strength.
  • the yield strength of the steel material is increased as deformation continues, and thus, the difference from the fracture strength becomes smaller, accompanied by decreased impact toughness.
  • an element e.g., titanium (Ti) , vanadium (V), etc.
  • Non-patent document 1 Effect of Ti addition on strain aging of low-carbon steel wire rod (Ikuo Ochiai, Hiroshi Ohba, Iron and Steel, Volume 75 (1989), issue 4, p. 642 -)
  • Non-patent document 2 The effect of processing variables on the mechanical properties and strain ageing of high-strength low-alloy V and V-N steels (V. K. Heikkinen and J. D. Boyd, CANADIAN METALLURGICAL QUARTERLY Volume 15 Number 3 (1976), p. 219 -)
  • KR 2013 0076569 A discloses a steel for use in manufacture of a pressure vessel and having good sulfide stress cracking resistance and low temperature toughness.
  • the steel includes, by weight %, 0.03 to 0.18%C, 0.05 to 0.5% Si, 0.5 to 2% Mn, 0.005 to 0.1% A1, 0.15 to 0.5% Cu, 0.15 to 1% Ni, 0.01 to 0.05% Nb, 0.01 to 0.05% Ti, 0.05 to 0.3% Mo, 0.05 to 0.5% Cr, 0.001 to 0.003% Ca, 0.001 to 0.01% N, not more than 0.0015% S, not more than 0.012% P and the balance Fe and inevitable impurities.
  • EP2764946 A1 discloses a welded steel pipe formed by cold-bending and welding a steel plate.
  • the steel plate has a composition that seeks to maintain toughness of the weld heat-affected zone at relatively low temperatures.
  • the composition of the steel plate is in weight % 0.03 to 0.08% C, 0.01 to 0.20% Si, 1.0 to 2.2% Mn, 0.015% or less P, 0.001 to 0.05% Al, 0.005 to 0.050% Nb, 0.005 to 0.030% Ti, 0.0020 to 0.0080% N and one or more selected from a group including Cu, Ni, Cr, Mo, V and B.
  • JP 2014 043627 A discloses a steel for use in a polyolefin-coated, low-temperature UOE steel pipe comprising, in weight %, 0.03 to 0.07% C, 0.05 to 0.50% Si, 1.4 to 2.2% Mn, 0.020% or less P, 0.003% or less S, 0.15 to 0.60% Cu, 0.15 to 0.80% Ni, 0.005 to 0.045% Nb, 0.005 to 0.030% Ti, 0.0070% or less N, 0.005 to 0.060% Al and the balance Fe and inevitable impurities.
  • An aspect of the present disclosure is to provide a steel material which may not only secure high strength and high toughness, but may also significantly avoid a strength decrease due to cold deformation, thereby being appropriately applied as a material of pressure vessels, offshore structures and the like, and a method for manufacturing the same.
  • a heat-treated steel material having excellent low-temperature stain aging impact properties, simultaneously with high strength may be provided, and the steel material may be appropriately applied as a material for pressure vessels, offshore structures and the like, following a trend of being larger and more complicated.
  • FIG. 1 is a graph representing lower yield strength and tensile strength in a tensile curve of a steel material according to an aspect of the present disclosure.
  • the present inventors conducted an intensive study on the development of a steel material which may prevent a toughness decrease of the steel material by strain aging, while having high strength and high toughness, and as a result, confirmed that a steel material having a microstructure advantageous for securing the above-described physical properties from optimization of a steel component composition and manufacturing conditions may be provided, thereby completing the present disclosure.
  • the steel material of the present disclosure may effectively prevent a toughness decrease by strain aging by optimizing the contents of the elements having an influence on MA phase formation in the steel component composition to significantly decrease the MA phase (martensite-austenite composite phase).
  • the high-strength steel material having excellent low-temperature strain aging impact properties includes 0.04-0.14 wt% of carbon (C), 0.05-0.60 wt% of silicon (Si), 0.6-1.8 wt% of manganese (Mn), 0.005-0.06 wt% of soluble aluminum (sol.
  • the content of each component refers to wt%.
  • Carbon (C) an element advantageous for securing strength of a steel is bonded to pearlite or niobium (Nb), nitrogen (N) and the like to exist as carbonitrides, and thus, is a main element for securing tensile strength. It is not preferable that the content of this C is less than 0.04%, since the tensile strength on a matrix may be lowered, and when the content is more than 0.14%, pearlite may be excessively produced, so that low-temperature strain aging impact properties may be deteriorated.
  • Silicon (Si) an element added for a deoxidation and desulfurization effect of a steel, and also for solid solution strengthening and is added at 0.05% or more for securing yield strength and tensile strength.
  • the content of silicon is more than 0 .60%, since weldability and low-temperature impact properties are lowered, and a steel surface is easily oxidized so that an oxide film may be severely formed.
  • the content of Si is limited to 0.05-0.60%.
  • Manganese (Mn) is added at 0.6% or more, since manganese has a large effect on strength increase by solid solution strengthening.
  • Mn Manganese
  • segregation becomes severe in the center of a steel plate in the thickness direction, and at the same time formation of MnS, a nonmetallic inclusion, is encouraged, together with segregated S.
  • the MnS inclusion produced in the center is stretched by rolling, and as a result, significantly deteriorates low-temperature toughness and lamella tear resistant properties, and thus, the content of Mn limited to 1.8% or less.
  • the content of Mn is limited to 0.6-1.8%.
  • Soluble aluminum (sol. Al) is used as a strong deoxidizing agent in a steel manufacturing process together with Si, and 0.005% of sol. Al is added in deoxidation alone or in combination.
  • the content is more than 0.06%, the above-described effect is saturated, and the fraction of Al 2 O 3 in the oxidative inclusion produced as a resultant product of deoxidation is increased more than necessary, and the size is larger.
  • it is not easy to be removed during refining, resulting in significant reduction in low-temperature toughness, and thus, is not preferable.
  • Niobium (Nb) has a large effect of being solid-solubilized in austenite when reheating a slab, thereby increasing hardenability of austenite, and being precipitated as fine carbonitrides (Nb,Ti)(C,N) upon hot rolling, thereby suppressing recrystallization during rolling or cooling to allow a final microstructure to be finely formed.
  • the content is more than 0.05%, since it is easy to form excessive MA, or a coarse precipitate in the center in the thickness direction, thereby deteriorating low-temperature toughness in the center of the steel.
  • the content of Nb is limited to 0.005-0.05%, more advantageously 0.02% or more, still more advantageously 0.022% or more.
  • V 0.01% or less (exclusive of 0%)
  • Vanadium (V) is almost all solid-solubilized again when heating a slab, and thus, there is little effect of strength increase by precipitation or solid solubilization after rolling, normalizing heat treatment.
  • V is an expensive element, and causes cost increase when adding it in a large amount, and thus, considering this, 0.01% or less of is added.
  • Titanium (Ti) is present as a hexagonal precipitate mainly in the form of TiN at high temperature, or forms carbonitride (Nb,Ti) (C,N) precipitates with Nb and the like to suppress crystal grain growth in the welding heat-affected zone. For this, 0.001% or more of Ti is added. However, when the content is excessive and more than 0.015%, coarse TiN is formed in the center of the steel in the thickness direction, which serves as a fracture crack initiation point, thereby greatly reducing strain aging impact properties.
  • the content of Ti is limited to 0.001-0.015%.
  • Copper (Cu) has an effect of greatly improving strength by solid solubilization and precipitation, and not greatly affecting strain aging impact properties. However, when excessively added, it causes cracks on a steel surface, and is an expensive element, and thus, considering this, the content of Cu is limited 0.01-0.4%.
  • Nickel (Ni) has little strength increase effect. However, it is effective in improving low-temperature strain aging impact properties, and in particular, when adding Cu, has an effect of suppressing a surface crack by selective oxidation which occurs upon reheating a slab. For this, 0.01% or more of Ni is added. However, considering the economic efficiency due to a high price, the content of Ni is limited to 0.6% or less.
  • Chromium (Cr) has a small effect of increasing yield strength and tensile strength by solid solubilization. However, it slows down a cementite decomposition rate during heat treatment after welding or tempering, thereby preventing a drop in strength. For this, 0.01% or more of Cr is added. However, it is not preferable that the content is more than 0.2%, since the manufacturing cost rises, and also low-temperature toughness of the welding heat-affected zone is deteriorated.
  • Molybdenum has an effect of delaying transformation in the course of cooling after heat treatment, resulting in a large increase in strength, and also, being effective in preventing a drop in strength during heat treatment after welding or tempering like Cr, and preventing toughness decrease by grain boundary segregation of impurities such as P. For this, 0.001% or more of molybdenum is added. However, it is also economically disadvantageous to excessively add molybdenum, an expensive element, and thus, the content of Mo is limited to 0.3% or less.
  • the content of Ca is limited to 0.0002-0.0040%.
  • Nitrogen (N) has an effect of being bonded to added Nb, Ti, Al, etc. to form a precipitate, thereby refining the crystal grains of the steel to improve the strength and toughness of a base metal.
  • Nb, Ti, Al, etc. has an effect of being bonded to added Nb, Ti, Al, etc. to form a precipitate, thereby refining the crystal grains of the steel to improve the strength and toughness of a base metal.
  • nitrogen is known as a representative element to decrease low-temperature toughness.
  • surface cracks are promoted by embrittlement at high temperature.
  • Phosphorus (P) has an effect of increasing strength when added.
  • it is an element which significantly impairs low-temperature toughness by grain boundary segregation, as compared with the effect of increasing strength, and thus it is preferable to keep the content of P as low as possible.
  • the content of P is limited to the range not affecting the physical properties, i.e., 0.02% or less.
  • S Sulfur
  • MnS Sulfur
  • the content of S is limited to the range not affecting the physical properties, i.e., 0.003% or less.
  • the remaining component of the steel material is iron (Fe) .
  • Fe iron
  • unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, they may not be excluded. Since these impurities are known to any person skilled in the common steel manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.
  • the high-strength steel material of the present disclosure satisfying the alloy component composition as described above consists of a mixed structure of ferrite, pearlite, bainite and a MA (martensite-austenite) composite phase.
  • ferrite is the most important since it allows the ductile deformation of the steel material, and it is included as a main phase, while finely controlling its average crystal grain size to 15 ⁇ m or less.
  • a grain boundary may be increased to suppress crack propagation, basic toughness of a steel material may be improved, and also strength increase by an effect of lowering a work hardening rate during cold deformation may be significantly reduced, thereby improving strain aging impact properties simultaneously.
  • Hard phases including the pearlite, bainite, MA other than the ferrite is advantageous for securing high strength by increasing the tensile strength of a steel material.
  • such phases serve as fracture initiation points or propagation paths due to high hardness, thereby deteriorating the strain aging impact properties. Therefore, the fraction is controlled, and the sum of fractions of the hard phases is more than 0% and is limited to 18% or less.
  • the MA phase since the MA phase has the highest strength, and is transformed from martensite having strong brittleness by deformation, it is a phase which deteriorates the low-temperature toughness most significantly. Therefore, the fraction of the MA phase is more than 0% and is limited to 3.5% or less, and more preferably to 1.0-3.5%.
  • the high-strength steel material of the present disclosure having the microstructure as described above includes carbonitrides produced by Nb, Ti, Al, etc., among the added elements, and the carbonitrides play an important role in inhibiting crystal grain growth in the course of rolling, cooling and heat treatment to allow the grains to be fine.
  • the steel material includes 0.01% or more, and preferably 0.01-0.06% of the carbonitrides having an average size of 300 nm or less by weight ratio.
  • a steel slab satisfying the above-described alloy component alloy is manufactured, and then in order to obtain a steel material satisfying microstructure, carbide conditions and the like aimed at in the present disclosure, hot rolling (controlled rolling), cooling and normalizing heat treatment are performed.
  • the manufactured steel slab Prior to this, the manufactured steel slab is subjected to a reheating process.
  • the reheating temperature is controlled to 1080-1250°C, and when the reheating temperature is less than 1080 °C, re-solid solubilization of carbides produced in the slab during continuous casting is difficult. Therefore, it is preferable to perform reheating to at least a temperature at which 50% or more of added Nb may be solid-solubilized again. However, when the temperature is more than 1250°C, the size of austenite crystal grains is unduly large, so that the mechanical physical properties such as strength and toughness of the finally manufactured steel material are greatly deteriorated.
  • the reheating temperature is limited to 1080-1250°C.
  • Manufacture of the hot-rolled steel plate includes finish rolling of the reheated steel slab as described above.
  • the finish rolling process is controlled rolling, and the rolling end temperature is controlled to be 780°C or more.
  • the rolling end temperature is about 820-1000°C.
  • the quenching property is lowered in the region in which Mn and the like are not segregated during rolling, thereby producing ferrite during rolling, and as the ferrite is produced as such, solid-solubilized C and the like are segregated into a remaining austenite region and concentrated. Accordingly, the region in which C and the like are concentrated during cooling after rolling is transformed into bainite, martensite or a MA phase, thereby producing a strong layered structure formed of ferrite and a hardened structure.
  • the hardened structure of the layer in which C and the like are concentrated has high hardness and also a greatly increased fraction of the MA phase. Since low-temperature toughness is decreased by an increase of hardened structure and arrangement of a layered structure, the rolled end temperature is controlled to be 780°C or more.
  • the hot-rolled steel plate obtained by controlled rolling according to the above is cooled by air cooling or water cooling, and then is subject to normalizing heat treatment in a constant temperature range, thereby manufacturing a steel material having the desired physical properties.
  • the normalizing heat treatment is performed by maintaining a temperature range of 850-960°C for a certain period of time, and then cooling in the air.
  • the normalizing heat treatment temperature is less than 850°C, the re-solidification solubilization of cementite and a MA phase in pearlite and bainite is not readily able to decrease the solid-solubilized C, so that it is difficult to secure strength, and also, a finally remaining hardened phase remains coarse, thereby significantly impairing strain aging impact properties.
  • the temperature is more than 960°C, crystal grain growth occurs to deteriorate the strain aging impact properties.
  • the normalizing heat treatment temperature range is maintained for ⁇ (1.3 ⁇ t)+(10-60) ⁇ minutes (wherein 't' denotes a steel material thickness (mm)).
  • 't' denotes a steel material thickness (mm)
  • the high-strength steel material obtained according to the above has excellent strength and toughness, and also may effectively prevent toughness decrease by strain aging upon cold deformation.
  • a yield ratio (YS (lower yield strength) /TS (tensile strength)) after heat treatment of 0.65-0.80 may be secured.
  • the steel slabs having the component composition shown in the following Table 1 were subjected to reheating, hot rolling and normalizing heat treatment under the conditions shown in the following Table 2, thereby manufacturing hot-rolled steel plates having a final thickness of 6 mm or more.
  • the microstructure fraction, size and carbonitride fraction of each of the manufactured hot-rolled steel plates were measured.
  • a Charpy impact transition temperature was measured in the state of being aged at 250°C for 1 hour after 5% stretching of a cold deformation amount, which may represent strength (tensile strength and lower yield strength) and strain aging impact properties of each hot-rolled steel plate, and represented in the following Table 3.
  • the steel plate section was polished with a mirror surface, and etched with Nital or Lepera as desired, thereby measuring an image for a certain area of a specimen at 100-500X magnification with an optical or scanning electron microscope, and then the fraction of each image was measured from the measured images using an image analyzer. In order to obtain a statistically significant value, the measurement was repeated for the same specimen but at the changed position, and the average value was calculated.
  • the fraction of the fine carbonitrides having an average size of 300 nm or less was measured by an extraction residue method.
  • tensile property values lower yield strength, tensile strength and a yield ratio (lower yield strength/tensile strength) were measured, respectively from a nominal strain-nominal stress curve obtained by a common tensile test, and a strain aging impact property value was measured by adding 0%, 5% and 8% in advance as a tensile strain, aging a stretched specimen at 250°C for 1 hour, and then performing a Charpy V-notch impact test.
  • 'F fraction' refers to a ferrite fraction
  • 'F size' refers to an average size of ferrite crystal grains
  • the represented hardened phase fraction (%) includes the carbonitride fraction (%).
  • the hot-rolled steel plate satisfying all of the component composition and manufacturing conditions of the present disclosure has high strength, and also secures excellent low-temperature toughness even after cold deformation, thereby being appropriately used in pressure vessels, offshore structures and the like, following a trend of being larger and more complicated.

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  • Materials Engineering (AREA)
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  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
EP16876051.0A 2015-12-15 2016-12-15 High-strength steel material having excellent low-temperature strain aging impact properties and method for manufacturing same Active EP3392367B1 (en)

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KR102364473B1 (ko) * 2017-08-23 2022-02-18 바오샨 아이론 앤 스틸 유한공사 저온 압력 용기용 강 및 그 제조 방법
KR102020434B1 (ko) * 2017-12-01 2019-09-10 주식회사 포스코 수소 유기 균열 저항성 및 저온 충격인성이 우수한 고강도 강재 및 그 제조방법
KR102218423B1 (ko) * 2019-08-23 2021-02-19 주식회사 포스코 저온인성 및 ctod 특성이 우수한 박물 강재 및 그 제조방법
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EP3392367A1 (en) 2018-10-24
US20180363111A1 (en) 2018-12-20
CN108368593B (zh) 2020-10-02
EP3392367A4 (en) 2019-02-27
KR20170071639A (ko) 2017-06-26
KR101758483B1 (ko) 2017-07-17
WO2017105109A1 (ko) 2017-06-22
US20240110267A1 (en) 2024-04-04

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