EP3696287B1 - Manufacturing method for a thick steel plate having excellent low-temperature strain aging impact properties - Google Patents

Manufacturing method for a thick steel plate having excellent low-temperature strain aging impact properties Download PDF

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Publication number
EP3696287B1
EP3696287B1 EP18865647.4A EP18865647A EP3696287B1 EP 3696287 B1 EP3696287 B1 EP 3696287B1 EP 18865647 A EP18865647 A EP 18865647A EP 3696287 B1 EP3696287 B1 EP 3696287B1
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EP
European Patent Office
Prior art keywords
rolling operation
strain aging
toughness
less
steel plate
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EP18865647.4A
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German (de)
English (en)
French (fr)
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EP3696287C0 (en
EP3696287A4 (en
EP3696287A1 (en
Inventor
Woo-Gyeom KIM
Kyung-Keun Um
Ki-Hyun Bang
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Posco Holdings Inc
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Posco Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • 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
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • C22C38/02—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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the present invention relates to a thick steel plate having excellent low-temperature strain aging impact properties and a manufacturing method therefor and, more particularly, to a thick steel plate having excellent low-temperature strain aging impact properties that may be used as a material in ship building, marine structures and the like; and a manufacturing method therefor.
  • the strain aging impact properties are evaluated by subjecting a steel plate to several percent of tensile strain, aging the steel plate at about 250°C for 1 hour, processing the aged steel plate to make an impact specimen, and then performing an impact test on the impact specimen.
  • the more severe the strain aging phenomenon the faster the toughness of the steel plate decreases, and the decrease in toughness may also increase.
  • the lifespan of the site and the structure to which the steel plate is applied may be reduced and stability may be affected. Therefore, in recent years, a steel plate having high resistance to the strain aging phenomenon has been required for the purpose of increasing the lifespan of the steel plate subjected to strain to increase the stability of the structure.
  • Deterioration in impact toughness by the strain aging phenomenon may occur when yield strength is greater than breaking strength.
  • the greater the difference between yield strength and breaking strength the greater the amount of strain of the steel materials in ductility, and the absorbed impact energy may increase. Therefore, when cold deformation is performed to apply the steel materials to the structure, the yield strength of the steel materials may increase, to decrease the difference between the yield strength and the breaking strength, which is accompanied by a decrease in impact toughness.
  • the decrease in toughness due to the increase in yield strength may be caused by subjecting strain of the steel materials to fix interstitial elements in the steel materials such as C, N, and the like to the dislocation over time.
  • Non-Patent Document 1 In order to prevent the decrease in toughness by cold deformation, conventionally, a method of significantly decreasing the amount of carbon (C) or nitrogen (N) dissolved in the steel materials for suppressing strength increase by an aging phenomenon after deformation, a method of adding an element such as nickel (Ni), or the like to lower stacking fault energy to facilitate the movement of dislocations, and the like have been applied. Alternatively, a method of performing stress relief heat treatment after cold deformation to decrease dislocation produced in the steel materials, thereby lowering the yield strength increased by work hardening, has been used, and, as an example thereof, Non-Patent Document 1 below is disclosed.
  • US 2010/258219 A1 discloses a high-strength steel plate having acicular ferrite and bainite as a main microstructure and an austenite/martensite (M & A) as a second phase under the control of a cooling rate above the austenite transformation temperature.
  • the high-strength steel plate comprises: carbon (C): 0.03 to 0.10 wt %, silicon (Si): 0.1 to 0.4 wt %, manganese (Mn): 1.8 wt % or less, nickel (Ni): 1.0 wt % or less, titanium (Ti): 0.005 to 0.03 wt %, niobium (Nb): 0.02 to 0.10 wt %, aluminum (Al): 0.01 to 0.05 wt %, calcium (Ca): 0.006 wt % or less, nitrogen (N): 0.001 to 0.006 wt %, phosphorus (P): 0.02 wt % or less, sulfur (S): 0.005 wt % or less, and the balance of iron (Fe) and other inevitable impurities.
  • the method for manufacturing a high-strength steel plate may be useful to economically and effectively manufacture a high strength steel, which is able to secure excellent properties such as high strength and high toughness since the acicular ferrite and bainite may be effectively formed without adding expensive elements such as molybdenum (Mo).
  • EP 3128029 Al discloses steel material for highly deformable line pipes that has specified strain aging resistance and HIC resistance and has a specific chemical composition and a metallographic structure composed mainly of ferrite and bainite.
  • the total area fraction of the ferrite and the bainite is 90% or more, and the difference in hardness between the ferrite and the bainite is 70 or more in terms of Vickers hardness.
  • the steel material has a uniform elongation of 9% or more and a yield ratio of 90% or less.
  • An aspect of the present invention is to provide a method of manufacturing a thick steel plate having excellent low-temperature strain aging impact properties.
  • a thick steel plate having excellent low-temperature strain aging impact properties and excellent yield strength may be provided as a result of practicing the method defined herein.
  • the content of the alloy composition described below means by weight.
  • C is an element which is effective for a solid solution strengthening, and may be present as carbonitride by Nb, and the like, to secure tensile strength.
  • the C content is 0.04% or more.
  • the C content is excessive, not only may formation of a martensite-austenite (MA) be promoted, but pearlite may also be generated to degrade impact and fatigue properties at low temperatures.
  • the C content is in the range of 0.04 to 0.08%.
  • Si may be an element necessary for assisting Al to deoxidize molten steel, and to secure yield and tensile strength.
  • the Si content is in the range of 0.4% or less to secure impact and fatigue properties at low temperatures.
  • Si may prevent diffusion of C to promote formation of the MA.
  • the Si content is in the range of 0.05 to 0.4%.
  • the Si content is more preferably in the range of 0.05 to 0.2% in order to more stably secure toughness by minimizing the formation of MA.
  • Mn may be added in an amount of 1.0% or more, since Mn has a relatively large effect on an increase in strength by solid solution strengthening.
  • the Mn content exceeds 2.0%, since toughness may be degraded due to formation of MnS inclusions or segregation of a central portion, the Mn content is in the range of 1.0 to 2.0%.
  • the Mn content is more preferably in the range of 1.3 to 1.7% in consideration of an effect of increasing strength and a decrease in toughness due to the segregation.
  • Phosphor (P) 0.01% or less
  • P may be an element causing grain boundary segregation and may cause embrittlement of steel, an upper limit thereof needs to be 0.01%.
  • S may be mainly combined with Mn to form MnS inclusions, decreasing toughness at low temperature. Therefore, in order to secure toughness at low temperature and fatigue properties at low temperature, it is necessary to limit the S content to 0.003% or less.
  • Al may be not only a major deoxidizer of steel, but also an element necessary for fixing N during strain aging.
  • Al is added in an amount of 0.015% or more.
  • a fraction and a size of Al 2 O 3 inclusions may increase to cause a decrease in the toughness at low temperature.
  • the Al content is in the range of 0.015 to 0.04%.
  • Al is more preferably in the range of 0.015 to 0.025% in order to more stably secure the toughness by minimizing the formation of MA.
  • Ti may be an element that reduces solid solution N by forming Ti nitride (TiN) in combination with N causing strain aging.
  • Ti nitride may serve to contribute to miniaturization by inhibiting coarsening of a microstructure, and to improve toughness.
  • Ti is added in an amount of at least 0.005%.
  • solid solution Ti which is not bonded with N, may remain to form Ti carbide (TiC), to degrade toughness of the base metal and toughness of the welded portion. Therefore, the Ti content is in the range of 0.005 to 0.02%. More preferably, Ti may have a range of 0.005 to 0.017% to prevent coarsening of nitride.
  • Cu may be an element that does not significantly degrade impact properties, and improves strength by solid solution and precipitation.
  • the Cu content exceeds 0.35%, surface cracking of the steel plate due to thermal shock may occur. Therefore, the Cu content is in the range of 0.35% or less.
  • Ni may be an element that may improve strength and toughness at the same time, although an effect of increasing strength is not great. Ni is added in an amount of 0.05% or more in order to sufficiently obtain the effect. Since Ni is a relatively expensive element, when the Ni content exceeds 0.8%, economic efficiency may be reduced. Therefore, the Ni content has a range of 0.05 to 0.8%. Ni has more preferably a range of 0.2 to 0.8% in a viewpoint of an increase in strength and toughness.
  • Nb may be an element staying in a solid solution state or precipitating carbonitrides, suppressing recrystallization during rolling or cooling, reducing a grain size of a microstructure, and increasing strength.
  • the Nb is added in an amount of at least 0.003%.
  • C concentration may occur due to C affinity, to promote the formation of MA phase, and to degrade the toughness and fracture properties at low temperatures. Therefore, the Nb content is in the range of 0.003-0.03%.
  • N may be a main element causing strain aging, and it is desirable to keep its content as low as possible.
  • the N content is 0.008% or less.
  • the N content is less than 0.002%, toughness of the base metal and toughness of the welded portion may be degraded by causing solid solution strengthening or forming other precipitates in a state in which elements for suppressing the strain aging impact properties are added. Therefore, the N content lies in the range of 0.002 to 0.008%.
  • Ca When Ca is added to molten steel during a steelmaking process after Al deoxidation, Ca may be bonded to S which exists mainly as MnS to inhibit production of MnS, simultaneously with formation of globular-shaped CaS, to have an effect of suppressing cracks in a central portion of the steel material. Therefore, in order to form S which is added in the present invention into CaS sufficiently, 0.0002% or more is added.
  • the Ca content is more than 0.0050%, Ca remaining after forming CaS is bonded to O to produce coarse oxidative inclusions, which are stretched and fractured in rolling to serve as a crack initiation point at low temperatures. Therefore, the Ca content is in the range of 0.0002-0.0050%.
  • Cr may be an element forming a strong carbide, may reduce fraction of ferrite, and may promote formation of hard phases, to degrade impact toughness. Therefore it is preferable to keep the Cr content as low as possible or not included, and in the invention an upper limit thereof is managed to 0.009%.
  • Mo in a similar manner to Cr, may be also an element for forming a strong carbide, may reduce a fraction of ferrite, and may promote formation of hard phases, to degrade impact toughness. Therefore it is preferable to keep the Mo content as low as possible or not included, and in the present invention, an upper limit thereof is managed to 0.0009%.
  • the other component of the steel sheet is iron (Fe).
  • Impurities of raw materials or manufacturing environments may be inevitably included in the steel sheet, and such impurities may not be removed from the steel sheet.
  • Such impurities are well-known to those of ordinary skill in manufacturing industries, and thus specific descriptions of the impurities will not be given herein.
  • the microstructure of the thick steel plate produced according to the method of the invention includes 95 area% or more of ferrite having an average grain size of 10 ⁇ m or less.
  • the crystal grains of the ferrite as described above may be miniaturized to improve the strain aging impact properties at low temperature.
  • the fraction of the ferrite is less than 95 area%, it may be difficult to secure the effect. More preferably, the fraction of ferrite is 98 area% or more.
  • the remainder of the microstructure may include at least one of cementite and MA, and the fraction thereof may be 5 area% or less, and more preferably 2 area% or less.
  • the ferrite may have a maximum grain size of 20um or less.
  • the maximum grain size of the ferrite exceeds 20 pm or less, it may be difficult to secure low-temperature strain aging impact properties targeted by the present invention.
  • the ferrite may consist of polygonal ferrite and acicular ferrite. Therefore, as described above, a hard phase that may be a starting point of the impact toughness may be minimized, and ferrite having good shock absorption may be configured as a microstructure, to secure shock and strain age shock properties at low temperature.
  • the thick steel plate may have a yield strength of 350MPa or more, a tensile strength of 450MPa or more, an impact toughness of 200J or more at -60°C, and a strain aging impact toughness of 100J or more at -60°C, and may secure excellent low-temperature strain aging impact properties, as well as high yield strength.
  • the strain aging impact toughness means an impact energy value measured after aging treatment at 250°C for 1 hour, after a tensile strain of 5 to 10% is applied.
  • the thick steel plate may have a thickness of 40mm or more.
  • an upper limit of the thickness of the thick steel plate is not particularly limited, but may have, for example, a thickness of 100mm or less.
  • the thick steel plate of the present invention may be applied to the shipbuilding and offshore structural industries that require a bending process, a cold deformation process, and the like, and may contribute to have excellent strain aging impact properties to secure stability and extend a lifespan of the structure.
  • a steel slab having the alloy composition described above is reheated at 1020 to 1150°C.
  • the reheating temperature exceeds 1150°C, grains of austenite may be coarsened to deteriorate toughness, and when the reheating temperature is lower than 1020°C, Ti, Nb, and the like may not be sufficiently employed to cause a decrease in strength.
  • the reheated steel slab is subjected to a recrystallization zone rolling operation in 5 passes or fewer (including 0 passes) to obtain a bar.
  • the recrystallization zone rolling operation during a hot-rolling process is performed only to match a width of the product.
  • the recrystallization zone rolling operation exceeds 5 passes, there may be a problem that the total reduction amount in the non-recrystallization zone rolling operation is reduced. Therefore, in the present invention, it is necessary to omit or minimize the recrystallization zone rolling operation.
  • the bar is subjected to a non-recrystallization zone rolling operation at Ar3 or higher, and preferably about 750°C or higher, to obtain a hot-rolled steel material.
  • a structure anisotropy may be formed due to stretching of ferrite, to have a problem of deteriorating impact toughness.
  • a reduction amount in the non-recrystallization zone rolling operation is 90% or more (including 100%) of the sum of a reduction amount in the recrystallization zone rolling operation and the reduction amount in the non-recrystallization zone rolling operation.
  • the recrystallization zone rolling operation may be performed in 5 passes or fewer (including 0 passes) as described above, the reduction amount in the non-recrystallization zone rolling operation may be performed at 90% or more, to realize grain refinement and secure excellent low temperature strain aging impact properties.
  • cooling the hot-rolled steel material to 300 to 500°C at a cooling rate of 2 to 15°C/s, by a water-cooling process and the like, is further included.
  • the cooling rate is less than 2°C/s, it may be difficult to secure the target strength.
  • the cooling rate exceeds 15°C/s, a relatively large amount of hard phase, such as MA, bainite, and the like, may be formed in a manner that degrades toughness.
  • the cooling may not be performed after the non-recrystallization zone rolling operation.
  • the tensile strength may drop slightly.
  • FIG. 1 is a captured photograph of a microstructure of Inventive Example 1. As can be seen in FIG. 1 , in the case of Inventive Example 1 that satisfies the conditions of the present invention, it can be confirmed that grains of the microstructure were fine.
  • FIG. 2 is a captured photograph of a microstructure of Comparative Example 1. As can be seen in FIG. 2 , in the case of Comparative Example 1 that does not satisfy the conditions of the present invention, it can be confirmed that grains of the microstructure were coarse.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
EP18865647.4A 2017-10-11 2018-10-04 Manufacturing method for a thick steel plate having excellent low-temperature strain aging impact properties Active EP3696287B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
KR1020170131605A KR101949036B1 (ko) 2017-10-11 2017-10-11 저온 변형시효 충격특성이 우수한 후강판 및 그 제조방법
PCT/KR2018/011722 WO2019074236A1 (ko) 2017-10-11 2018-10-04 저온 변형시효 충격특성이 우수한 후강판 및 그 제조방법

Publications (4)

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EP3696287A4 EP3696287A4 (en) 2020-08-19
EP3696287A1 EP3696287A1 (en) 2020-08-19
EP3696287B1 true EP3696287B1 (en) 2023-12-06
EP3696287C0 EP3696287C0 (en) 2023-12-06

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EP (1) EP3696287B1 (enrdf_load_stackoverflow)
JP (1) JP7022822B2 (enrdf_load_stackoverflow)
KR (1) KR101949036B1 (enrdf_load_stackoverflow)
CN (1) CN111225987B (enrdf_load_stackoverflow)
ES (1) ES2971876T3 (enrdf_load_stackoverflow)
WO (1) WO2019074236A1 (enrdf_load_stackoverflow)

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KR102218423B1 (ko) * 2019-08-23 2021-02-19 주식회사 포스코 저온인성 및 ctod 특성이 우수한 박물 강재 및 그 제조방법
KR102255822B1 (ko) * 2019-12-06 2021-05-25 주식회사 포스코 저온충격인성이 우수한 노말라이징 열처리 강판 및 제조방법
CN113088834B (zh) * 2021-02-26 2022-12-09 舞阳钢铁有限责任公司 一种高品质海上石油建设用钢板及其生产方法

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CN111225987B (zh) 2022-06-10
KR101949036B1 (ko) 2019-05-08
JP2020537047A (ja) 2020-12-17
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JP7022822B2 (ja) 2022-02-18
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