EP3395987B1 - Acier à haute résistance et à faible taux d'élasticité présentant une excellente résistance à la fissuration par corrosion sous contrainte et une excellente ténacité à basse température - Google Patents

Acier à haute résistance et à faible taux d'élasticité présentant une excellente résistance à la fissuration par corrosion sous contrainte et une excellente ténacité à basse température Download PDF

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EP3395987B1
EP3395987B1 EP16879393.3A EP16879393A EP3395987B1 EP 3395987 B1 EP3395987 B1 EP 3395987B1 EP 16879393 A EP16879393 A EP 16879393A EP 3395987 B1 EP3395987 B1 EP 3395987B1
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Prior art keywords
steel
less
temperature
rolling
strength
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German (de)
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EP3395987A1 (fr
EP3395987A4 (fr
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Sung-Ho Jang
Hak-Cheol 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/001—Heat treatment of ferrous alloys containing 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • 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
    • C22C38/08—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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—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 disclosure relates to a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness.
  • a temperature for liquefying a gas is generally low (-52°C in the case of LPG) at normal pressure, and thus, steel used in a liquefied gas storage tank has been required to have excellent low temperature toughness in a welded part, as well as in a base material.
  • IGC CODE International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk
  • methods for removing stress from a welded part include a post welding heat treatment (PWHT) based on a heat treatment and a mechanical stress relief (MSR) method of removing stress by adding hydrostatic pressure, or the like, to the welded part, or the like.
  • PWHT post welding heat treatment
  • MSR mechanical stress relief
  • yield strength and tensile stress are limited, to be significantly different.
  • gas tanks are basically required to be enlarged in size, it may be difficult to remove stress by the PWHT method and most shipbuilders prefer the MSR method, and thus, steel for manufacturing gas tanks is required to have low yield ratio characteristics.
  • Patent document 1 proposes a technique of adding 6.5 to 12.0% of Ni to achieve excellent low temperature toughness.
  • Patent document 2 proposes a technique of mixedly using tempered martensite and bainite by performing quench tempering on steel having a specific composition.
  • Patent document 1 has a problem of low economical efficiency due to high-priced Ni content and has a problem of degrading stress corrosion cracking (SCC) resistance.
  • Patent document 3 proposes a technique of only softening a surface layer of a steel sheet to realize a low-yield ratio. This technique, however, may achieve low temperature toughness and low yield ratio separately but cannot obtain both low temperature toughness and low yield ratio together.
  • precipitation strengthening, solid solution strengthening, and martensite strengthening may be used but these methods degrade toughness and elongation, while enhancing strength.
  • WO2009072753 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 economically and effectively to 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).
  • EP2940172 A1 discloses a high strength steel sheet having low yield ratio properties and cryogenic temperature toughness and that are suitable to be applied to the steel material for a gas tank used for the storage of gas or the like.
  • An aspect of the present disclosure is to provide a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness, and a manufacturing method thereof.
  • the low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness and the manufacturing method thereof may be provided.
  • the inventors of the present application recognized that it is difficult to make both ammonia stress corrosion cracking resistance and low temperature toughness excellent and have studied to solve the problem.
  • the inventors confirmed that it is possible to provide a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness by controlling an alloy composition and a microstructure and a manufacturing method thereof, thereby completing the present disclosure.
  • C is the most important element for securing basic strength, it is necessary to be contained within an appropriate range in the steel, and in order to obtain an additive effect, preferably, C is added in an amount of 0.02% or more.
  • the C content is less than 0.02%, strength may be reduced and the yield ratio may be lowered, which is not preferable. If the C content exceeds 0.10%, a large amount of low temperature transformation phases such as bainite, or the like, is generated to exceed an upper limit of yield strength that may cause ammonia stress corrosion cracking (SCC).
  • SCC stress corrosion cracking
  • the content of C is limited to 0.02 to 0.10%. Preferably, it is 0.05 to 0.08%.
  • Si has an effect of increasing strength due to the effect of solid solution strengthening and is advantageously used as a deoxidizing agent in steel making process.
  • the Si content is less than 0.05%, the deoxidation effect and the strength improving effect may be insufficient. If the Si content exceeds 0.5%, the low-temperature toughness is lowered and weldability is deteriorated.
  • the silicon content is limited to 0.05 to 0.5%. Preferably, it is 0.05 to 0.3%.
  • Manganese contributes to ferrite grain refinement and is an element useful for improving strength by solid solution strengthening.
  • manganese In order to obtain the effect of manganese, manganese is required to be added in an amount of 0.5% or more. If, however, the content exceeds 2.0%, hardenability may be excessively increased, which promotes formation of upper bainite and martensite to significantly reduce impact toughness and ammonia stress corrosion cracking (SCC) resistance and to reduce toughness of weld heat-affected zone as well.
  • SCC stress corrosion cracking
  • the Mn content is limited to 0.5 to 2.0%. Preferably, it is 1.0 to 1.5%.
  • Ni is an important element for facilitating cross slip of dislocations at low temperatures to improve impact toughness and hardenability and to improve strength. In order to obtain such an effect, Ni is preferably added in an amount of 0.05% or more. If the Ni content exceeds 1.0%, ammonia stress corrosion cracking (SCC) may occur and manufacturing costs may be increased due to the high cost of Ni relative to other hardenable elements.
  • SCC stress corrosion cracking
  • the Ni content is limited to 0.05 to 1.0%, and preferably, 0.2 to 0.5%.
  • Nb (niobium) 0.003% or less
  • Nb dissolved in reheating at high temperatures is precipitated very finely in the form of NbC to inhibit the recrystallization of austenite, thereby making the structure finer.
  • Nb is controlled to 0.003% or less.
  • Titanium forms oxides and nitrides in the steel to inhibit growth of crystal grains during reheating, thereby significantly improving low temperature toughness, and is also effective in refining the microstructure of a welded portion.
  • titanium In order to obtain such an effect, titanium needs to be added in an amount of 0.005 wt% or more. If the content exceeds 0.1 wt%, low temperature toughness may be reduced due to clogging of a nozzle or crystallization of a central portion.
  • the titanium content is 0.005 to 0.1%. Preferably, it is 0.01 to 0.03%.
  • Aluminum is an element useful for deoxidizing molten steel, and to this end, aluminum needs to be added in an amount of 0.005 wt% or more. If the content exceeds 0.5 wt%, nozzle clogging may occur during continuous casting. Therefore, the aluminum content is 0.005 to 0.5%. Preferably, it is 0.005 to 0.05%.
  • Phosphorus is an element that causes grain boundary segregation in a base material and a welded portion. Since phosphorus causes a problem of embrittling steel, an amount of phosphorus needs to be actively reduced. However, reducing phosphorus to an extreme limit may deepen a load of a steel making process and since the aforementioned problem does not significantly arise as long as the content of phosphorus is 0.015% or less, an upper limit thereof is limited to 0.015%, more preferably, to 0.010%.
  • S Sulfur
  • MnS metal-oxide-semiconductor
  • sulfur is preferably controlled to as low as possible and the content is limited to 0.015 wt% or less, more preferably, to 0.005 wt%.
  • the balance of the present disclosure is iron (Fe) .
  • impurities may be inevitably incorporated from a raw material or a surrounding environment, which may not be excluded. These impurities are known to any one skilled in the art in the ordinary manufacturing process and thus not specifically mentioned in this disclosure.
  • the microstructure of the steel of the present disclosure includes, in area %, 60% or more of acicular ferrite and a balance of at least one phase of bainite, polygonal ferrite and martensite-austenite constituent (MA).
  • the area fraction of the acicular ferrite is 60% or more.
  • the inclusion of pearlite may lower tensile strength and low-temperature impact toughness, and thus, the microstructure of the steel of the present disclosure may not contain pearlite.
  • the acicular ferrite measured in terms of the equivalent of a circle diameter is 30 ⁇ m or less. If the size exceeds 30pm, impact toughness may be lowered.
  • the bainite is granular bainite and upper bainite.
  • an area fraction of the bainite is 30% or less. If the area fraction of the bainite exceeds 30%, an upper limit (440 MPa) of yield strength (440 MPa) which may cause ammonia stress corrosion cracking (SCC) may be exceeded, and thus, it is necessary to limit the fraction of the bainite.
  • the MA phase is 10% by area or less and the size measured by the equivalent of a circle diameter is preferably 5 ⁇ m or less.
  • MA Martensite-Austenite constituent
  • MA martensitic island.
  • the steel of the present disclosure satisfying the above conditions may have a yield ratio (YS/TS) of 0.85 or less, preferably, 0.8 or less.
  • the steel may have tensile strength of 490 MPa or greater, for example, about 510 to 610 MPa, having excellent tensile strength.
  • an upper limit of yield strength of the steel is 440 MPa or less and does not exceed the upper limit of yield strength which causes ammonia stress corrosion cracking (SCC), and thus, ammonia stress corrosion cracking (SCC) resistance may be excellent.
  • an impact transition temperature of the 1/4t portion in a thickness direction of the steel is -60°C or lower, low temperature toughness may be excellent.
  • t represents a thickness of the steel.
  • the steel has a thickness of 6 mm or greater, and preferably, 6 to 50 mm.
  • the steel of the present disclosure may secure all of high strength, low yield ratio, excellent low temperature toughness, and ammonia stress corrosion cracking (SCC) resistance.
  • the method of manufacturing a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness includes : heating a slab having the above-described alloy composition to 1000 to 1200°C; rough-rolling the heated slab at a temperature of 1100 to 900°C; finishing-rolling at a temperature between Ar3 + 100°C and Ar3 + 30°C on the basis of a center temperature after the rough rolling; and cooling to a temperature of 300°C or lower after the finishing-rolling.
  • the slab having the above-described alloy composition is heated to 1000 to 1200°C.
  • the heating temperature of the slab is 1000°C or higher, and this is to dissolve a Ti carbonitride formed during casting. If the heating temperature of the slab is too low, deformation resistance during rolling is too high, so that a reduction ratio per rolling pass may not be increased in a follow-up rolling process, and thus, a lower limit thereof is limited to 1000°C. However, if heating is carried out at an excessively high temperature, austenite may be coarsened to lower toughness, and thus, an upper limit of the heating temperature is 1200°C.
  • the heated slab is subjected to rough rolling at a temperature of 1100 to 900°C.
  • the rough rolling temperature is set to be not lower than a temperature (Tnr) at which recrystallization of the austenite is stopped.
  • Tnr a temperature at which recrystallization of the austenite is stopped.
  • An effect of breaking a cast structure such as dendrites formed during casting and reducing the size of austenite may be obtained through rolling.
  • the rough rolling temperature is limited to 1100 to 900°C.
  • the rough rolling may be performed so that the last three rolling passes have a reduction ratio of 10% or greater per pass.
  • the reduction ratio per pass is at least 10% and a total cumulative reduction ratio is at least 30% for the last three rolling passes during rough rolling.
  • the reduction ratio per pass in rough rolling is lowered, sufficient deformation is not transferred to the central portion, which may cause toughness degradation due to center coarsening. Therefore, the reduction ratio per pass of the last three passes is preferably limited to 10% or greater.
  • a cumulative rolling reduction ratio at the time of rough rolling it is preferable to set a cumulative rolling reduction ratio at the time of rough rolling to 30% or greater.
  • finishing rolling is performed at a temperature between Ar3 + 100°C and Ar3 + 30°C on the basis of a temperature of the central portion.
  • finishing rolling is carried out at a temperature between Ar3 +100°C and Ar3 +30°C and a microstructure of the steel sheet to be subjected to finishing rolling under such conditions may be a composite structure having the features mentioned above.
  • the cumulative reduction ratio at 60% or greater during finishing rolling and to maintain the reduction ratio per pass, excluding the final shape sizing phase, at 10% or more .
  • the steel sheet After the finishing rolling, the steel sheet is cooled to a temperature of 300°C or lower.
  • the cooling is started at a temperature of Ar3+30°C to Ar3 and cooled to a finish cooling temperature (FCT) of 300°C or lower, for example, about 100 to 300°C.
  • FCT finish cooling temperature
  • the finish cooling temperature is higher than 300°C, the fine MA phase may be decomposed due to a tempering effect to make it difficult to realize a low yield ratio.
  • the finish cooling temperature is 300°C or lower.
  • first cooling is performed such that a cooling rate at the central portion is 15°C/s or greater up to Bs-10°C to Bs+10
  • second cooling is performed up to 300°C or lower such that a cooling rate at the central portion is 10 to 50°C/s.
  • the cooling start temperature is Ar3 + 30°C to Ar3.
  • the above-mentioned first cooling starts, after finishing rolling, to perform cooling at a temperature of Ar3 + 30°C to Ar3 up to Bs-10°C at a cooling rate of 15°C/s or higher, for example, 30°C/s or higher, in the central portion of the steel sheet.
  • the cooling rate of the central portion of the steel sheet is lower than 15°C/s up to Bs-10°C to Bs+10°C in the first cooling, it is possible to form a coarse polygonal ferrite to lower tensile strength and impact toughness.
  • the second cooling is performed after the first cooling up to the finish cooling temperature of 300°C or lower, for example, 100 to 300°C, at a cooling rate of 10°C/s to 50°C/s in the central portion of the steel sheet.
  • the bainite fraction is formed to be 30% or greater by area as in the microstructure of 1- (1) of FIG. 1 to exceed the yield strength upper limit (440 MPa) causing ammonia stress corrosion cracking (SCC), and the excessive increase in strength may lower elongation and impact toughness .
  • a coarse polygonal ferrite and pearlite rather than the fine acicular ferrite like the microstructure of 1-(3) of FIG. 1 , may be formed, leading to a possibility that tensile strength is 490 MPa or less and Charpy transition temperature is -60°C or higher.
  • a 300 mm-thick steel slab having the composition shown in Table 1 below was reheated to a temperature of 1100°C and then subjected to rough rolling at a temperature of 1050°C to prepare a bar. A cumulative reduction ratio during rough rolling was applied equally as 30%. Also, Ar3 and Bs temperatures according to compositions of each steel were calculated and are shown in Table 1 below.
  • finishing rolling was performed to satisfy the difference between the finishing rolling temperature and the Ar3 temperature shown in Table 2 below to obtain a steel sheet having the thickness shown in Table 2, and thereafter, cooling performed at various cooling rates through multistage cooling.
  • a finish cooling temperature of first cooling was equal to the Bs temperature of each steel.
  • microstructure The microstructure, yield strength, tensile strength, yield ratio, Charpy impact transition temperature, and ammonia stress corrosion cracking (SCC) test were performed on the steel sheet prepared as described above, and the results are shown in Table 3.
  • a sample of the microstructure was taken from the 1/4t portion of the steel sheet, mirror-polished, corroded using a Nital corrosion solution, and observed using an optical microscopy, and thereafter, a phase ratio was obtained through an image analysis.
  • a sample was taken from a 1/4t portion of the steel sheet, mirror-polished, corroded using a LePera corrosion solution, and observed using an optical microscope, and thereafter, a phase ratio of the MA phase was obtained through an image analysis.
  • a sample of No. JIS4 was taken from a 1/4t portion of the steel sheet in a direction perpendicular to a rolling direction and subjected to a tensile test at room temperature to measure yield strength, tensile strength and A yield ratio.
  • low-temperature impact toughness a sample was taken from a 1/4t portion of the steel sheet in a direction perpendicular to the rolling direction to manufacture a V-notch test sample and Charpy impact test was performed three times at each temperature at temperatures from -20 to -100°C at an internal of 20°C to derive a regression equation of each temperature average value, and low-temperature impact toughness was obtained at a temperature of 100J as a transition temperature.
  • ammonia stress corrosion cracking (SCC) test was carried out using the test solution under the test conditions described in Table 4 by making proof ring test samples. 80% of actual yield stress was applied, and samples which were not broken for 720 hours were evaluated as pass and samples which were broken before 720 hours were evaluated as fail.
  • SCC ammonia stress corrosion cracking
  • inventive examples satisfying the compositions and manufacturing conditions proposed in the present disclosure are steel having excellent ammonia stress corrosion cracking (SCC) resistance, as well as having high strength and high toughness, and having a yield ratio of 0.8 or less, low yield ratio characteristics.
  • microstructure of the inventive example A-1 was observed with a microscope and the results showed that the microstructure was a mixed structure including, in area %, 60% of more of acicular ferrite and the balance including at least one phase of bainite, polygonal ferrite and martensite-austenite constituent (MA) as illustrated in 1-(2) of FIG. 1 .

Claims (9)

  1. Acier à faible rapport d'élasticité et à haute résistance présentant une excellente résistance à la fissuration par corrosion sous contrainte et une ténacité à basse température, comprenant :
    en pourcentage en poids, 0,02 à 0,10 % de carbone (C), 0,5 à 2,0 % de manganèse (Mn), 0,05 à 0,5 % de silicium (Si), 0,05 à 1,0 % de nickel (Ni), 0,005 à 0,1 % de titane (Ti), 0,005 à 0,5 % d'aluminium (Al), au plus 0,003 % de niobium (Nb), au plus 0,015 % de phosphore (P), au plus 0,015 % de soufre (S), un complément de Fe et d'autres impuretés inévitables,
    dans lequel une microstructure comprend, en pourcentage de surface (%), au moins 60 % de ferrite aciculaire et le complément comprenant au moins une phase de bainite, de ferrite polygonale et de martensite-austénite (MA),
    dans lequel une taille de ferrite aciculaire mesurée en termes d'équivalent d'un diamètre de cercle est d'au plus 30 µm,
    dans lequel la bainite a au plus 30 % en surface,
    dans lequel la phase MA est d'au plus 10 % en surface, et dans lequel l'acier a une épaisseur d'au moins 6 mm.
    Les procédés de mesure de tous les paramètres revendiqués sont présentés dans la description.
  2. Acier selon la revendication 1, dans lequel une taille de la phase MA mesurée en termes d'équivalent d'un diamètre de cercle est d'au plus 5 µm.
  3. Acier selon la revendication 1, dans lequel un rapport d'élasticité de l'acier est d'au plus 0,85 et où la résistance à la traction de l'acier est d'au moins 490 Mpa, le procédé de mesure de la résistance à la traction étant présenté dans la description.
  4. Acier selon la revendication 1, dans lequel la limite d'élasticité de l'acier est d'au plus 440 MPa. Le procédé de mesure de la limite d'élasticité est présenté dans la description.
  5. Acier selon la revendication 1, dans lequel une température de transition d'impact de l'acier est d'au plus -60 °C.
    Le procédé de mesure de la température de transition d'impact est présenté dans la description.
  6. Procédé de production d'un acier à faible rapport d'élasticité et à haute résistance présentant une ténacité à basse température selon la revendication 1, le procédé comprenant :
    le chauffage d'une dalle comprenant, en pourcentage en poids, 0,02 à 0,10 % de carbone (C), 0,5 à 2,0 % de manganèse (Mn), 0,05 à 0,5 % de silicium (Si), 0,05 à 1,0 % de nickel (Ni), 0,005 à 0,1 % de titane (Ti), 0,005 à 0,5 % d'aluminium (Al), au plus 0,003 % de niobium (Nb), au plus 0,015 % de phosphore (P), au plus 0,015 % de soufre (S), un complément de Fe et d'autres impuretés inévitables, à 1 000 à 1 200 °C ;
    le laminage grossier de la dalle chauffée à une température de 1 100 à 900 °C ;
    le laminage de finition à une température comprise entre Ar3 + 100 °C et Ar3 + 30 °C en fonction d'une température centrale après le laminage grossier ; et
    le refroidissement à une température d'au plus 300 °C après le laminage de finition, et lors du refroidissement, le premier refroidissement est effectué de telle manière qu'une vitesse de refroidissement au niveau de la partie centrale soit d'au moins 15 °C/s jusqu'à Bs - 10 °C à Bs + 10 °C et un second refroidissement est effectué jusqu'à au plus 300 °C, de sorte qu'une vitesse de refroidissement au niveau de la partie centrale soit de 10 à 50 °C/s.
    Ar3 et Bs sont calculés comme suit : Ar 3 = 910 − 310 * C − 80 * N − 55 * Ni ;
    Figure imgb0005
     Bs = 830 − 270 * C − 90 * Mn − 37 * Ni
    Figure imgb0006
  7. Procédé selon la revendication 6, dans lequel une température de début de refroidissement est Ar3 + 30 °C à Ar3.
  8. Procédé selon la revendication 6, dans lequel le laminage grossier est effectué de telle manière que les trois dernières passes de laminage aient un taux de réduction d'au moins 10 % par passe.
  9. Procédé selon la revendication 6, dans lequel le laminage de finition est effectué de telle manière qu'un taux de réduction par passe soit d'au moins 10 % et qu'un taux de réduction cumulé soit d'au moins 60 %.
EP16879393.3A 2015-12-23 2016-12-23 Acier à haute résistance et à faible taux d'élasticité présentant une excellente résistance à la fissuration par corrosion sous contrainte et une excellente ténacité à basse température Active EP3395987B1 (fr)

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