EP3081662B1 - Tôle d'acier et procédé permettant de fabriquer cette dernière - Google Patents

Tôle d'acier et procédé permettant de fabriquer cette dernière Download PDF

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EP3081662B1
EP3081662B1 EP14869973.9A EP14869973A EP3081662B1 EP 3081662 B1 EP3081662 B1 EP 3081662B1 EP 14869973 A EP14869973 A EP 14869973A EP 3081662 B1 EP3081662 B1 EP 3081662B1
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steel plate
toughness
steel
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EP3081662A1 (fr
EP3081662A4 (fr
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Katsuyuki Ichimiya
Shigeki KITSUYA
Kazukuni Hase
Shigeru Endo
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JFE Steel Corp
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • 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
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations

Definitions

  • This disclosure relates to a high tensile strength steel plate, and a method for manufacturing the same, used in steel structures such as marine structures and ships, pressure vessels, and penstocks.
  • this disclosure relates to a thick, high tensile strength steel plate, and a method for manufacturing the same, that not only has a yield stress (YS) of 460 MPa or greater and excellent strength and toughness, but that also has excellent low-temperature toughness in a multilayer weld (CTOD property) and that, after Post Weld Heat Treatment (PWHT), has excellent strength and toughness (PWHT property).
  • YS yield stress
  • COD property multilayer weld
  • PWHT Post Weld Heat Treatment
  • steel plates used in ships, marine structures, pressure vessels, and the like are subjected to welding to form structures with desired shapes. Therefore, from the perspective of structural safety, these steel plates are not only required to have high strength and excellent toughness but also to have excellent toughness in weld joints (weld metal and Heat-Affected Zone (HAZ)) upon welding.
  • weld joints weld metal and Heat-Affected Zone (HAZ)
  • COD test Crack Tip Opening Displacement test
  • the bond (the boundary between weld metal and base material) and a region in which the bond is formed into a dual phase region by reheating (a region in which coarse grains are formed in the first cycle of welding and which is heated into a ferrite and austenite dual phase region by the subsequent welding pass, hereinafter referred to as a dual phase reheating area) are also prone to becoming local brittle zones.
  • JP H03-053367 B2 (PTL 1) and JP S60-184663 A (PTL 2) disclose techniques in which, by dispersing fine grains in steel plates by means of combined addition of rare-earth elements (REM) and Ti, grain growth of austenite is suppressed, thereby improving the toughness of the weld zone.
  • REM rare-earth elements
  • PTL 1 and PTL 2 also disclose a technique for dispersing Ti oxides, a technique for combining the capability of ferrite nucleation of BN with oxide dispersion, a technique for adding Ca and a REM to control the morphology of sulfides so as to increase the toughness, and other such techniques.
  • JP 3697202 B2 discloses a technique for dispersing Ti oxides into steel so as to improve the HAZ toughness.
  • the technique disclosed in PTL 4 sets the cooling rate after rolling to 0.1 °C/s or less and adopts a method to precipitate Cu particles during this process. Such a technique is problematic, however, in terms of manufacturing stability.
  • the N/A1 ratio is set between 0.3 and 3.0, thereby suppressing toughness degradation due to the adverse effect of coarsened AlN and of solute N, but it is easier to control solute N with Ti.
  • PWHT Post Weld Heat Treatment
  • the base material is also heated at the same time. Therefore, the base material properties need to be maintained even upon PWHT.
  • an element that forms a precipitate at that temperature is generally added.
  • a further steel plate having a low yield ratio and high strength excellent in terms of strain ageing resistance and a method for manufacturing the same is disclosed in PTL 6.
  • the steel material used in these steel structures is often thick material, for example with a plate thickness of 35 mm or greater. Therefore, in order to ensure a strength such that the yield stress is at least 460 MPa grade, a steel chemical composition including a large amount of added alloying elements has become advantageous.
  • a thick, high tensile strength steel plate, and a method for manufacturing the same that is suitable for use in large steel structures such as marine structures, has a yield stress (YS) of 460 MPa or greater, and has an excellent CTOD property in a multilayer weld and an excellent PWHT property can be obtained. Therefore, this disclosure is extremely useful in industrial terms.
  • C is an essential element for ensuring the strength of a high tensile strength steel plate.
  • quench hardenability is degraded, and it becomes necessary to add a large amount of quench hardenability-improving elements, such as Cu, Ni, Cr, or Mo, in order to ensure strength, resulting in a rise in costs.
  • quench hardenability-improving elements such as Cu, Ni, Cr, or Mo
  • the toughness of the weld zone degrades. Therefore, the C content is set in the range of 0.020 % to 0.090 %, and preferably 0.020 % to 0.080 %.
  • the Si is added as a deoxidizing element and in order to obtain the steel plate strength. To obtain these effects, the Si content needs to be at least 0.01 %. On the other hand, a large Si content exceeding 0.35 % leads to deterioration in weldability and toughness of the weld joint. Therefore, the Si content needs to be set in the range of 0.01 % to 0.35 %, preferably 0.01 % to 0.23 %.
  • the Mn content In order to ensure the steel plate strength and the weld joint strength, the Mn content needs to be at least 1.40 %. Conversely, when the amount of Mn content exceeds 2.00 %, the weldability deteriorates, quench hardenability becomes excessive, and the toughness of the steel plate and the toughness of the weld joint deteriorate. Therefore, the Mn content is set in the range of 1.40 % to 2.00 %, more preferably 1.40 % to 1.95 %.
  • the P content is set to 0.008 % or less, preferably 0.006 % or less.
  • the P content is preferably as small as possible, but considering factors such as refining cost, the lower limit may be approximately 0.002 %.
  • the S content is set to 0.0035 % or less, preferably 0.0030 % or less.
  • the S content is preferably as small as possible, but considering factors such as refining cost, the lower limit may be approximately 0.0004 %.
  • Al is an element to be added in order to deoxidize molten steel, and the Al content needs to be set to 0.010 % or more.
  • the Al content exceeds 0.060 %, however, the toughness of the steel plate and the toughness of the weld zone are degraded, and Al is mixed into the weld metal by dilution due to welding, which degrades toughness. Therefore, the Al content is limited to 0.060 % or less, preferably 0.017 % to 0.055 %.
  • the Al content is specified in terms of acid-soluble Al (also referred to as "Sol. Al" or the like).
  • Ni is an element useful for improving the strength and toughness of the steel plate and is also useful for improving the CTOD property of the weld zone.
  • the added content of Ni needs to be 0.40 % or more.
  • Ni is an expensive element, however, and excessive addition thereof also increases the likelihood of damage to the surface of the slab at the time of casting. Therefore, the upper limit of the Ni content is set to 2.00 %.
  • Mo is an element that fulfills an important function in this disclosure, and adding an appropriate amount thereof is useful for increasing the strength of the steel plate. This effect is obtained by improving the quench hardenability and the temper softening resistance at the time of tempering. Furthermore, Mo maintains the complex precipitate formed by Mo, Ti, and Nb in a fine state, thereby strengthening thick material and controlling a reduction in toughness. In order to obtain these effects, the Mo content needs to be at least 0.05 %. An excessive Mo content, however, adversely affects the toughness of the thick material. Hence, the upper limit on the Mo content is set to 0.50 %. The Mo content is more preferably in the range of 0.08 % to 0.40 %, and even more preferably in the range of 0.16 % to 0.30 %.
  • Nb forms an unrecrystallized zone of austenite in the low temperature region. Therefore, by performing rolling in such a temperature region, the structure of the steel plate can be refined and the toughness of the steel plate can be increased. Furthermore, Nb has the effect of improving the quench hardenability, and by being added in combination with Mo and Ti, Nb has the effect of improving the softening resistance at the time of tempering. Nb is also a useful element for improving the strength of the steel plate. In order to obtain these effects, the Nb content needs to be at least 0.005 %. When the Nb content exceeds 0.040 %, however, the toughness deteriorates. Hence, the upper limit on the Nb content is set to 0.040 %, preferably 0.035 %.
  • Ti is precipitated as TiN when molten steel solidifies, which suppresses coarsening of austenite in the weld zone, thus contributing to improvement in the toughness of the weld zone. Furthermore, by being added in combination with Mo and Nb, Ti has the effect of improving the softening resistance at the time of tempering. When the Ti content is less than 0.005 %, however, such an effect is small. On the other hand, when the Ti content exceeds 0.025 %, TiN coarsens, and it is not possible to obtain the effect of improving the toughness of the steel plate and the weld zone. Therefore, the Ti content is set in the range of 0.005 % to 0.025 %.
  • N reacts with Ti and Al to form precipitates. Crystal grains are thereby refined, and the toughness of the steel plate is improved. Furthermore, N is a necessary element for forming TiN which suppresses coarsening of the structure of the weld zone. In order to obtain such effects, the N content needs to be set to 0.0020 % or more. On the other hand, when the N content exceeds 0.0050 %, solute N markedly degrades the toughness of the steel plate and the weld zone and leads to a deterioration in strength due to a reduction in solute Nb caused by generation of complex precipitates of Ti and Nb. Therefore, the upper limit on the N content is set to 0.0050 %.
  • Ca is an element that improves toughness by fixing S. In order to obtain this effect, the Ca content needs to be at least 0.0005 %. On the other hand, Ca content exceeding 0.0050 % causes saturation of the effect. Therefore, Ca is added in the range of 0.0005 % to 0.0050 %.
  • the O content is set to 0.0035 % or less, preferably 0.0028 % or less.
  • the O content is preferably as small as possible, but considering factors such as refining cost, the lower limit may be approximately 0.0010 %.
  • Ceq is set to 0.520 % or less. Ceq is preferably in the range of 0.440 % to 0.520 %.
  • the value of [Ti]/[N] is set in the range of 1.5 to 4.0, and preferably 1.8 to 3.5.
  • ⁇ [Ca] - (0.18 + 130 ⁇ [Ca]) ⁇ [O] ⁇ /1.25/[S] is a value representing the Atomic Concentration Ratio (ACR) of Ca and S, which are effective for sulfide morphological control, and can be adjusted by controlling the amount of Ca added and the amount of dissolved oxygen in the molten steel at the time of addition to be within appropriate ranges.
  • ACR Atomic Concentration Ratio
  • the sulfide morphology can be estimated by this ACR value, but in this disclosure, the ACR value is specified as an index in order to finely disperse CaS which does not dissolve even at high temperatures and which acts as nuclei for ferrite transformation.
  • the ACR value When the ACR value is 0 or less, CaS is not crystallized. Consequently, S is precipitated in the form of MnS only, which easily dissolves in the heat-affected zone, making it impossible to obtain ferrite product nuclei. Furthermore, the MnS precipitated alone is elongated during rolling and causes degradation in the toughness of the steel plate. Therefore, in this disclosure, the ACR value needs to exceed zero.
  • the ACR value when the ACR value is 1.5 or more, the proportion of oxides among Ca-based inclusions increases, and the proportion of sulfides that function as nucleation sites decreases, making it impossible to obtain the effect of improving toughness. Therefore, in this disclosure, the ACR value needs to be less than 1.5.
  • the ACR value is preferably in the range of 0.15 to 1.30, and more preferably in the range of 0.20 to 1.00. 5.5 C 4 / 3 + 15 P + 0.90 Mn + 0.12 Ni + 7.9 Nb 1 / 2 + 0.53 Mo ⁇ 3.70
  • the value of the left-hand side of the formula above (5.5[C] (4/3) + 15[P] + 0.90[Mn] + 0.12[Ni] + 7.9[Nb] (1/2) + 0.53[Mo]) is the hardness index of the central segregation area formed by components that are likely to be concentrated in the central segregation area and is referred to below as the Ceq* value.
  • test pieces Since the CTOD test is carried out over the entire thickness of a steel plate, test pieces include central segregation. In the case where the composition concentration in the central segregation is significant, a hardened region occurs in the heat-affected zone. Therefore, a good value cannot be obtained in the CTOD test.
  • the Ceq* value is set to be 3.70 or less, and preferably 3.50 or less. No lower limit is placed on the Ceq* value, but considering factors such as productivity, preferably the lower limit is approximately 2.2.
  • At least one selected from the group consisting of Cu: less than 0.7 %, Cr: 0.1 % to 1.0 %, and V: 0.005 % to 0.05 % may be added to increase quench hardenability.
  • Adding Cu improves the strength of the steel plate. If the content of Cu exceeds 0.7 %, however, the hot ductility deteriorates. Therefore, the Cu content is limited to 0.7 % or less.
  • the Cu content is preferably 0.1 % to 0.6 %.
  • the Cr content is an element effective in increasing the strength of the steel plate.
  • the Cr content is set to 0.1 % or more.
  • V 0.005 % to 0.05 %
  • V is an element that is effective in improving the strength and toughness of the steel plate at a content of 0.005 % or more. Setting the V content to exceed 0.05 %, however, leads to deterioration of toughness. Therefore, the V content is preferably 0.005 % to 0.05 % when included.
  • At least one selected from the group consisting of Mg: 0.0002 % to 0.0050 % and a REM: 0,0010 % to 0.0200 % may be added to increase the HAZ toughness.
  • Mg and REM are elements having the effect of improving the toughness of steel via the dispersion of oxides.
  • the Mg content is set to 0.0002 % or more, and the REM content to 0.0010 % or more.
  • Mg content exceeding 0.0050 % and REM content exceeding 0.0200 % merely causes saturation of this effect. Accordingly, when adding these elements, the content is preferably set in the above-mentioned ranges.
  • the Mg content is more preferably 0.0005 % to 0.0020 %, and the REM content is more preferably 0.0020 % to 0.0150 %.
  • Components other than the above-mentioned chemical composition are Fe and incidental impurities.
  • B exists in a segregated manner at austenite grain boundaries, suppresses ferrite transformation, and generates bainite structures that include a large amount of M-A.
  • M-A M-A
  • B has the disadvantage of making the structure brittle particularly in the heat-affected zone. Accordingly, in this disclosure, the content of B in the steel plate needs to be kept below 0.0003 %.
  • FIG. 1 illustrates the relationship between (i) the precipitate size and precipitate composition after PWHT and (ii) the change in strength and toughness before and after PWHT ( ⁇ TS, ⁇ vTrs), and
  • FIG. 2 illustrates TEM replica observation and EDX analysis results for precipitates in steel.
  • the change in strength and in toughness before and after PWHT respectively need to satisfy the following ranges: ⁇ TS of 5 MPa to -15 MPA, and ⁇ vTrs of 10 °C to -5 °C.
  • ⁇ TS 5 MPa to -15 MPA
  • ⁇ vTrs 10 °C to -5 °C.
  • the mean size of the precipitates needs to be kept to 20 nm or less, and that the Ti content (represented as [Ti]), the Nb content (represented as [Nb]), and the Mo content (represented as [Mo]) in the precipitates need to satisfy the relationship [Nb]/([Ti] + [Nb] + [Mo]) ⁇ 0.3.
  • the above-mentioned precipitates are Ti, Nb, and Mo precipitates. Since it suffices for the relationship [Nb]/([Ti] + [Nb] + [Mo]) ⁇ 0.3 to be satisfied, the precipitates need to be at least Nb precipitates, whereas Ti and Mo precipitates may be included in any range that satisfies this relationship.
  • having an excellent PWHT property refers to ⁇ TS being in the range of 5 MPa to -15 MPa, and ⁇ vTrs being in the range of 10 °C to -5 °C.
  • the precipitates in this disclosure are Mo, Ti, and Nb precipitates. Specifically, these precipitates are carbides, nitrides, or carbonitrides of Mo, Ti, and Nb, or a mixture thereof.
  • the method of determining the precipitate particle size in this disclosure is in accordance with the TEM replica method. Specifically, after appropriately collecting the precipitate zone of Ti, Nb, and Mo carbides from the steel, the average equivalent circular diameter was determined using image processing on an observation made at 100,000X over four fields of view and taken as the particle size of the precipitates. In this disclosure, the lower limit on the measurement target for precipitate size was set to 2 nm. The reason is that precipitates with a precipitate size smaller than 2 nm are hard to measure.
  • Our steel is preferably manufactured with the method of manufacturing described below.
  • Molten steel adjusted to have a chemical composition within the above-described ranges is prepared by steelmaking with an ordinary method using a converter, an electric heating furnace, a vacuum melting furnace, or the like.
  • the slab is hot rolled to a desired plate thickness.
  • the result is then cooled, and as necessary, tempered.
  • the slab heating temperature and rolling reduction are prescribed.
  • the temperature conditions on the steel plate are prescribed by the temperature at the central portion in the plate thickness direction of the steel plate.
  • the temperature at the central portion in the plate thickness direction is determined from the plate thickness, the surface temperature, the cooling conditions, and the like by simulation calculation or the like.
  • the temperature at the central portion in the plate thickness direction may be determined by calculating the temperature distribution in the plate thickness direction using the finite difference method.
  • the slab reheating temperature is set to 950 °C or higher in order to remove casting defects in the slab reliably with hot rolling. If the slab is reheated to a temperature exceeding 1150 °C, however, the austenite crystallized grains coarsen, causing the toughness of the steel plate to degrade. Hence, the upper limit on the slab reheating temperature is set to 1150 °C.
  • the cumulative rolling reduction of hot rolling in a temperature range of 900 °C or higher is set to 30 % or higher. The reason is that if the cumulative rolling reduction is less than 30 %, coarse grains formed during reheating remain and adversely affect the toughness of the steel plate. No upper limit is placed on the cumulative rolling reduction of hot rolling in a temperature range of 900 °C or higher, but in industrial terms, the upper limit is approximately 95 %.
  • austenite grains In this temperature range, the rolled austenite grains do not sufficiently recrystallize. Therefore, austenite grains that remain flattened after rolling constitute a state of high internal distortion that includes numerous defects, such as an internal distortion zone. These austenite grains act as the driving force for ferrite transformation and encourage phase transformation.
  • the cumulative rolling reduction of hot rolling in a temperature range of less than 900 °C is set in the range of 30 % to 70 %.
  • Cooling rate at least to 500 °C: 1.0 °C/s or higher
  • accelerated cooling is performed at least to 500 °C at a cooling rate of 1.0 °C/s or higher. The reason is that if the cooling rate is less than 1.0 °C/s, sufficient strength of the steel plate is not obtained. Furthermore, if cooling is stopped at a higher temperature than 500°C, the proportion of ferrite and pearlite structure increases, making high strength and high toughness of thick material incompatible. While no lower limit is placed on the stop temperature of accelerated cooling, the steel is preferably cooled to room temperature.
  • Tempering temperature 450 °C to 650 °C
  • the tempering in this disclosure more preferably uses induction heating, which suppresses coarsening of carbides during tempering.
  • the tempering is preferably performed so that the temperature at the center of the steel plate calculated by a simulation using the finite difference method or the like is from 450 °C to 650 °C.
  • Thick material in this disclosure has a thickness of 15 mm or greater. Accordingly, the thickness in this disclosure refers to a steel thickness of 15 mm or greater, but the effects of this disclosure are best obtained when the steel thickness is in a range of 40 mm to 100 mm. Manufacturing conditions other than the above-described manufacturing conditions on thick, high tensile strength steel may be in accordance with conventional methods.
  • the thick, high tensile strength steel of this disclosure coarsening of austenite grains in the heat-affected zone is suppressed while finely dispersing nuclei for ferrite transformation that do not dissolve even at high temperatures, thereby refining the structure of the heat-affected zone. High toughness is thus obtained. Also in an area reheated to a dual phase by the thermal cycle at the time of multilayer welding, the structure of the heat-affected zone due to initial welding is refined. Therefore, in the dual phase reheating area, the toughness of the non-transformed area can be improved, the austenite grains that undergo retransformation can be refined, and the extent of reduction in toughness can be reduced. Additionally, by generating fine complex precipitates of Ti, Nb, and Mo, a thick, high tensile strength steel plate is provided with an excellent CTOD property and PWHT property.
  • a Charpy impact test was also performed by collecting JIS No. 4 V-notch test pieces measuring 2 mm from the 1/2 position along the thickness of the steel plates, so that the longitudinal direction of each test piece was perpendicular to the rolling direction of the steel plate. The absorbed energy vE -40 °C at -40 °C was then measured.
  • the base metal properties were evaluated as being good when all of the following relationships were satisfied: YS ⁇ 460 MPa, TS ⁇ 570 MPa, and vE -40 °C ⁇ 200 J.
  • the toughness of the weld zone was evaluated by producing a multilayer fill weld joint, using a single bevel groove, by submerged arc welding having a welding heat input of 35 kJ/cm and measuring the absorbed energy vE -40 °C at -40 °C with a Charpy impact test, using the weld bond on the straight side at the 1/2 position along the thickness of the steel plates as the notch position for the test.
  • the toughness of the weld zone was determined to be good when the mean for three tests satisfied the relationship vE -40 °C ⁇ 150 J.
  • the CTOD value at -10 °C i.e. ⁇ -10 °C
  • the CTOD property of the weld joint was determined to be good when the minimum of the CTOD value ( ⁇ -10 °C) over three tests was 0.5 mm or greater.
  • the precipitate zone in the steel was collected by the TEM replica method, and the average equivalent circular diameter was determined using image processing on an observation made at 100,000X over four fields of view.
  • Precipitates with a particle size near the mean were selected by EDX, the precipitate composition thereof was determined, and [Nb]/([Ti]+[Nb]+[Mo]) was determined as the mean of three precipitates.
  • ⁇ TS TS(after PWHT) - TS(before PWHT)
  • ⁇ vTrs vTrs(after PWHT) - vTrs(before PWHT)
  • Table 3 illustrates the hot rolling conditions, heat treatment conditions, base metal properties, the results of the above-described Charpy impact test and CTOD test on the weld zone, the precipitate size/composition, and the change in base metal properties after PWHT.
  • steel codes A to E represent steel conforming to this disclosure
  • steel codes F to Z represent comparative steel in which one of the steel components is outside of the range of this disclosure.
  • Sample Numbers 1, 2, 5, 6, 8, and 11 in Table 3 are all Examples for which the results of the Charpy impact test on the weld bond, the results of the three-point bending CTOD test on the weld bond, the precipitate size/composition in the steel plate, and the PWHT property all satisfy the targets.
  • the steel of the Examples according to this disclosure has both excellent strength and toughness of the steel plate, as the yield stress (YS) of the steel plate is 460 MPa or higher, and the Charpy absorbed energy (vE -40 °C) is 200 J or higher. Furthermore, in the weld joint bond, vE -40 °C is 150 J or higher, and the CTOD value is 0.5 mm or higher, thus also providing excellent toughness in the heat-affected zone.
  • the mean particle size of the precipitates is 20 ⁇ m or less and [Nb]/([Ti]+[Nb]+[Mo]) ⁇ 0.3, then the base metal property after PWHT is also excellent.
  • the Comparative Examples outside of the ranges of this disclosure only steel plates for which one of the above-described properties was inferior could be achieved.

Claims (2)

  1. Plaque d'acier, comprenant :
    une composition chimique constituée de, en % en masse,
    C : 0,020 % à 0,090 %,
    Si : 0,01 % à 0,35 %,
    Mn : 1,40 % à 2,00 %,
    P : 0,008 % ou moins,
    S : 0,0035 % ou moins,
    Al : 0,010 % à 0,060 %,
    Ni : 0,40 % à 2,00 %,
    Mo : 0,05 % à 0,50 %,
    Nb : 0,005 % à 0,040 %,
    Ti : 0,005 à 0,025 %,
    N : 0,0020 % à 0,0050 %,
    Ca : 0,0005 % à 0,0050 %, et
    O : 0,0035 % ou moins,
    Ceq spécifié par la formule (1) ci-dessous se situant dans une plage de 0,420 % à 0,520 %, les formules (2), (3) et (4) ci-dessous étant satisfaites, facultativement au moins l'un choisi parmi le groupe constitué de, en % en masse,
    Cu : moins de 0,7 %,
    Cr : 0,1 % à 1,0 %,
    V : 0,005 % à 0,05 %
    Mg : 0,0002 % à 0,0050 % et
    REM: 0,0010 % à 0,0200 %, le reste étant Fe et des impuretés accidentelles incluant B à moins de 0,0003 % ; et
    de précipités comprenant Ti, Nb, et Mo et ayant une taille moyenne de particules de 2 nm ou plus et de 20 nm ou moins, dans laquelle [Nb]/([Ti] + [Nb] + [Mo]) ≥ 0,3,
    où [Ti] est la teneur en Ti dans les précipités, [Nb] est la teneur en Nb dans les précipités, et [Mo] est la teneur en Mo dans les précipités : Ceq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5
    Figure imgb0015
     1,5 ≤ Ti / N ≤ 4,0
    Figure imgb0016
     0 < Ca − 0,18 + 130 × Ca × O / 1,25 / S < 1,5
    Figure imgb0017
     5,5 C 4 / 3 + 15 P + 0,90 Mn + 0,12 Ni + 7,9 Nb 1 / 2 + 0,53 Mo ≤ 3,70
    Figure imgb0018
     où [M] est la teneur en l'élément M en % en masse, et
    une épaisseur de la plaque d'acier est de 15 mm ou plus, et dans laquelle la plaque d"acier présente
    une YS de 460 MPa ou plus,
    une TS de 570 MPa ou plus,
    une énergie absorbée par Test Charpy vE-40 °C de 200 J ou plus,
    un ΔTS de 5 MPa à -15 MPa, et
    un ΔvTrs de 10 °C à -5 °C,
    où ΔTS est le changement de résistance à la traction dans le métal de base avant et après traitement thermique post-soudure, et ΔvTrs est le changement de robustesse dans le métal de base avant et après traitement thermique post-soudure.
  2. Procédé de fabrication d'une plaque d'acier selon la revendication 1, le procédé comprenant les étapes consistant à :
    chauffer un acier ayant la composition chimique selon la revendication 1 entre 950 °C et 1150 °C ;
    soumettre ensuite l'acier à un laminage à chaud à une réduction de laminage cumulative de 30 % ou plus dans une plage de température de 900 °C ou plus et une réduction de laminage cumulative de 30 % à 70 % dans une plage de température inférieure à 900 °C ;
    refroidir ensuite l'acier à au moins 500 °C au niveau de la partie centrale dans la direction d'épaisseur de plaque de la plaque d'acier à une vitesse de refroidissement de 1,0 °C/s ou plus,
    et comprenant facultativement en outre une trempe entre 450 °C et 650 °C après le refroidissement.
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