EP3239331B1 - Acier à haute résistance ayant une excellente résistance à la propagation de fissures et son procédé de production - Google Patents

Acier à haute résistance ayant une excellente résistance à la propagation de fissures et son procédé de production Download PDF

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EP3239331B1
EP3239331B1 EP15873589.4A EP15873589A EP3239331B1 EP 3239331 B1 EP3239331 B1 EP 3239331B1 EP 15873589 A EP15873589 A EP 15873589A EP 3239331 B1 EP3239331 B1 EP 3239331B1
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steel
steel sheet
thickness
central portion
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EP3239331A4 (fr
EP3239331A1 (fr
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Hak-Cheol Lee
Sung-Ho Jang
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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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, rods, wire, tubes, profiles or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/02—Manufacture of metal sheets, rods, wire, tubes, profiles or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of sheets
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/021—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving particular fabrication steps or treatments of ingots or slabs
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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
    • C22C38/08—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
    • 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
    • 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
    • C21D2201/00—Treatment for obtaining particular effects
    • C21D2201/05—Grain orientation
    • 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
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling

Definitions

  • the present disclosure relates to a high-strength steel having excellent brittle crack arrestability and a method of manufacturing the same.
  • brittle crack arrestability referring to stability of a structure
  • a high-strength steel is applied to a major structure such as a ship, or the like
  • cases in which guaranteed levels of brittle crack arrestability are required have increased.
  • a low temperature transformation phase is generated in a central portion, a phenomenon in which brittle crack arrestability is significantly reduced occurs. Therefore, it may be difficult to improve brittle crack arrestability of an extremely thick high-strength steel.
  • EP 2 660 346 describes a high-strength steel sheet superior toughness at cryogenic temperatures, and a method for manufacturing the same.
  • KR 100 723 201 B1 describes thick steel plates for ship building.
  • An aspect of the present disclosure is to provide a high-strength steel having excellent brittle crack arrestability.
  • Another aspect of the present disclosure is to provide a method of manufacturing a high-strength steel having excellent brittle crack arrestability.
  • a high-strength steel sheet having excellent brittle crack arrestability includes 0.05 wt% to 0.1 wt% of carbon (C), 0.9 wt% to 1.5 wt% of manganese (Mn), 0.8 wt% to 1.5 wt% of nickel (Ni), 0.005 wt% to 0.1 wt% of niobium (Nb), 0.005 wt% to 0.1 wt% of titanium (Ti), 0.1 wt% to 0.6 wt% of copper (Cu), 0.1 wt% to 0.4 wt% of silicon (Si), 100 ppm or less of phosphorous (P), 40 ppm or less of sulfur (S), and the remainder being iron (Fe) and other inevitably contained impurities, the high-strength steel having a microstructure being one structure selected from the group consisting of a single-phase structure of ferrite, a single-phase structure of bainite, a complex
  • a fraction of pearlite may be 20% or less.
  • the steel thickness may be 80mm to 100mm.
  • a method of manufacturing a high-strength steel sheet having excellent brittle crack arrestability includes: reheating a slab at 950°C to 1100°C, the steel of the slab consisting of 0.05 wt% to 0.1 wt % of carbon (C), 0.9 wt% to 1.5 wt% of manganese (Mn), 0.8 wt% to 1.5 wt% of nickel (Ni), 0.005 wt% to 0.1 wt% of niobium (Nb), 0.005 wt% to 0.1 wt% of titanium (Ti), 0.1 wt% to 0.6 wt% of copper (Cu), 0.1 wt% to 0.4 wt% of silicon (Si), 100 ppm or less of phosphorous (P), 40 ppm or less of sulfur (S), and the remainder being iron (Fe) and other inevitably contained impurities, and then rough rolling the slab at a temperature of 1100°C to
  • a reduction ratio per pass with respect to three final passes may be 5% or more, and a total cumulative reduction ratio may be 40% or more.
  • a crystal grain size of a central portion of the bar in a thickness direction before the finish rolling after the rough rolling may preferably be 150 ⁇ m or less, and more preferably 100 ⁇ m or less.
  • a reduction ratio during the finish rolling may be set such that a ratio of a slab thickness (mm) / a steel sheet thickness (mm) after finish rolling may be 3.5 or above, preferably 3.8 or above.
  • the cooling of the steel sheet is performed at a cooling rate of a central portion of the steel sheet in a thickness direction of 2°C/s or more.
  • the cooling of the steel sheet may be performed at an average cooling rate from 3°C/s to 300°C/s.
  • a high-strength steel having high yield strength and excellent brittle crack arrestability is obtained.
  • FIG. 1 is an image of a central portion of Inventive steel 1 in a thickness direction, captured with an optical microscope.
  • the inventors conducted research and experiments into improving yield strength and brittle crack arrestability of a thick steel having a thickness of 50 mm or more, and the present disclosure has been proposed based on the results thereof.
  • a steel composition, a structure, a texture, and manufacturing conditions of a steel are controlled, so yield strength and brittle crack arrestability of a steel having a thick thickness are further improved.
  • a main concept in the present disclosure is as follows.
  • a pressing condition is controlled, and a sufficient temperature difference between a central portion and a surface is secured, so a fine structure is secured in a central portion of a steel.
  • a high-strength steel having excellent brittle crack arrestability includes the features as defined in claim 1.
  • C is the most important element in securing basic strength, so C needs to be contained in a steel in an appropriate range. In order to obtain such an additive effect, C is added in an amount of 0.05% or more.
  • the content of C exceeds 0.10%, due to the generation of a large amount of martensite-austenite constituent (MA), high strength of ferrite itself, and generation of a large amount of a low temperature transformation phase, or the like, low temperature toughness may be reduced.
  • the content of C is limited to 0.05% to 0.10%, preferably limited to 0.061% to 0.091%, and more preferably limited to 0.065% to 0.085%.
  • Mn is a useful element for improving strength due to solid solution strengthening and for improving hardenability to allow a low temperature transformation phase to be generated.
  • Mn is added in an amount of 0.9% or more.
  • the content of Mn is limited to 0.9% to 1.5%, preferably limited to 0.97% to 1.39%, and more preferably limited to 1.15% to 1.30%.
  • Ni is an important element, allowing dislocation cross-slip to be easily performed at a low temperature to improve impact toughness, and improving hardenability to improve strength.
  • Ni is added in an amount of 0.8% or more.
  • hardenability is excessively increased, so a low temperature transformation phase is generated.
  • toughness may be reduced and manufacturing costs may be increased.
  • an upper limit of the content of Ni is limited to 1.5%.
  • the content of Ni is preferably limited to 0.89% to 1.42% and more preferably limited to 1.01% to 1.35 %.
  • Nb is precipitated in the form of NbC or NbCN, thereby improving strength of a base material.
  • Nb dissolved during reheating at a high temperature, is significantly finely precipitated in the form of NbC during rolling, so recrystallization of austenite is suppressed.
  • a structure may be refined.
  • Nb is added in an amount of 0.005% or more.
  • an upper limit of the content of Nb is limited to 0.1%.
  • the content of Nb is preferably limited to 0.012% to 0.028% and more preferably limited to 0.018% to 0.024%.
  • Ti is precipitated as TiN during reheating, and thus inhibits growth of a crystal grain in a base material and in a weld heat affected zone, thereby significantly improving low temperature toughness.
  • Ti is added in an amount of 0.005% or more.
  • Ti when Ti is added in an amount exceeding 0.1%, a continuous casting nozzle may be clogged, or low temperature toughness may be reduced by crystallization of a central portion. In this case, the content of Ti is limited to 0.005% to 0.1%.
  • the content of Ti is preferably limited to 0.009% to 0.024% and more preferably limited to 0.011% to 0.018%.
  • P and S are elements causing brittleness in a grain boundary or causing brittleness by forming a coarse inclusion.
  • P is limited to 100 ppm or less and S is limited to 40 ppm or less.
  • Si improves strength of a steel and has a strong deoxidizing effect, and thus is an essential element in producing clean steel.
  • Si is added in an amount of 0.1% or more.
  • an upper limit of the content of Si is limited to 0.4%.
  • the content of Si is preferably limited to 0.22% to 0.32% and more preferably limited to 0.25% to 0.3 %.
  • Cu is a main element in improving strength of a steel by improving hardenability and causing solid solution strengthening, and is a main element in increasing a yield strength due to generation of ⁇ (epsilon) Cu precipitate when tempering is applied.
  • ⁇ epsilon
  • Cu is added in an amount of 0.1% or more.
  • an upper limit of the content of Cu is limited to 0.6%.
  • the content of Cu is preferably limited to 0.21% to 0.51% and more preferably is limited to 0.18% to 0.3%.
  • the contents of Cu and Ni are set such that a Cu/Ni weight ratio is 0.6 or less and preferably 0.5 or less.
  • iron (Fe) is provided as a remainder thereof.
  • the impurities may be known to those skilled in the art, and thus, may not be particularly described in this specification.
  • the steel according to an exemplary embodiment has a microstructure including a single structure selected from the group consisting of a single-phase structure of ferrite, a single-phase structure of bainite, a complex-phase structure of ferrite and bainite, a complex-phase structure of ferrite and pearlite, and a complex-phase structure of ferrite, bainite, and pearlite.
  • the ferrite is polygonal ferrite or acicular ferrite, and the bainite is preferably granular bainite.
  • fraction of pearlite is preferably limited to 20% or less.
  • a grain size having a high-angle boundary of 15 degrees or more, measured in an EBSD method, in a central portion is 30 ⁇ m or less.
  • the grain size of a structure of a central portion of the steel is refined to be 30 ⁇ m or less, so strength is improved due to strengthening by grain refinement, while brittle crack arrestability is improved by significantly reducing the generation and propagation of cracks.
  • An area ratio of a (100) plane forming an angle within 15 degrees with respect to a plane perpendicular to a rolling direction in a region of the high-strength steel in a range of 20% of an overall steel thickness based on a position equal to 1/2 of the steel thickness is 40% or less.
  • a crack is propagated in a width direction of a steel, that is, in a direction perpendicular to a rolling direction, and a brittle fracture surface of a body-centered cubic structure (BCC) is a (100) plane.
  • BCC body-centered cubic structure
  • a texture of an area of a central portion in which a microstructure is relatively coarse in comparison with a surface is controlled.
  • a texture of a steel and, particularly, an area ratio of a (100) plane forming an angle within 15 degrees with respect to a plane perpendicular to a rolling direction in a region of the high-strength steel in a range of 20% of an overall steel thickness based on a position equal to 1/2 of the steel thickness is controlled to be 40% or less. Even when a crack is generated, propagation of the crack is significantly reduced, so brittle crack arrestability is improved.
  • the steel has a yield strength of 390 MPa or more.
  • the steel has a thickness of 50 mm or more, preferably has a thickness of 50 mm to 100 mm, and more preferably has a thickness of 80 mm to 100 mm.
  • a method of manufacturing a high-strength steel is defined in claim 4.
  • a slab is reheated before rough rolling.
  • a reheating temperature of a slab is set to be 950 °C or more, to dissolve carbonitride of Ti and/or Nb formed during casting.
  • it is preferably to heat at 1000°C or more.
  • austenite may be coarsened.
  • an upper limit of the reheating temperature is limited to 1100°C.
  • the slab, having been reheated, is rough rolled.
  • a rough rolling temperature is a temperature (Tnr) at which recrystallization of austenite is stopped or more. Due to rolling, effects in which a casting structure such as a dendrite formed during casting or the like is destroyed and a size of austenite is reduced may be obtained. To obtain the effects described above, the rough rolling temperature is limited to 1100°C to 900°C.
  • a temperature difference between a central portion and a surface of the slab or bar immediately before the rough rolling should be 100°C to 300°C.
  • a surface of the slab or bar maintains a temperature lower than that of a central portion. While the temperature difference exists, when rough rolling is performed, more deformation occurs in a central portion in which a temperature is relatively high than in the surface in which a temperature is relatively low. Thus, a crystal grain size of a central portion is more refined. In this case, an average grain size of a central portion is maintained to be 30 ⁇ m or less.
  • a phenomenon in which the surface in which a temperature is relatively low has strength higher than that of a central portion in which a temperature is relatively high, so more deformation occurs in a central portion having relatively low strength, is used.
  • a temperature difference between a central portion and a surface is 100°C to 300°C.
  • the temperature difference between a central portion and a surface of the slab or bar indicates a difference between a temperature of a surface of the slab or bar measured immediately before rough rolling, and a temperature of a central portion, calculated in consideration of a cooling condition and a thickness of the slab or bar immediately before rough rolling.
  • Measuring of a temperature of a surface and a thickness of the slab is performed before first rough rolling, and measuring of a temperature of a surface and a thickness of the bar is performed before rough rolling, starting from a second process of rough rolling.
  • a temperature difference between a central portion and a surface of the slab or bar indicates that a temperature difference, in which a temperature difference for each pass of rough rolling is measured and a total average value is calculated, is 100°C to 300°C.
  • a reduction ratio per pass is 5% or more, and a total cumulative reduction ratio is preferably 40% or more.
  • a reduction ratio per pass of rough rolling is lowered, sufficient deformation is not transferred to a central portion, so toughness degradation caused by coarsening of a central portion may occur.
  • a reduction ratio per pass of three final passes is preferably limited to 5% or more.
  • a total cumulative reduction ratio during rough rolling is preferably set to be 40% or more.
  • the bar having been rough rolled is finish rolled at 850°C to Ar 3 (a ferrite transformation start temperature), so a steel sheet is obtained.
  • a finish rolling temperature of finish rolling is 850°C or less.
  • a crystal grain size of a central portion of a bar before finish rolling after the rough rolling is 200 ⁇ m or less, preferably 150 ⁇ m or less, and more preferably 100 ⁇ m or less.
  • the crystal grain size of a central portion of a bar before finish rolling after the rough rolling may be controlled according to a rough rolling condition, or the like.
  • a reduction ratio during the finish rolling may be set such that a ratio of a slab thickness (mm)/a steel sheet thickness (mm) after finish rolling is 3.5 or above, preferably 3.8 or more.
  • a steel sheet After finish rolling, a steel sheet has a thickness of 50 mm or more, preferably has a thickness of 50 mm to 100 mm, and more preferably has a thickness 80 mm to 100 mm.
  • a steel sheet After finish rolling, a steel sheet is cooled to 700°C or less When a cooling end temperature exceeds 700°C, a microstructure is not properly formed, so yield strength may be 390 Mpa or less.
  • Cooling of the steel sheet is performed at a cooling rate of a central portion of the steel sheet in a thickness direction of 2°C/s or more.
  • a cooling rate of a central portion of the steel sheet in a thickness direction is less than 2°C/s, a microstructure is not properly formed, so yield strength may be 390 Mpa or less.
  • cooling of the steel sheet may be performed at an average cooling rate from 3°C/s to 300°C/s.
  • the average temperature difference between a central portion and a surface during rough rolling of Table 2 refers to a difference between a temperature of a surface of a slab or bar measured immediately before rough rolling, and a temperature of a central portion calculated in consideration of an amount of water sprayed to a bar and a slab thickness immediately before rough rolling, and is a result in which a temperature difference for each pass of rough rolling is measured and a total average value is calculated.
  • a thickness of the bar having been rough rolled was 180 mm, and a crystal grain size before finish rolling after rough rolling was 80 ⁇ m.
  • finish rolling was performed at a finish rolling temperature of 770°C, so a steel sheet having a thickness of Table 2 was obtained, and cooling was performed to a temperature of 700°C or less at a cooling rate of 5°C/sec thereafter.
  • a microstructure, yield strength, an average grain size of a central portion measured in EBSD, an area ratio of a (100) plane forming an angle within 15 degrees with respect to a plane perpendicular to a rolling direction in a region of the high-strength steel in a range of 20% of an overall steel thickness based on a position equal to 1/2 of the steel thickness, a Kca value (a brittle crack arrestability coefficient) were investigated, and a result thereof is illustrated in Table 2.
  • a Kca value of Table 2 is a value evaluated by performing an ESSO test with respect to a steel sheet.
  • Steel Grade Steel Composition (Weight%) C Si Mn Ni Cu Ti Nb P (ppm) S (ppm) Cu/Ni weight ratio Inventive steel 1 0.061 0.23 1.25 0.89 0.35 0.015 0.019 75 16 0.39 Inventive steel 2 0.082 0.31 1.36 0.95 0.44 0.016 0.017 77 25 0.46 Inventive steel 3 0.054 0.32 1.09 1.26 0.36 0.009 0.023 82 34 0.29 Inventive steel 4 0.072 0.22 1.39 1.13 0.21 0.024 0.012 65 19 0.19 Inventive steel 5 0.069 0.29 1.17 1.21 0.45 0.02 0.019 68 22 0.36 Inventive steel 6 0.091 0.31 0.97 1.42 0.51 0.019 0.028 71 31 0.36 Comparative steel 1 0.072 0.25 1.21 0.97 0.36 0.017 0.026 69 16 0.37 Comparative steel 2 0.
  • an average temperature difference between a central portion and a surface during rough rolling presented in the present disclosure is controlled to be less than 70°C.
  • a grain size of a central portion is 35.4 pm
  • an area ratio of a (100) plane forming an angle within 15 degrees with respect to a plane perpendicular to a rolling direction in a region of the high-strength steel in a range of 20% of an overall steel thickness based on a position equal to 1/2 of the steel thickness is 40% or more
  • a Kca value measured at -10°C does not exceed 6000, required for steel for shipbuilding according to the related art.
  • the content of C has a value higher than an upper limit of the content of C according to the present disclosure.
  • a grain size of austenite of a central portion is refined, but upper bainite is generated.
  • a grain size of a final microstructure is 38.3 pm
  • an area ratio of a (100) plane forming an angle within 15 degrees with respect to a plane perpendicular to a rolling direction in a region of the high-strength steel in a range of 20% of an overall steel thickness based on a position equal to 1/2 of the steel thickness is 40% or more.
  • upper bainite in which brittleness may easily occur is included as a base structure, so a Kca value is a value of 6000 or less at -10°C.
  • the content of Si has a value higher than an upper limit of the content of Si according to the present disclosure.
  • a grain size of austenite of a central portion is refined, but upper bainite is partially generated in a central portion.
  • a Kca value is a value of 6000 or less at -10°C.
  • the content of Mn has a value higher than an upper limit of the content of Mn according to the present disclosure.
  • a microstructure of a base material is upper bainite.
  • an area ratio of a (100) plane forming an angle within 15 degrees with respect to a plane perpendicular to a rolling direction in a region of the high-strength steel in a range of 20% of an overall steel thickness based on a position equal to 1/2 of the steel thickness is 40% or more
  • a Kca value is a value of 6000 or less at -10°C.
  • the content of Ni has a value higher than an upper limit of the content of Ni according to the present disclosure. Due to high hardenability, a microstructure of a base material is granular bainite and upper bainite. When rough rolling is performed, through cooling, a grain size of austenite of a central portion is refined, but a grain size of a final microstructure is 31.2 ⁇ m, and a Kca value is a value of 6000 or less at -10°C.
  • a Kca value is a value of 6000 or less at -10°C.
  • Inventive steel 1 through 6 satisfying a composition range according to the present disclosure and in which a grain size of austenite of a central portion is refined through cooling during rough rolling, yield strength satisfies 390 MPa or more, and a grain size of a central portion satisfies 30 ⁇ m or less.
  • a complex-phase structure of ferrite and pearlite, a single-phase structure of acicular ferrite, or a complex-phase structure of acicular ferrite and granular bainite is included as a microstructure.
  • an area ratio of a (100) plane forming an angle within 15 degrees with respect to a plane perpendicular to a rolling direction in a region of the high-strength steel in a range of 20% of an overall steel thickness based on a position equal to 1/2 of the steel thickness is 40% or less, and a Kca value satisfies a value of 6000 or more at -10°C.
  • FIG. 1 is an image of a central portion of Inventive steel 1 in a thickness direction captured with an optical microscope. As illustrated in FIG. 1 , it is confirmed that a structure of a central portion in a thickness direction is refined.

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Claims (7)

  1. Tôle d'acier à haute résistance présentant une excellente capacité d'arrêt de fissures cassantes, constituée par :
    0,05 % en poids à 0,1 % en poids de carbone (C), 0,9 % en poids à 1,5 % en poids de manganèse (Mn), 0,8 % en poids à 1,5 % en poids de nickel (Ni), 0,005 % en poids à 0,1 % en poids de niobium (Nb), 0,005 % en poids à 0,1 % en poids de titane (Ti), 0,1 % en poids à 0,6 % en poids de cuivre (Cu), 0,1 % en poids à 0,4 % en poids de silicium (Si), 100 ppm ou moins de phosphore (P), 40 ppm ou moins de soufre (S), et le reste étant du fer (Fe) et d'autres impuretés inévitablement présentes, l'acier à haute résistance présentant une microstructure étant une structure choisie dans le groupe constitué par une structure monophasée de ferrite, une structure monophasée de bainite, une structure à phase complexe de ferrite et de bainite, une structure à phase complexe de ferrite et de perlite et une structure à phase complexe de ferrite, de bainite et de perlite, et présentant une épaisseur de 50 mm ou plus,
    les teneurs en Cu et Ni étant fixées de sorte qu'un rapport pondéral de Cu/Ni est de 0,6 ou moins,
    la ferrite étant de la ferrite aciculaire ou de la ferrite polygonale et la bainite étant une bainite granulaire,
    dans la tôle d'acier à haute résistance, une granulométrie ayant une limite d'angle élevé de 15 degrés ou plus, mesurée selon une méthode de diffraction d'électrons rétrodiffusés (EBSD), sur une partie centrale d'une épaisseur d'acier, étant de 30 µm ou moins,
    un rapport de surface (100) d'un plan formant un angle à moins de 15 degrés par rapport à un plan perpendiculaire à une direction de laminage dans une région de la tôle d'acier à haute résistance dans une plage de 20 % d'une épaisseur totale sur la base d'une position égale à 1/2 de l'épaisseur d'acier étant de 40 % ou moins, et
    la limite d'élasticité de l'acier à haut résistance étant de 390 MPa ou plus.
  2. Tôle d'acier à haute résistance présentant une excellente capacité d'arrêt de fissures cassantes selon la revendication 1, dans laquelle lorsque la microstructure de l'acier est une structure à phase complexe comportant de la perlite, une fraction de la perlite est de 20 % ou moins.
  3. Tôle d'acier à haute résistance présentant une excellente capacité d'arrêt de fissures cassantes selon la revendication 1, dans laquelle une épaisseur d'acier est de 80 mm à 100 mm.
  4. Procédé de fabrication d'une tôle d'acier à haute résistance présentant une excellente capacité d'arrêt de fissures cassantes selon la revendication 1, le procédé comprenant :
    le réchauffage d'une brame à une température comprise entre 950 °C et 1 100 °C, la brame comprenant 0,05 % en poids à 0,1 % en poids de carbone (C), 0,9 % en poids à 1,5 % en poids de manganèse (Mn), 0,8 % en poids à 1,5 % en poids de nickel (Ni), 0,005 % en poids à 0,1 % en poids de niobium (Nb), 0,005 % en poids à 0,1 % en poids de titane (Ti), 0,1 % en poids à 0,6 % en poids de cuivre (Cu), 0,1 % en poids à 0,4 % en poids de silicium (Si), 100 ppm de phosphore ou moins (P), 40 ppm de soufre ou moins (S) et le reste étant du fer (Fe) et d'autres impuretés inévitablement présentes, puis le laminage brut de la brame à une température comprise entre 1 100 °C et 900 °C ;
    l'obtention d'une tôle d'acier présentant une épaisseur de 50 mm ou plus par laminage de finition d'une barre laminée de façon brute à une température de 850 °C à Ar3; et
    le refroidissement de la tôle d'acier à une température de 700 °C ou moins, dans lequel les teneurs en Cu et Ni sont définies de sorte qu'un rapport pondéral de Cu/Ni est de 0,6 ou moins, le laminage brut étant effectué en deux passages ou plus, la surface extérieure de la brame ou de la barre maintenant une température inférieure à celle d'une partie centrale dans le sens de l'épaisseur avant chaque passage du laminage brut, une valeur moyenne des différences de température entre la partie centrale dans une direction d'épaisseur de la brame ou de la barre et une surface extérieure de la brame ou de la barre immédiatement avant chaque passage du laminage brut est comprise entre 100 °C et 300 °C, les différences de température étant des différences entre une température d'une surface extérieure de la brame ou de la barre mesurée immédiatement avant chaque passage du laminage brut, et une température d'une partie centrale calculée en tenant compte d'une condition de refroidissement et d'une épaisseur de la brame ou de la barre immédiatement avant chaque passage du laminage brut, et une granulométrie cristalline d'une partie centrale de la barre avant le laminage de finition après le laminage brut étant de 200 µm ou moins, et le refroidissement de la tôle étant effectué à une vitesse de refroidissement d'une partie centrale de la tôle d'acier de 2 °C/s ou plus.
  5. Procédé de fabrication d'une tôle d'acier à haute résistance présentant une excellente capacité d'arrêt de fissures cassantes selon la revendication 4, dans lequel un rapport de réduction par passage, par rapport à trois passages finaux lorsque le laminage brut est effectué, est de 5 % ou plus, et un rapport de réduction cumulative totale est de 40 % ou plus.
  6. Procédé de fabrication d'une tôle d'acier à haute résistance présentant une excellente capacité d'arrêt de fissures cassantes selon la revendication 4, dans lequel un rapport de réduction pendant le laminage de finition est établi de sorte qu'un rapport d'une épaisseur de brame en mm/d'une épaisseur de tôle d'acier en mm après le laminage de finition est de 3,5 ou plus.
  7. Procédé de fabrication d'une tôle d'acier à haute résistance présentant une excellente capacité d'arrêt de fissures cassantes selon la revendication 4, dans lequel le refroidissement de la tôle d'acier est effectué à une vitesse de refroidissement moyenne de 3 °C/s à 300 °C/s.
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US10822671B2 (en) 2014-12-24 2020-11-03 Posco High-strength steel having superior brittle crack arrestability, and production method therefor
JP6536514B2 (ja) * 2016-08-09 2019-07-03 Ｊｆｅスチール株式会社 脆性亀裂伝播停止特性に優れた構造用高強度厚鋼板およびその製造方法
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JP6788589B2 (ja) 2020-11-25
CN107109597A (zh) 2017-08-29
CN107109597B (zh) 2020-01-31
KR101746999B1 (ko) 2017-06-15
WO2016105062A8 (fr) 2016-11-24
EP3239331A4 (fr) 2017-11-08
JP2018503744A (ja) 2018-02-08
KR20160078926A (ko) 2016-07-05
US20170327922A1 (en) 2017-11-16
EP3239331A1 (fr) 2017-11-01
WO2016105062A1 (fr) 2016-06-30

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