US20170327922A1 - High-strength steel having superior brittle crack arrestability, and production method therefor - Google Patents

High-strength steel having superior brittle crack arrestability, and production method therefor Download PDF

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US20170327922A1
US20170327922A1 US15/535,582 US201515535582A US2017327922A1 US 20170327922 A1 US20170327922 A1 US 20170327922A1 US 201515535582 A US201515535582 A US 201515535582A US 2017327922 A1 US2017327922 A1 US 2017327922A1
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steel
strength steel
brittle crack
central portion
crack arrestability
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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 OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/02—Manufacture of metal sheets, bars, wire, tubes 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling

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.
  • 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 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 including one structure selected from the group consisting of a single-phase structure of ferrite,
  • the contents of Cu and Ni may be set such that a Cu/Ni weight ratio may be 0.6 or less, in detail, 0.5 or less.
  • a grain size having a high-angle boundary of 15 degrees or more, measured in an EBSD method, in a central portion of the steel may be 30 ⁇ m (micrometers) or less.
  • 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 may be 40% or less.
  • yield strength may be 390 MPa or more.
  • a method of manufacturing a high-strength steel having excellent brittle crack arrestability includes: reheating a slab at 950° C. to 1100° C., the slab including 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 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, preferably.
  • a crystal grain size of a central portion of the bar in a thickness direction before the finish rolling after the rough rolling may be 200 ⁇ m or less, preferably 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 may be 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 may be obtained.
  • FIG. 1 is an image of a central portion of Inventive steel 1 in a thickness direction, captured with an optical microscope.
  • 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 steel composition is appropriately controlled.
  • the content of each of Mn, Ni, Cu, and Si is optimized.
  • a steel composition is appropriately controlled.
  • the content of each of Mn, Ni, and Cu is optimized.
  • a structure of a steel is refined.
  • a structure of a central portion of a steel is refined.
  • the structure of a central portion of a steel is refined, 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.
  • a texture of an area of a central portion in which a microstructure is relatively coarse in comparison with a surface is controlled.
  • the texture of a steel in detail, the texture of an area of a central portion of a steel, is controlled. Even when a crack is generated, propagation of the crack is significantly reduced, so brittle crack arrestability is improved.
  • 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 to a central portion of a steel.
  • 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 preferably 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 preferably limited to 0.05% to 0.10%, more preferably limited to 0.061% to 0.091%, and most 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 preferably added in an amount of 0.9% or more.
  • the content of Mn is preferably limited to 0.9% to 1.5%, more preferably limited to 0.97% to 1.39%, and most 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 preferably added in an amount of 0.8% or more.
  • an upper limit of the content of Ni is preferably limited to 1.5%.
  • the content of Ni is more preferably limited to 0.89% to 1.42% and most 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 preferably added in an amount of 0.005% or more.
  • an upper limit of the content of Nb is preferably limited to 0.1%.
  • the content of Nb is more preferably limited to 0.012% to 0.028% and most 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 preferably added in an amount of 0.005% or more.
  • the content of Ti is preferably limited to 0.005% to 0.1%.
  • the content of Ti is more preferably limited to 0.009% to 0.024% and most preferably limited to 0.011% to 0.018%.
  • Si Si (Silicon): 0.1% to 0.4%
  • Si improves strength of a steel and has a strong deoxidizing effect, and thus is an essential element in producing clean steel.
  • Si is preferably added in an amount of 0.1% or more.
  • an upper limit of the content of Si is preferably limited to 0.4%.
  • the content of Si is more preferably limited to 0.22% to 0.32% and most 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.
  • Cu is preferably added in an amount of 0.1% or more.
  • an upper limit of the content of Cu is preferably limited to 0.6%.
  • the content of Cu is more preferably limited to 0.21% to 0.51% and most 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) may be 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 may have 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 preferably 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 may be preferably 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 may be 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
  • an area ratio of a (100) plane forming an angle within 15 degrees with respect to a plane perpendicular to a rolling direction is significantly reduced.
  • 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 preferably 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 having excellent brittle crack arrestability includes: reheating a slab including 0.05 wt % to 0.1 wt % of C, 0.9 wt % to 1.5 wt % of Mn, 0.8 wt % to 1.5 wt % of Ni, 0.005 wt % to 0.1 wt % of Nb, 0.005 wt % to 0.1 wt % of Ti, 0.1 wt % to 0.6 wt % of Cu, 0.1 wt % to 0.4 wt % of Si, 100 ppm or less of P, 40 ppm or less of S, and the remainder being iron (Fe) and other inevitably contained impurities at 950° C.
  • a temperature difference between a central portion and a surface of the slab or bar before the rough rolling is 70° C. or more.
  • a slab is reheated before rough rolling.
  • a reheating temperature of a slab is preferably set to be 950° C. or more, to dissolve carbonitride of Ti and/or Nb formed during casting.
  • it is more preferably to heat at 1000° C. or more.
  • austenite may be coarsened.
  • an upper limit of the reheating temperature is preferably limited to 1100° C.
  • the slab, having been reheated, is rough rolled.
  • a rough rolling temperature is preferably 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 preferably 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 70° C. or more.
  • 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, preferably, an average grain size of a central portion may be 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 preferably 100° C. or more and more preferably 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.
  • 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 70° C. or more.
  • 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 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 preferably 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.
  • 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.
  • 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.
  • 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 Kca value of Table 2 is a value evaluated by performing an ESSO test with respect to a steel sheet.
  • 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 ⁇ m
  • 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.
  • 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.

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US10883159B2 (en) 2014-12-24 2021-01-05 Posco High-strength steel having superior brittle crack arrestability, and production method therefor
EP3901309A4 (en) * 2018-12-19 2022-03-09 Posco ULTRA-THICK STRUCTURAL STEEL HAVING EXCELLENT RESISTANCE TO INITIAL BRITTLE CRACKS AND METHOD OF MANUFACTURING IT
US11634784B2 (en) 2016-12-22 2023-04-25 Posco Co., Ltd Ultra-thick steel material having excellent surface part NRL-DWT properties and method for manufacturing same

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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
US10883159B2 (en) 2014-12-24 2021-01-05 Posco High-strength steel having superior brittle crack arrestability, and production method therefor
US11634784B2 (en) 2016-12-22 2023-04-25 Posco Co., Ltd Ultra-thick steel material having excellent surface part NRL-DWT properties and method for manufacturing same
EP3901309A4 (en) * 2018-12-19 2022-03-09 Posco ULTRA-THICK STRUCTURAL STEEL HAVING EXCELLENT RESISTANCE TO INITIAL BRITTLE CRACKS AND METHOD OF MANUFACTURING IT

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JP6788589B2 (ja) 2020-11-25
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JP2018503744A (ja) 2018-02-08
KR20160078926A (ko) 2016-07-05
EP3239331A1 (en) 2017-11-01
CN107109597A (zh) 2017-08-29
CN107109597B (zh) 2020-01-31
EP3239331A4 (en) 2017-11-08
WO2016105062A1 (ko) 2016-06-30

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