Classifications

• • 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
• • 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
  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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

Definitions

• the present invention relates to a structural steel sheet suitable for ships or steel structures and, more particularly, to a high-strength steel sheet having excellent ductility and low-temperature toughness and a method for manufacturing the same.
• a ship, a steel structure, or the like may experience accidents such as flooding or sinking as a steel plate is fractured by external impacts such as a collision.
• cracks may occur due to forming processes during the manufacture of the ship or steel structure. In this case, there may be a problem such as the increase in a construction period or manufacturing costs.
• Patent Document 1 discloses a steel plate having excellent collision absorption property while having a tensile strength of 490 MPa or more and a uniform elongation of 15% or more by controlling an average grain diameter of ferrite as a main phase between 3 and 12 ⁇ m and making the ferrite fraction 90 % or more while refining an average equivalent circle diameter of a second phase to 0.8 ⁇ m or less.
• Patent Document 2 discloses a steel sheet having a microstructure made of ferrite and a hard second phase, a volume fraction of the ferrite of 75% or more over the entire sheet thickness, a hardness of Hv 140 or more and 160 or less, and an average crystal grain size of 2 ⁇ m or more by applying a process including front ooling, air cooling, and rear ooling after rolling.
• Patent Document 3 discloses a thick steel plate in which a microstructure is mainly composed of ferrite and pearlite in order to increase energy absorption capability during a collision, and an average dislocation density of the ferrite is lowered to a certain level or less while a hardness, a fraction, an average area, and an average circumferential length of the phase satisfying certain conditions. Further, in order to obtain the above-described thick steel plate, a process of heating a steel material to the temperature higher than a normal reheating temperature, and then performing controlled rolling on the steel material and air cooling or weak water cooling on the rolled steel material is disclosed.
• EP 2 752 499 A1 and EP 2 940 172 A1 disclose steel sheet with low temperature toughness.
• Patent Document 1 discloses only uniform elongation, and but does not substantially disclose the effect of suppressing defects such as the fracture due to external impacts or the like.
• Patent Document 2 also discloses only the uniform elongation, and therefore, the total elongation or the like of the steel plate disclosed in Patent Document 2 is unclear.
• Patent Document 3 discloses the total elongation, but does not disclose securing the toughness at all, which is a very important property of the structural steel sheet.
• An aspect of the present invention is to provide a high-strength steel sheet having excellent ductility and low-temperature toughness and a method for manufacturing the same in providing a steel sheet suitable for a structural use .
• the steel sheet of the present invention has an effect that may be advantageously applied as a structural steel sheet.
• the ductility of the steel is relatively reduced. Accordingly, it is not easy to manufacture steel having high strength and excellent elongation.
• the high elongation of steel does not necessarily mean that the steel has excellent low-temperature toughness, so it is more difficult to secure excellent low-temperature toughness as well as high strength and high ductility.
• the content of each element is based on a weight, and the fraction of a microstructure is based on an area.
• Carbon (C) 0.05 to 0.12%
• Carbon (C) is an element that affects the fraction of pearlite in a steel microstructure, and is advantageous in securing strength.
• the carbon (C) may be contained in an amount of 0.05% or more.
• C is added in an amount of 0.05% or more.
• the content exceeds 0.12%, the fraction of the pearlite in the steel microstructure becomes excessive, so low-temperature toughness decreases.
• C is contained in an amount of 0.05 to 0.12%, and advantageously, may be contained in an amount of 0.06 to 0.10%.
• Silicon (Si) is an element that helps deoxidation of steel, increases hardenability, and iscontained in an amount of 0.2% or more in order to secure a target level of strength. However, when the content exceeds 0.5%, there is a problem that the strength is excessively increased, thereby impairing total elongation and low-temperature impact toughness.
• Si is contained in an amount of 0.2 to 0.5%.
• Manganese (Mn) is an element that is useful for increasing the strength without significantly reducing the elongation of the steel. In order to secure the target level of strength in the present disclosure, Mn is contained in an amount of 1.2% or more, but when the content exceeds 1.8%, the strength of the steel increases significantly, thereby making it difficult to secure ductility.
• Mn is contained in an amount of 1.2 to 1.8%, and advantageously, may be contained in an amount of 1.4 to 1.7%.
• Phosphorus (P) is an impurity that is inevitably mixed in steel, and needs to be minimized because the phosphorus (P) reduces the ductility and low-temperature impact toughness of the steel.
• P Phosphorus
• an upper limit of P is limited to 0.012%.
• 0% may be excluded in consideration of a load during a process of manufacturing steel.
• S Sulfur
• S is an impurity that is inevitably mixed in steel, such as P, and is necessary to minimize its content since the sulfur (S) forms sulfides and significantly reduces ductility.
• an upper limit of S may be limited to 0.005%.
• 0% may be excluded in consideration of a load during the process of manufacturing steel.
• Aluminum (Al) is an essential element for deoxidation of steel, and is contained in an amount of 0.01% or more in order to secure cleanliness of the steel. However, when the content is excessive, since the toughness of a welded joint may be impaired, the content is limited to 0.06% or less in consideration of the impairment of the toughness.
• Titanium (Ti) is an element useful for refining grains of ferrite during austenite-ferrite transformation by suppressing excessive growth of austenite during a heating process in the process of manufacturing steel.
• Ti is contained in an amount of 0.005% or more, but when the content exceeds 0.02%, coarse nitrides are formed, thereby reducing the effect of grain refinement and deteriorating impact toughness.
• Ti is contained in an amount of 0.005 to 0.02%.
• Niobium is effective in refining grains of austenite by being precipitated as carbonitride during a rolling process in the process of manufacturing steel, and contributes to the improvement in the strength.
• Nb is added in an amount of 0.01% or more, but when the content exceeds 0.03%, the strength excessively increases, thereby making it difficult to secure the ductility and impairing the toughness of a welded joint.
• Nb is contained in an amount of 0.01 to 0.03%.
• N Nitrogen
• Nb is advantageous in obtaining an effect of suppressing the growth of the grains of the austenite during the heating of the steel by being combined with the Ti, Nb, or the like and refining grains by forming fine carbonitrides during the rolling.
• N is added in an amount of 0.002% or more, but when the content exceeds 0.006%, the surface quality of steel cast and sheet may be deteriorated.
• N is contained in an amount of 0.002 to 0.006%.
• Nickel (Ni) is an element that does not significantly impair the elongation while improving strength by refining grains of ferrite, similar to the Mn. By adding such Ni in a certain amount, the strength, ductility, and low-temperature toughness targeted in the present disclosure may be more advantageously secured. However, when the content exceeds 0.5%, the elongation decreases and the manufacturing cost increases, so Ni is contained in an amount of 0.5% or less.
• Ni may be 0%.
• the remaining component of the present invention is iron (Fe).
• Fe iron
• unintended impurities may inevitably be mixed from a raw material or the surrounding environment, and thus, these impurities may not be excluded. Since these impurities are known to anyone with ordinary skill in the manufacturing process, all the contents are not specifically mentioned in the present specification.
• the present invention forms a ferrite-pearlite microstructure of the steel sheet in order to secure a balance between the strength and ductility of the steel sheet, and secure the intended physical properties by minimizing the fraction of the bainite which may be partially contained during the process of manufacturing a steel sheet.
• the pearlite is contained in an area fraction of 5 to 25%, and the bainite is contained in an area fraction of 2% or less (including 0%) .
• the fraction of the pearlite is less than 5%, it is difficult to secure the target level of strength, and when the fraction exceeds 25%, the elongation decreases and the target level of toughness may not be achieved.
• the fraction of the bainite exceeds 2%, the post elongation is lowered, and thus it is difficult to secure the target level of total elongation in the present disclosure.
• the relationship between the average grain size and elongation of the polygonal ferrite is not linear, and when the average grain size of the polygonal ferrite is smaller than 2 ⁇ m, the elongation tends to decrease rapidly.
• the average grain size of the polygonal ferrite by controlling the average grain size of the polygonal ferrite to 2 to 8 ⁇ m, it is possible to secure the balance between the strength and ductility from appropriate refinement.
• the average grain size of the polygonal ferrite is less than 2 ⁇ m, the uniform elongation is significantly reduced, thereby making it difficult to secure the total elongation.
• the size exceeds 8 ⁇ m, the fraction of the pearlite should be increased to secure the target level of strength, but the low-temperature impact toughness is deteriorated.
• the steel sheet of the present invention having a microstructure as described above has a yield strength of 355 MPa or more, a tensile strength of 490 MPa or more, an elongation of 30% or more, and an impact toughness of 100 J or more at -40°C, and therefore, may secure the low-temperature toughness as well as the strength and ductility.
• the steel sheet of the present invention has a thickness of 8 to 15mm.
• the high-strength steel sheet according to the present invention is manufactured through a series of processes of [heating-hot rolling-cooling] a steel slab that satisfies the alloy compositions proposed in the present invention.
• the steel slab is subjected to the heating to homogenizing followed by the hot rolling.
• the heating process is preferably performed at 1100 to 1200°C.
• the heating temperature is less than 1100°C, the steel slab is not sufficiently uniform, and Nb carbonitride or the like present in the center of the thickness of the steel slab is not sufficiently dissolved, thereby making it difficult to secure the target level of strength.
• the temperature exceeds 1200°C, the elongation and low-temperature toughness are degraded due to the abnormal grain growth of the grains of the austenite, which is not preferable.
• the heating time may be set differently according to the thickness of the steel slab, and it is preferable to set it so that the steel slab may be sufficiently uniform from the surface to the center of the thickness of the steel slab.
• heating may be performed for 1 minute or more per 1 mm of the thickness of the steel slab.
• the hot-rolled steel plate is manufactured by hot rolling the heated steel slab according to the above.
• the two-step rolling may be performed.
• the rough rolling is performed in the first rolling, which may be performed immediately after the extraction of the heated steel slab from the heating furnace.
• the rough rolling may include broadside rolling to secure the width of the final steel plate, and the rolling may be carried out up to the thickness at which the finish rolling, which is the subsequent second rolling, begins.
• the finish rolling is performed as the second rolling, and the rolling is performed so as to have an intended thickness.
• the finish rolling is performed in a temperature range of Ar3 + 70°C to Ar3 + 170°C.
• the lower the temperature during the finish rolling the smaller the grain size of the ferrite in the final microstructure, so that the strength and low-temperature toughness may be improved and the elongation may be reduced.
• the finish rolling needs to be performed in an appropriate temperature range.
• the temperature range may be very narrow, in this case, there is a problem that it is difficult to industrially manufacture the steel sheet.
• the present inventors have deeply studied the relationship between the alloy compositions and the manufacturing process, the present inventors found that it is possible to expand the temperature range advantageous for securing the intended physical properties during the finish rolling by appropriately adding Mn or Mn and Ni in the alloy compositions.
• the Mn and Ni lower the ferrite transformation temperature to induce the ferrite grain refinement, thereby improving the strength and low-temperature toughness and not significantly impairing the elongation.
• the steel sheet having excellent strength and ductility as well as low-temperature toughness may be obtained.
• Ar3 may be represented by the following formula.
• Ar3 910 ⁇ 310 C ⁇ 80 Mn ⁇ 20 Cu ⁇ 55 Ni ⁇ 15 Cr ⁇ 80 Mo each element is represented by weight percent
• the finish rolling such that the cumulative reduction ratio is 60 to 90% during the finish rolling in the above-described temperature range.
• the cumulative reduction ratio during the finish rolling is less than 60%, the average grain size of the ferrite becomes coarse, and thus, it is difficult to secure the strength of the target level, whereas when the cumulative reduction ratio exceeds 90%, the average grain size of the ferrite becomes too fine, and thus, it is advantageous for securing strength but the elongation is deteriorated.
• the hot-rolled steel plate manufactured by performing the hot rolling is cooled.
• cooling is performed to room temperature through air cooling, which means cooling in the atmosphere.
• the steel sheet of the present invention manufactured through the series of manufacturing processes described above has a thickness of 8 to 15 mm, and the microstructure intended in the present invention may be uniformly formed, regardless of any thickness within the thickness range.
• the steel slab having a thickness of 250 mm was obtained by a continuous casting method. Thereafter, a steel plate having a thickness of 8 to 15 mm was manufactured through heating, rolling, and cooling under the conditions shown in Table 2 below. When it comes to cooling, air and water cooling were applied, and in the case of the water cooling, the cooling was performed at a cooling rate of about 20°C/s, the water cooling was terminated at 650°C, and then the air cooling was performed to room temperature.
• Table 1 Steel Alloy Composition (wt%) Ar3 No.
• the tensile specimen was machined into a proportional specimen with a gauge length of 5.65 ⁇ V (specimen width ⁇ specimen thickness) by setting a specimen width to 25 mm and setting the thickness of the specimen to the thickness of the steel plate such that the specimen length was perpendicular to the rolling direction of the steel sheet, and the yield strength (YS), tensile strength (TS), and total elongation (E1) values were measured through a room temperature tensile test.
• YS yield strength
• TS tensile strength
• E1 total elongation
• the impact specimen was machined into an ASTM E 23 Type A standard specimen (however, a steel plate with a thickness of 8 mm was machinedinto subsize specimens (10 mm ⁇ 7.5 mm)) such that the length of the specimen was perpendicular to the rolling direction of the steel plate, and then subjected to an impact test at -40°C, which was represented as the average of the energy values measured from three specimens.
• Comparative Example 1 in which the content of C in the alloy compositions was excessive and the temperature when heating the slab was too high, the fraction of the pearlite was high, and the average grain size of the ferrite was coarse, so the elongation and impact energy value were inferior.
• Comparative Example 2 in which the content of C in the alloy compositions was insufficient was not able to secure the target level of strength due to the low fraction of pearlite.
• Comparative Example 6 the ferrite particle diameter was too small, so the strength was high, but the ductility was inferior.
• Comparative Example 7 the ferrite particle diameter was too large, so the strength did not reach the target level.
• Comparative Example 8 the thickness of the final steel plate was 23 mm, and the air cooling was applied after the hot rolling, but the air cooling rate was relatively slow, so that the strength of the target level could not be secured.

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• 2018
  • 2018-11-29 KR KR1020180150707A patent/KR102164112B1/ko active IP Right Grant
• 2019
  • 2019-11-29 US US17/297,740 patent/US20220042132A1/en active Pending
  • 2019-11-29 CN CN201980077747.8A patent/CN113166885B/zh active Active
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