EP3730644A1 - Hochfester stahl mit ausgezeichneter zähigkeit von durch schweissen wärmebeaufschlagten zonen und verfahren zu seiner herstellung - Google Patents

Hochfester stahl mit ausgezeichneter zähigkeit von durch schweissen wärmebeaufschlagten zonen und verfahren zu seiner herstellung Download PDF

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EP3730644A1
EP3730644A1 EP18891918.7A EP18891918A EP3730644A1 EP 3730644 A1 EP3730644 A1 EP 3730644A1 EP 18891918 A EP18891918 A EP 18891918A EP 3730644 A1 EP3730644 A1 EP 3730644A1
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steel
heat
affected zone
excellent toughness
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French (fr)
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EP3730644A4 (de
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Jae-Yong CHAE
Sang-Deok Kang
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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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • C21D8/0221—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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0062—Heat-treating apparatus with a cooling or quenching zone
    • 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
    • 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
    • C22C38/12—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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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 disclosure relates to a structural steel used as a material for storage tanks, pressure vessels, building structures, ship structures, or the like, and more particularly, to a high strength steel having excellent toughness in a heat-affected zone and a method of manufacturing the same.
  • Patent Document 1 is a representative technique using a precipitate of TiN and relates to a structural steel material having an impact toughness of about 200J at 0°C (about 300J in a base material) when a heat input of 100J/cm (a highest heating temperature of 1400°C) is applied.
  • Ti/N is practically managed to be 4-12, and thus, TiN precipitates of 0.05 ⁇ m or less are 5.8 pr 3 pieces/mm 2 to 8.1 ⁇ 10 4 pieces/mm 2 , and in addition, TiN precipitates of 0.03 to 0.2 ⁇ m are 3.9 ⁇ 10 3 pieces/mm 2 to 6.2 ⁇ 10 4 pieces/mm 2 , in refining ferrite to secure toughness of a weld portion.
  • Patent Document 1 a problem in that cracks may be severely generated on the slab surface during continuous casting by forming excessive carbon and nitride, is caused.
  • a thick plate product is produced using the slabs having a large number of surface cracks as above, there is also a problem in which cracks or the like also occur in the surface of the final product. Therefore, there is a great possibility that problems such as surface repair or the like may occur, or defective products incapable of being repaired may be manufactured.
  • Patent Document 1 Japanese Patent Laid-open Publication No. 1999-140582
  • An aspect of the present disclosure is to provide a steel material capable of securing an excellent heat-affected zone (HAZ) while having excellent strength and toughness of a base material even after welding and a stress relief heat treatment, and a method of manufacturing the same.
  • HZ heat-affected zone
  • a high-strength steel having excellent toughness in a heat-affected zone includes:
  • a method of manufacturing a high-strength steel having excellent toughness in a heat-affected zone includes:
  • a steel material having excellent toughness in a heat-affected zone during large heat input welding without lowering strength and toughness of a base material even after a stress relief heat treatment after welding since the strength of the base material is maintained even when stress annealing is performed, the steel may be suitably used in a storage tank, a pressure vessel, a structure, and the like. In addition, since the steel according to an exemplary embodiment of the present disclosure has no defects such as surface cracking, the steel may be suitably used as a structural steel material.
  • the inventors have studied in depth to fundamentally solve the problem of defects such as cracks on the steel surface when manufacturing thick steel materials for use as existing structural steel materials, and have confirmed that the heat-affected zone having excellent toughness could be secured by controlling the microstructure of the heat-affected zone during welding, as well as securing base material strength and toughness, when optimizing the steel composition and manufacturing conditions, in completing the present disclosure.
  • a steel according to an exemplary embodiment of the present disclosure may have an effect in which the steel may be suitably applied as a structural steel.
  • the steel according to an exemplary embodiment of the present disclosure includes, by weight% (hereinafter, %), carbon (C): 0.16 to 0.20%, manganese (Mn): 1.0 to 1.5%, silicon (Si) : 0.3% or less (excluding 0), aluminum (Al): 0.005 to 0.5%, phosphorus (P) : 0.02% or less, sulfur (S) : 0.01% or less, titanium (Ti) : 0.005 to 0.02%, niobium (Nb) : 0.01 to 0.1%, and nitrogen (N): 0.006 to 0.01%.
  • the steel may include, if necessary, one or more selected from the group consisting of calcium (Ca): 0.006% or less, vanadium (V) : 0.03% or less, nickel (Ni) : 2.0% or less, copper (Cu) : 1.0% or less, chromium (Cr) : 1.0% or less, and molybdenum (Mo): 1.0% or less.
  • C is an element having a greatest influence on the slab solidification behavior, it needs to be contained in the steel within an appropriate range. If the content of C is less than 0.16%, the strength of a solidified layer increases when the phase transformation occurs during slab solidification. Therefore, there is a problem in which the occurrence of cracking on the slab surface may be facilitated by causing shrinkage and forming a non-uniform solidification layer. On the other hand, if the content thereof exceeds 0.20%, the carbon equivalent becomes too large. Therefore, in this case, there is a problem in that the toughness of the weld portion deteriorates as the hardenability of the weld portion is greatly increased. Therefore, in the present disclosure, the content of C may be preferably 0.16 to 0.20%.
  • the Mn is an element useful for securing the strength of the steel sheet by increasing the hardenability of the steel, but in the present disclosure, it is necessary to appropriately limit the content thereof to secure toughness of the heat-affected zone (HAZ).
  • HZ heat-affected zone
  • Mn does not significantly deteriorate the toughness of the heat-affected zone, but tends to be segregated in the center of the thickness of the steel sheet.
  • the Mn segregated portion as described above has a very high Mn content, compared to the average content, and thus, there is a problem of easily generates a brittle structure that greatly harms the toughness of the weld-heat-affected zone.
  • the Si increases the strength of the steel sheet and is an element necessary for deoxidation of molten steel, but Si inhibits the formation of cementite when unstable austenite is decomposed, and thus promotes a martensite austenite constituent (MA) , in which there is a problem in which toughness of the heat-affected zone (HAZ) is significantly lowered.
  • the Si content may be preferably 0.3% or less, and if it exceeds 0.3%, coarse Si oxide is formed, and unpreferably, brittle fracture may occur around such inclusions.
  • the Al is an element capable of deoxidizing molten steel inexpensively, and for this use, it may be preferable to add 0.005% or more. However, if the content exceeds 0.5%, there is a problem of causing nozzle clogging during continuous casting, and the solidified Al may form the martensite austenite constituent in the weld portion and may result in a decrease in toughness of the welding portion.
  • the Al content may be preferably 0.005 to 0.5%.
  • Phosphorus (P) 0.02% or less
  • the P is an element that is advantageous for strength improvement and corrosion resistance, but since it is an element that greatly inhibits impact toughness, the content thereof may be advantageously managed to be as low as possible, and thus, the upper limit may be preferably 0.02%.
  • S is an element that greatly inhibits impact toughness by forming MnS or the like, the content thereof may be advantageously managed to be as low as possible, and thus, it may be preferable to set the upper limit thereof to 0.01%.
  • the Ti is combined with nitrogen (N) to form a fine nitride, thereby reducing grain coarsening that may occur near the welding melting line, to suppress a decrease in toughness.
  • N nitrogen
  • the content of Ti is too low, the number of Ti nitrides is insufficient, and thus, the effect of suppressing coarsening is not sufficiently exhibited, and thus, it may be preferable to include Ti in an amount of 0.005% or more.
  • the upper limit thereof may be preferably 0.02%.
  • the Nb is precipitated in the form of NbC or Nb(C, N) to greatly improve the strength of the base material and the weld portion.
  • solidified Nb during reheating at a high temperature suppresses recrystallization of austenite and transformation of ferrite or bainite, thereby exhibiting an effect that the structure is refined. Therefore, it may be preferable to include 0.01% or more to secure the strength of the base material even after undergoing stress relief heat treatment after welding, such as for a storage container. However, if the content exceeds 0.1% and is excessively added, brittle cracks may appear at the corners of the steel and greatly reduce the toughness of the heat-affected zone, and thus, it may be preferable not to exceed 0.1%.
  • the N is combined with the above-described Ti to form a fine nitride to alleviate grain coarsening that may occur near the weld melting line to prevent toughness from deteriorating.
  • it is necessary to contain N in an amount of 0.006% or more.
  • the content is too excessive, there is a problem of significantly reducing toughness, and thus, it may be preferable not to exceed 0.01%.
  • the steel sheet according to an exemplary embodiment of the present disclosure may further include elements capable of securing advantageous physical properties in the present disclosure.
  • the steel sheet may further include calcium (Ca): 0.006% or less, vanadium (V): 0.03% or less, nickel (Ni): 2.0% or less, copper (Cu): 1.0% or less, chromium (Cr) : 1.0% or less, molybdenum (Mo) : 1.0% or less, and the like, which will be described below in detail.
  • the Ca is mainly used as an element that controls the shape of the MnS inclusion and improves low-temperature toughness.
  • excessive Ca addition causes a large amount of CaO-CaS to form and combine to form coarse inclusions, thus impairing the cleanliness of the steel and spoiling weldability in the field. Therefore, it may be preferable that the Ca is 0.006% or less.
  • the V has a solid-solution temperature lower than other alloying elements, and has an excellent effect of preventing a decrease in strength by depositing in a heat-affected zone (HAZ).
  • HZ heat-affected zone
  • Ni is almost the only element capable of simultaneously improving the strength and toughness of the base material, but since it is an expensive element, exceeding 2.0% is not only very disadvantageous in terms of economy, but also has a problem of deterioration of weldability. Therefore, it may be preferable not to exceed 2.0% when the Ni is added.
  • the Cu is an element capable of improving the strength of steel while significantly reducing a decrease in toughness of the base material. However, if it is added excessively, there is a problem of significantly deteriorating the surface quality of the product, and therefore, it may be preferable to include the copper in an amount of 1.0% or less.
  • the Cr has a great effect on strength improvement by increasing hardenability. However, if it is added excessively, there is a problem that the weldability is greatly deteriorated, and thus, it may be preferable that the content does not exceed 1.0%.
  • the Mo has an effect of inhibiting the formation of a ferrite phase by greatly improving the hardenability even with a relatively small amount, and is an element capable of greatly improving the strength.
  • it if it is added excessively, there is a problem of significantly increasing hardness of the weld portion and inhibiting the toughness, and thus, it may be preferable that the content does not exceed 1.0%.
  • the steel sheet according to an exemplary embodiment of the present disclosure includes an iron (Fe) component in addition to the above-mentioned alloying elements.
  • Fe iron
  • unintended impurities may be inevitably mixed from the raw material or a surrounding environment, and thus cannot be excluded.
  • the steel according to an exemplary embodiment of the present disclosure preferably has a surface crack sensitivity index (Cs) of 0.3 or less, which is defined by the following relational expression 1.
  • Cs 71.4 ⁇ C 2 ⁇ 30.3 ⁇ C + 3.32 , where [C] indicates the weight percent value that is the content of C.
  • C is an element that has a greatest influence on the slab solidification behavior, and if the C content is less than 0.16% in an embodiment of the present disclosure, the surface crack sensitivity index (Cs) of the relational expression 1 exceeds 0.3.
  • the surface crack sensitivity index (Cs) of the relational expression 1 is 0.3 or less.
  • the Cs value of the relational expression 1 is preferably as low as possible, but since C is present in the steel, the Cs value may be preferably greater than 0.
  • the value of Free-N defined by the following relational expression 1 may preferably be greater than zero.
  • Free-N N ⁇ Ti / 47.887 ⁇ 14.01 ⁇ B / 10.81 ⁇ 14.01
  • the [N], [Ti], and [B] respectively indicate the content weight percent value of each of N, Ti and B.
  • Nb precipitates generated by the addition of Nb, NbC, Nb(C)N-type precipitates, etc. play a major role in securing strength after a stress relief heat treatment.
  • N is combined with Ti, Al, B, etc., to preferentially form another type of precipitate, such as TiN, BN, etc., thereby negatively affecting securing the intended Nb precipitate.
  • the free-N is less than 0, Ti and B that do not form sufficient nitrogen-based precipitates may be combined with C to form coarse precipitates. Therefore, it may be preferable that the value of free N defined by the following relational expression 2 is greater than zero.
  • the upper limit of the Free-N is not particularly limited, but may be preferably 0.008148 or less.
  • the steel according to an exemplary embodiment of the present disclosure has a ferrite-pearlite composite structure as a microstructure, as a main structure.
  • second phases such as bainite, martensite, etc are not produced.
  • the bainite or martensite structure has pearlite of 50 to 75% in area fraction, and the rest thereof is ferrite.
  • the steel according to an exemplary embodiment of the present disclosure it may be preferable that, after the stress relief heat treatment performed after high-input heat welding, precipitates of at least 1.27 ⁇ 10 6 precipitates per 1 mm 2 having a diameter of 100 nm or less, and 900 or more precipitates in a single crystal grain, are distributed. Through the distribution of the precipitates, the strength and toughness of the base material may be prevented from being deteriorated even after a stress relief heat treatment.
  • a most neighboring part is rapidly heated to a high temperature close to the melting point, and then rapidly cooled to room temperature.
  • a low-temperature phase such as bainite or martensite may be generated, and even when ferrite is generated, a microstructure type having a high stress therein, such as acicular ferrite, is generated.
  • the microstructure of the heat-affected zone has a problem of easily breaking in the processing or use environment of steel due to embrittlement occurrence.
  • the stress relief heat treatment of the weld portion is performed, which relieves stress of the weld portion and the heat-affected zone to reduce embrittlement, to lower possibility of breakage occurrable in the use environment.
  • the stress relief heat-treatment conditions are diverse depending on the welding conditions and the thickness of the steel. For example, in the case of A516-70, a pressure vessel steel material for medium and normal temperature, heat treatment is performed at a temperature of 620°C for 120 minutes.
  • the stress relief heat treatment may have a negative effect on the base material itself, not the weld portion or the heat-affected zone.
  • a steel material composed of microstructures such as ferrite and pearlite when stress relief heat treatment at a level of 400 to 800°C is performed, generation and coarsening of precipitates containing carbides may occur actively.
  • carbide coarsening occurs in proportion to time, and a decrease in carbonization concentration in the matrix structure occurs, thereby causing a decrease in overall strength. Therefore, it is necessary to appropriately manage the formation of precipitates containing carbides to prevent the strength from being deteriorated by the welding and stress relief heat treatment.
  • the matrix structure of the steel according to an exemplary embodiment of the present disclosure has ferrite-pearlite, a relatively soft ferrite structure is susceptible to fracture, but in many cases, the fracture also proceeds along the pearlite band, and thus, it may be preferable that that the fine precipitates are evenly distributed regardless of the matrix structure.
  • precipitates are produced in coarse form, such as Fe 3 C, VC, MoC, Ce 23 C 6 , or the like, or even if the precipitates are formed in a fine size
  • precipitates do not contribute significantly to the disturbance of propagation, and furthermore, rather may act as a starting point for fracture, to serve as reducing strength and toughness.
  • it may be important that the size of the precipitate is fine and the precipitates are properly distributed.
  • the precipitate according to an exemplary embodiment of the present disclosure is an Nb-based carbide, in more detail, NbC.
  • the Nb-based carbide is mainly produced and grown in a relatively low temperature zone of 600 to 700°C (in a temperature zone directly below the ferrite transformation point in austenite), and serves to suppress a decrease in strength and ferrite grain growth in the process thereof.
  • the steel according to an exemplary embodiment of the present disclosure is a steel material with improved quenchability, as compared to a related art steel material, and a required structure may be formed inside the steel material without rapid water cooling or the like.
  • a required structure may be formed inside the steel material without rapid water cooling or the like.
  • low-temperature toughness deteriorates in most cases. Therefore, in an exemplary embodiment of the present disclosure, by defining the preferred structure shape of the steel material, even if the quenchability of the steel material is improved, there is an effect of preventing deterioration of low-temperature toughness characteristics.
  • the steel according to an exemplary embodiment of the present disclosure has excellent tensile strength of 500 MPa or more and Charpy impact energy at 0°C of 150 J or more, even after the stress relief heat treatment (for example, 120 minutes at 620°C) after fabrication of the welded structure. Furthermore, the steel has excellent impact toughness in which the fraction of martensite austenite constituent in the microstructure of the heat-affected zone (HAZ) is 3% or less and the Charpy impact energy at 0°C is 100 J or more.
  • HZ fraction of martensite austenite constituent in the microstructure of the heat-affected zone
  • the manufacturing method according to an exemplary embodiment of the present disclosure includes preparing a steel slab that satisfies the above-described alloy composition, heating, hot rolling and cooling the steel slab.
  • preparing a steel slab that satisfies the above-described alloy composition heating, hot rolling and cooling the steel slab.
  • a steel slab having the above-described alloy composition is prepared, and then the steel slab is heated. At this time, it may be preferable to heat the steel slab in the temperature range of 1050 to 1250°C.
  • the heating may be preferably performed at 1050°C or higher, to solidify Ti and/or Nb carbon/nitride formed during casting. For example, it is necessary to heat the steel slab at 1050°C or higher to sufficiently solidify Ti and/or Nb carbon and nitride formed during casting.
  • austenite may be coarsened, and thus, it may be preferable to limit the reheating temperature to 1250°C or lower in consideration thereof.
  • the heated steel slab is hot rolled.
  • the hot rolling may be preferably performed to produce a hot rolled steel sheet by performing hot finish rolling at a predetermined temperature after roughly rolling the heated steel slab under normal conditions. At this time, the hot finish rolling is performed at 910°C or lower.
  • the hot finish rolling is for transforming the austenite structure into a non-uniform microstructure, and if the hot finish rolling temperature exceeds 910°C, a coarse structure is formed, and thus impact toughness is deteriorated. More advantageously, the hot finish rolling may be more preferably performed at a temperature in a range of 850 to 910°C. If the rolling termination temperature is lowered to less than 850°C, there is a problem that it is difficult to control the shape of a plate material.
  • the hot rolled steel sheet obtained by the hot finish rolling may be preferable to perform cooling at a low speed lower than a normal air cooling level.
  • the temperature range is a main temperature section in which precipitates are generated and grown.
  • the cooling rate may be preferably 1°C/Hr or more.
  • a minimum thermal driving force may be secured through the cooling process.
  • a method of implementing the slow cooling as described above there is also a method of using a separate cold storage facility, or a method of stacking, in multiple stages, steel sheets of similar dimensions after hot rolling without additional thermal insulation.
  • the microstructure of the base material and the distribution of precipitates of 100 nm or less and the number of precipitates in the crystal grains were measured, and the tensile strength and impact toughness were measured, and the results are illustrated in Table 3.
  • the impact toughness of the heat-affected zone and the fraction of the martensite austenite constitute were measured and the results are illustrated in Table 3.
  • the impact toughness was measured by performing a Charpy V-notch impact test at 0°C.
  • Le-Pera etching a point-counting method was used to measure the estimated position and relative area fraction of the martensite austenite constituent.
  • F refers to ferrite and P refers to pearlite.
  • FIG. 2 illustrates the size (nm) of the precipitate by observing the precipitate of Inventive example 1 by TEM. As illustrated in FIG. 2 , it can be seen that in Inventive Example 1 of the present disclosure, NbC precipitates of 100 nm or less are evenly formed.
  • FIG. 3 illustrates the size (nm) of the precipitate by observing the precipitates of Comparative Example 6 by the TEM, and it can be seen that in Comparative Example 6, a coarse FeC precipitate was formed.
  • the base material not only secures high strength and impact toughness, but also the heat-affected zone (HAZ) may secure high impact toughness.
  • HAZ heat-affected zone
  • the steel according to an exemplary embodiment of the present disclosure may ensure excellent toughness of HAZ even during large heat input welding, and may be produced as a steel material without defects such as surface cracks.
  • Comparative Examples 1 and 4 satisfy the alloy composition of the present disclosure, but the hot finish rolling temperature is too high, and thus, that sufficient toughness of the base material cannot be secured due to coarsening of the microstructure.
  • FIGS. 1A and 1B are images of the base material microstructures of Inventive Example 1 and Comparative Example 1, respectively. Although all of the microstructures are formed of ferrite and pearlite in the same manner, in the case of Comparative Example 1, it is considered that the grain size is coarse and thus, the impact toughness was lowered.
  • Comparative Examples 2 and 3 also satisfy the alloy composition of the present disclosure, but the slab heating temperature is outside the range of the present disclosure.
  • an element that inhibits austenite grain growth at high temperature such as Nb or the like, is not sufficiently solidified, or the austenite grain size is excessively coarse due to high temperature, resulting in a decrease in strength and impact toughness of the base material.
  • Comparative Example 5 after hot rolling, the cooling rate of the steel was outside the range proposed in the present disclosure during cooling, and thus, the precipitation of the steel was not secured. Thus, the strength in the present disclosure was not obtained.

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EP18891918.7A 2017-12-24 2018-12-21 Hochfester stahl mit ausgezeichneter zähigkeit von durch schweissen wärmebeaufschlagten zonen und verfahren zu seiner herstellung Pending EP3730644A4 (de)

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PCT/KR2018/016522 WO2019125075A1 (ko) 2017-12-24 2018-12-21 용접열영향부 인성이 우수한 고강도 강재 및 그 제조방법

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KR100368242B1 (ko) * 2000-08-02 2003-02-06 주식회사 포스코 용접열영향부 인성이 우수한 용접구조용 강재 및 그제조방법, 이를 이용한 용접구조물
JP3842707B2 (ja) * 2002-08-30 2006-11-08 株式会社神戸製鋼所 低合金耐熱鋼用溶接金属
JP2005171326A (ja) * 2003-12-11 2005-06-30 Nippon Steel Corp 耐表面損傷性および耐内部疲労損傷性に優れた高炭素鋼レール
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JP6235402B2 (ja) * 2014-04-17 2017-11-22 株式会社神戸製鋼所 強度、靭性および耐ｓｒ割れ性に優れた溶接金属
KR20160078772A (ko) * 2014-12-24 2016-07-05 주식회사 포스코 용접열영향부 인성이 우수한 강재 및 그 제조방법

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US20200325562A1 (en) 2020-10-15
KR20190077186A (ko) 2019-07-03

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