US20190055620A1 - Cu-containing low-alloy steel having excellent balance between strength and low-temperature toughness and method for producing same - Google Patents

Cu-containing low-alloy steel having excellent balance between strength and low-temperature toughness and method for producing same Download PDF

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US20190055620A1
US20190055620A1 US16/079,769 US201716079769A US2019055620A1 US 20190055620 A1 US20190055620 A1 US 20190055620A1 US 201716079769 A US201716079769 A US 201716079769A US 2019055620 A1 US2019055620 A1 US 2019055620A1
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low
steel
strength
temperature
alloy steel
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Yuta Honma
Kunihiko Hashi
Rinzo Kayano
Gen Sasaki
Kokichi UNO
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Japan Steel Works Ltd
Japan Steel Works M&E Inc
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Japan Steel Works Ltd
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Assigned to THE JAPAN STEEL WORKS. LTD. reassignment THE JAPAN STEEL WORKS. LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HASHI, KUNIHIKO, HONMA, YUTA, KAYANO, RINZO, SASAKI, GEN, UNO, KOKICHI
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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
    • 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
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • 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
    • C21D1/78—Combined heat-treatments not provided for above
    • 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
    • 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
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and 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
    • 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
    • C22C38/002—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
    • C22C38/06—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
    • C22C38/18—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
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the present invention relates to a Cu-containing low-alloy steel which has an excellent balance between strength and low-temperature toughness and is for use in applications where low-temperature toughness is required, and relates to a process for producing the Cu-containing low-alloy steel.
  • Petroleum and natural gas are extensively used as main energy sources. In recent years, exploitation of these resources is shifting from the land to the sea. Especially in the exploitation of marine resources, digging at depths of water deeper than continental shelves is coming to be mainly performed. Steels for marine structures for use in this very-large-depth exploitation are required to have not only excellent low-temperature toughness but also high yield strength from the standpoint of ensuring safety.
  • Steels for marine structure use which are for ensuring an excellent balance between strength and toughness are known. These steels include steel plates containing 1.0-1.3% by mass Cu, as provided for in, for example, ASTM A710, and forged steel materials containing up to 0.43% by mass Cu, as provided for in, for example, ASTM A707.
  • These steels are based on a low-carbon low-carbon-equivalent composition system in which strength is ensured by causing Cu precipitation by aging, and thus combine strength and low-temperature toughness.
  • Non-Patent Document 1 describes an improvement of a composition system on the basis of ASTM A707 Grade L5, and indicates that the improved material was quenched and tempered and evaluated for mechanical property. The results of the evaluation are explained therein.
  • the steel of Non-Patent Document 1 has an FATT of ⁇ 60° C., and a further improvement in low-temperature toughness is necessary from the standpoint of insuring safety.
  • Patent Document 1 proposes a production process in which an M* value determined by C, Si, Al, N, and B is specified and direct quenching is conducted after rolling, in order to produce a high-strength steel plate having excellent CTOD (crack tip opening displacement) characteristics.
  • Patent Document 1 indicates the following M* value.
  • Patent Document 2 proposes a process for producing a low-C high-tension steel of the Cu precipitation hardening type excellent in terms of low-temperature toughness and weldability, the process including rolling a steel plate containing 0.7-1.5% by mass Cu at a temperature of 900-700° C. and at a rolling reduction of 30% or more and then subjecting the steel plate to a Cu precipitation treatment at a temperature in the range of 500-650° C. to thereby produce the high-tension steel.
  • Patent Document 3 proposes a method for the intercritical quenching of a B-containing steel, wherein the contents of B, N, and Ti are specified and the temperature for the intercritical quenching is specified, thereby stably producing a high-tension steel having a low yield ratio.
  • Patent Document 4 proposes that a Ni-containing steel plate excellent in terms of low-temperature toughness and balance between strength and toughness is produced by intercritical quenching.
  • Patent Documents 1 and 2 each necessitate a step for refining rolling and cannot hence be applied to the case where no rolling is performed or where the plate is too thick to roll.
  • the plate thickness is 120 mm at the most. Consequently, the proposed production processes cannot be applied to processes in which no rolling is performed or to large structures including, for example, a flange part having a thickness of 150 mm or larger.
  • Patent documents 3 and 4 do not define Cu content, and do not clearly show a production process for obtaining a Cu-containing low-alloy steel which, although changing in strength upon aging, has an excellent balance between strength and toughness.
  • An object of the present invention which has been achieved under the circumstances described above, is to provide a Cu-containing low-alloy steel having an excellent balance between strength and low-temperature toughness.
  • a proper composition range in the present invention is clarified.
  • proper conditions for thermal refining including intercritical quenching for producing a Cu-containing low-alloy steel having an excellent balance between strength and low-temperature toughness are shown.
  • first embodiment is a Cu-containing low-alloy steel having an excellent balance between strength and low-temperature toughness, having a chemical composition including, in terms of % by mass, 0.01-0.08% C, 0.10-0.40% Si, 0.80-1.80% Mn, 0.80-2.50% Ni, 0.50-1.00% Cr, 0.80-1.50% Cu, 0.20-0.60% Mo, 0.010-0.050% Al, 0.030-0.080% Nb, and 0.005-0.020% N, with the balance being Fe and unavoidable impurities,
  • a 0.2% proof stress is 525 MPa or higher and a ductile/brittle fracture appearance transition temperature (FATT), as measured through a 2-mm V-notched Charpy impact test, is ⁇ 70° C. or lower.
  • FATT ductile/brittle fracture appearance transition temperature
  • the chemical composition further includes up to 0.010% by mass Ca.
  • the Cu-containing low-alloy steel has an absorbed energy of 130 J or higher in a 2-mm V-notched Charpy impact test at ⁇ 80° C.
  • an average EBSD grain diameter is 10 ⁇ m or less and a maximum EBSD grain diameter is 120 ⁇ m or less in cases when boundaries having a misorientation of 15° or larger are taken as grain boundaries.
  • first embodiment is a process for producing the Cu-containing low-alloy steel having an excellent balance between strength and low-temperature, including thermal refining which includes:
  • the thermal refining is applied to a steel for a large structure, the steel having a thick portion with a thickness of 150-500 mm.
  • the steel is produced by hot forging and then subjected to the thermal refining.
  • C is a necessary additive element from the standpoint of ensuring strength, and a lower limit is hence 0.01%.
  • inclusion thereof in an amount exceeding 0.08% results not only in a decrease in toughness due to strength enhancement but also in precipitation of a hard phase during intercritical quenching and a decrease in weldability. Consequently, an upper limit is 0.08%.
  • the lower limit is desirably 0.02% and the upper limit is desirably 0.05%.
  • Si is used as a deoxidizing element in performing melting/smelting for alloy production.
  • Si is an element necessary for ensuring strength.
  • a lower limit is hence 0.10%.
  • An upper limit is hence 0.40%.
  • the lower limit is desirably 0.20% and the upper limit is desirably 0.35%.
  • Mn is a useful deoxidizing element like Si, and contributes to an improvement in quench hardenability.
  • the content thereof must be 0.80% or higher.
  • An upper limit is hence 1.80%.
  • the lower limit is desirably 1.00% and the upper limit is desirably 1.50%. More preferably, the lower limit is 1.20% and the upper limit is 1.45%.
  • Ni is an element necessary for improving quench hardenability and for thereby ensuring strength and low-temperature toughness.
  • a lower limit is hence 0.80%. However, excessive inclusion thereof stabilizes the retained ⁇ , resulting in a decrease in toughness.
  • An upper limit is hence 2.50%.
  • the lower limit is desirably 1.50% and the upper limit is desirably 2.30%.
  • the lower limit is 2.00% and the upper limit is 2.20%. More preferably, the lower limit is 2.10% and the upper limit is 2.15%.
  • Cr is an important element for ensuring quench hardenability and ensuring strength and toughness.
  • a lower limit is hence 0.50%. However, excessive inclusion thereof enhances quench hardenability, resulting in a decrease in toughness and enhanced susceptibility to weld cracking.
  • An upper limit is hence 1.00%.
  • the lower limit is desirably 0.60% and the upper limit is desirably 0.80%.
  • the lower limit is 0.70% and the upper limit is 0.75%.
  • a lower limit is hence 0.80%. However, excessive inclusion thereof results in a decrease in toughness or a decrease in hot workability.
  • An upper limit is hence 1.50%.
  • the lower limit is desirably 1.10% and the upper limit is desirably 1.30%.
  • the lower limit is 1.20% and the upper limit is 1.25%.
  • Mo contributes to an improvement in quench hardenability and is an important element for ensuring strength and toughness.
  • a lower limit is hence 0.20%. However, excessive inclusion thereof results in a decrease in toughness or a decrease in weldability.
  • An upper limit is hence 0.60%.
  • the lower limit is 0.30% and the upper limit is 0.50%. More preferably, the lower limit is 0.40% and the upper limit is 0.45%.
  • Al combines with N to form AlN, thereby inhibiting the growth of crystal grains. Formation of finer crystal grains is essential for improving the toughness.
  • a lower limit of Al content is hence 0.010%. However, excessive inclusion thereof results in a decrease in toughness due to coarse AlN grains.
  • An upper limit is hence 0.050%.
  • the lower limit is desirably 0.010% and the upper limit is 0.030%.
  • the lower limit is 0.020% and the upper limit is 0.030%.
  • Nb forms carbonitrides to inhibit the growth of crystal grains, and is an important element for forming finer crystal grains.
  • a lower limit is hence 0.030%. However, excessive addition thereof accelerates the aggregation or enlargement of the carbonitride grains, resulting in a decrease in toughness.
  • An upper limit is hence 0.080%.
  • the lower limit is desirably 0.04% and the upper limit is desirably 0.060%.
  • the lower limit is 0.040% and the upper limit is 0.050%.
  • N forms AlN and carbonitrides to inhibit the growth of crystal grains, and is contained because N is an important element for forming finer crystal grains.
  • a lower limit is set at 0.005% in order to sufficiently obtain the effect. However, excessive addition thereof accelerates the precipitation of a large amount of AlN and carbonitrides and the aggregation or enlargement thereof, resulting in a decrease in toughness.
  • An upper limit is hence 0.020%. Preferably, the lower limit is 0.005% and the upper limit is 0.011%.
  • Ca forms oxides and sulfides and is hence used as a deoxidizing or desulfurizing element according to need.
  • excessive addition thereof results in a decrease in toughness.
  • the content thereof is hence 0.010% or less.
  • the upper limit is desirably 0.005%.
  • the chemical composition should contain Ca in an amount of 0.0005% or larger.
  • the chemical composition may contain Ca as an unavoidable impurity in an amount less than 0.0005%.
  • EBSD electron backscatter diffractometry
  • the low-temperature toughness decreases. More preferably, the average EBSD grain diameter is 10 ⁇ m or less and the maximum EBSD grain diameter is 110 ⁇ m or less.
  • a C3 transformation point temperature at which austenite transformation occurs. Even in cases when the heating temperature for quenching is not lower than the A C3 transformation point, quench hardenability is not ensured if the temperature is still low.
  • a lower-limit temperature is hence 850° C.
  • too high temperatures for quenching cause the enlargement of ⁇ grains during the heating, resulting later in a decreased in toughness.
  • An upper limit is hence 950° C.
  • This quenching can be repeatedly conducted multiple times according to need.
  • the present invention is not particularly limited in means for heating or cooling to be used in this quenching, and means having desired heating or cooling ability can be suitably selected.
  • the steel which has undergone the quenching is subsequently subjected to intercritical quenching, in which the steel is heated to a temperature in the range of [(A C3 transfoiuiation point) ⁇ 80° C.] to [(A C3 transformation point) ⁇ 10° C.] and then cooled.
  • the intercritical quenching is a heat treatment method in which a steel is heated to a temperature (intercritical temperature) which lies between the A C1 point and the A C3 point and at which the ⁇ phase and the ⁇ phase are both present, and is then cooled.
  • the present invention is not particularly limited in means for heating or cooling to be used in this intercritical quenching, and means having desired heating or cooling ability can be suitably selected. This heat treatment is the most important in the invention.
  • the heating temperature in this intercritical quenching is limited to a temperature in the range of [(A C3 transformation point) ⁇ 80° C.] to [(A C3 transformation point) ⁇ 10° C.], as stated above.
  • the heating temperature is lower than [(A C3 transformation point) ⁇ 80° C.]
  • transformation to the ⁇ phase occurs in an insufficient amount and a large amount of the ⁇ phase suffers isothermal tempering, resulting in an enlarged Cu precipitate. Consequently, a 0.2% proof stress cannot be ensured.
  • the later size reduction of crystal grains does not proceed, making it difficult to ensure low-temperature toughness.
  • tempering is given to the steel at a temperature in the range of 560-660° C.
  • the heating temperature is lower than 560° C.
  • an increase in 0.2% proof stress occurs due to the aging effect of the Cu precipitate, resulting in a decrease in toughness.
  • the tempering temperature exceeds 660° C.
  • overaging occurs, making it impossible to ensure a 0.2% proof stress. Consequently, the temperature for the tempering is in the range of 560-660° C.
  • the present invention is applicable to production of a material having a thick portion.
  • Examples of the material include ones having a thick portion with a maximum thickness of 150-500 mm.
  • the present invention can produce the following effects.
  • the ductile/brittle fracture appearance transition temperature is the temperature at which the mode of fracture changes from ductile fracture to brittle fracture with declining temperature. The lower the ductile/brittle fracture appearance transition temperature, the lower the temperature down to which the steel has toughness.
  • the ductile/brittle fracture appearance transition temperature is more preferably ⁇ 80° C. or lower.
  • FIG. 1 is a diagram showing a heat pattern for thermal refining in one embodiment of the invention.
  • FIG. 2 s are drawing-substitute photomicrographs of specimens of an Example according to the invention.
  • FIG. 3 s are drawings showing high-angle boundary maps of the specimens, the boundary maps indicating grain boundaries having a misorientation of 15° or larger and obtained from the results of an examination by EBSD.
  • a steel having the chemical composition specified in the invention can be produced as an ingot through melting in an ordinary way so that the composition is attained.
  • the present invention is not particularly limited in methods for producing the ingot.
  • the steel ingot produced through melting is hot-forged into any desired shape and then subjected to the thermal refining, which includes quenching (Q), intercritical quenching (L), and tempering (T).
  • the thermal refining which includes quenching (Q), intercritical quenching (L), and tempering (T).
  • the hot-forged material can be a thick one.
  • the hot-forged material can have a thick portion having a thickness of 150-500 mm.
  • the Cu-containing low-alloy steel is heated to a temperature in a range of 850-950° C. to conduct quenching. Thereafter, the steel is subjected to intercritical quenching at a temperature in a range of [(A C3 transformation point) ⁇ 80° C.] to [(A C3 transformation point) ⁇ 10° C.] and then to tempering at 560-660° C.
  • a heat treatment such as, for example, normalizing (N) may be conducted between the hot forging and the thermal refining.
  • Conditions for the normalizing can include heating conditions of, for example, 950-1.000° C.
  • the specified composition ranges and the production process described above make it possible to produce a thick Cu-containing low-alloy forged steel which has excellent low-temperature toughness and, in particular, has an excellent balance between strength and low-temperature toughness and which is suitable for use as a steel for marine structures such as mooring equipment, risers, flowlines, etc.
  • the Cu-containing low-alloy steel thus obtained has a 0.2% proof stress of 525 MPa or higher and a ductile/brittle fracture appearance transition temperature (FATT), as measured through a 2-mm V-notched Charpy impact test, of ⁇ 70° C. or lower.
  • FATT ductile/brittle fracture appearance transition temperature
  • this low-alloy steel has an absorbed energy of 130 J or higher in a 2-mm V-notched Charpy impact test at ⁇ 80° C.
  • the absorbed energy is preferably 140 J or higher.
  • the low-alloy steel after the thermal refining, has an average EBSD grain diameter of 10 ⁇ m or less and a maximum EBSD grain diameter of 120 ⁇ m or less in cases when boundaries having a misorientation of 15° or larger are taken as grain boundaries. It is preferable that the average EBSD grain diameter be 10 ⁇ m or less and the maximum EBSD grain diameter be 110 ⁇ m or less.
  • the low-alloy steel preferably has a 0.2% proof stress of 525 MPa or higher and a tensile strength of 600 MPa or higher.
  • the low-alloy steel preferably has a ductile/brittle fracture appearance transition temperature (FATT), as measured through a 2-mm V-notched Charpy impact test, of ⁇ 80° C. or lower.
  • FATT ductile/brittle fracture appearance transition temperature
  • Specimens respectively having the compositions shown in Table 1 were each produced as a 50-kg steel ingot through melting with a vacuum induction melting furnace. Each steel ingot produced was hot-forged at 1,250° C. into a plate having a thickness of 45 mm and a width of 130 mm (forging ratio: 3.1 s or higher), subsequently normalized (960° C.), and then subjected to thermal refining under the refining conditions (Q treatment, L treatment, T treatment) shown in Table 2.
  • the Q treatment quenching
  • the quenching temperature is not particularly limited so long as the temperature is in the range of 850-950° C., for the reasons shown above.
  • the cooling in the Q treatment and L treatment was conducted at a cooling rate of 10° C./min, as a simulation of the water cooling of a plate having a thickness of 450 mm.
  • the T treatment (tempering) conditions for each specimen are shown in Table 2.
  • Test pieces were taken out of each test material obtained, and were subjected to a tensile test and a Charpy impact test to evaluate the strength and low-temperature toughness.
  • the test methods are as follows.
  • EBSD orientation imaging microscopy
  • TSL TexSEM Laboratories, Inc.
  • the evaluation of the samples by EBSD is as follows. An electron beam is caused to strike on one site in the surface of each sample and the resultant backscatter diffraction is examined. Thus, the orientation angles of the crystal grains in the site can be determined.
  • the field of view having a size of 300 ⁇ M ⁇ 400 ⁇ m is scanned while minutely shifting the position of irradiation with the electron beam (at an examination pitch of 0.3 ⁇ m).
  • a map of the orientation angles of the crystal grains within the filed can be obtained.
  • a boundary line is drawn between the regions of any adjacent examination sites which differ in orientation angle by 15° or more, thereby obtaining a map concerning boundaries having a misorientation of 15° or larger, such as those shown in FIG. 3 .
  • the boundary lines can be regarded as crystal grain boundaries, and each region surrounded by such boundary lines can be regarded as one crystal grain.
  • the area of each region surrounded by such boundary lines was calculated, and the diameter of a circle having the same area was calculated and taken as the diameter (EBSD grain diameter) of the crystal grain.
  • EBSD grain diameter diameter of the crystal grain.
  • five different fields of view of 300 ⁇ m ⁇ 400 ⁇ m were arbitrarily selected and EBSD grain diameters were calculated for each field of view. An average value thereof was taken as average EBSD grain diameter, and the largest of those values was taken as maximum EBSD grain diameter.
  • the specimens of steel No. 1 and steel No. 2 were of the steel kind C.
  • Steels Nos. 1 and 2 are Comparative Examples for which a QT process, which is a common production process, was used.
  • Steel No. 1 failed to have reduced EBSD grain diameters by the mere QT process and had low low-temperature toughness.
  • satisfactory low-temperature toughness was not obtained. It is hence clear that satisfactory low-temperature toughness is difficult to ensure by merely performing the QT process.
  • the microstructures of steel No. 1 (QT process) and steel No. 2 (QLT process) are shown in FIG. 2
  • high-angle boundary maps concerning boundaries having a misorientation of 15° or larger, obtained from the results of the EBSD examination are shown in FIG. 3 .
  • the results of an examination of the microstructures and the high-angle boundary maps showed that the L treatment had brought about a complicated microstructure in which the meandering of high-angle boundaries was observed.
  • fine crystal grains were observed in grains.
  • the meandering of high-angle boundaries and the dispersed inclusion of fine grains contribute to an improvement in low-temperature toughness.
  • the specimens of steels Nos. 5 to 7 were of the steel kind A and had excellent strength and toughness due to the use of the heat treatment process according to the invention.
  • the specimens of steel Nos. 8 to 19 were of the steel kind B.
  • Steel No. 8 (Comparative Example) underwent an L treatment but did not undergo T treatment.
  • the aging effect of a Cu precipitate was insufficient and, hence, a decrease in 0.2% proof stress was observed.
  • Steel No. 23 (Comparative Example) was of the steel kind F, which was a comparative material. Use of the steel kind F also failed to obtain sufficient low-temperature toughness even when the QLT process, which is recommenced in the present invention, had been applied. The reasons for this include that since the steel kind F had too high a C content, C concentrated in the ⁇ phase during the L heating to cause precipitation of a hard phase. Steels Nos. 10, 11, 17 to 19, 22, and 23 shown in Table 32 were not examined for EBSD grain diameter.
  • the present invention is suitable for use as a steel for marine structures such as mooring equipment, risers, and flowlines.
  • uses of the invention are not limited to these.

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US16/079,769 2016-02-25 2017-02-08 Cu-containing low-alloy steel having excellent balance between strength and low-temperature toughness and method for producing same Abandoned US20190055620A1 (en)

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CN114134404A (zh) * 2021-05-20 2022-03-04 江阴兴澄特种钢铁有限公司 一种经济型破冰船用fh36钢板及其制备方法
CN114892072A (zh) * 2022-04-08 2022-08-12 上海大学 一种高强高韧抗氢脆钢板及其成分优选和制备方法

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JP7025950B2 (ja) * 2018-02-16 2022-02-25 日本製鋼所Ｍ＆Ｅ株式会社 高強度高靱性を有するＣｕ含有低合金鋼およびその製造方法
JP7402055B2 (ja) 2020-01-07 2023-12-20 日本製鋼所Ｍ＆Ｅ株式会社 溶接熱影響部の靱性が優れたＣｕ含有低合金鋼およびその製造方法

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JP3968011B2 (ja) * 2002-05-27 2007-08-29 新日本製鐵株式会社 低温靱性および溶接熱影響部靱性に優れた高強度鋼とその製造方法および高強度鋼管の製造方法
JP5076423B2 (ja) 2006-09-27 2012-11-21 Ｊｆｅスチール株式会社 Ｎｉ含有鋼板の製造方法

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CN114134404A (zh) * 2021-05-20 2022-03-04 江阴兴澄特种钢铁有限公司 一种经济型破冰船用fh36钢板及其制备方法
CN114892072A (zh) * 2022-04-08 2022-08-12 上海大学 一种高强高韧抗氢脆钢板及其成分优选和制备方法

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