US20210207237A1 - Cu-CONTAINING LOW ALLOY STEEL EXCELLENT IN TOUGHNESS OF WELD HEAT AFFECTED ZONE, AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Cu-CONTAINING LOW ALLOY STEEL EXCELLENT IN TOUGHNESS OF WELD HEAT AFFECTED ZONE, AND METHOD FOR MANUFACTURING THE SAME Download PDF

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US20210207237A1
US20210207237A1 US16/809,881 US202016809881A US2021207237A1 US 20210207237 A1 US20210207237 A1 US 20210207237A1 US 202016809881 A US202016809881 A US 202016809881A US 2021207237 A1 US2021207237 A1 US 2021207237A1
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low alloy
alloy steel
containing low
toughness
weld heat
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Yuta Honma
Gen Sasaki
Kunihiko Hashi
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Japan Steel Works M&E Inc
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Japan Steel Works Ltd
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    • 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
    • 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
    • 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
    • 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
    • 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
    • C22C38/001—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
    • C22C38/44—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
    • 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
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a Cu-containing low alloy steel suitable as a steel for offshore structures to be used for mooring facilities, risers, flowlines and the like, and a method for manufacturing the same.
  • Petroleum and natural gas are broadly used as the core of energy.
  • the exploitation thereof is being shifted from the land to the sea, and particularly with regard to the sea resource exploitation, mining in deep water rather than continental shelves is becoming the mainstream.
  • For steels for offshore structures to be used in the ultradeep-water exploitation from the viewpoint of securing safety, not only the demand on the toughness of members themselves but also the demand on the toughness of HAZ is increased in severity, and the CTOD value has been demanded.
  • steels for offshore structures there is known, for example, a steel containing 0.43% by mass or lower of Cu, specified by ASTM A707.
  • the steel is one in which by precipitating Cu by aging treatment, the strength is secured due to a low-carbon and low-carbon equivalent composition in consideration of weldability, and the strength and the low-temperature toughness are simultaneously satisfied.
  • the above steel has a low Cu content and in the case of adopting two phase region quenching, cannot secure the strength of members even when having been subjected to aging treatment.
  • Patent Literature 1 proposes a method of improving the strength-toughness balance of a thick-wall forged steel which is used as a steel for offshore structures, in order to utilize two phase region quenching treatment and secure a low-temperature toughness by age hardening according to Cu.
  • Patent Literature 3 Furthermore, formation of a fine structure of HAZ is achieved, in Patent Literature 3, by utilizing a complex precipitate of fine Ti nitride and MnS, and in Patent Literature 4, by utilizing fine Ti oxide or Mg oxide.
  • Patent Literature 5 the suppression of island martensite (MA) is achieved by a value of M1* and a value of M2* based on the chemical composition, and the improvement of the CTOD characteristic is achieved with the thickness being 1 inch or larger and the tensile strength being 700 MPa.
  • Patent Literature 1 since much of MA is produced in HAZ by multi-layer welding with the welding heat input being set at 1.6 kJ/mm or higher, the toughness is reduced and the CTOD characteristic cannot stably be secured.
  • Patent Literature 2 rolling is needed in the manufacturing process and the manufacturing method of Patent Literature 2 cannot be applied to large structures containing thick-wall flanges of 150 mm or larger and the like.
  • Patent Literatures 3 and 4 By technologies proposed in Patent Literatures 3 and 4, it is very difficult to uniformly control micro-inclusions in thick-wall forged steels and the like in the steel manufacturing process thereof, and a stable effect cannot be attained.
  • Patent Literature 5 since it uses a steel plate as its object, the content of Al and N is low; and in a thick-wall forged steel, the crystal grain diameter cannot be made fine during thermal refining, so the toughness of the member itself cannot be secured. Furthermore, although there is a prescription of adding B in the range of 0.0005 to 0.0015% for strength enhancement, it is difficult to make the B to be homogeneously contained in the whole in the thick-wall forged steel manufactured from a steel ingot. Besides, depending on heat handling during the thermal refining, borides detrimental to the toughness are precipitated.
  • the present invention has been achieved with the above situation as the background, and has an object to provide a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, and a manufacturing method thereof, in which the composition of the steel is optimized and the low-temperature toughness of the weld HAZ is enabled to be improved.
  • a first aspect thereof is a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, wherein the steel comprises a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 to 0.60%, Al: 0.010 to 0.050%, Nb: 0.020 to 0.080% and N: 0.005 to 0.020%, the balance consisting of Fe and unavoidable impurities.
  • a second aspect of the present invention is the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to the above aspect, wherein the chemical composition further contains, in % by mass, Ca: 0.010% or less.
  • a third aspect of the present invention is the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to the above aspects, wherein a structure of the weld heat affected zone (HAZ) after affected by weld heat at a heat input of 3.5 kJ/mm has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 4%.
  • HZ weld heat affected zone
  • MA island martensite
  • a fourth aspect of the present invention is the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, wherein a structure of an two phase region coarse grain HAZ (ICCGHAZ) present in the above weld heat affected zone (HAZ) has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 5%.
  • ICCGHAZ two phase region coarse grain HAZ
  • MA island martensite
  • a fifth aspect of the present invention is the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, wherein the Cu-containing low alloy steel is a thermally refined Cu-containing low alloy steel, and the thermally refined Cu-containing low alloy steel has a 0.2% yield strength of 525 MPa or higher and a ductile-brittle fracture appearance transition temperature (FAIT) measured by a V-notch Charpy impact test of ⁇ 70° C. or lower.
  • FIT ductile-brittle fracture appearance transition temperature
  • a first aspect thereof is a method comprising thermal refining conditions for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to any one of the above aspects, wherein the thermal refining conditions comprises quenching a Cu-containing low alloy steel by heating in the temperature range of 850 to 950° C., thereafter two phase region quenching the resultant by heating in the temperature range of the (Ac 3 transformation temperature ⁇ 50° C.) or higher to the (Ac 3 transformation temperature ⁇ 10° C.) or lower, and further tempering the resultant at 500 to 600° C.
  • a method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone is a method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to any one of the above aspects, wherein by hot forging the Cu-containing low alloy steel having a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 to 0.60%, Al: 0.010 to 0.050% and Nb: 0.020 to 0.080%, and one or two of N: 0.005 to 0.020% and Ca: 0.010% or less, the balance consisting of Fe and unavoidable impurities, the Cu-containing low alloy steel is applied as a steel for a large structure having a thick-wall part of 150 mm to 450 mm in plate thickness.
  • a method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone comprises a step of the above thermal refining conditions in the above aspect.
  • any of component contents in the composition is indicated in % by mass.
  • Si is used as a deoxidizing element when melting and/or smelting alloys. Then since Si is a necessary element for securing the strength, its lower limit is set at 0.05%. However, since an excessive incorporation thereof increases the generating amount of MA formed of the weld heat affected zone and reduces the toughness, its upper limit is set at 0.40%.
  • Mn is, similarly to Si, an element useful as a deoxidizing element, and since Mn also contributes to the improvement of hardenability, its lower limit is set at 0.20%. However, since an excessive incorporation thereof increases the generating amount of MA (Martensite Austenite) formed of the weld heat affected zone and reduces the toughness, its upper limit is set at 0.70%. Then, for the same reasons, it is desirable that its lower limit is set at 0.30%, and its upper limit is set at 0.60%, and it is more desirable that its upper limit is set at 0.50%.
  • MA Martensite Austenite
  • Ni is a necessary element for securing the strength owing to an improvement in hardenability and securing the low-temperature toughness
  • its lower limit is set at 1.20%.
  • its upper limit is set at 2.50%. Then, for the same reasons, it is desirable that its lower limit is set at 1.50%, and its upper limit is set at 2.30%.
  • Cr is an important element for securing the hardenability and securing the strength and the toughness
  • its lower limit is set at 0.50%.
  • an excessive incorporation thereof raises the hardenability, reduces the toughness and raises the weld cracking sensitivity
  • its upper limit is set at 1.00%.
  • Cu is precipitated during aging treatment and improves the strength of the steel.
  • it is very important to secure the strength owing to a Cu precipitate.
  • its lower limit is set at 0.80%.
  • its upper limit is set at 1.50%. Then, for the same reasons, it is desirable that its lower limit is set at 1.10%, and its upper limit is set at 1.30%.
  • Mo contributes to the improvement of the hardenability and is an important element for securing the strength and the toughness, its lower limit is set at 0.20%. However, since an excessive incorporation thereof reduces the toughness and decreases the weldability, its upper limit is set at 0.6%.
  • Al combines with N to form AlN, thereby suppressing crystal grain growth. Grain refining is essential for improving the toughness, and with regard to the content of Al, its lower limit is set at 0.010%. However, since an excessive incorporation thereof reduces the toughness due to coarse AlN, its upper limit is set at 0.050%.
  • Nb is an important element for suppressing crystal grain growth as a carbonitride and refining the crystal grains
  • its lower limit is set at 0.020%.
  • its upper limit is set at 0.080%.
  • N is an important element for suppressing crystal grain growth and refining the crystal grains as AlN and carbonitride, and its lower limit is set at 0.005%. However, since an excessive addition thereof promotes precipitating and aggregating and coarsening of a large amount of the AlN and carbonitride and reduces the toughness, its upper limit is set at 0.020%.
  • Ca forms an oxide or a sulfide as Ca—Si, therefore, as desired, Ca is used as a deoxidizing and desulfurizing element.
  • the addition is set at 0.010% or less. For the same reason, it is desirable that its upper limit is further set at 0.005%.
  • MA contains high-carbon martensite and retained ⁇
  • MA is very hard, and behaves as a hard phase.
  • the presence of the hard phase makes a strength difference from a matrix phase and becomes a starting point of the stress concentration during brittle fracture.
  • the generating amount of MA is increased and the toughness, particularly the CTOD characteristic is reduced.
  • a reduction in the area ratio of MA is needed.
  • the region in HAZ of the present kind of steel where the toughness is most reduced is ICCGHAZ (inter critically reheated CGHAZ, CGHAZ: coarse grain HAZ), and the area ratio of MA in this region lower than 5% and also the area ratio of MA in the whole HAZ lower than 4% are observed to provide an improvement in the toughness.
  • the area ratio of MA is, in the case of the quantity of heat input being 3.5 kJ/mm or lower, obtained from an average value thereof obtained in the whole HAZ or in the region of the ICCGHAZ.
  • the quantity of heat input is presented as a condition of the evaluation of characteristics, and the quantity of heat input during welding in the present invention is not limited to the above range.
  • the steel needs at least to be heated at a temperature of the Ac 3 transformation temperature or higher. Then, even when the heating temperature of quenching is the Ac 3 point or higher, since hardenability cannot be secured in the case where the temperature is low, the lower limit temperature is set at 850° C. However, since making the quenching temperature high coarsens the ⁇ particle diameter during heating and reduces the toughness thereafter, its upper limit is set at 950° C. Here, the quenching can be repeated several times as required. Then, heating means and cooling means during the quenching are not especially limited, in the present invention, and means capable of providing desired heating capability and cooling capability can suitably be selected.
  • the quenched steel is then subjected to two phase region quenching treatment being heated in the temperature range of the (Ac 3 transformation temperature ⁇ 50° C.) or higher to the (Ac 3 transformation temperature ⁇ 10° C.) or lower and thereafter cooled.
  • Heating means and cooling means during the two phase region quenching are also not especially limited, in the present invention, and means capable of providing desired heating capability and cooling capability can suitably be selected. This heat treatment is the most important one in the manufacturing method according to the present invention.
  • the heating temperature in the above heat treatment is specified to the temperature range of the (Ac 3 transformation temperature ⁇ 50° C.) or higher to the (Ac 3 transformation temperature ⁇ 10° C.) or lower.
  • the heating temperature is a temperature of lower than (Ac 3 transformation temperature ⁇ 50° C.)
  • the amount of transformation to the ⁇ phase is insufficient; the amount of the ⁇ phase subjected to the high-temperature tempering is large; and the Cu precipitate is coarsened, therefore the 0.2% yield strength cannot be secured.
  • the crystal grains thereafter are also not refined; and the constituent concentration to the transformed ⁇ phase is caused, and the ⁇ phase remains even at room temperature, making it difficult for the toughness to be secured.
  • the heating temperature is set at a high temperature exceeding (Ac 3 transformation temperature ⁇ 10° C.)
  • the amount of transformation to the ⁇ phase becomes excessive, and since the transformed ⁇ phase reduces hardenability and makes granular bainitic ferrite, a good metal structure cannot be obtained. Additionally, the crystal grain diameter becomes coarsened and a sufficient strength and the low-temperature toughness cannot be secured.
  • the heating temperature is specified to the temperature range of the (Ac 3 transformation temperature ⁇ 50° C.) or higher to the (Ac 3 transformation temperature ⁇ 10° C.) or lower.
  • the resultant steel is tempered in the temperature range of 500 to 600° C.
  • the heating temperature is lower than 500° C.
  • the 0.2% yield strength is increased due to the aging effect of the Cu precipitate and the toughness is reduced.
  • the temperature range of the tempering treatment is set at 500 to 600° C.
  • the present invention can suitably be applied to manufacture of materials having thick-wall parts.
  • a material whose thick-wall part has a maximum wall thickness of 150 mm or larger and 450 mm or smaller.
  • the temper rolling is difficult, and the advantageous effects of the present invention can remarkably be attained.
  • the wall thickness exceeds 450 mm, in the cooling process of quenching and two phase region quenching, the cooling rate is decreased and the strength is decreased.
  • the method for manufacturing the Cu-containing low alloy steel according to the present invention by specification of the compositional range and thermal refining and the like, it is enabled the manufacture of a thick-wall Cu-containing low alloy forged steel which is suitable as a steel for offshore structures used for mooring facilities, risers, flowlines and the like, and is excellent in the low-temperature toughness, and particularly has a high strength and high toughness.
  • FIG. 1 is a schematic explanatory diagram showing an example of a quenching method of the present invention.
  • FIG. 2 is a schematic diagram of a weld heat affected zone of a multi-layer welded joint.
  • FIG. 3 is photographs as a substitute of drawings showing SEM images after Le Pera etching of an Invention Example and a Comparative Example.
  • FIG. 4 is a graph showing relations between the CTOD value of welded joints and the notch position of an Invention Example and a Comparative Example.
  • a Cu-containing low alloy steel to be used in the present invention if a composition specified by the present invention is set to the target, can be ingoted by a common method, and in the present invention, the method is not particularly limited.
  • the manufactured steel ingot is hot forged into an arbitrary shape, and thereafter, subjected to the above-mentioned quenching (Q), two phase region quenching (L) and tempering (T) treatment.
  • the content and a method of the hot forging are not especially limited, and the forging ratio and the like are also not especially limited.
  • a material can be hot forged into one having a thick wall, for example, into a material having a thick-wall part of 150 mm to 450 mm in wall thickness.
  • the Cu-containing low alloy steel according to the present invention brings about an especially suitable effect on a material applied as a steel for the above-mentioned large structure having a thick-wall part
  • the present invention is not especially limited in the wall thickness, and can also be used in applications whose thickness is smaller than the above.
  • the Cu-containing low alloy steel is quenched by being heated in the temperature range of 850 to 950° C. Thereafter, the quenched steel is two phase region quenched in the temperature range of the (Ac 3 transformation temperature ⁇ 50° C.) or higher to the (Ac 3 transformation temperature ⁇ 10° C.) or lower, and furthermore tempered at 500 to 600° C.
  • a heat treatment such as normalizing (N) can also be carried out.
  • N normalizing
  • the heating condition of 950 to 1,000° C. can be exemplified.
  • a heat treatment such as normalizing (N) can also be carried out.
  • N normalizing
  • the heating condition of 950 to 1,000° C. can be exemplified.
  • FIG. 1 A heating pattern of the above thermal refining conditions is shown in FIG. 1 .
  • the steel is, in the first quenching, heated at a temperature of the Ac 3 or higher, and heat treatment is performed so that the heating temperature during the two phase region quenching falls in the specified range. Then in the tempering treatment, the steel is heated at a temperature of the Ac 1 or lower for heat treatment.
  • compositional range and the manufacturing method thereof According to the specification of the compositional range and the manufacturing method thereof according to the above, it is enabled the manufacture of a thick-wall Cu-containing low alloy forged steel which is suitable as a steel for offshore structures used for mooring facilities, risers, flowlines and the like, and is excellent in the low-temperature toughness, particularly in the balance between the strength and the low-temperature toughness.
  • the Cu-containing low alloy steel obtained in the above has characteristics of a 0.2% yield strength of 525 MPa or higher and a ductile-brittle fracture appearance transition temperature (FATT) measured by a V-notch Charpy impact test of ⁇ 70° C. or lower.
  • FATT ductile-brittle fracture appearance transition temperature
  • a welding method to be applied to the formation of a multi-layer welded joint is not especially limited.
  • a structure of the weld heat affected zone (HAZ) after affected by heat at a quantity of heat input of 3.5 kJ/mm has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 4%.
  • a structure of an two phase region coarse grain HAZ (ICCGHAZ) present in the above weld heat affected zone (HAZ) has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 5%.
  • the weld heat affected zone is defined as a region where a microstructure or macrostructure of a base metal has been changed due to the welding heat effect.
  • a weld cross section is etched by using a mixed aqueous solution of ammonium copper chloride and hydrochloric acid, a 10% nital or the like; and in a microstructure observation, a weld cross section is etched by using a 2% nital, a 5%-nital, a 5%-nitric acid alcohol solution, a mixed alcohol solution of a 5%-nitric acid and a 1%-picric acid, 5%-picral or the like.
  • an etchant used for the etching is not limited to the above as long as being one capable of observing differences in the structure.
  • Test materials having compositions indicated in Table 1 were ingoted into 50 kg steel ingots by a vacuum induction melting furnace.
  • the ingoted steel ingots were each hot forged at 1,250° C. into 45 mm in thickness ⁇ 130 mm in width (forging ratio: 3.1 s or more), and furthermore subjected to N treatment (960° C.), thereafter, subjected to Q treatment (900° C.), L treatment (800° C.) and T treatment (580° C.)
  • cooling of the Q treatment and the L treatment was set at a cooling rate (20° C./min), which equivalently simulated that of water cooling of a plate thickness of 350 mm. Thereafter, a heat cycle for reproducing a multi-layer weld heat affected zone was imparted.
  • a coarse grain region of the first pass which is the region that the toughness was most reduced in the present steel, was made into an ICCG (Inter-Critically Coarse Grain) HAZ two phase region reheated in the second and subsequent passes, as shown in FIG. 2 .
  • ICCG Inter-Critically Coarse Grain
  • CG Coarse Grain
  • the quantity of heat input was obtained by the following expression and was determined at the conditions providing a quantity of heat input of 3.5 kJ/mm.
  • the CGHAZ structure was prepared in simulation of a structure of the coarse grain region in the vicinity of a welding line of the first pass being treated at a maximum heating temperature of 1,350° C. for a holding time of 5 sec. Furthermore, after the preparation of the CGHAZ structure, the ICCGHAZ structure was prepared in simulation under the conditions of two phase region reheating of the second pass being carried out at a maximum heating temperature of 780° C. for a holding time of 5 sec.
  • the temperature rise rate was set at 70° C./sec; and the cooling time in the range of 800° C. to 500° C. was set to 50 sec.
  • heating in the heat cycle was carried out by high-frequency heating; and cooling was carried out by spraying of carbon dioxide gas or helium gas, and the temperature rise, the temperature holding and the cooling were controlled based on a measurement value of a couple attached on the test piece surface.
  • Test pieces were sampled from the reproducing heat cycle test pieces of the CGHAZ and the ICCGHAZ, and the area ratios of MA were measured. Furthermore, a CTOD test was carried out for the reproducing heat cycle test pieces of the ICCGHAZ to evaluate the low-temperature toughness.
  • the test method was as follows.
  • the CTOD test involved sampling a test piece of 10 mm ⁇ 10 mm ⁇ 46 mm from the reproducing heat cycle test piece, and being carried out according to the specification of ISO12135.
  • the test temperature was set at ⁇ 20° C. and the number of the test pieces tested was three; and the low-temperature toughness was evaluated by using a critical CTOD value which was the lowest CTOD value among the obtained results.
  • the fracture morphology was also evaluated from a stable crack extension amount.
  • the fracture morphology is indicated by parentheses in Table, and ⁇ m indicates that no stable crack extension occurred until a maximum load point; and ⁇ c indicates that the stable crack extension occurred in 0.2 mm or less and then, unstable crack extension occurred.
  • a test piece of 15 mm ⁇ 30 mm ⁇ 138 mm was sampled and the test was carried out according to the specification of ISO15653. The test temperature was set at ⁇ 20° C. and the number of the test pieces tested was three.
  • the optimal thermal refining conditions capable of providing characteristics as thick-wall forged steels of steels for offshore structures was inspected by using Invention Steels A to D.
  • Each of the materials was ingoted, forged, subjected to N (960° C.) and Q (900° C.), and thereafter thermally refined under the thermal refining conditions indicated in Table 3.
  • the Q treatment of Examples was all carried out at a temperature of 900° C., but for the above-mentioned reason, when the quenching temperature was in the range of 850 to 950° C., the Q treatment temperature was not especially limited.
  • the cooling of the Q treatment and the L treatment was set at a cooling rate (20° C./min), which equivalently simulated that of water cooling of a plate thickness of 350 mm.
  • Test pieces were sampled from the obtained test materials, and were subjected to a tensile test and a Charpy impact test and evaluated for the strength and the low-temperature toughness. Test methods were as follows.
  • round bar tensile test pieces (parallel part diameter: 12.5 mm, G.L.: 50 mm) were sampled from the obtained test materials; and the tensile test was carried out at room temperature according to the specification of JIS 22241 and the 0.2% yield strength (Y.S.) and the tensile strength (T.S.) were determined.

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JP2940647B2 (ja) 1991-08-14 1999-08-25 新日本製鐵株式会社 溶接用低温高靱性鋼の製造方法
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