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  • 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
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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  • 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
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • 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
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  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
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  • C22C—ALLOYS
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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  • 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
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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  • C22C—ALLOYS
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  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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  • 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
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  • 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
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  • 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
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  • 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
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  • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
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  • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/16—Controlling or regulating processes or operations
  • B22D11/22—Controlling or regulating processes or operations for cooling cast stock or mould
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  • 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
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
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  • 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

Definitions

• the present invention relates to an ultrahigh tensile strength steel plate for a large welded structure such as an offshore structure or the like which requires a high degree of safety, having excellent weldability and toughness of a heat-affected zone.
• weldability and toughness of a heat-affected zone of a steel plate tend to deteriorate as the thickness and the strength increase. This is because a large amount of an alloy element which deteriorates the toughness of the heat-affected zone has to be added to ensure strength.
• weldability has a wide range of meanings, the weldability generally indicates the hardenability of a heat-affected zone and weld cold crack sensitivity and is indicated by various component parameters such as a carbon equivalent Ceq and a weld crack sensitivity composition P CM in many cases. The greater the amount of the alloy components is, the higher the indices are.
• the hardenability of the heat-affected zone and weld cold crack sensitivity increase and generally, the weldability deteriorates. It is known that the toughness of the heat-affected zone is not always consistent with the values of the indices of the weldability but has a high correlation with the values of the indices.
• thermo-mechanical treatment that is, thermo-mechanical control process (TMCP) and thermal refining treatment (quenching-tempering treatment) for boron (B)-added steel, which are widely known for any person skilled in the art needless to particularly disclose techniques here.
• TMCP thermo-mechanical control process
• thermal refining treatment quenching-tempering treatment
• TMCP is for controlling the overall processes of heating-rolling-cooling for producing steel and a water cooling process that is also referred to as accelerated cooling or control cooling is effective to increase the strength of thick material after rolling.
• thermal conduction due to a physical phenomenon called thermal conduction, a sufficient cooling rate cannot be obtained at a thickness center portion of the thick material by water cooling, and it is difficult to ensure an increase in thickness and strength with a low composition.
• the hardenability of the heat-affected d zone has a high correlation with the weld cold crack sensitivity and the toughness of the heat-affected zone as described above, and there has been a problem in the unconditional utilization of boron (B).
• B boron
• the effect is exhibited only under the presence of boron (B) in a solid solution state, and thus, a component for controlling precipitation of a boron compound, and process control, are necessary.
• TMCP there has been a case to which findings in the refining treatment cannot be applied as they are.
• refining treatment that is, quenching-tempering treatment is disadvantageous in terms of a heat treatment period or cost compared to TMCP. Further, it is fact that non-refining, that is, the attainment of TMCP has been actually socially requested from the viewpoint of environmental load and energy saving in recent years.
• Patent Document 1 there is disclosed an invention related to Cu-precipitated steel including 0.8% or more of Cu, which is relatively high.
• Cu when a large amount of Cu alone is added, Cu cracking occurs at the time of heating or hot-rolling, and thus, there is a problem of having a difficulty in production.
• Patent Document 1 Japanese Unexamined Patent Application, First Publication No. 2011-1625
• the present invention is made in consideration of the above circumstances and an object thereof is to provide an ultrahigh tensile strength steel plate for a large welded structure such as an offshore structure or the like which requires a high degree of safety, having excellent weldability and toughness of a heat-affected zone.
• the main target is a steel plate used for an offshore structure which requires the crack tip opening displacement (CTOD) properties of a welded joint zone with properties of a thickness of 50 mm to 100 mm, a tensile strength of 600 MPa to 700 MPa, a yield strength of 500 MPa to 690 MPa, and the lowest CTOD value of crack tip opening displacement of 0.25 mm or more in a heat-affected zone. It is preferable that the lowest CTOD value is high to ensure sufficient safety against fracture.
• CTOD property evaluation is severer evaluation method compared to Charpy impact properties evaluation, and thus, the steel plate for an offshore structure is set as the main target. Accordingly, it is needless to say that the present invention can be widely applied as a steel plate for welded structures such as vessels, steel frames, bridges, various tanks, or the like.
• the main features are (a) B-N-Ti amount balance optimization for ensuring solid soluted boron (B), (b) C content ultra reduction for alleviating the hardenability of the heat-affected zone by (solid soluted) B, (c) P CM optimization for ensuring the strength, weldability and toughness of the heat-affected zone, (d) Al-free Ti deoxidation for ensuring the toughness of the heat-affected zone, (e) oxide (O) reduction under the Al-free statement for suppressing a coarse oxide, and the like. Since the features are not independent events and have a close relationship among each other, the features are not easily attained at the same time and can be realized by a systematic and precise test conducted by the inventors for the first time and thus, the present invention has been completed.
• the gist of the present invention is as follows.
• the ultrahigh tensile strength steel having excellent weldability and toughness of the heat-affected zone at a low cost and also possible to further increase safety while the size of a welded structure such as an offshore structure can be increased.
• An object of the present invention is to provide an ultrahigh tensile strength steel for a large welded structure such as an offshore structure or the like which requires a high degree of safety, having excellent weldability and toughness of the heat-affected zone, and a steel plate having a thickness of 50 mm to 100 mm, a tensile strength of 600 MPa to 700 MPa, a yield strength of 500 MPa to 690 MPa, and the lowest CTOD value of crack tip opening displacement of 0.25 mm or more in the heat-affected zone is a main target.
• the lower limit thereof may be 0.018%, 0.020%, 0.023% or 0.025%.
• the upper limit is set to 0.045%.
• the upper limit thereof may be set to 0.042%, 0.040%, 0.037%, or 0.035%.
• Si 0% to 0.40% or less
• Si is unavoidably included in the steel and particularly promotes formation of hard and brittle martensite-austenite (MA) constituent (hereafter, abbreviated to MA) and deteriorates the toughness of the heat-affected zone. Therefore, the smaller the Si is, the more preferable it is.
• MA martensite-austenite
• the amount of C is limited to be relatively small, when Si is included in an amount of up to 0.40%, the MA formation amount is small, and the amount is allowable from the viewpoint of the toughness of the heat-affected zone.
• the upper limit thereof may be 0.30%, 0.25%, 0.20%, 0.15%, or 0.10% or less.
• the lower limit of Si does not need to be defined and the lower limit thereof is set to 0%.
• Si may be included to improve the toughness of the base metal of the steel plate or deoxidize the steel, and as necessary, the lower limit thereof may be set to 0.01%, 0.02%, or 0.03%.
• Mn is a relatively inexpensive element, but has a great effect to increase the strength and the toughness of the base metal and the heat-affected zone is relatively less adversely affected by Mn.
• it is important to form intragranular ferrite with Ti oxides or the like as nuclei in the heat-affected zone to improve the toughness of the heat-affected zone.
• Mn also plays an important role. The role is to promote ferrite transformation in such a manner that MnS is precipitated in the Ti oxides to form a Mn depletion region in the vicinity thereof and the transformation temperature is increased higher than that of the matrix.
• the amount of Mn is limited to 1.80% or more in the present invention.
• the lower limit thereof does not have a critical metallurgical and technical meaning and is limited to make component properties clear within a range in which excellent properties that are targeted in the present invention are exhibited.
• the lower limit may be set to 1.85% or 1.90%.
• Mn is an inexpensive element and is attempted to be maximally utilized.
• the amount of Mn is limited to 2.20% or less.
• the upper limit thereof may be 2.15% or 2.10%.
• P and S are unavoidably included as impurities and it is preferable that the amount of P and S be small in terms of the toughness of the base metal and the toughness of the HAZ.
• the upper limits of P and S are respectively set to 0.008% and 0.005%.
• the upper limit of P may be 0.006%, 0.005%, or 0.004%
• the upper limit of S may be 0.004%, 0.003%, or 0.002%, respectively.
• P and S are unavoidable impurities and the lower limits of P and S do not need to be defined. If needed, the lower limits of P and S may be 0%.
• the ultrahigh tensile strength steel which is the target of the present invention, it is preferable to add 0.40% or more of Cu.
• the lower limit thereof may be 0.45%, 0.50%, or 0.55%.
• the amount of Cu exceeds 0.70%, a precipitation hardening phenomenon is exhibited and the material properties of the steel, particularly, the strength is significantly changed in a discontinuous manner. Therefore, in the present invention, the amount of Cu is limited to 0.70% or less as a range in which the strength change is easily controlled in a continuous manner.
• the upper limit thereof may be limited to 0.65%, 0.60%, or 0.55%.
• Ni is known as a highly toughening element and is effective to improve the strength and the toughness of the base metal with less deterioration in the toughness of the heat-affected zone. Therefore, in the ultrahigh tensile strength steel as in the present invention, Ni is a very useful element. Particularly, in the ultralow carbon chemical composition as in the present invention, strength compensation by an alloy element is essential and it is necessary to include at least 0.80% or more of Ni. In order to improve the toughness of the heat-affected zone, the lower limit thereof may be 0.90%, 1.00%, 1.05%, or 1.10%. On the other hand, Ni is an expensive alloy and thus, the content is preferably suppressed to the minimum in which required properties such as strength, toughness, and the like can be obtained.
• the upper limit thereof is set to 1.80%.
• the upper limit is not a property or metallurgical limit.
• the upper limit thereof may be limited to 1.75%, 1.70%, 1.65%, 1.60%, 1.55%, or 1.50%.
• the steel of the present invention including a slightly large amount of Cu as described above it is effective to include Ni exceeding 2.0 with respect to the amount of Cu to suppress Cu cracking of a cast piece, and Ni is limited to [Ni]/[Cu] > 2.0 in Claim 1.
• Nb is an effective element to obtain a controlled rolling effect which is effective for structure refinement by expanding an austenite non-recrystallization temperature region to a high temperature region in a rolling process.
• the structure refinement is an effective method for improving both strength and toughness.
• At least 0.005% of Nb is necessarily included to reliably obtain the effect.
• the lower limit thereof may be 0.006%, 0.007%, or 0.008%.
• Nb which exhibits such a very useful effect in a base metal increases the hardness of the heat-affected zone to promote MA formation, and thus, is harmful to the toughness. Therefore, the upper limit has to be suppressed to 0.015%.
• the upper limit thereof may be 0.013%, 0.011%, or 0.010%.
• Mo is extremely effective from the viewpoint of improving the strength of the base metal, and is an essential element in the thick high strength steel plate as in the present invention.
• B is utilized, a higher hardenability improving effect is exhibited by including both Mo and B simultaneously.
• at least 0.05% of Mo is necessarily included.
• the lower limit thereof may be 0.07%, 0.09%, 0.11%, or 0.13% to reliably exhibit the hardenability improving effect.
• the effect is significant but excessive addition causes a remarkable increase in the hardness and remarkably promotion of MA formation, and thus, it is necessary to limit the amount of Mo to 0.25% or less.
• the upper limit thereof may be 0.23%, 0.21%, 0.19%, or 0.17%.
• the present invention is an Al-free and Ti-deoxidized steel. It is necessary to include at least 0.005% of Ti since the steel needs to be deoxidizes and the microstructure is refined by forming Ti oxides, and forming intragranular ferrite using the Ti oxides as nuclei in the heat-affected zone.
• the lower limit thereof may be 0.006% or 0.007%.
• the upper limit is set to 0.015%.
• the stoichiometrical relationship between Ti and N is limited to [N] - [Ti]/3.4 ⁇ 0%, which represents excess N (lack of Ti) in Claim 1.
• N excess N
• Ti consumption by deoxidization has to be considered exactly, but it is experimentally confirmed that complication is avoided and also, there is no great influence substantially.
• the upper limit of Ti may be 0.013%, 0.012%, 0.011%, or 0.010%.
• B is one of important elements in the present invention.
• the hardenability improving effect of B is very high and an alloy element can be significantly suppressed by utilizing B. Therefore, it is necessary that the amount of B is at least 0.0004%.
• the lower limit thereof may be 0.0005%, 0.0006%, or 0.0007%.
• the FB may be set to 0.0004% or more, or 0.0005% to more effectively utilize B.
• the upper limit of FB [B] - 0.77 ⁇ ([N] - 0.29 ⁇ ([Ti] - 2 ⁇ ([O] - 0.89 ⁇ [Al]))) is not particularly limited, but is naturally limited from the limitation range of each element.
• the Expression 3 is an expression for obtaining the amount of the solid soluted B (the amount of effective B: FB) in the steel obtained from a stoichiometric ratio in consideration of a bonding force strength between the respective elements.
• the upper limit of the FB does not need to be defined particularly but may be 0.0010%.
• a B parameter Bp which is defined by Expression 5 as a parameter for avoiding an increase the hardness of the HAZ due to B is preferably 0.09% to 0.30%.
• Bp 884 ⁇ C ⁇ 1 - 0.3 ⁇ C 2 + 294 ⁇ FB
• Bp is an empirical equation derived from the analysis by testing plural pieces of molten steel at an experimental laboratory and is parameterized by (the highest hardness estimated by amount of C) x (FB contribution). As the FB increases, the hardness of the HAZ tends to be increased. Particularly, the CTOD properties are significantly affected as this case. When Bp exceeds 0.30%, the hardness of a fusion line (FL) is remarkably increased in some cases, and thus, it has been found that the Bp is preferably limited to 0.30% or less to satisfy 0.25 mm or more that is the target value of the CTOD properties. As necessary, the upper limit of the Bp may be 0.27% or 0.25%.
• the Bp has to be 0.09% or more, and thus, Bp of less than 0.09% within a region in which the effect of the solid soluted B targeted in the welded steel according to the embodiment cannot be obtained. Therefore, the Bp may be 0.09% or more. As necessary, the lower limit of the Bp may also be 0.12% or 0.15%.
• N is an element which is unavoidably included in steel production and the reduction of N more than needed increases a load on steel production and is not preferable in terms of industrial production.
• N rather forms nitrides with the addition of Ti.
• the nitrides are stable at a high temperature and thus, have an effect of pinning austenite grain growth coarsening at the time of heating before hot-rolling the steel or in the heat-affected zone that is slightly away from the fusion line. Therefore, it is preferable that 0.0020% or more of N be included.
• N is bonded with B to form nitrides as described above, and thus, the hardenability improving effect of B is highly likely to be reduced.
• the upper limit is naturally limited, but additionally, when the amount of N exceeds 0.0060%, a surface defect occurs at the time of producing a steel piece.
• the upper limit is set to 0.0060%.
• the upper limit is preferably 0.0055% or less, and more preferably 0.005% or less.
• the amount of O is preferably 0.0030% or less, and more preferably 0.0028% or less, or 0.0025% or less.
• Al is one of unavoidable impurities.
• the reason for limiting the upper limit in Claim 1 is that when the content of Al exceeds 0.004% even being unavoidably included, the composition of the oxides is changed and is highly likely not to function as the nuclei for intragranular ferrite, and thus, the amount of Al is limited to 0.004% or less. As necessary, the upper limit thereof may be 0.003% or 0.002%.
• the lower limit of the amount of Al does not need to be defined particularly, and the lower limit thereof is 0%. However, Al is mixed during the refining process of the steel in some cases, and thus, the lower limit thereof may be 0.0001% or 0.0003%.
• the steel according to the embodiment also includes a balance consisting of Fe and unavoidable impurities in addition to the above components.
• the impurities are components that are mixed on account of various factors in the production process including raw materials such as ore or scrap when steel is produced on an industrial scale and are allowed to be contained within the range such that the components do not exert an adverse influence on the present invention.
• the steel plate according to the embodiment may include one or two types or more of Cr, V, Ca, Mg and REM, in addition to the above components.
• the lower limits of the components do not need to be defined particularly and the lower limits thereof are 0%.
• the alloy components are intentionally added or mixed as impurities, it is interpreted that the steel is within the scope of the Claims as long as the contents thereof are within the scope of the Claims.
• Cr is set to 0.30% or less since Cr deteriorates the CTOD properties of the heat-affected zone.
• the upper limit thereof may be 0.20%, 0.15%, 0.10%, or 0.05%.
• the lower limit of the amount of Cr does not need to be defined particularly, and the lower limit thereof is 0%. However, there is a case that Cr is mixed as an impurity and thus, the lower limit thereof may be 0.001%.
• V is an effective element to improve the strength of the base metal.
• the upper limit is set to 0.06% or less.
• the upper limit thereof may be 0.04%, 0.02%, or 0.01%.
• the lower limit of the amount of V does not need to be defined particularly, and the lower limit thereof is 0%. However, even when V is mixed as an impurity, the lower limit thereof may be 0.001%.
• Mg can be included as necessary. When Mg is included, fine Mg-containing oxides are formed and thus, it is effective in y grain size refinement. However, when the Mg content exceeds 0.0050%, the number of oxides is excessively increased and ductility is reduced. Thus, the upper limit thereof is set to 0.0050%. The upper limit thereof may be limited to 0.0030%, 0.0020%, 0.0010%, or 0.0003%. The lower limit of the Mg content does not need to be defined particularly, and the lower limit thereof is 0%.
• the welded steel according to the embodiment may include the following alloy elements for the purpose of further improving the strength and the toughness of the steel itself, or as impurities from auxiliary raw material such as scrap, in addition to the above components.
• Ca is mixed as an impurity in some cases and thus, the upper limit thereof may be limited to 0.0010%, 0.0005%, or 0.0003%.
• Rare earth metal is mixed as impurities in some cases and thus, the upper limit thereof may be limited to 0.0010%, 0.0005%, or 0.0003%.
• the REM is a general term of 17 elements including 15 lanthanoid elements and Y and Sc.
• the upper limit of the Sb content may be 0.03%.
• the upper limit of the Sb content may be 0.01%, 0.005%, 0.003%, or 0.001%.
• the upper limits of the As and Sn contents may be 0.02%. As necessary, the upper limits of the As and Sn contents may be 0.005%, 0.003%, or 0.001%.
• the lower limits of Ca, REM, Sb, As, and Sn do not need to be defined particularly and the lower limits are 0%.
• each of Pb, Zr, Zn, and W content is set to 0.1% or less, 0.01% or 0.005% or less.
• the lower limits do not need to be defined particularly and are 0%.
• Co is included as an impurity in Ni in some cases. Since Co deteriorates the toughness of the HAZ, the upper limit of the Co content may be 0.05% or 0.002%. The lower limit thereof does not need to be defined particularly, and the lower limit thereof is 0%.
• P CM C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 B
• each of the elements represents the amounts included in the steel by mass%.
• the number of oxides having an equivalent circle diameter of 2 ⁇ m or more is 20 particles/mm 2 or less, and the number of Ti oxides included as transformation nuclei in the steel and having an equivalent circle diameter of 0.05 ⁇ m to 0.5 ⁇ m is 1.0 ⁇ 10 3 particles/mm 2 to 1.0 ⁇ 10 5 particles/mm 2 to satisfy the CTOD properties.
• the oxides act as a starting point of fracture and the CTOD properties are deteriorated.
• the number of Ti oxides having an equivalent circle diameter of 0.05 ⁇ m to 0.5 ⁇ m is less than 1.0 ⁇ 10 3 particles/mm 2
• the number of Ti oxides as nuclei for forming intragranular transformation ferrite is insufficient and when the number of Ti oxides exceeds 1.0 ⁇ 10 5 particles/mm 2 , the Ti oxides act as a starting point of fracture, and in both the cases, the CTOD properties are deteriorated.
• the steel of the present invention is produced industrially by a continuous casting method.
• the reason is that the solidification cooling rate of molten steel is high and a large amount of fine Ti oxides and Ti nitrides can be formed in a slab.
• an average cooling rate at the center portion of the slab to 800°C from around a solidifying point is 5 °C/min or higher.
• the reason is to obtain oxides having an equivalent circle diameter of 2 ⁇ m or more of 20 particles/mm 2 or less and Ti oxides having an equivalent circle diameter of 0.05 ⁇ m to 0.5 ⁇ m of 1.0 ⁇ 10 3 particles/mm 2 to 1.0 ⁇ 10 5 particles/mm 2 in the steel.
• the average cooling rate of the slab When the cooling rate of the slab is lower than 5 °C/min, fine oxides are hardly obtained and coarse oxides increase. On the other hand, even when the average cooling rate exceeds 50 °C/min, the number of fine Ti oxides does not significantly increase and the cost of production rather increases, and thus, the average cooling rate may be 50 °C/min or lower.
• the average cooling rate at the center portion of the slab can be obtained by measuring the cooling rate of the surface of the slab and performing thermal conduction calculation.
• the average cooling rate can also be obtained by measuring a casting temperature and an amount of cooling water and performing thermal conduction calculation.
• the re-heating temperature thereof is preferably 1000°C to 1100°C.
• the re-heating temperature exceeds 1100°C, the Ti oxides are coarsened and an effect of improving deterioration in toughness of the base metal or the toughness of the HAZ cannot be expected.
• the productivity is impaired.
• TMCP After the re-heating, production in TMCP is required.
• rolling is performed at a temperature of 950°C or higher under a cumulative reduction of 30% or more.
• the grains of heated coarse austenite are sized and refined, and thus, the more the cumulative reduction, the more preferable it is.
• the rolling in a high temperature region is regulated by the thickness of the slab and the subsequent rolling conditions. It is difficult to grasp the rolled structure in a high temperature state, but in the factory or laboratory test of the inventors, when the cumulative reduction is 30% or more and the subsequent rolling-cooling conditions are within an appropriate range, it has been confirmed that the properties are stable.
• the rolling is performed under a cumulative reduction of 40% or more at a temperature of 720°C to 950°C and a total cumulative reduction of 60% or more, and the rolling is finished at a temperature of 700°C to 750°C.
• the temperature regions are mostly austenite non-recrystallization temperature regions.
• thick material has temperature distribution in a thickness direction and has a high temperature in the vicinity of the thickness center portion, rolling may not be sufficiently performed in the non-recrystallization temperature region. Therefore, the temperature and the cumulative reduction are limited in two stages the in the present invention.
• the rolling under a cumulative reduction of 40% or more at a temperature of 720°C to 950°C is a minimum required rolling reduction in austenite non-recrystallization by a depth of about 1/4 of the thickness from the surface layers of the front and rear surfaces. Further, the reason for finishing the rolling with a total cumulative reduction of 60% or more at a temperature of 700°C to 750°C is that the rolling is applied to even to the thickness center portion in the austenite non-recrystallization temperature region to a degree that enables structure refinement.
• the thickness center portion In the thickness center portion, a relatively small reduction in the austenite non-recrystallization temperature region is unavoidable, but the structure can be refined to a degree at which satisfactory a balance between strength and toughness can be ensured with a relatively low heating temperature and an appropriate reduction in a high temperature region limited in the present invention. In the rolling conditions deviated from the limitation ranges, it has been experimentally confirmed that the toughness of the thickness center portion is particularly deteriorated.
• cooling after the rolling it is necessary to perform cooling to 280°C or lower by starting water cooling within 80 seconds after the rolling is finished. It is preferable that water cooling start rapidly after the rolling. However, it is unavoidable that a certain period of time is required for transportation from an end of a rolling mill to a cooling facility in large real production facilities. Even in this case, the precipitation of ferrite during air cooling until the cooling after the rolling is not preferable in terms of strength, and the ferrite is highly likely to be coarsened due to precipitation in the air cooling, which is not preferable in terms of toughness. Therefore, it is necessary to start water cooling within 80 seconds after the rolling is finished. The water cooling preferably starts within 60 seconds.
• tempering After the cooling, tempering has to be performed in a temperature range of 400°C to 550°C.
• the balance between strength and toughness of the base metal are improved and also can be stably controlled with high accuracy.
• nonuniformity at the time of cooling is alleviated, and thus, an effect of releasing residual stress in the steel is obtained.
• shape change caused by the nonuniformity and the residual stress at the time of cutting is suppressed.
• the tempering is performed at a temperature of lower than 400°C, the effects are reduced and when the tempering is performed at a temperature of higher than 550°C, the strength is significantly reduced and it is difficult to ensure high strength that is targeted in the present invention.
• all the aforementioned temperatures are steel surface temperatures.
• a steel piece or a cast piece having the steel components described (1) is heated at a temperature of 1000°C to 1100°C, rolling is finished such that the cumulative reduction is 30% or more at a temperature of 950°C or higher, the cumulative reduction is 40% or more at a temperature of 720°C to 950°C, and the cumulative reduction is 60% or more at a temperature of 700°C to 750°C, water cooling is performed within 80 seconds after the rolling is finished to cool the steel to 280°C or lower, and then, further tempering is performed in a temperature range of 400°C to 550°C.
• the welding was performed by the submerged arc welding method that is generally used as test welding and multilayer welding was performed with a weld heat input of 4.5 kJ/mm at a K groove so that the weld fusion line (FL) became vertical.
• a CTOD test was performed according to American Petroleum Institute (API) standard RP 2Z and British Standard (BS) 7448. Notches were made on the fusion line that is referred to as coarse grain HAZ (CGHAZ) and six pieces were tested at a test temperature of -10°C on each.
• API American Petroleum Institute
• BS British Standard
• the yield strength (YS) was 526 MPa to 611 MPa at a depth position of 1/4 of the steel plate and the yield strength was 516 MPa to 594 MPa at a depth position of 1/2 of the steel plate
• the tensile strength (TS) was 616 MPa to 680 MPa at a depth position of 1/4 of the steel plate and the tensile strength was 604 MPa to 656 MPa at a depth position of 1/2 of the steel plate.
• the fracture transition temperatures were -48°C to -81°C at a depth position of 1/4 of the steel plate and the fracture transition temperatures were -40°C to -68°C at a depth position of 1/2 of the steel plate, and the lowest CTOD value at -10°C was 0.29 mm to 0.94 mm, and thus, satisfactory fracture toughness was exhibited. Further, satisfactory weldability was exhibited by the P CM values and the CTOD properties of the invention steels.
• Comparative examples a to c the steel components in Comparative examples a to c, Comparative examples e to o, and Comparative examples q to v were out of the range of the present invention and the above-described mechanical properties were not satisfied.
• Comparative example f with the steel component No. 21 did not satisfy Ni/Cu > 2.0, cracking occurred at the time of hot rolling and thus the production was difficult.
• Comparative examples d, w, and x whose steel components are within the range of the present invention but the FB or P CM values are out of the range of the present invention did not satisfy FB ⁇ 0.0003%, or the P CM value was not within a range of 0.18% to 0.23%, and thus, the strength of the base metal was decreased or increased, the toughness of the base metal was deteriorated, or the toughness of the heat-affected zone was deteriorated.
• the ultrahigh tensile strength steel having excellent weldability and toughness of the heat-affected zone at a low cost and also possible to further increase safety while the size of a welded structure such as an offshore structure can be increased.

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