EP1221493B1 - Dicke stahlplatte mit hervorragenden ctod-eigenschaften der durch schweissen beeinflussten bereiche und mit einer streckgrenze von 460 mpa oder mehr - Google Patents

Dicke stahlplatte mit hervorragenden ctod-eigenschaften der durch schweissen beeinflussten bereiche und mit einer streckgrenze von 460 mpa oder mehr Download PDF

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EP1221493B1
EP1221493B1 EP01930007A EP01930007A EP1221493B1 EP 1221493 B1 EP1221493 B1 EP 1221493B1 EP 01930007 A EP01930007 A EP 01930007A EP 01930007 A EP01930007 A EP 01930007A EP 1221493 B1 EP1221493 B1 EP 1221493B1
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haz
particles
less
grains
steel
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EP1221493A1 (de
EP1221493A4 (de
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Akihiko Nippon Steel Corp. Kimitsu Works KOJIMA
Yoshio Nippon Steel Corp. Kimitsu Works TERADA
Akihito Nippon Steel Corp. Techn. Devel. KIYOSE
Yuzuru Nippon Steel Corp. Kimitsu Works YOSHIDA
Toshihiko Nippon Steel Corporation ADACHI
Kazuaki Nippon Steel Corp. Kimitsu Works TANAKA
Ryuji Nippon Steel Corp. Techn. Devel. UEMORI
Shiro Nippon Steel Corporation IMAI
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Nippon Steel Corp
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Priority claimed from JP2001049838A external-priority patent/JP3699657B2/ja
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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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—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
    • C22C38/04—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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations

Definitions

  • the present invention relates to a steel plate having an excellent CTOD (Crack Tip Opening Displacement) property in a weld heat-affected zone (HAZ) and a yield strength not lower than 460 MPa, preferably in the class of 500 to 550 MPa, mainly used for offshore structures, and also applicable to other welded structures of which strengths and HAZ toughness (CTOD property) on a comparable level are required.
  • CTOD Cross Tip Opening Displacement
  • a CTOD property at -10°C is required of welded joints in offshore structures used in the Northern Sea.
  • Ti-oxide steel is used as is described, for example, in "Proceedings of 12th International Conference on OMAE, 1993, Glasgow, UK, ASME, Volume III-A, pp.207 - 214.” Because portions of a HAZ in close proximity to a fusion line are heated to 1,400°C or higher, a pinning effect by TiN particles becomes lost, austenite ( ⁇ ) grains are markedly coarsened, and also the structure of the HAZ is coarsened, thus deteriorating the toughness thereof.
  • the Ti-oxide steel described above has been developed as a steel that solves such a problem.
  • This technology provides a steel in which a HAZ structure is fined by using acicular ferrite grains that are generated with thermally stable Ti oxide particles used as transformation nuclei in ⁇ grains coarsened due to the loss of the pinning effect of TiN particles, as is described in Japanese Unexamined Patent Publication Nos. S63-210235 and H6-075599, for example.
  • the acicular ferrite grains which effectively fine coarse ⁇ grains are called intragranular transformed ferrite (IGF) grains.
  • the yield strength of this Ti-oxide steel is not higher than 420 MPa, and no steel plate has been developed that assures a CTOD property in its HAZ while having a yield strength higher than 420 MPa.
  • a steel plate having a higher yield strength is required in order to reduce the weight of offshore structures. That is, a steel plate is strongly required which is able to assure a CTOD property while having a yield strength not lower than 460 MPa, a strength higher than any available.
  • JP-A-09-157787 discloses a high tensile strength steel for welding, excellent in toughness in very large heat input welded heat affected zone.
  • This steel contains Mg-containing oxides having a grain size of 0.01 to 0.20 ⁇ m with a number of 40,000 to 1000,000 pieces/mm 2 and Ti-containing oxides and MnS having a grain size of 0.20 to 5.0 ⁇ m with a number of 20 to 400 pieces/mm 2 .
  • JP-A-11-279684 also discloses a high tensile strength steel for welding, excellent in toughness in very large heat input welded heat affected zone.
  • This steel contains Ti-Mg-Al composite oxides having a grain size of 0.20 to 0.50 ⁇ m with a number of 10 to 500 pieces/mm 2 and composites of Al-Mg oxides with TiN having a grain size of 0.005 to 0.1 ⁇ m.
  • the object of the present invention is to provide a steel plate having a yield strength not lower than 460 MPa, preferably in the class of 500 to 550 MPa, and a CTOD not less than 0.2 mm in a HAZ at -10°C.
  • Figs. 1 (a) to (d) are illustrations schematically showing the concept of HAZ structure control for a steel plate having an excellent CTOD property in its weld heat-affected zone and a yield strength not lower than 460 MPa according to the present invention.
  • Fig. 1 (a) illustrates a HAZ structure in a conventional Ti-oxide steel
  • Fig. 1 (d) illustrates a HAZ structure in a steel of the present invention.
  • reference numeral 1 stands for a weld metal
  • 2 for a weld heat-affected zone (HAZ)
  • 3 for a fusion line.
  • HAZ structure 4 stands for a ⁇ grain boundary, GBF for a grain-boundary ferrite grain, FSP for a ferrite side-plate grain, IGF for an intragranular'transformed ferrite grain, Bu for upper bainite, and MA for a martensite-austenite constituent.
  • a HAZ in close proximity to a fusion line becomes hard, thus making it difficult to secure a sufficient CTOD property.
  • a HAZ structure in such a state is schematically shown in Fig. 1 (a).
  • the primary cause' to embrittle a HAZ is that coarse grain-boundary ferrite (GBF) grains and a ferrite side-plate (FSP) grains generated along the grain boundary of a coarse ⁇ grain increase the susceptibility of the HAZ to brittle fracture with the increase in HAZ hardness, even though the interior of the coarse ⁇ grain is fined by intragranular transformed ferrite (IGF) grains formed therein. Accordingly, it is necessary to decrease the susceptibility to brittle fracture by fining the GBF grains and FSP grains.
  • GBF grain-boundary ferrite
  • FSP ferrite side-plate
  • the secondary cause of the embrittlement is that increased amounts of alloy elements added for strength enhancement increase the hardenability of the HAZ and a large number of microscopic brittle phases called MA (martensite-austenite constituent) are generated, which accelerate the occurrence of brittle fracture. Also, when a yield strength not lower than 460 MPa is to be achieved, it is necessary to decrease MA to the greatest possible extent. From what is described above, it is a guideline for achieving a satisfactory CTOD property of welded joints under a high yield strength to remove the two causes of embrittlement described above while maintaining the metallographic effect (IGF effect) of Ti-oxide steel. In other words, the gist of the present invention is to control a HAZ structure so as to satisfy the following three points at the same time:
  • the TiN particles precipitated in the compounded form are thermally stable without growing or dissolving even in close proximity to the fusion line and therefore they can strongly pin ⁇ grain boundaries to not allow their movement. Even if welding is carried out with a large amount of weld heat input, the size of the ⁇ grains in close proximity to the fusion line can be kept on the order of 100 ⁇ m. In some cases, these pinning particles existing on the ⁇ grain boundaries directly function by themselves as transformation nuclei for the GBF grains and FSP grains, and therefore an increase of transformation places also contributes to the fining of the GBF grains and FSP grains.
  • the existence of not less than 10,000 pieces/mm 2 of such TiN particles precipitated in the compounded form fines the GBF grains and FSP grains to a size not adversely affecting a CTOD property. If the TiN particles precipitated in the compounded form are less than 10,000 pieces/mm 2 , the fining of the ⁇ grains and the number of transformation nuclei on the ⁇ grain boundaries become insufficient, and consequently, the GBF grains and FSP grains are not sufficiently fined, thus deteriorating the CTOD property. In some cases, sulfide is precipitated in a compounded form on the TiN particles, but this does not adversely affect their above described functions as pinning particles or transformation nuclei.
  • Fig. 1 (b) is a schematic diagram showing a HAZ structure to which only the technique of the item (1) explained above is applied. Although the GBF grains and FSP grains are fined, the interior of the ⁇ grains is covered by a brittle structure including MA, called upper bainite, and a sufficient CTOD property cannot be acquired by this technique only. Therefore, the technique of the item (2) explained hereunder must be jointly used.
  • Mg is intentionally added in order to generate the above ultrafine oxide particles in large numbers. Since Mg is also contained in oxide particles of an ordinary size (several ⁇ m), a study was made, in the process of the present invention, to generate IGF grains by utilizing such relatively large Mg-containing oxide particles. As a result, the following three conditions were found to be important for IGF transformation nuclei:
  • Mn Mn oxide particles of 0.5 to 10 ⁇ m.
  • Mg, Al and Ti are essential for the present invention in order to generate the ultrafine pinning particles, which consist of (Mg, Al) oxide and Ti nitride, explained in the item (1). Since these elements have deoxidizing power stronger than Mn has, the oxide particles of 0.5 to 10 ⁇ m are mainly composed of Mg, Al and Ti. Therefore it is difficult to stably incorporate not less than 0.3 wt% of Mn into the oxide particles of 0.5 to 10 ⁇ m.
  • Mn-containing sulfide is precipitated in a compounded form on the oxide particles.
  • Mn content in the compounded particles can be stably increased to 0.3 wt% or higher and the particles can effectively function as IGF transformation nuclei.
  • the sulfide was not compounded therewith but existed separately. To sum up, it was found that the Mn-containing sulfide was able to be stably compounded with, and precipitated on, the oxide particles of 0.5 to 10 ⁇ m by incorporating not less than 10 wt% of Mg into the oxide particles. As a result of this, IGF transformation nuclei at not less than 10 pieces/mm 2 of 0.5 to 10 ⁇ m and containing not less than 0.3 wt% of Mn can be secured in the compounded form of the oxide and the sulfide.
  • Fig. 1 (c) is a schematic diagram showing a HAZ structure to which the techniques of the items (1) and (2) explained above are jointly applied.
  • the HAZ structure is fined by generating a large number of IGF grains in addition to the fining of GBF grains and FSP grains. If the amount of added alloy constituents is inappropriate, the amount of MA generated is increased to deteriorate a CTOD property. Therefore, it is necessary to stably enhance the CTOD property by jointly using the technique of the item (3) explained below.
  • a steel of the present invention in which ⁇ grains are smaller and besides IGF generating ability is higher to cause places of transformation to increase on ⁇ grain boundaries and within ⁇ grains, has a feature that the hardenability of a HAZ is remarkably lowered relative to a conventional steel having the same chemical compositions.
  • the effect of alloy contents on MA generation was intensively studied with the cooling rate (cooling time from 800°C to 500°C being about 15 sec) used in the execution of welding for offshore structures and the ranges of C and Mn of the present invention taken as preconditions. As a result, the following two points have become apparent:
  • Nb is the upper limit of Nb for a 460 MPa-class yield strength in the case of "Proceedings of 12th International Conference on OMAE, 1993, Glasgow, UK, ASME, Volume III-A, pp. 199 - 205”
  • Nb is 0.024 wt% for a 420 MPa-class yield strength in the case of "Proceedings of 13th International Conference on OMAE, 1994, Houston, ASME, Volume III, pp. 307 - 314.”
  • an Nb content on the order of 0.02 wt% has been substantially an upper limit.
  • the present invention has the advantage of allowing Nb up to 0.05 wt% to be effectively used.
  • Fig. 1 (d) is a schematic diagram showing a HAZ structure to which the technique of the item (3) explained above and the techniques of the items (1) and (2) are jointly applied.
  • the HAZ structure is sufficiently fined and besides the amount of MA is stably reduced and, therefore, a satisfactory CTOD property of welded joints is achieved for a higher strength.
  • the present invention becomes feasible by implementing the techniques of items (1), (2) and (3) at the same time.
  • % means weight %.
  • C at not less than 0.04 % is necessary in order to secure the strength and toughness of a base metal and a HAZ.
  • the C content exceeds 0.14 %, the toughness of the base metal and the HAZ is deteriorated and also the weldability is deteriorated, and therefore 0.14 % is the upper limit.
  • Si can be added for deoxidation. However, if the Si content exceeds 0.4 %, HAZ toughness is deteriorated.
  • Al, Ti, or Mg can also be used for deoxidation and, therefore, the Si content should be as little as possible from the viewpoint of HAZ toughness. Since Si accelerates MA generation in a HAZ, it is an undesirable element for the present invention.
  • Mn at not less than 1 % is necessary to secure the strength and toughness of a base metal and a HAZ. Also, Mn is important for forming a sulfide that constitutes IGF transformation nuclei. However, if the Mn content exceeds 2.0 %, the base metal and HAZ are embrittled and also the weldability is deteriorated, and therefore 2.0 % is the upper limit.
  • P is an impurity element for the present invention and has to be reduced below 0.02 % in order to secure the satisfactory quality of a base metal and a HAZ.
  • S is a necessary element for the present invention. S at not less than 0.001 % has to be secured in order to precipitate sulfide on oxide particles as IGF transformation nuclei in a compounded form. However, if the S content exceeds 0.005 %, the toughness of a base metal and a HAZ is deteriorated, and therefore 0.005 % is the upper limit.
  • Nb is extremely effective in enhancing the strength of a base metal while minimizing the deterioration of the toughness of a HAZ. Also, Nb is effective in enhancing toughness through the fining of the structure of a base metal. For example, Nb at not less than 0.005 % is essential in order to acquire satisfactory base metal toughness while achieving a yield strength of the 500 MPa class for a plate thickness of 76.2 mm. However, if the Nb content exceeds 0.05 %, the toughness of a HAZ is deteriorated by an increase in the amount of MA or by precipitation hardening, and therefore 0.05 % is the upper limit. Nb is an element that should be positively used for successfully manufacturing a base metal according to the present invention, and it is desirable to make effective use of Nb at not less than 0.02 %.
  • Al together with Mg, forms ultrafine oxide particles of 0.01 to 0.1 ⁇ m and functions as pinning particles in company with TiN precipitated in a compounded form on the oxide particles and further as transformation nuclei for GBF particles and FSP grains, thereby fining a HAZ structure.
  • Al at not less than 0.001 % is necessary. If the Al content is less than 0.001 %, it is impossible to secure the required number of ultrafine oxide particles for obtaining not less than 10,000 pieces/mm 2 of compounded TiN particles, and therefore the fining of ⁇ grains and the number of transformation nuclei on ⁇ grain boundaries become insufficient. As a result, neither GBF grains nor FSP grains are sufficiently fined, thus deteriorating the HAZ toughness.
  • the Al content in the oxide constituting IGF transformation nuclei is increased and, as a counteraction to this, the Mg content in the oxide falls below 10 wt%.
  • Mn-containing sulfide is hardly precipitated on the oxide particles to cause them to lose their ability as IGF transformation nuclei, and therefore it becomes difficult to secure not less than 10 pieces/mm 2 of IGF transformation nuclei in a stable manner.
  • the upper limit of the Al is 0.01 %.
  • Ti forms TiN which precipitates on ultrafine (Mg, Al) oxide particles in a compounded form of 0.01 to 0.5 ⁇ m in size and functions as pinning particles and further as transformation nuclei for GBF grains and FSP grains, thus fining a HAZ structure.
  • Ti at not less than 0.005 % is necessary. If the Ti is less than 0.005 %, it is impossible to secure not less than 10,000 pieces/mm 2 of TiN particles in the compounded form. As a result, neither GBF grains nor FSP grains are sufficiently fined, thus deteriorating the HAZ toughness.
  • both Si and Al are close to their lower limits, deoxidizing elements sometimes become insufficient, and therefore it is desirable to add Ti at not less than 0.01 % in order to cause Ti to take-over the deoxidation.
  • the Ti content exceeds 0.03 %, TiC is precipitated or TiN particles are coarsened to a size as large as several ⁇ m, thereby embrittling a base metal and a HAZ.
  • the upper limit of the Ti is 0.03 %.
  • Mg plays the most important role in the present invention.
  • the primary role of Mg is to form, together with Al, ultrafine oxide particles of 0.01 to 0.1 ⁇ m, to function as pinning particles in company with TiN precipitated in a compounded form on the oxide particles and further to function as transformation nuclei for GBF grains and FSP grains, thereby fining the HAZ structure.
  • the secondary role of Mg is to accelerate the precipitation of Mn-containing sulfide in the compounded form on oxide particles of 0.5 to 10 ⁇ m by being incorporated therein at 10 wt% or more to provide a function as transformation nuclei to the oxide particles, thereby fining the HAZ structure.
  • Mg at not less than 0.0003 %, preferably at not less than 0.0005 %, is necessary. If the Mg is less than 0.0003 %, the contents of Si, Al, Ti, and the like in the oxide are increased and, as a counteraction to this, the Mg content in the oxide falls below 10 wt%. As a result, the Mn-containing sulfide is hardly precipitated on the oxide particles to cause them to lose their ability as IGF transformation nuclei, and therefore the number of IGF transformation nuclei becomes insufficient. At the same time, it becomes difficult to secure the required number of ultrafine (Mg, Al) oxide particles for obtaining not less than 10,000 pieces/mm 2 of compounded TiN particles. However, if the Mg exceeds 0.005 %, its metallographic effect is saturated, and therefore this value is set as its upper limit.
  • O constitutes ultrafine (Mg, Al) oxide particles that have a HAZ pinning effect while it constitutes Mg-containing oxide particles of 0.5 to 10 ⁇ m that function as IGF transformation nuclei in a HAZ.
  • O at not less than 0.001 % is necessary. If the O content is less than 0.001 %, it becomes difficult to secure the necessary number of ultrafine oxide particles for obtaining not less than 10,000 pieces/mm 2 of compounded TiN particles and securing not less than 10 pieces/mm 2 of oxide particles of 0.5 to 10 ⁇ m.
  • N forms TiN which precipitates on ultrafine (Mg, Al) oxide particles in a compounded form of 0.01 to 0.5 ⁇ m in size and functions as pinning particles and further as transformation nuclei for GBF grains and FSP grains, thereby fining the HAZ structure.
  • N at not less than 0.001 % is necessary. If the N content is less than 0.001 %, it is impossible to secure not less than 10,000 pieces/mm 2 of TiN particles in the compounded form. However, if the N content exceeds 0.01 %, solute N is increased to cause a base metal and a HAZ to be embrittled and surface properties of a cast slab to be deteriorated, and therefore this value is set as the upper limit.
  • Ca, REM and Zr can be added as deoxidizing agents or desulfurizing agents. They contribute to the reduction of the O content by acting as deoxidizing agents. As desulfurizing agents, they contribute to the reduction of S content while they control the shape of sulfide. In order to improve the material quality of a base metal and a HAZ through these effects, the content of each element is required to be 0.0005 % or more. If these elements are too large in amount, they are mixed in IGF transformation nuclei to decrease the Mg content and Mn content in oxide and sulfide constituting the IGF transformation nuclei, and thus the IGF transformation nuclei lose their function.
  • the upper limits of Ca, REM and Zr are 0.005 %, 0.01 % and 0.01 %, respectively, and it is necessary to limit the total amount of the three elements to 0.02 % or less.
  • REM here indicates lanthanoid elements such as La and Ce, and even if a misch metal made up of these elements mixed together is alternatively added, the effect described above can be obtained.
  • Cu, Ni, Cr and Mo can be utilized to enhance the strength, toughness, corrosion resistance and the like, of a base metal.
  • any content of these elements has to be 0.05 % or more.
  • these elements have been positively used in cases where it is necessary to attain the enhancement of the strength and toughness of a base metal and the enlargement of plate thickness range at the same time.
  • the upper limits of Cu, Ni, Cr and Mo have to be controlled to 1.5 %, 3.0 %, 0.5 % and 0.5 %, respectively, and further, the total amount of these elements must be adjusted not to exceed 3.0 %. If any of these elements exceeds its upper limit or if the total amount of these elements exceeds 3.0 %, the CTOD property of a HAZ is remarkably deteriorated.
  • V effectively enhances the strength of a base metal and a HAZ by the precipitation strengthening.
  • V at not less than 0.005 % is necessary.
  • 0.05 % is set as the upper limit.
  • B is effective in enhancing the strength and toughness of a base metal.
  • not less than 0.0001 % of B is necessary.
  • the B content exceeds 0.003 %, weldability is remarkably deteriorated, and therefore, 0.003 % is set as the upper limit.
  • a steel of the present invention undergoes the adjustment of chemical compositions to specified values and is continuously cast into a slab in a steelmaking process in the steel industry, and the slab is manufactured into a steel plate through processes of reheating, rolling, cooling, and heat-treatment, these being controlled in various ways.
  • reheating preferably the 500 to 550 MPa class
  • a thick plate having a thickness such as 76.2 mm
  • strength and toughness can be adjusted by tempering. Also, it is possible to apply hot-charge rolling without cooling a cast slab once.
  • the toughness of a HAZ is determined not only by chemical compositions but also by the state of the dispersion of pinning particles and that of IGF transformation nuclei. The state of the dispersion of these particles is not greatly changed in the course of manufacturing a base metal. Accordingly, the toughness of a HAZ does not greatly depend on the manufacturing processes of a base metal and therefore each of the reheating, rolling, and heat-treating processes can be of any type.
  • the state of the dispersion of inclusions specified in the present invention is quantitatively measured by methods such as those described below.
  • the number of TiN particles of 0.01 to 0.5 ⁇ m including oxide composed of Mg and Al is determined by: preparing a sampling replica specimen taken from an arbitrary position of a base-metal steel plate, observing the specimen using a transmission electron microscope (TEM) under the magnification of 10,000 to 50,000 so as to cover an area of at least 1,000 ⁇ m 2 , measuring the number of TiN particles having sizes falling within the targeted range, and converting it into the number of particles per unit area (pieces/mm 2 ).
  • TEM transmission electron microscope
  • the identification of (Mg, Al) oxide particles with TiN particles is performed by means of composition analysis using energy-dispersive X-ray spectroscopy (EDS) annexed to the TEM and crystalline structure analysis of electron diffraction images using the TEM.
  • EDS energy-dispersive X-ray spectroscopy
  • the number of particles of 0.5 to 10 ⁇ m formed by compounding oxide with Mn-containing sulfide can be measured by a method such as that described below.
  • a polished specimen with a mirror finished surface is prepared by cutting a small piece of specimen out of an arbitrary position of a base-metal steel plate, the specimen is observed using an optical microscope under the magnification of 1,000 so as to cover an area of at least 3 mm 2 , the number of particles having sizes falling within the targeted range is measured, and the measured number is converted into the number of particles per unit area (pieces/mm 2 ).
  • At least 10 randomly selected particles taken from the same specimen and having sizes falling within the targeted range undergo composition analysis using a wavelength-dispersive X-ray spectroscope (WDS) annexed to the scanning electron microscope (SEM).
  • WDS wavelength-dispersive X-ray spectroscope
  • SEM scanning electron microscope
  • Table 1 shows the chemical compositions of the continuous-cast steels
  • Table 2 shows the thickness of each steel plate, the manufacturing method thereof, the number of pinning particles, the number of IGF transformation nuclei, the material quality of base metals, the welding conditions, and the toughness of each HAZ.
  • the steels of the present invention have plate thicknesses of 38.1 to 76.2 mm, base-metal yield strengths (YS) of 510 to 570 MPa, and satisfactory CTOD exceeding 0.2 mm at -10°C in a multilayered joint bond part (CGHAZ) made by submerged arc welding with weld heat input of 3.5 to 10.0 kJ/mm.
  • YS base-metal yield strengths
  • CGHAZ multilayered joint bond part
  • comparative steels are inferior in base metal quality or HAZ quality for the plate thickness of 76.2 mm because of the inappropriate chemical composition.
  • Steel 12 has an insufficient number of IGF transformation nuclei because the amount of S is too small, and is inferior in HAZ toughness.
  • Steel 13 is inferior in base metal toughness and HAZ toughness because the amount of S is too large.
  • Steel 14 is inferior in the strength and toughness of the base metal because the amount of Nb is too small.
  • Steel 15 is inferior in HAZ toughness because the amount of Nb is too large.
  • Steel 16 has an insufficient number of pinning particles because the amount of Al is too small, and is inferior in HAZ toughness.
  • Steel 17 has an insufficient number of IGF transformation nuclei because the amount of Al is too large, and is inferior in HAZ toughness.
  • Steel 18 has an insufficient number of pinning particles because the amount of Ti is too small, and is inferior in HAZ toughness.
  • Steel 19 is inferior in base metal toughness and HAZ toughness because the amount of Ti is too large.
  • Steel 20 and Steel 21 are insufficient in the number of pinning particles and in the number of IGF transformation nuclei because of too small amounts of Mg and O, respectively, and are inferior in HAZ toughness.
  • Steel 22 has an insufficient number of pinning particles because the amount of N is too small, and is inferior in HAZ toughness.
  • Steel 23 is inferior in HAZ toughness because the total amount of Cu, Ni, Cr and Mo is too large.
  • Steel 24 has an insufficient number of IGF transformation nuclei because the total amount of Ca, REM and Zr is too large, and is inferior in HAZ toughness.
  • the present invention remarkably improves the CTOD property of welded joints of a high-strength, ultra-heavy steel plate, and as a result, it paves the way for a weight reduction and upsizing of offshore structures. This allows the construction cost of offshore structures to be sharply reduced and energy development in much deeper seas area to be carried out.

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  • Heat Treatment Of Steel (AREA)

Claims (1)

  1. Stahlplatte mit einer hervorragenden CTOD-Eigenschaft in einer durch Schweißen wärmebeeinflußten Zone und einer Streckgrenze von nicht weniger als 460 MPa, dadurch gekennzeichnet, daß sie eine chemische Zusammensetzung aufweist, die folgendes in Gewichtsprozent enthält: C 0,04 bis 0,14%, Si 0,4% order weniger, Mn 1.0 bis 2.0%, P 0,02 % oder weniger, S 0,001 bis 0,005 %, Al 0,001 bis 0,01 % Ti 0,005 bis 0,03 % Nb 0,005 bis 0,05 %, Mg 0,0003 bis 0,005 %, O 0,001 bis 0,005 %, N 0,001 bis 0,01 %, und optional eine oder mehrere der folgenden Komponenten: Ca 0,0005 bis 0,005 % REM 0,0005 bis 0,01 % und Zr 0,0005 bis 0,01 %, Zr 0.0005 to 0.01%,
    wobei die Gesamtmenge von Ca, REM und Zr nicht mehr als 0,02 % beträgt, und weiterhin optional eine oder mehrere der Komponenten: Cu 0,05 bis 1,5 %, Ni 0,05 bis 3,0 %, Cr 0,05 bis 0,5 % Mo 0,05 bis 0,5 % V 0,005 bis 0,05 % und B 0,0001 bis 0,003 %,
       wobei die Gesamtmenge von Cu, Ni, Cr, Mo und V nicht mehr als 3,0 % beträgt, Rest bestehend aus Eisen und unvermeidbaren Verunreinigungen, wobei die Stahlplatte zusammengesetzte, ultrafeine 0,01 bis 0,5 µm große Partikel mit nicht weniger als 10.000 Teilchen/mm2 aus TiN mit Oxiden von Mg und Al, und zusammengesetzte Partikel aus ausgeschiedenem Sulfid aufweist, die nicht weniger als 0,3 Gewichtsprozent Mn auf 0,5 bis 10 µm großen Oxidpartikeln mit nicht weniger als 10 Teilchen/mm2 enthalten, wobei die Oxidpartikel nicht weniger als 10 Gewichtsprozent Mg enthalten.
EP01930007A 2000-05-09 2001-05-09 Dicke stahlplatte mit hervorragenden ctod-eigenschaften der durch schweissen beeinflussten bereiche und mit einer streckgrenze von 460 mpa oder mehr Expired - Lifetime EP1221493B1 (de)

Applications Claiming Priority (9)

Application Number Priority Date Filing Date Title
JP2000136105 2000-05-09
JP2000136105 2000-05-09
JP2000348105A JP2002143880A (ja) 2000-11-15 2000-11-15 生物学的排水処理装置
JP2000348257 2000-11-15
JP2000348105 2000-11-15
JP2000348257 2000-11-15
JP2001049838 2001-02-26
JP2001049838A JP3699657B2 (ja) 2000-05-09 2001-02-26 溶接熱影響部のＣＴＯＤ特性に優れた４６０ＭＰａ以上の降伏強度を有する厚鋼板
PCT/JP2001/003876 WO2001086013A1 (fr) 2000-05-09 2001-05-09 Tole d'acier epaisse excellente du point de vue de ses caracteristiques ctod dans la zone affectee par la chaleur du soudage et dont la limite conventionnelle d'elasticite est superieure ou egale a 460 mpa

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EP1221493A4 EP1221493A4 (de) 2003-01-15
EP1221493B1 true EP1221493B1 (de) 2005-01-12

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CA2676781C (en) 2003-10-17 2012-04-10 Nippon Steel Corporation High-strength steel sheets excellent in hole-expandability and ductility
JP4660250B2 (ja) * 2004-04-07 2011-03-30 新日本製鐵株式会社 大入熱溶接による溶接熱影響部の低温靭性に優れた厚手高強度鋼板
JP4252974B2 (ja) * 2005-05-25 2009-04-08 株式会社日本製鋼所 クラッド鋼用母材および該クラッド鋼用母材を用いたクラッド鋼の製造方法
EP2218800B1 (de) * 2007-12-07 2012-05-16 Nippon Steel Corporation Stahl mit schweisshitzebeeinflusstem bereich mit hervorragenden ctod-eigenschaften und verfahren zu herstellung des stahls
US8668784B2 (en) 2009-05-19 2014-03-11 Nippon Steel & Sumitomo Metal Corporation Steel for welded structure and producing method thereof
TWI365915B (en) 2009-05-21 2012-06-11 Nippon Steel Corp Steel for welded structure and producing method thereof
JP2011246804A (ja) 2010-04-30 2011-12-08 Nippon Steel Corp 電子ビーム溶接継手及び電子ビーム溶接用鋼材とその製造方法
KR101425761B1 (ko) * 2010-11-22 2014-08-01 신닛테츠스미킨 카부시키카이샤 전자빔 용접 조인트 및 전자빔 용접용 강재와 그 제조 방법
JP5015360B2 (ja) * 2010-11-22 2012-08-29 新日本製鐵株式会社 電子ビーム溶接継手及び電子ビーム溶接用鋼材とその製造方法
US9403242B2 (en) 2011-03-24 2016-08-02 Nippon Steel & Sumitomo Metal Corporation Steel for welding
CN109072383B (zh) * 2016-04-21 2021-02-09 日本制铁株式会社 厚钢板

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JP3256118B2 (ja) * 1995-12-06 2002-02-12 新日本製鐵株式会社 超大入熱溶接熱影響部の靱性に優れた溶接用高張力鋼

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