EP2666880A1 - Matériau d'acier à ténacité supérieure de zone soudée, touchée par la chaleur, et son procédé de fabrication - Google Patents

Matériau d'acier à ténacité supérieure de zone soudée, touchée par la chaleur, et son procédé de fabrication Download PDF

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EP2666880A1
EP2666880A1 EP12736520.3A EP12736520A EP2666880A1 EP 2666880 A1 EP2666880 A1 EP 2666880A1 EP 12736520 A EP12736520 A EP 12736520A EP 2666880 A1 EP2666880 A1 EP 2666880A1
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oxides
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EP2666880A4 (fr
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Takashi SUGITANI
Tetsushi Deura
Yoshitomi Okazaki
Hidenori Nako
Hiroki Ohta
Masaki Shimamoto
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Kobe Steel 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/06—Deoxidising, e.g. killing
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/08—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
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the present invention relates: to a steel material used for bridges, high-rise buildings, marine vessels, etc.; and in particular to a steel material having a superior toughness at a zone affected by heat when the steel material is welded (hereunder referred to as "welded heat-affected zone” or “HAZ” occasionally) and a method for manufacturing the steel material.
  • Patent Literatures 1 to 3 a steel material that can inhibit the toughness of a HAZ from deteriorating when a high heat input welding method is adopted.
  • a steel material is characterized by containing oxides of REM and/or CaO and ZrO 2 as oxides acting as nuclei of intragranular ferrite transformation.
  • the oxides disperse finely in steel because they exist in the state of a liquid in molten steel.
  • the oxides are thermally stable, are solute, and do not disappear even when they are exposed for a long period of time at a high temperature of about 1,400°C for example, and hence contribute largely to the improvement of HAZ toughness.
  • Patent Literature 4 discloses that the size and the number of all oxide-based inclusions (not being limited to oxides acting as nuclei of intragranular ferrite transformation but including all oxides) in a steel material are greatly related to the improvement of HAZ toughness and in particular, by reducing the number of coarse oxides having circle equivalent diameters exceeding 5.0 ⁇ m to five or less, it is possible to obtain a steel material excellent in HAZ toughness even when high heat input welding is applied with a high input heat of about 50 kJ/mm.
  • Patent Literature 4 since the number of coarse oxides can conspicuously be suppressed, it is possible to enhance HAZ toughness even when a material is welded with an input heat larger than the input heat adopted in a HAZ toughness evaluation method disclosed in an example of Patent Literature 1. Specifically, whereas a heat cycle of cooling a steel material for 300 sec. in the temperature range of 800°C to 500°C after the steel material is retained for 5 sec.
  • Patent Literature 4 At a heating temperature of 1,400°C (heat input conditions: 1,400°C x 5 sec., cooling time Tc is 300 sec.) is applied and an absorption energy at -40°C (vE- 40 ) is measured in Patent Literature 1, in Patent Literature 4 an absorption energy is measured likewise when a heat cycle in which the retention time at 1,400°C is prolonged to 30 sec. (heat input conditions: 1,400°C x 30 sec., cooling time Tc is 300 sec.) is applied and it is confirmed that a good HAZ toughness is obtained even on this occasion.
  • Patent Literatures 5 to 7 disclose: not such a technology of using both REM oxides and ZrO 2 as disclosed in Patent Literatures 1 to 4; but that HAZ toughness can be improved by adding REM into molten steel having an adjusted dissolved oxygen content when a high heat input welding exceeding about 300 kJ/cm (about 30 kJ/mm) is applied.
  • the improvement of welding operation efficiency is demanded inevitably and a welding input heat increases in accordance with the demand.
  • a steel material exhibiting excellent performance even under a high heat input condition of 50 kJ/mm or more that has not been much studied heretofore is desired.
  • the present invention is established in view of the above situation and an object thereof is to provide: a steel material excellent in HAZ toughness even when high heat input welding is applied with a input heat of 50 kJ/mm or more; and a method for manufacturing the steel material.
  • a steel material excellent in toughness at a welded heat-affected zone according to the present invention that has solved the above problem is a steel material containing C: 0.02% to 0.15% (in terms of % by mass, the same shall apply hereafter), Si: 0.5% or less (excluding 0%), Mn: 2.5% or less (excluding 0%), P: 0.03% or less (excluding 0%), S: 0.02% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), Ti: 0.005% to 0.10%, Zr: 0.0005% to 0.050%, REM: 0.0003% to 0.015%, Ca: 0.0003% to 0.010%, and O: 0.0005% to 0.010%, with the balance consisting of Fe and unavoidable impurities.
  • the gist thereof is to satisfy the conditions that (a) the ZrO 2 content is 5% to 50%, the REM oxide (M 2 O 3 when REM is expressed by the symbol M) content is 5% to 50%, and the CaO content is 50% or less (excluding 0%) in average when the composition of all oxide-based inclusions included in the steel material is measured and then expressed in terms of the mass of the individual oxides, (b) among all the inclusions included in the steel material, the number of inclusions having circle equivalent diameters of 0.1 to 2 ⁇ m is not less than 120 in an observation visual field area of 1 mm 2 , the number of oxides having circle equivalent diameters exceeding 3 ⁇ m is not more than 5.0 in an observation visual field area of 1 mm 2 , and the number of oxides having circle equivalent diameters exceeding 5 ⁇ m is not more than 5.0 in an observation visual field area of 1 mm 2 , and (c-1) the proportion of the number of REM and Zr containing inclusions I satisfying the condition that the molar ratio of REM to Z
  • the steel material may further contain the following elements as other elements;
  • a steel material according to the present invention can be manufactured: by adding REM so that a dissolved oxygen content Q of and an added REM amount Q REM in molten steel may satisfy the following expression (1), 2 ⁇ log ⁇ Q REM + 3 ⁇ log Q Of ⁇ - 12.00 when REM is added to the molten steel having the dissolved oxygen content Q of adjusted in the range of 0.0003% to 0.01% by mass; and by controlling the condition of adding elements so as to satisfy the following condition (2) and/or (3) when REM and Zr are regarded as a-group elements and Ti, Ca, and Al are regarded as b-group elements in the case of adding REM, Zr, Ti, Ca, and Al to the molten steel having the dissolved oxygen content Q of adjusted in the above range,
  • oxides oxides containing Zr, REM, and Ca acting as nuclei of intragranular ⁇ transformation ( ⁇ means ferrite or a mixed structure comprising ferrite and bainite, the same shall apply hereafter) are formed and the sizes and the numbers (namely grain size distributions) of inclusions and oxides existing in a steel material and the proportion of the number of inclusions containing prescribed elements in specific relationship to the number of all inclusions are controlled appropriately, it is possible to provide a steel material excellent in HAZ toughness at high heat input welding.
  • the present invention relates to a technology for obtaining a steel material having a HAZ toughness not deteriorating even when the steel material is welded with a higher input heat by improving the technology of using oxides acting as nuclei of intragranular ⁇ transformation disclosed in Patent Literatures 1 to 4.
  • the present inventors have studied in order to provide a steel material excellent in HAZ toughness at high heat input welding of a higher level even after Patent Literature 4 is proposed. As a result, the present inventors have found that, in order to provide a steel material excellent in HAZ toughness even when "heat cycle of cooling a steel material for 400 sec. in the temperature range of 800°C to 500°C after the steel material is retained for 5 sec.
  • Patent Literature 4 At a heating temperature of 1,450°C" (heat input conditions: 1,450°C x 5 sec., cooling time Tc is 400 sec.) that are conditions of a higher input heat than Patent Literature 4 is applied, it is insufficient to merely reduce the number of oxides having circle equivalent diameters exceeding 5.0 ⁇ m to not more than 5 like Patent Literature 4.
  • the present inventors have found that it is extremely important that: the number of oxides exceeding 3.0 ⁇ m to which documents including Patent Literature 4 have not heretofore paid attention at all is reduced and, when the composition of all inclusions included in a steel material is measured, the proportion of the number of REM and Zr containing inclusions I satisfying the condition that the molar ratio of REM to Zr (REM/Zr) is 0.6 to 1.4 to the number of all the inclusions included in the steel material is 30% or more; or the proportion of the number of REM, Zr, Al, Ca, and Ti containing inclusions II satisfying the condition that the ratio of the total mole number of REM and Zr to the total mole number of Al, Ca, and Ti [(REM+Zr)/(Al+Ca+Ti)] is 0.5 to 1.2 to the number of all the inclusions included in the steel material is 40% or more; and have completed the present invention.
  • the present invention is characterized by stipulating the requirements (c) and (d) in addition to the requirements (a) and (b) in relation to Patent Literature 4.
  • oxides acting as nuclei of intragranular ⁇ transformation namely oxides containing Zr, REM, and Ca from all oxides included in a steel material
  • the former is particularly referred to as "Zr-REM-Ca-based oxides” and the latter is particularly referred to as " all oxide-based inclusions” occasionally.
  • the oxides here mean to include both individual oxides and composite oxides formed by combining inclusions other than the oxides (for example, sulfides, nitrides, carbides, or a composite compound of those).
  • indispensable components (Zr, REM, and Ca) constituting Zr-REM-Ca-based oxides are particularly referred to as "intragranular ⁇ transformation forming elements" occasionally.
  • Zr-REM-Ca-based oxides acting as the origins of intragranular ⁇ transformation are explained hereunder.
  • the Zr-REM-Ca-based oxides mean substances indispensably including oxides of Zr, oxides of REM, and oxides of Ca.
  • Elements (intragranular ⁇ transformation forming elements) constituting the Zr-REM-Ca-based oxides are Zr, REM, and Ca but, besides those, may also include oxide forming elements such as Ti, Mn, Si, and Al or other components existing in steel.
  • the existence form of the Zr-REM-Ca-based oxides is not particularly limited and the Zr-REM-Ca-based oxides may exist as either individual oxides containing intragranular ⁇ transformation forming elements individually or composite oxides containing two or more intragranular ⁇ transformation forming elements.
  • Examples of individual oxides are ZrO 2 in the case of Zr, CaO in the case of Ca, and M 2 O 3 , MgO 5 , and MO 2 in the case of REM when REM is represented by the symbol "M".
  • the oxides may exist either in the state where they agglutinate or in the state where other compounds such as sulfides and nitrides precipitate compositely in the oxides.
  • the Zr-REM-Ca-based oxides further contain oxides of Ti.
  • oxides of Ti may exist either as individual oxides (Ti 2 O 3 , Ti 3 O 5 , or TiO 2 for example) or in the form of composite oxides including at least one kind of the Zr-REM-Ca-based oxides and Ti.
  • oxides, sulfides, nitrides, carbides, or a composite compound of those are also included and oxides, sulfides, nitrides, carbides, and a composite compound of those contained in a steel material are collectively referred to as "all inclusions" in the present description.
  • oxides having circle equivalent diameters of 0.1 to 2 ⁇ m are referred to as "fine oxides", oxides having circle equivalent diameters exceeding 3 ⁇ m as “coarse oxides”, and oxides having circle equivalent diameters exceeding 5 ⁇ m as “ultra-coarse oxides” respectively and they are distinguished from each other in some cases.
  • oxides having circle equivalent diameters exceeding 5 ⁇ m are defined as “coarse oxides” in Patent Literature 4
  • oxides having circle equivalent diameters exceeding 3 ⁇ m are defined as “coarse oxides” in the present description.
  • a steel material excellent in HAZ toughness at high heat input welding means a steel material having an absorption energy at -40°C (vE- 40 ) of 130 J or more when a heat cycle (heat history) of retaining the steel material for 5 sec. at 1,450°C and thereafter cooling the steel material for 400 sec. from 800°C to 500°C. (heat input conditions: 1,450°C x 5 sec., cooling time Tc is 400 sec.) is given to the steel material.
  • the larger a value of vE -40 the better and a preferable vE -40 value is 150 J or more.
  • the above heat cycle may sometimes be referred to as "high heat input heat history".
  • the input heat given by the heat cycle is larger than the input heat given by a heat cycle described in Patent Literatures 1 and 4 and in that sense the input heat level at "high heat input welding" in the present invention is different from the heat input level at "high heat input welding” described in Patent Literatures 1 and 4.
  • the temperature in the heat cycle is set at 1,450°C in the present invention in consideration of the fact that heating temperature of a site particularly adjacent to weld metal (referred to as a bond site occasionally) in a HAZ exceeds 1,400°C and reaches about 1,450°C.
  • a steel material according to the present invention satisfies the conditions that the ZrO 2 content is 5% to 50%, the REM oxide (M 2 O 3 when REM is expressed by the symbol M) content is 5% to 50%, and the CaO content is 50% or less (excluding 0%) in average when the composition of all oxide-based inclusions included in the steel material is measured and then expressed in terms of the mass of the individual oxides (100% in total) and thereby they function effectively as nuclei of intragranular ⁇ transformation. If the respective oxide contents are lower than the relevant lower limits, the amount of oxides acting as nuclei of intragranular ⁇ transformation during welding is insufficient and the function of improving HAZ toughness is not exhibited. On the other hand, if the respective oxide contents exceed the relevant upper limits, oxides coarsen, the number of fine oxides effectively acting as nuclei of intragranular ⁇ transformation reduces, and the function of improving HAZ toughness is not effectively exhibited.
  • the ZrO 2 content is 5% or more, preferably 8% or more, and yet preferably 10% or more.
  • the upper limit thereof is 50%, preferably 45%, and yet preferably 40%.
  • the REM oxide content is 5% or more, preferably 10% or more, and yet preferably 13% or more.
  • the upper limit thereof is 50%, preferably 45%, and yet preferably 40%.
  • the REM oxides exist in the forms of M 2 O 3 , M 3 O 5 , and MO 2 in a steel material when REM is expressed by the symbol M and a REM oxide content means an amount obtained by converting all REM oxides into M 2 O 3 in the present invention.
  • CaO functions effectively as nuclei of intragranular ⁇ transformation but, if it is contained excessively, the ability of intragranular ⁇ transformation rather deteriorates. Further, if CaO is contained excessively, a nozzle used during casting melts and is damaged. Consequently, the upper limit thereof is set at 50%, preferably 45% or less, yet preferably 40% or less, and still yet preferably 30% or less. In order to effectively exhibit the above function, CaO is contained by preferably 3% or more, yet preferably 5% or more, and still yet preferably 10% or more.
  • the other components in the composition of the all oxide-based inclusions are not particularly limited and the examples are oxides of oxide forming elements (SiO 2 , Al 2 O 3 , and MnO for example) included in a steel material according to the present invention.
  • composition of all oxide-based inclusions included in a steel material can be measured by observing the surface of the steel material with an electron probe X-ray micro analyzer (EPMA) for example and quantitatively analyzing oxides recognized in an observation visual field. Details of the measurement conditions are explained in the section of the examples that will be described later.
  • EPMA electron probe X-ray micro analyzer
  • a steel material according to the present invention is a steel material satisfying all of the following conditions that;
  • Nos. 1 to 32 in Table 5 are the cases of satisfying all the requirements stipulated in the present invention. According to study focused on the requirements (ii) and (iii), the number of oxides exceeding 3 ⁇ m is controlled to 4.64 even in the case of No. 5 where the number of oxides exceeding 5 ⁇ m is largest (1.440 oxides) in Nos. 1 to 32 and as a result a good HAZ toughness is secured.
  • Nos. 35 to 38, 49, 53, 54, and 61 in Table 6 are the cases of satisfying the requirement (iii) but not satisfying the requirement (ii). Specifically, although the oxides exceeding 5 ⁇ m are suppressed to 0.440 to 2.250 pieces, namely not more than 5.0 pieces, the oxides exceeding 3 ⁇ m are more than 5.0 pieces and increase to 5.71 to 10.65 pieces and as a result a desired HAZ toughness is not obtained.
  • Nos. 35 to 38, 49, 53, 54, and 61 stated above are the cases included in the range of Patent Literature 4 on the point that they satisfy the requirement (iii) but a desired HAZ toughness stipulated in the present invention is not attained in the cases of not satisfying the requirement (ii) even when the cases are included in the range of Patent Literature 4.
  • the requirement (ii) is further stipulated as a requirement for securing a desired HAZ toughness in addition to the requirement (iii) in the present invention.
  • oxides exceeding 5.0 pieces may exist in the very narrow range of more than 3 ⁇ m to not more than 5 ⁇ m in some manufacturing conditions and a desired HAZ toughness is not obtained only by the existence of coarse oxides exceeding 5.0 pieces in the range of more than 3 ⁇ m to not more than 5 ⁇ m even if the number of fine oxides in the region of the requirement (i) is greatly increased and the number of ultra-coarse oxides in the region of the requirement (iii) is reduced.
  • the number of coarse oxides having grain sizes exceeding 3 ⁇ m is set at 5.0 or less and the number of ultra-coarse oxides having grain sizes exceeding 5 ⁇ m is also set at 5.0 or less.
  • the numbers are preferably 3.0 or less, yet preferably 2.0 or less, still yet preferably 1.0 or less, and ultimately preferably zero.
  • the number of ultra-coarse oxides having grain sizes exceeding 5 ⁇ m rather than coarse oxides having grain sizes exceeding 3 ⁇ m in the range of the present invention (not more than 5.0 pieces in both the cases). More specifically, whereas a better HAZ toughness is obtained as the number of ultra-coarse oxides approaches the lower limit (zero), then the number is preferably about 1.0 or less, and a number ultimately closer to zero is the best, the number of coarse oxides may be acceptable even if the number is close to the upper limit (5.0) and even the number of 4.0 or less is preferably accepted.
  • the number of oxides having circle equivalent diameters exceeding 3 ⁇ m and the number of oxides having circle equivalent diameters exceeding 5 ⁇ m can be obtained by observing a cross-section of a steel material for example with an EPMA, quantitatively analyzing the component composition of inclusions recognized in an observation visual field, regarding the inclusions having oxygen contents of 5% or more as oxides, and observing and measuring the circle equivalent diameters of the oxides for example with a scanning-type electron microscope (SEM).
  • SEM scanning-type electron microscope
  • the number of fine inclusions having circle equivalent diameters of 0.1 to 2 ⁇ m is set at not less than 120 in an observation visual field area of 1 mm 2 .
  • the number of fine inclusions is set at not less than 120 in an observation visual field area of 1 mm 2 , preferably not less than 200 per 1 mm 2 , yet preferably not less than 500 per 1 mm 2 , and still yet preferably not less than 700 per 1 mm 2 .
  • the number of fine inclusions having circle equivalent diameters of 0.1 to 2 ⁇ m may be obtained by observing and measuring a cross-section of a steel material for example with SEM.
  • inclusions having circle equivalent diameters of less than 0.1 ⁇ m scarcely contribute to the improvement of HAZ toughness by the dispersion of inclusions and hence are not included in the number of the aforementioned inclusions.
  • a “circle equivalent diameter” stated above is a diameter of a circle assumed so as to have the same area as inclusions (including oxides) and can be recognized on an observation plane of an SEM.
  • the requirement (c-1) specifies a molar ratio of REM/Zr and the proportion of the number of the inclusions I required for realizing a desired HAZ toughness with regard to inclusions containing REM and Zr in intragranular ⁇ transformation forming elements (REM, Zr, and Ca).
  • the requirement (c-2) specifies a molar ratio of (REM+Zr)/(Al+Ca+Ti) and the proportion of the number of the inclusions II required for realizing a desired HAZ toughness with regard to inclusions containing intragranular ⁇ transformation forming elements (Zr, REM, and Ca) and other elements constituting inclusions (Ti and Al).
  • the inclusions I satisfying the condition that a REM/Zr ratio is 0.6 to 1.4: are superior in the ability of intragranular ⁇ transformation to inclusions having REM/Zr ratios of less than 0.6 or more than 1.4; and hence contribute to further miniaturizing a metallographic structure at a HAZ and improving HAZ toughness.
  • the proportion of the number of the inclusions I is set at 30% or more, it is possible to attain an absorption energy at -40°C (vE- 40 ) of 130 J or more even when high heat input welding is applied.
  • the larger the proportion of the number of the inclusions I to the number of all inclusions, the better and the proportion is preferably 40% or more and yet preferably 50% or more.
  • the larger the proportion of the number of the inclusions I, the better and the proportion is ultimately preferably 100%.
  • the present inventors have investigated, with regard to inclusions containing REM, Zr, and Ti, the behavior of the melting point of inclusions with a high temperature laser microscope in order to find a component composition region where the melting point lowers.
  • the melting point of inclusions containing REM, Zr, and Ti is influenced by the contents of Ca and Al and, when a (REM+Zr)/(Al+Ca+Ti) ratio in terms of the mole number of the elements is in the range of 0.5 to 1.2, the melting point of the inclusions lowers locally and the ability of intragranular ⁇ transformation increases.
  • the inclusions II satisfying the condition that the (REM+Zr)/(Al+Ca+Ti) ratios are 0.5 to 1.2 are excellent in intragranular ⁇ transformation in comparison with inclusions having (REM+Zr)/(Al+Ca+Ti) ratios of lower than 0.5 or higher than 1.2, hence further miniaturize a metallographic structure at a HAZ, and contribute to the improvement of HAZ toughness.
  • the composition of inclusions included in a steel material may be obtained by observing a cross-section of the steel material for example with an EPMA and quantitatively analyzing the component composition of the inclusions recognized in an observation visual field and the proportion of the number of the inclusions I and the proportion of the number of the inclusions II to the number of all inclusions may be obtained after the composition of all inclusions included in the steel material is measured.
  • the composition of inclusions having circle equivalent diameters of 0.1 ⁇ m or more is quantitatively analyzed. The reason is that inclusions having circle equivalent diameters of less than 0.1 ⁇ m are too small to be quantitatively analyzed with a high degree of accuracy.
  • a steel material according to the present invention contains, as the basic components, C: 0.02% to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 2.5% or less (excluding 0%), P: 0.03% or less (excluding 0%), S: 0.02% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), Ti: 0.005% to 0.10%, Zr: 0.0005% to 0.050%, REM: 0.0003% to 0.015%, and Ca: 0.0003% to 0.010%.
  • C 0.02% to 0.15%
  • Si 0.5% or less
  • Mn 2.5% or less
  • P 0.03% or less
  • S 0.02% or less
  • Al 0.050% or less
  • N 0.010% or less
  • Ti 0.005% to 0.10%
  • Zr 0.0005% to 0.050%
  • REM 0.0003% to 0.015%
  • Ca 0.0003% to 0.010%.
  • C is an element indispensable for securing the strength of a steel material (base material) and must be contained by 0.02% or more.
  • a C content is set at preferably 0.04% or more and yet preferably 0.05% or more. If a C content exceeds 0.15% however, insular martensite (MA) is formed abundantly at a HAZ during welding and not only causes the toughness of a HAZ to deteriorate but also adversely affects the weldability. Consequently, a C content is set at 0.15% or less, preferably 0.10% or less, and yet preferably 0.08% or less.
  • Si is an element that has a deoxidation function and contributes to improving the strength of a steel material (base material) by solid-solution strengthening. It is preferable to contain Si by 0.01% or more in order to effectively exhibit such functions.
  • An Si content is set at yet preferably 0.05% or more and still yet preferably 0.1% or more. If an Si content exceeds 0.5% however, the weldability and toughness of a steel material deteriorate and hence an Si content has to be suppressed to not more than 0.5%.
  • An Si content is set at preferably 0.3% or less, yet preferably 0.25% or less, and still yet preferably 0.21% or less.
  • Mn is an element contributing to improving the strength of a steel material (base material). If an Mn content exceeds 2.5% however, the weldability of a steel material (base material) deteriorates. Consequently, an Mn content must be suppressed to not more than 2.5%.
  • An Mn content is set at preferably 2.30% or less and yet preferably 2.0% or less. Meanwhile, it is preferable to contain Mn by 0.2% or more in order to effectively exhibit the above effect.
  • An Mn content is set at yet preferably 0.40% or more, still yet preferably 0.60% or more, and particularly preferably 0.8% or more.
  • P is an element likely to segregate and deteriorates HAZ toughness particularly by segregating at crystal grain boundaries in a steel material. Consequently, a P content has to be suppressed to 0.03% or less.
  • a P content is set at preferably 0.02% or less and yet preferably 0.015% or less. Meanwhile, usually P is unavoidably contained by about 0.001%.
  • S is a harmful element that forms sulfides (MnS) by combining with Mn and deteriorates the toughness and ductility in the plate thickness direction of ⁇ base material. Further, if S combines with REM such as La and Ce and forms sulfides of REM (LaS and CeS for example), oxides of REM are inhibited from forming and hence HAZ toughness deteriorates. Consequently, an S content has to be suppressed to 0.02% or less. An S content is set at preferably 0.015% or less, yet preferably 0.010% or less, and still yet preferably 0.006% or less. Meanwhile, usually S is unavoidably contained by about 0.0005%.
  • Al is an element functioning as a deoxidizing agent. If Al is excessively added however, Al reduces oxides, coarse Al oxides are formed, and HAZ toughness deteriorates. Consequently, an Al content has to be suppressed to 0.050% or less.
  • An Al content is set at preferably 0.04% or less, yet preferably 0.03% or less, still yet preferably 0.025% or less, and particularly preferably 0.010% or less. Meanwhile, usually Al is unavoidably contained by about 0.0005%.
  • N is an element precipitating nitrides (ZrN and TiN for example) and the nitrides prevent austenite grains formed at a HAZ during welding from coarsening by a pinning effect, accelerate intragranular ⁇ transformation, and contribute to the improvement of HAZ toughness.
  • N forms nitrides more and accelerates the miniaturization of austenite grains more as it increases and hence N acts effectively on the improvement of toughness at a HAZ. If an N content exceeds 0.010% however, the content of solute N increases, the toughness of a base material itself deteriorates, and HAZ toughness also deteriorates. Consequently, an N content has to be suppressed to 0.010% or less.
  • An N content is set at preferably 0.0090% or less and yet preferably 0.008% or less. Meanwhile, it is preferable to contain N by 0.003% or more in order to effectively exhibit the above effect.
  • An N content is set at yet preferably 0.004% or more and still yet preferably 0.005% or more.
  • Ti is an element that forms nitrides such as TiN and oxides containing Ti in a steel material and contributes to the improvement of HAZ toughness.
  • Ti has to be contained by 0.005% or more in order to exhibit the effect.
  • a Ti content is preferably 0.007% or more and yet preferably 0.010% or more. If Ti is excessively added however, a base material itself hardens by solid-solution strengthening of Ti, the lowering of HAZ toughness is caused, and hence Ti has to be suppressed to 0.10% or less.
  • a Ti content is set at preferably 0.07% or less and yet preferably 0.06% or less.
  • Zr is an element that forms composite oxides including Zr and contributes to the improvement of HAZ toughness.
  • Zr has to be contained by 0.0005% or more in order to exhibit the function.
  • a Zr content is preferably 0.0015% or more and yet preferably 0.0020% or more. If Zr is excessively added however, coarse Zr oxides (ZrO 2 for example) are formed abundantly and HAZ toughness deteriorates. Consequently, a Zr content is suppressed to 0.050% or less.
  • a Zr content is set at preferably 0.04% or less, yet preferably 0.03% or less, and still yet preferably 0.01% or less.
  • REM rare-earth elements
  • Ca are elements necessary for forming respective oxides.
  • the oxides are likely to disperse finely and the finely dispersing oxides act as nuclei of intragranular ⁇ transformation and hence contribute to the improvement of HAZ toughness.
  • REM should be contained by 0.0003% or more and a REM content is set at preferably 0.001% or more and yet preferably 0.0020% or more. If REM is excessively added however, solute REM forms and segregates and thereby the toughness of a base material deteriorates. Consequently, a REM content should be suppressed to 0.015% or less.
  • a REM content is set at preferably 0.010% or less and yet preferably 0.007% or less.
  • REM in the present invention includes lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium). Among those elements, it is preferable to contain at least one element selected from the group consisting of La, Ce, and Y and it is yet preferable to contain La and/or Ce.
  • Ca should be added by 0.0003% or more and a Ca content is set at preferably 0.0005% or more, yet preferably 0.0008% or more, and still yet preferably 0.001% or more. If Ca is excessively added however, coarse Ca sulfides form and the toughness of a base material deteriorates. Further, if Ca is excessively added, CaO forms excessively, inclusions of high CaO concentrations form, the composition deviates from an optimum inclusion composition range, hence the effect of the inclusions in functioning as intragranular transformation nuclei weakens, and HAZ toughness rather deteriorates. Consequently, a Ca content is suppressed to 0.010% or less. A Ca content is preferably 0.009% or less, yet preferably 0.008% or less, and still yet preferably 0.005% or less.
  • a steel material according to the present invention contains the above elements as indispensable components and an O (oxygen) content is 0.0005% to 0.010%.
  • An oxygen content here is a total oxygen content and means the total content of oxygen constituting oxides and free oxygen dissolving in a steel material.
  • the residual components of a steel material may be iron and unavoidable impurities (Mg, As, Se, etc. for example).
  • a steel material according to the present invention further contains the following elements as other elements;
  • Both Cu and Ni are elements contributing to enhancing the strength of a steel material and can be added individually or compositely.
  • a Cu content is set at preferably 2% or less, yet preferably 1.8% or less, and still yet preferably 1.5% or less. Meanwhile, in order to effectively exhibit the function by the addition of Cu, it is preferable to contain Cu by 0.05% or more.
  • a Cu content is set at yet preferably 0.1% or more and still yet preferably 0.20% or more.
  • an Ni content is set at preferably 3.5% or less, yet preferably 3.0% or less, and still yet preferably 2.5% or less. Meanwhile, in order to effectively exhibit the function by the addition of Ni, it is preferable to contain Ni by 0.05% or more.
  • An Ni content is set at yet preferably 0.1% or more and still yet preferably 0.2% or more.
  • Both Cr and Mo are elements contributing to enhancing the strength of a steel material and can be added individually or compositely.
  • a Cr content is set at preferably 3% or less, yet preferably 2% or less, and still yet preferably 1.0% or less. Meanwhile, in order to effectively exhibit the function by the addition of Cr, it is preferable to contain Cr by 0.05% or more.
  • a Cr content is set at yet preferably 0.1% or more and still yet preferably 0.15% or more.
  • an Mo content is set at preferably 1% or less, yet preferably 0.9% or less, and still yet preferably 0.8% or less. Meanwhile, in order to effectively exhibit the function by the addition of Mo, it is preferable to contain Mo by 0.05% or more.
  • An Mo content is set at yet preferably 0.1% or more and still yet preferably 0.15% or more.
  • Nb and V are elements that precipitate as carbonitrides, prevent austenite grains from coarsening during welding by the pinning effect of the carbonitrides, and have the function of improving HAZ toughness. Nb and V can be added individually or compositely.
  • an Nb content is set at preferably 0.25% or less, yet preferably 0.2% or less, and still yet preferably 0.15% or less. Meanwhile, in order to effectively exhibit the function by the addition of Nb, it is preferable to contain Nb by 0.002% or more.
  • An Nb content is set at yet preferably 0.010% or more and still yet preferably 0.02% or more.
  • V content exceeds 0.1%, in the same manner as Nb, precipitating carbonitrides coarsen and HAZ toughness rather deteriorates. Consequently, a V content is set at preferably 0.1% or less, yet preferably 0.09% or less, and still yet preferably 0.08% or less. Meanwhile, in order to effectively exhibit the function by the addition of V, it is preferable to contain V by 0.002% or more. A V content is set at yet preferably 0.005% or more and still yet preferably 0.01% or more.
  • B is an element that suppresses the formation of grain boundary ferrite and improves toughness. If a B content exceeds 0.005% however, B precipitates as BN at austenite grain boundaries and toughness is caused to lower. Consequently, a B content is set at preferably 0.005% or less, yet preferably. 0.004% or less, and still yet preferably 0.0030% or less. Meanwhile, in order to effectively exhibit the functions by the addition of B, it is preferable to contain B by 0.001% or more. A B content is set at yet preferably 0.0015% or more.
  • REM, Zr, Ti, Ca, and Al are added to the molten steel having the dissolved oxygen content Q Of adjusted in the above range so that the conditions of adding the elements may satisfy the following requirements (2) and/or (3) when REM and Zr are regarded as a-group elements and Ti, Ca, and Al are regarded as b-group elements;
  • the expression (1) is set in order to secure a desired HAZ toughness stipulated in the present invention and it is possible to secure a desired HAZ toughness by appropriately controlling an added REM amount Q REM in accordance with a dissolved oxygen content Q Of in molten steel on the basis of the expression (1) (refer to examples that will be described later).
  • That a dissolved oxygen content Q Of and an added REM amount Q REM in molten steel satisfy the expression (1) means that an added REM amount Q REM relating to the formation of oxides of REM is set so as to be reduced. It is estimated that, as a result, the number of formed REM oxides also reduces, resultantly the numbers of coarse and ultra-coarse oxides reduce in the ranges of the present invention, and a desired HAZ toughness is secured.
  • a Z value is set at -12.00 or less.
  • a Z value is preferably -12.25 or less, yet preferably -12.50 or less, and still yet preferably -12.75 or less.
  • the lower limit of a Z value is not particularly limited but about -15 in consideration of a REM content in steel and others.
  • Patent Literature 4 the expression (1) is not taken into consideration at all in Patent Literature 4.
  • an added REM amount Q REM is increased so that the value on the left side of the expression (1) (Z value) may exceed -12.00.
  • Patent literatures 5 to 7 although to add REM to molten steel having an adjusted dissolved oxygen content Q Of is described, to decide an added REM amount Q REM in accordance with a dissolved oxygen content Q Of and add REM is not taken into consideration at all.
  • an added REM amount Q REM may be added arbitrarily in accordance with a dissolved oxygen content Q Of as stated above.
  • an added REM amount Q REM is set so as to be larger than the amount of REM contained in a steel material according to the present invention. The reason is that the REM added before casting volatilizes during a casting process and the like and disperses into slag and the amount of the REM contained in a steel material reduces.
  • a dissolved oxygen content Q Of in molten steel is set in the range of 0.0003% to 0.01% by mass.
  • Dissolved oxygen means oxygen that does not form oxides and exists in molten steel in a free state. That is, in order to manufacture a steel material according to the present invention, firstly as a prerequisite, a dissolved oxygen content Q Of in molten steel is adjusted in the range of 0.0003% to 0.01% by mass. If a dissolved oxygen content Q Of in molten steel is less than 0.0003% by mass, the dissolved oxygen content Q Of in the molten steel is insufficient, hence a prescribed amount of Zr-REM-Ca-based oxides acting as nuclei of intragranular ⁇ transformation are not secured, and HAZ toughness is not improved.
  • a dissolved oxygen content Q Of is set at 0.0003% or more by mass.
  • a dissolved oxygen content Q Of is preferably 0.001% or more by mass and yet preferably 0.0020% or more by mass.
  • a dissolved oxygen content Q Of exceeds 0.01% by mass, the dissolved oxygen content in molten steel is too much, hence not only reaction between oxygen and the aforementioned elements in the molten steel intensifies and refining operation is unfavorably influenced but also coarse oxides and ultra-coarse oxides form and HAZ toughness rather deteriorates. Consequently, a dissolved oxygen content Q Of should be suppressed to 0.01% or less by mass.
  • a dissolved oxygen content Q Of is set at preferably 0.008% or less by mass and yet preferably 0.007% or less by mass.
  • a dissolved oxygen content Q Of in molten steel refined primarily in a converter or an electric furnace usually exceeds 0.01% by mass.
  • a method of adjusting a dissolved oxygen content Q Of in molten steel a method of applying vacuum deoxidation with an RH-type degassing refiner or a method of adding deoxidizing elements such as Si, Mn, Ti, and Al is named for example and a dissolved oxygen content Q Of may be adjusted by arbitrarily combining those methods. Further, it is also possible to adjust a dissolved oxygen content Q Of with a ladle heating type refining apparatus or a simple molten steel processing apparatus in place of an RH-type degassing refiner. On this occasion, since a dissolved oxygen content Q Of cannot be adjusted by vacuum deoxidation, a method of adding an deoxidizing element such as Si may be adopted for the adjustment of the dissolved oxygen content Q Of . In the case of adopting a method of adding a deoxidizing element such as Si, it is also possible to add a deoxidizing element when molten steel is tapped from a converter to a ladle.
  • REM and Zr When REM and Zr are added separately, either Zr may be added after REM is added or REM may be added after Zr is added and in any of the cases it is necessary to control the interval from the time when REM (or Zr) is added to the time when Zr (or REM) is added to not longer than 5 min.
  • the interval is preferably within 4 min. and yet preferably within 3 min.
  • Ti to molten steel before REM is added with the aim of further improving HAZ toughness by the miniaturization of Ti oxides. Since the interface energy of Ti oxides with molten steel is smaller than that of Zr-REM-Ca-based oxides, it is possible to miniaturize the Ti oxides and resultantly form fine oxides contributing to HAZ toughness by adding Ti before Zr, REM, and Ca are added to the molten steel. Then by adding Zr, REM, and Ca as stated above after Ti is added, desired Zr-REM-Ca-based oxides acting as nuclei of intragranular ⁇ transformation can be obtained.
  • the requirement (3) stipulates the addition conditions of a-group elements and b-group elements and thereby the proportion of the number of the inclusions II can be adjusted.
  • the time period to the time when the addition of a first element in the a-group elements commences means the time period to the time when a first a-group element is added.
  • it means the time period to the time of simultaneous addition in the case where REM and Zr are added simultaneously and the time period to the time when REM (an element added first in the a-group elements) is added in the case where Zr is added after REM is added.
  • the time period to the time when the addition of a last element in the a-group elements commences means the time period to the time when the addition of all the a-group elements terminates. For example, it means the time period to the time of simultaneous addition in the case where REM and Zr are added simultaneously and the time period to the time when Zr (an element added last in the a-group elements) is added in the case where Zr is added after REM is added.
  • Fig. 1 shows an example of a sequence in the addition of elements when the b-group elements are added before and after the a-group elements are added.
  • a 1 and a 2 represent the a-group elements and a symbol ⁇ shows the time when the addition of each of the elements commences.
  • b 1 to b 4 represent the b-group elements and a symbol ⁇ shows the time when the addition of each of the elements commences.
  • elements are added in the sequence of b 1 , b 2 , a 1 , a 2 , b 3 , and b 4 , an element added first in the a-group elements is a 1 , an element added last in the a-group elements is a 2 , an element added first in the b-group elements is b 1 , and an b-group element added first after the a-group elements are added is b 3 .
  • t1 is a time period from the time when the addition of a first element in the b-group elements commences to the time when the addition of a first element in the a-group elements commences
  • the time period from the time when the addition of b 1 commences to the time when the addition of a 1 commences is t1 in Fig. 1
  • t2 is a time period from the time when the addition of a last element in the a-group elements commences to the time when the addition of a first element in the b-group element commences
  • the time period from the time when the addition of a 2 commences to the time when the addition of b 3 commences is t2 in Fig. 1 .
  • a 1 and a 2 may be added simultaneously and on this occasion the time when the addition of a first element in the a-group elements commences and the time when the addition of a last element in the a-group elements commences are the same.
  • t1 means the time period from the time when the addition of Al (a b-group element added first) commences to the time when the addition of REM (an a-group element added first) commences and t2 means the time period from the time when the addition of Zr (an a-group element added last) commences to the time when the addition of Ca (a b-group element added first in the remaining b-group elements) commences.
  • t1 means the time period from the time when the addition of Al (a b-group element added first) commences to the time when the simultaneous addition of REM and Zr commences
  • t2 means the time period from the time when the simultaneous addition of REM and Zr commences to the time when the addition of Ca (a b-group element added first in the remaining b-group elements) commences.
  • the sum of t1 and t2 is set at 3 min. or longer. By setting the sum of t1 and t2 at 3 min. or longer, it is possible to form inclusions containing REM and Zr and containing appropriate amounts of Al, Ca, and Ti.
  • the sum of t1 and t2 is set at preferably 5 min. or longer and yet preferably 7 min. or longer.
  • the upper limit of the sum of t1 and t2 is not particularly limited but, if the time is too long, productivity lowers and hence the upper limit is about 20 min.
  • either the b-group elements may be added after the a-group elements are added or the a-group elements may be added after the b-group elements are added. Otherwise, the a-group elements may be added after the b-group elements are added and successively the b-group elements may be added.
  • the b-group elements are added both before and after the a-group elements are added, it is acceptable as long as the kinds and the contents of all the b-group elements are controlled both before and after the a-group elements are added.
  • either the a-group elements may be added after some of the b-group elements are added and successively the remainder of the b-group elements may be added or an identical element may be redundantly added before and after the a-group elements are added.
  • the a-group elements and the b-group elements may be added simultaneously or individually in the ranges satisfying the requirement (b-2).
  • the forms of REM, Ca, Zr, Al and Ti added in molten steel are not particularly limited and for example pure La, pure Ce, or pure Y as REM, pure Ca, pure Zr, pure Al, pure Ti, Fe-Si-La alloy, Fe-Si-Ce alloy, Fe-Si-Ca alloy, Fe-Si-La-Ce alloy, Fe-Ca alloy, Fe-Zr alloy, Fe-Ti alloy, Fe-Al alloy, or Ni-Ca alloy may be added. Otherwise, misch metal may be added in molten steel. Misch metal is a mixture of rare earth elements and specifically contains Ce by about 40% to 50% and La by about 20% to 40%. Since misch metal contains Ca as an impurity in many cases however, when misch metal includes Ca, the range stipulated in the present invention has to be satisfied.
  • Molten steel obtained by adjusting the components in this way is continuously casted in accordance with an ordinary method to manufacture a slab and successively the slab is hot-rolled in accordance with an ordinary method.
  • a steel material according to the present invention can secure an absorption energy at -40°C (vE- 40 ) of 130 J or more even when a heat cycle of cooling the steel material for 400 sec. from 800°C to 500°C after the steel material is retained for 5 sec. at 1,450°C (heat input conditions: 1,450°C x 5 sec., cooling time Tc is 400 sec.) is given.
  • a steel material according to the present invention can be used as a material for structures such as bridges, high-rise buildings, and marine vessels and can prevent the toughness of a welded heat-affected zone from deteriorating not only at low-to-middle heat input welding but also at high heat input welding with a welding input heat of 50 kJ/mm or more.
  • a steel material according to the present invention is applied to a heavy steel plate 3.0 mm or more in thickness or the like.
  • Test steels having the component compositions (% by mass) shown in Tables 3 and 4 below (the balance consists of iron and unavoidable impurities) are melted and refined under the conditions shown in Tables 1 and 2 below with a vacuum melting furnace (capacity 150 kg), cast into ingots of 150 kg, and cooled. Successively, the ingots are heated and rolled and heavy steel plates are manufactured. It is confirmed here that the total O content of each of the test steels satisfying the requirements stipulated in the present invention among the test steels shown in Tables 3 and 4 is in the range of 0.0005% to 0.010%.
  • the components of elements other than Ti, Zr, REM, and Ca are adjusted and the dissolved oxygen content Q Of in the molten steel is adjusted by deoxidizing the molten steel with at least one element selected from the group consisting of C, Si, Mn, and Al.
  • the dissolved oxygen contents Q Of after adjusted are shown in Table 1.
  • Ti is added in molten steel having an adjusted dissolved oxygen content Q Of , successively Zr and REM are added, and then Ca or Ca and Al is/are added.
  • the addition sequence of Zr and REM is shown in Tables 1 and 2.
  • Zr is added after REM is added or REM is added after Zr is added
  • the time spent from the time when either of the elements is added to the time when the other of the elements is added (interval between the additions) is shown in Tables 1 and 2.
  • the sum (t1 + t2) of the intervals between the addition of the a-group elements (REM and Zr) and the addition of the b-group elements (Ti, Ca, and Al) is shown in Tables 1 and 2.
  • Ti is added in the form of Fe-Ti alloy
  • Zr is in the form of Fe-Zr alloy
  • REM in the form of a misch metal containing La by about 25% and Ce by about 50%
  • Ca in the form of Ni-Ca alloy
  • Al in the form of pure Al, respectively.
  • REM is not added in the form of a misch metal but only Ce is added.
  • the elements After the elements are added, they are casted into ingots and cooled.
  • the obtained ingots are hot-rolled and heavy steel plates 30 to 80 mm in thickness are manufactured.
  • a sample is cut out from each of the obtained heavy steel plates on a transverse section at a position of t/4 (here, t is the thickness of a steel plate), the component composition of all oxide-based inclusions included in the sample is measured and expressed in terms of the mass of the individual oxides, and the average composition of the oxides is computed.
  • the component composition of all oxide-based inclusions is measured through the following procedure.
  • the surface of a cutout sample is observed with an electron probe microanalyzer (EPMA: "JXA-8500F (equipment name)" made by JEOL DATUM Ltd. and the component composition of the inclusions having circle equivalent diameters of 0.1 ⁇ m or more is measured quantitatively.
  • the acceleration voltage is set at 20 kV
  • the specimen current is set at 0.01 ⁇ A
  • the number of analysis is set at 100 or more as the observation conditions and the component compositions at the center parts of the inclusions are analyzed quantitatively by wavelength dispersive spectrometry of characteristic X-rays.
  • the elements subjected to analysis are Si, Mn, S, Al, Ti, Zr, La, Ce, Ca and O (oxygen), the relationship between an X-ray strength and an element concentration of each element is obtained as a calibration curve with a known material beforehand, and the content of an element contained in an inclusion is measured quantitatively from the X-ray strength of the inclusion subjected to the analysis and the calibration curve.
  • an inclusion having an oxygen content of 5% or more by mass is defined as an oxide.
  • each of the elements is expressed in terms of the mass of the individual oxide from the ratio of the X-ray strengths showing the existence of the elements and thereby the composition of the oxide is computed.
  • a value obtained by expressing the elements in terms of the mass of the individual oxides and averaging them is regarded as the average composition of the oxides.
  • the average compositions of ZrO 2 , oxides of REM, and CaO are shown in Tables 5 and 6 below.
  • oxides of REM exist in the forms of M 2 O 3 , M 3 O 5 , and MO 2 in a steel material when a metallic element is represented by the symbol M but the composition is computed by converting all the oxides to M 2 O 3 .
  • "Others" shown in Tables 5 and 6 are oxides (AL 2 O 3 , MnO, and SiO 2 for example) other than ZrO 2 , oxides of REM, and CaO.
  • the circle equivalent diameters of the quantitatively analyzed inclusions are measured by SEM observation and the number of inclusions having circle equivalent diameters (grain sizes) of 0.1 to 2.0 ⁇ m is measured.
  • the numbers in an observation visual field area of 1 mm 2 are shown in Tables 5 and 6 as the observation results.
  • the circle equivalent diameters of the oxides having the oxygen contents of 5% or more by mass in the obtained quantitative analysis result are measured by SEM observation and the numbers of oxides having circle equivalent diameters (grain sizes) exceeding 3 ⁇ m and the numbers of oxides having circle equivalent diameters (grain sizes) exceeding 5 ⁇ m are measured.
  • the numbers of oxides in an observation visual field area of 1 mm 2 are shown in Tables 5 and 6.
  • Fig. 2 The relationship between a Z value and the number of oxides having circle equivalent diameters exceeding 3 ⁇ m in an observation visual filed area of 1 mm 2 is shown in Fig. 2 .
  • Fig. 2 the results of Nos. 1 to 32 ( ⁇ in Fig. 2 ) and the results in which the Z values are in the range of -12.50 to -11.50 ( ⁇ in Fig. 2 ) in the results of Nos. 35 to 40, 53, 54, and 61 shown in Tables 5 and 6 are plotted in order to represent the critical significance of Z values.
  • the molar ratio of REM to Zr is computed, the proportion of the number of the REM and Zr containing inclusions I (proportion of the number of the inclusion I) satisfying the condition that a REM/Zr ratio is 0.6 to 1.4 to the number of all inclusions is computed, and the results are shown in Tables 5 and 6.
  • the ratio of the total mole number of REM and Zr to the total mole number of Al, Ca, and Ti [(REM+Zr)/(Al+Ca+Ti)] is computed, the proportion of the number of the REM, Zr, Al, Ca, and Ti containing inclusions II (proportion of the number of the inclusion II) satisfying the condition that a (REM+Zr)/(Al+Ca+Ti) ratio is 0.5 to 1.2 to the number of all the inclusions is computed, and the results are shown in Tables 5 and 6.
  • weld reproducing test shown below simulating high heat input welding is carried out in order to evaluate the toughness of a HAZ affected by heat during welding.
  • a sample of a heavy steel plate cut out from the position of t/4 (here, t is the plate thickness) is subjected to a heat cycle of heating the sample to 1,450°C and, retaining the sample for 5 sec. at the temperature, and thereafter cooling the sample.
  • the cooling rate is adjusted so that the cooling time from 800°C to 500°C may be 400 sec. (heat input conditions: 1,450°C x 5 sec., cooling time Tc is 400 sec.).
  • the impact characteristic of a sample after cooled is evaluated by obtaining three V-notched Charpy test pieces in the rolling direction from a sample subjected to the heat cycle and applying impact test in accordance with JIS Z2242.
  • an absorption energy at -40°C (vE- 40 ) is measured and the average of the three measurements is computed.
  • vE- 40 absorption energy at -40°C
  • a case where the average of vE- 4 o is 130 J or more is rated as acceptable (HAZ toughness is good).
  • the measurement results are shown in Tables 5 and 6.
  • Nos. 1 to 32 are the cases of satisfying the requirements stipulated in the present invention and a steel material having a good HAZ toughness is obtained by adjusting ZrO 2 , REM oxides, and CaO so as to be contained by prescribed amounts when the composition of all oxide-based inclusions included in the steel material is measured and then expressed in terms of the mass of the individual oxides, then inhibiting the oxides having circle equivalent diameters exceeding 3 ⁇ m and the oxides having circle equivalent diameters exceeding 5 ⁇ m from forming, forming the inclusions having circle equivalent diameters of 0.1 to 1.2 ⁇ m abundantly, and further controlling the proportion of the number of the inclusions I to the number of all the inclusions to 30% or more and/or the proportion of the number of the inclusions II to the number of all the inclusions to 40% or more. Further, it is understood that the HAZ toughness tends to improve as the Si content increases.
  • Nos. 33 to 64 are the cases of deviating from any one of the requirements stipulated in the present invention.
  • the time from the addition of REM to the addition of Zr in the cases of Nos. 33, 51, 52, 55 to 58, 62, and 64 and the time from the addition of Zr to the addition of REM in the cases of Nos. 34 and 60 do not satisfy the requirements stipulated in the present invention and hence the proportion of the number of the inclusions I is lower than 30%. Consequently, the HAZ toughness deteriorates.
  • the REM content contained in a steel material is large, the amount of oxides of REM obtained by measuring the composition of all oxide-based inclusions included in the steel material and being expressed in terms of the mass of individual oxides is larger than the range stipulated in the present invention, hence the oxides coarsen, the number of fine oxides acting as nuclei of intragranular ⁇ transformation reduces, and the function of improving HAZ toughness is not exhibited.
  • the Zr content contained in a steel material is too little, hence the ZrO 2 content in the composition of all oxide-based inclusions reduces, and the amount of the Zr-REM-Ca-based oxides acting as nuclei of intragranular ⁇ transformation also reduces.
  • the HAZ toughness deteriorates.
  • the Zr content contained in a steel material is too much and hence the ZrO 2 content in the composition of all oxide-based inclusions increases.
  • the amount of oxides acting as nuclei of intragranular ⁇ transformation during welding is insufficient, a fine structure is not obtained, and the HAZ toughness deteriorates.
  • the Ti content contained in a steel material is too much, hence the base material is solute-strengthened by the solid solution of Ti, and resultantly the HAZ toughness deteriorates.
  • the Ti content contained in a steel material is too little and hence the amount of the formed inclusions having circle equivalent diameters of 0.1 to 2 ⁇ m acting as nuclei of intragranular ⁇ transformation is not secured. Consequently, the HAZ toughness deteriorates.
  • the Al content contained in a steel material is too much, hence coarse oxides having circle equivalent diameters exceeding 3 ⁇ m form abundantly, and the HAZ toughness deteriorates.
  • No. 50 is the case where the N content contained in a steel material is too much and it is estimated that the solute N content contained in the steel material is excessive and the HAZ toughness deteriorates.
  • Fig. 3 Successively, the relationship between the number of oxides having circle equivalent diameters exceeding 3 ⁇ m in an observation visual filed area of 1 mm 2 and an absorption energy at -40°C (vE- 40 ) is shown in Fig. 3 .
  • the results of Nos. 1 to 32 shown in Table 5 are represented with the symbols ⁇ and the results of Nos. 35 to 40, 49, 53, 54, and 61 shown in Table 6 (the cases of exceeding 5.0 pieces in the comparative examples) are represented with the symbols ⁇ .
  • vE- 40 is not less than 130 J and the HAZ toughness improves in the case where the proportion of the number of the inclusion I to the number of all inclusions is not less than 30% (No. 23).
  • a steel material according to the present invention is excellent in HAZ toughness even when a high heat input welding method is applied and can be used for bridges, high-rise buildings, and marine vessels.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Treatment Of Steel In Its Molten State (AREA)
EP12736520.3A 2011-01-18 2012-01-17 Matériau d'acier à ténacité supérieure de zone soudée, touchée par la chaleur, et son procédé de fabrication Withdrawn EP2666880A4 (fr)

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PCT/JP2012/050852 WO2012099119A1 (fr) 2011-01-18 2012-01-17 Matériau d'acier à ténacité supérieure de zone soudée, touchée par la chaleur, et son procédé de fabrication

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EP2977479A4 (fr) * 2013-03-22 2016-11-30 Kobe Steel Ltd Matériau acier présentant une résistance supérieure dans une zone affectée par la chaleur de soudage
EP3081663A4 (fr) * 2013-12-11 2017-06-14 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) Plaque d'acier dotée d'une excellente résistance à l'acidité, d'une excellente ténacité haz et d'une excellente dureté haz et tuyau en acier pour tuyau de canalisation
US10023946B2 (en) 2013-03-12 2018-07-17 Jfe Steel Corporation Thick steel sheet having excellent CTOD properties in multilayer welded joints, and manufacturing method for thick steel sheet
US10036079B2 (en) 2013-03-12 2018-07-31 Jfe Steel Corporation Thick steel sheet having excellent CTOD properties in multilayer welded joints, and manufacturing method for thick steel sheet
US10081042B2 (en) 2013-08-16 2018-09-25 Nippon Steel & Sumitomo Metal Corporation Electric resistance welded steel pipe excellent in weld zone and method of production of same

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CN102912221B (zh) * 2012-10-12 2014-05-21 舞阳钢铁有限责任公司 一种大厚度高层建筑用结构钢板及其生产方法
JP6211296B2 (ja) * 2013-04-30 2017-10-11 株式会社神戸製鋼所 耐サワー性とｈａｚ靭性に優れた鋼板
JP6301805B2 (ja) * 2014-10-17 2018-03-28 株式会社神戸製鋼所 溶接熱影響部の靭性に優れたタンク用厚鋼板
JP2018016890A (ja) * 2017-09-26 2018-02-01 株式会社神戸製鋼所 溶接熱影響部の靱性に優れたタンク用厚鋼板
KR102648171B1 (ko) 2019-06-27 2024-03-19 닛폰세이테츠 가부시키가이샤 강재 및 그 제조 방법
CN111321348B (zh) * 2020-03-30 2022-01-11 南京钢铁股份有限公司 一种lng船用肋板l型钢及其制造方法

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10023946B2 (en) 2013-03-12 2018-07-17 Jfe Steel Corporation Thick steel sheet having excellent CTOD properties in multilayer welded joints, and manufacturing method for thick steel sheet
US10036079B2 (en) 2013-03-12 2018-07-31 Jfe Steel Corporation Thick steel sheet having excellent CTOD properties in multilayer welded joints, and manufacturing method for thick steel sheet
EP2977479A4 (fr) * 2013-03-22 2016-11-30 Kobe Steel Ltd Matériau acier présentant une résistance supérieure dans une zone affectée par la chaleur de soudage
US10081042B2 (en) 2013-08-16 2018-09-25 Nippon Steel & Sumitomo Metal Corporation Electric resistance welded steel pipe excellent in weld zone and method of production of same
EP3081663A4 (fr) * 2013-12-11 2017-06-14 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) Plaque d'acier dotée d'une excellente résistance à l'acidité, d'une excellente ténacité haz et d'une excellente dureté haz et tuyau en acier pour tuyau de canalisation

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KR101512257B1 (ko) 2015-04-14
WO2012099119A1 (fr) 2012-07-26
JP5651090B2 (ja) 2015-01-07
CN103328672B (zh) 2015-06-03
JP2012162797A (ja) 2012-08-30
KR20130105713A (ko) 2013-09-25
EP2666880A4 (fr) 2015-02-25

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