WO2013190975A1 - Steel material having excellent toughness in weld-heat-affected zone - Google Patents

Steel material having excellent toughness in weld-heat-affected zone Download PDF

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WO2013190975A1
WO2013190975A1 PCT/JP2013/065142 JP2013065142W WO2013190975A1 WO 2013190975 A1 WO2013190975 A1 WO 2013190975A1 JP 2013065142 W JP2013065142 W JP 2013065142W WO 2013190975 A1 WO2013190975 A1 WO 2013190975A1
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oxide
steel material
amount
rem
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PCT/JP2013/065142
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French (fr)
Japanese (ja)
Inventor
正樹 島本
崇 杉谷
哲史 出浦
秀徳 名古
朗 伊庭野
裕己 太田
進佑 佐藤
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株式会社神戸製鋼所
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Priority to KR1020147035298A priority Critical patent/KR101697845B1/en
Priority to CN201380031755.1A priority patent/CN104411849B/en
Priority to EP13806413.4A priority patent/EP2862953A4/en
Publication of WO2013190975A1 publication Critical patent/WO2013190975A1/en

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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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • 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

Definitions

  • the present invention relates to a steel material used for a bridge, a high-rise building, a ship, and the like, and in particular, a part that is affected by heat when welded (hereinafter referred to as “welding heat affected zone” or “HAZ”).
  • welding heat affected zone a part that is affected by heat when welded
  • the present applicants have proposed steel materials that suppress HAZ toughness deterioration when the high heat input welding method is employed in Patent Documents 1 to 3.
  • These steel materials are characterized in that they contain at least one of a REM oxide and CaO, and ZrO 2 as an oxide that becomes the nucleus of the intragranular ferrite transformation. Since the oxide exists in a liquid state in molten steel, it is finely dispersed in the steel. In addition, the oxide is thermally stable, and, for example, it does not dissolve and disappear even when exposed to a high temperature of 1400 ° C. for a long time, and thus greatly contributes to the improvement of HAZ toughness.
  • the present applicant improves the technique using the oxide that becomes the nucleus of the intragranular ferrite transformation disclosed in Patent Document 1, and provides a steel material in which the HAZ toughness does not deteriorate even when welding is performed with a larger amount of heat input. Therefore, research was repeated and the technique of Patent Document 4 was proposed.
  • Patent Document 4 the size and number of all oxides in steel (not limited to oxides that become the core of intragranular ferrite transformation, but all oxides) are deeply involved in improving HAZ toughness.
  • the steel material has excellent HAZ toughness even if large heat input welding with a heat input of about 50 kJ / mm is performed. Is disclosed.
  • the present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to produce a steel material having excellent HAZ toughness even when large heat input welding with a heat input of 60 kJ / mm or more is performed. It is to provide.
  • the steel material excellent in the toughness of the weld heat affected zone according to the present invention that has solved the above problems is C: 0.02 to 0.15% (meaning mass%, the same applies to the following components), 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, Ti: 0.005 to 0.10%, REM : 0.0003 to 0.015%, Ca: 0.0003 to 0.010%, Zr: 0.0010 to 0.050%, N: 0.010% or less, O: 0.0005 to 0.010% And the balance is steel made of iron and inevitable impurities.
  • the steel material includes an oxide containing Zr, REM, and Ca
  • B Of all the inclusions contained in the steel material, there are 120 or more inclusions with an equivalent circle diameter of 0.1 to 2 ⁇ m per 1 mm 2 of the observation field area, and an oxide with an equivalent circle diameter of more than 3 ⁇ m is the observation field area.
  • the number density of inclusions defined in (b) above is a value obtained by observation with an electron probe X-ray microanalyzer (EPMA; Electron Probe X-ray Micro Analyzer).
  • Insol.X is a filter having an aperture of 0.1 ⁇ m or an aperture of 2.0 ⁇ m after electrolytic extraction of the steel material.
  • the amount of element Ti and Al in the extraction residue remaining on the filter is respectively determined by inductively coupled plasma emission spectrometry (ICP emission analysis), and the amount of element N is determined by indophenol blue spectrophotometry. Calculation was performed by subtracting the element X amount Insol.X 2.0 in the extraction residue remaining on the filter having the aperture of 2.0 ⁇ m from the element X amount Insol.X 2.0 remaining in the extraction residue remaining on the filter having the aperture of 0.1 ⁇ m. Value.
  • the steel material as another element, [1] At least one element selected from the group consisting of Cu: 2% or less, Ni: 3.5% or less, Cr: 3% or less, and Mo: 1% or less, [2] At least one of Nb: 0.25% or less and V: 0.1% or less, [3] B: 0.005% or less, Etc. may be contained.
  • an oxide an oxide containing Zr, REM, and Ca that forms the nucleus of the intragranular ⁇ transformation ( ⁇ means a ferrite or a mixed structure of ferrite and bainite; the same applies hereinafter) is formed.
  • ⁇ means a ferrite or a mixed structure of ferrite and bainite; the same applies hereinafter
  • the size and number of inclusions and oxides (ie, particle size distribution) present in the steel material are also appropriately controlled. Therefore, it is possible to provide a steel material excellent in HAZ toughness during large heat input welding with a heat input amount of 60 kJ / mm or more.
  • the steel material of the present invention not only has a predetermined amount or more of fine inclusions having an equivalent circle diameter of 0.1 to 2 ⁇ m useful for improving HAZ toughness, but also has an equivalent circle diameter that adversely affects the improvement of HAZ toughness.
  • the HAZ toughness is excellent.
  • the HAZ toughness evaluation method disclosed in the example of Patent Document 4 is used. Even if welding is performed with a large heat input, the HAZ toughness can be increased.
  • the inventors of the present invention have been conducting research to provide a steel material having higher HAZ toughness at the time of higher heat input welding even after proposing the above-mentioned Patent Document 4.
  • an oxide having an equivalent circle diameter of more than 5.0 ⁇ m as described in Patent Document 4 is used.
  • the characteristic part of the present invention is (A) Increasing the number of fine inclusions having an equivalent circle diameter of 0.1 to 2 ⁇ m useful for improving HAZ toughness (120 pieces / mm 2 or more), (B) The number of oxides having an equivalent circle diameter of more than 3 ⁇ m that adversely affects the HAZ toughness improvement is reduced (5.0 pieces / mm 2 or less), (C) The composition ratio of Ti oxide and Al oxide contained in fine inclusions having a circle-equivalent diameter of 0.1 to 2 ⁇ m is set within a predetermined range (specific measurement means is an electrolytic extraction method). When the calculated value satisfies the above formula (1)), the HAZ toughness can be improved even if welding is performed with a larger amount of heat input than that of Patent Document 4.
  • the electrolytic extraction method the ICP emission analysis method, Measured in combination with indophenol blue spectrophotometry. Therefore, in the above (C), the composition ratio of Ti oxide and Al oxide contained in the extraction residue that passes through the filter having a mesh size of 2.0 ⁇ m but does not pass through the filter having a mesh size of 0.1 ⁇ m is defined. is doing. Therefore, the characteristic part of the present invention is that the number density of inclusions having an equivalent circle diameter of 0.1 to 2 ⁇ m (above (A)) and the composition ratio of Ti oxide and Al oxide contained in the inclusion (above ( C)).
  • Patent Document 4 the number of oxides having an equivalent circle diameter of more than 5.0 ⁇ m is controlled.
  • the equivalent circle diameter is more than 3 ⁇ m as defined in (B) above.
  • the HAZ toughness can be further improved by controlling the number of oxides.
  • the composition ratio of Ti oxide and Al oxide contained in the fine inclusions is controlled as in Patent Document 4 above. Further, it has been clarified that it is not necessary to pay special attention to an oxide having an equivalent circle diameter of more than 3 ⁇ m and not more than 5 ⁇ m, and the number of oxides having an equivalent circle diameter of more than 3 ⁇ m may be controlled.
  • oxide that is, an oxide containing Zr, REM, and Ca
  • Zr / REM / Ca-based oxide the former is particularly referred to as “Zr / REM / Ca-based oxide”
  • total oxide the oxide in addition to oxides composed solely of oxides, oxides include oxides and inclusions other than oxides (for example, sulfides, nitrides, carbides, or composite compounds thereof). It is also meant to include complex oxides.
  • the essential components (Zr, REM, and Ca) constituting the Zr / REM / Ca-based oxide may be particularly referred to as “intragranular ⁇ -transformation nucleation elements”.
  • the steel material of the present invention includes non-oxides such as sulfides, nitrides and carbides, or composite compounds thereof in addition to the oxides described above, but in this specification, the oxidation material contained in the steel material is included.
  • Materials, sulfides, nitrides, carbides, or composite compounds thereof are collectively referred to as “total inclusions”.
  • total inclusions In the present specification, among all the inclusions contained in the steel material, an inclusion having an equivalent circle diameter of 0.1 to 2 ⁇ m is referred to as a “fine inclusion”.
  • an oxide having an equivalent circle diameter of 0.1 to 2 ⁇ m is referred to as “fine oxide”, and an oxide having an equivalent circle diameter of more than 3 ⁇ m is referred to as “coarse”. These are sometimes referred to as “oxides”.
  • an oxide having a circle equivalent diameter of more than 5 ⁇ m is defined as “coarse oxide”.
  • an oxide having a circle equivalent diameter of more than 3 ⁇ m is defined as “coarse oxidation”. Things ".
  • “steel material having excellent HAZ toughness of high heat input welding” means a heat cycle in which a steel material is held at 1400 ° C. for 60 seconds and then cooled from 800 ° C. to 500 ° C. in 450 seconds.
  • the thermal history means that the absorbed energy (vE -40 ) at -40 ° C satisfies 100 J or more.
  • This heat history corresponds to the heat history received when high heat input welding with a heat input of 60 kJ / mm or more is performed.
  • the above heat history is sometimes called “large heat input heat history”.
  • the amount of heat input by this heat cycle is higher than the amount of heat input by the heat cycle described in Patent Document 4 (about 50 kJ / mm).
  • the Zr / REM / Ca-based oxide serving as the starting point of the intragranular ⁇ transformation will be described.
  • the Zr / REM / Ca-based oxide means an oxide containing all of an oxide of Zr, an oxide of REM, and an oxide of Ca.
  • a part of the Zr / REM / Ca-based oxide may exist as a single oxide containing an intragranular ⁇ -transformation nucleation element alone, or two or more intragranular ⁇ -transformation nucleation elements may be present. It may exist as a complex oxide containing. Examples of the single oxide include ZrO 2 for Zr; CaO for Ca; and REM for REM represented by the symbol “M”, such as M 2 O 3 , M 3 O 5 , and MO 2 . These oxides may exist in an aggregated state, or may exist in a form in which other compounds such as sulfides and nitrides are complex-deposited on the oxides.
  • the Zr / REM / Ca-based oxide needs to contain Ti oxide and Al oxide.
  • a Ti oxide and an Al oxide into a fine Zr / REM / Ca-based oxide having an equivalent circle diameter of 0.1 to 2 ⁇ m, the intragranular ⁇ transformation is promoted and the HAZ toughness is further improved. It becomes like this. Details of the composition ratio of the Ti oxide and the Al oxide contained in the fine Zr / REM / Ca oxide will be described later.
  • a part of the Ti oxide may exist as a single oxide (for example, Ti 2 O 3 , Ti 3 O 5 , TiO 2 ).
  • a part of the Al oxide may exist as a single oxide (for example, Al 2 O 3 ).
  • the number of oxides having a circle equivalent diameter exceeding 3 ⁇ m is controlled, and as defined in Patent Document 4 above, an oxide having a circle equivalent diameter of more than 3 ⁇ m and an equivalent circle diameter. It is not necessary to distinguish and control oxides greater than 5 ⁇ m. This is because in the steel material of the present invention, the composition ratio of Ti oxide and Al oxide contained in the fine inclusions is appropriately controlled.
  • the number of coarse oxides having an equivalent circle diameter of more than 3 ⁇ m needs to be 5.0 or less per 1 mm 2 of the viewing field area. This number may as small as possible, preferably not more than 3 per 1 mm 2, more preferably less than 1 per 1 mm 2, substantially zero and most preferably per 1 mm 2.
  • the number of coarse oxides having a diameter equivalent to a circle exceeding 3 ⁇ m is determined by observing the cross section of the steel material with, for example, EPMA, quantitatively analyzing the component composition of inclusions observed in the observation field, and determining the oxygen content.
  • the inclusion equivalent to 5% by mass or more may be used as an oxide, and the equivalent circle diameter of the oxide may be obtained by observing and measuring with, for example, a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • the number of fine inclusions having an equivalent circle diameter of 0.1 to 2 ⁇ m needs to be 120 or more per 1 mm 2 of the viewing field area. is there.
  • the number of the fine inclusions is preferably 200 or more per 1 mm 2 , more preferably 500 or more per 1 mm 2, and still more preferably 1000 or more per 1 mm 2 .
  • the number of fine inclusions having a circle equivalent diameter of 0.1 to 2 ⁇ m may be obtained by measuring the cross section of the steel material by, for example, TEM observation.
  • inclusions having an equivalent circle diameter of less than 0.1 ⁇ m hardly contribute to the HAZ toughness improving effect by inclusion dispersion, and thus are not included in the number of inclusions.
  • the “equivalent circle diameter” is a diameter of a circle assumed to have the same area of the inclusion (in the case of oxide, meaning of oxide), and is recognized on the TEM observation surface.
  • the steel material of the present invention is a Ti oxide for fine inclusions having an equivalent circle diameter of 0.1 to 2 ⁇ m that contributes to the improvement of HAZ toughness.
  • the greatest feature is that the composition ratio of the material and the Al oxide is contained so as to satisfy a predetermined range. That is, if the Ti oxide and Al oxide contained in the fine Zr, REM, and Ca-based oxides that become the nucleus of the intragranular ⁇ transformation are controlled within a predetermined range, the HAZ when high heat input welding is performed.
  • a part of the Zr / REM / Ca-based oxide becomes a liquid phase, and this liquid material is crystallized into a crystal structure that effectively acts as a nucleus of intragranular ⁇ transformation in the subsequent cooling process. Therefore, the interfacial energy between the intragranular ⁇ and the austenite ( ⁇ ) serving as the parent phase is reduced, and the interfacial energy between the intragranular ⁇ and the Zr / REM / Ca-based oxide is further reduced. Transformation is further promoted. As a result, the HAZ toughness of the steel material is improved.
  • Insol.Ti, Insol.N, and Insol.Al indicate the concentrations of compound-type Ti, N, and Al contained in the steel material, and are values calculated by the following procedure. That is, the steel material is subjected to electrolytic extraction, and the extracted electrolytic solution is filtered using a filter having an opening of 0.1 ⁇ m or an opening of 2.0 ⁇ m, respectively, and the extraction residue remaining on the filter is recovered. Next, among the amounts of Ti, N, and Al contained in the extraction residue (hereinafter, these elements are represented by X), the amounts of element Ti and Al are the ICP emission analysis method, and the amount of element N is the indophenol blue absorbance.
  • the amount of element X contained in the extraction residue remaining on the filter having a mesh opening of 0.1 ⁇ m was determined by the method of Insol.X 0.1 and the amount of element X contained in the extraction residue remaining on the filter having a mesh opening of 2.0 ⁇ m.
  • Insol.X 2.0, and Insol.X 2.0 is calculated by subtracting Insol.X 2.0 from Insol.X 0.1 (see the following formula).
  • Insol.X Insol.X 0.1 -Insol.X 2.0
  • Insol.Ti, Insol.N, and Insol.Al pass through a 2.0 ⁇ m aperture filter, and Ti, N, contained in inclusions that do not pass 0.1 ⁇ m aperture. And the amount of Al is shown. In the present invention, those measured in this way are regarded as the amounts of Ti, N, and Al contained in the fine inclusions having an equivalent circle diameter of 0.1 to 2 ⁇ m.
  • Insol.Ti-3.4 ⁇ Insol.N means the amount of Ti contained as a Ti oxide in the electrolytic extraction residue.
  • the above Insol.Ti means a compound-type Ti amount existing as a compound in the steel material, and Ti is a Ti oxide (for example, TiO 2 ), Ti nitride (TiN), or these It exists as a complex compound (for example, oxynitride).
  • Ti present as a compound includes carbides, but since there is almost no Ti carbide having a particle size exceeding 0.1 ⁇ m that remains on a filter having an aperture of 0.1 ⁇ m, Insol. Ti does not include the amount of Ti derived from Ti carbide.
  • the above Insol.N means a compound-type N amount present as a compound in the steel material, and N exists as a nitride.
  • the nitride include TiN, ZrN, BN, and AlN.
  • Insol.N means the amount of N that substantially constitutes TiN. Since ZrN, BN, and AlN hardly grow to a size that remains on a filter having an aperture of 0.1 ⁇ m, the above-mentioned Insol.N does not include the amount of N derived from ZrN, BN, or AlN.
  • the ratio of the atomic weight of Ti to the atomic weight of N is approximately 3.4. Therefore, by calculating 3.4 ⁇ Insol.N, the amount of Ti forming TiN can be obtained. Further, by subtracting the amount of Ti forming TiN (3.4 ⁇ Insol.N) from Insol.Ti, the amount of Ti present as Ti oxide in the steel material can be calculated.
  • Insol.Al means the amount of Al present as a compound in the steel material, and substantially means the amount of Al constituting the Al oxide (Al compound typified by Al 2 O 3 ). is doing.
  • Al may exist as nitrides as well as oxides, but as described above, there is almost no Al nitride that grows in such a size that it remains on a filter with an aperture of 0.1 ⁇ m.
  • Insol.Al does not contain Al nitride-derived Al content.
  • the above formula (1) indicates that Ti contained in an extraction residue that passes through a 2.0 ⁇ m aperture filter and does not pass through a 0.1 ⁇ m aperture (ie, equivalent to an inclusion having a circle equivalent diameter of 0.1 to 2 ⁇ m).
  • the composition ratio (mass basis) of the oxide and Al oxide is shown, and the composition of only inclusions effective for improving the HAZ toughness is shown.
  • Nos. 32 and 33 are steel materials having almost the same composition.
  • No. 32 has a good HAZ toughness because the value of the above formula (1) is controlled in the range of 1.0 to 8.
  • no. No. 33 has not improved the HAZ toughness because the value of the above formula (1) is less than 1.0.
  • the electrolytic solution a solution capable of dissolving the matrix (matrix) of the steel material by electrolysis can be used.
  • a methanol solution of 10% acetylacetone-1% tetramethylammonium chloride can be used.
  • the electrolytic conditions may be those which can dissolve the parent phase of the steel material.
  • the current density is preferably 100 to 200 A / m 2 .
  • inclusions contained in the steel material are recovered by electrolytic extraction, and the recovered inclusions are separated using filters having different openings, and the equivalent circle diameter is 0.1 to 2 ⁇ m.
  • the composition of fine inclusions is measured by ICP emission analysis and indophenol blue spectrophotometry. Therefore, the amount of Ti constituting the Ti oxide contained in the fine inclusions can be accurately quantified. That is, for the analysis of the component composition of inclusions contained in steel materials, it is common to identify inclusions using EPMA and quantitatively analyze the component composition of inclusions.
  • the component composition of fine inclusions of about 1 to 2 ⁇ m is analyzed by EPMA, for example, the amount of Ti constituting Ti oxide and the amount of Ti constituting Ti nitride are distinguished and accurately determined. It was difficult to do. Inclusions effective in improving HAZ toughness have a fine equivalent circle diameter of 0.1-2 ⁇ m, and Ti oxide and Ti nitride rarely exist alone in steel. This is because they usually exist as complex compounds. Therefore, even if analyzed by EPMA, it is not possible to accurately quantify only the amount of Ti constituting the Ti oxide from the composite compound of Ti oxide and Ti nitride.
  • the composition of inclusions is measured by combining electrolytic extraction, ICP emission analysis, and indophenol blue absorptiometry, so that Ti oxide and Al oxidation in fine inclusions are measured.
  • the composition ratio of the product can be accurately determined.
  • the left side value of the above formula (1) is 1.0 or more, preferably 1.5 or more, more preferably 2.0 or more.
  • the value on the left side of the above formula (1) exceeds 8 or less, preferably 7.5 or less, more preferably 7.0 or less.
  • the steel material of the present invention has an average composition of ZrO 2 of 5 to 50 when the composition of all oxides contained in the steel material is measured and converted to mass as a single oxide (total is 100%).
  • %, REM oxide (representing M 2 O 3 when REM is represented by the symbol M): 5 to 50%, CaO: 50% or less are preferably satisfied.
  • the oxide effectively acts as a nucleus of intragranular ferrite transformation. If the lower limit value of each oxide is not reached, the amount of oxides that form intragranular ferrite nuclei at the time of welding becomes insufficient, and the effect of improving HAZ toughness is hardly exhibited. On the other hand, when the upper limit value of each oxide is exceeded, the oxide becomes coarse, the number of fine oxides that effectively act as the nuclei of intragranular ferrite decreases, and the HAZ toughness improving effect is hardly exhibited effectively. .
  • the ZrO 2 is more preferably 8% or more, and still more preferably 10% or more.
  • a more preferable upper limit is 45%, and a more preferable upper limit is 40%.
  • the REM oxide is more preferably 10% or more, and still more preferably 13% or more. On the other hand, a more preferable upper limit is 45%, and a more preferable upper limit is 40%.
  • the REM oxide is present in the steel material in the form of M 2 O 3 , M 3 O 5 , MO 2, etc., but all the REM oxide is converted to M 2 O 3 . It means the amount when converted.
  • the CaO effectively acts as a nucleus of intragranular ferrite transformation, but if contained excessively, the intragranular ferrite transformation ability may be deteriorated. Further, if CaO is excessively contained, the nozzle used for casting may be melted. Therefore, the upper limit of CaO is preferably 50%, more preferably 45% or less, still more preferably 40% or less, and particularly preferably 30% or less. In order to effectively exhibit the above action, CaO is preferably contained in an amount of 3% or more, more preferably 5% or more, and further preferably 10% or more.
  • the remaining components of the total oxide composition are not particularly limited, and examples thereof include oxides of oxide-forming elements contained in the steel material of the present invention (for example, SiO 2 , Al 2 O 3 , MnO, etc.).
  • composition of the total oxide contained in the steel material is measured by observing the surface of the steel material with, for example, EPMA, and quantitatively analyzing the oxide found in the observation field. Details of the measurement conditions will be described in the column of Examples described later.
  • the steel material of the present invention has, as basic components, C: 0.02 to 0.15%, Si: 0.5% or less, Mn: 2.5% or less, P: 0.03% or less, S: 0.02 %: Al: 0.050% or less, Ti: 0.005 to 0.10%, REM: 0.0003 to 0.015%, Ca: 0.0003 to 0.010%, Zr: 0.0010 to It contains 0.050% and N: 0.010% or less.
  • C is an element indispensable for securing the strength of the steel material (base material), and needs to be contained by 0.02% or more.
  • the amount of C is preferably 0.04% or more, and more preferably 0.05% or more.
  • the amount of C exceeds 0.15%, a large amount of island martensite (MA) is generated in the HAZ during welding, leading to deterioration of the toughness of the HAZ, and also adversely affecting the weldability.
  • the C content is 0.15% or less, preferably 0.10% or less, and more preferably 0.08% or less.
  • Si is an element that has a deoxidizing action and contributes to improving the strength of the steel (base metal) by solid solution strengthening.
  • Si is preferably contained in an amount of 0.01% or more.
  • the Si content is more preferably 0.02% or more, still more preferably 0.05% or more, and particularly preferably 0.10% or more.
  • the Si content is 0.5% or less, preferably 0.45% or less, more preferably 0.40% or less.
  • Si is recommended to be 0.30% or less, preferably 0.05% or less, more preferably 0.01% or less.
  • the HAZ toughness is improved, but the strength of the steel material may be reduced.
  • Mn is an element that contributes to improving the strength of the steel (base material). In order to exhibit such an effect effectively, it is preferable to make it contain 0.4% or more.
  • the amount of Mn is more preferably 0.50% or more, still more preferably 0.7% or more, and particularly preferably 0.8% or more. However, if the amount of Mn exceeds 2.5%, the weldability of the steel material (base material) is deteriorated. Therefore, the amount of Mn needs to be suppressed to 2.5% or less.
  • the amount of Mn is preferably 2.3% or less, more preferably 2.0% or less.
  • P is an element that is easily segregated, and particularly segregates at the grain boundaries in the steel material to deteriorate the HAZ toughness. Therefore, the P amount needs to be suppressed to 0.03% or less.
  • the amount of P is preferably 0.020% or less, more preferably 0.015% or less. In general, P is unavoidably contained in an amount of about 0.001%.
  • S is a harmful element that combines with Mn to produce sulfide (MnS) and degrades the toughness of the base metal and the ductility in the thickness direction. Further, when S is combined with REM such as La or Ce to generate REM sulfide (for example, LaS or CeS), generation of oxide of REM is inhibited, and thus HAZ toughness is deteriorated. Therefore, the S amount needs to be suppressed to 0.02% or less.
  • the amount of S is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.006% or less. Note that S is usually unavoidably contained in an amount of about 0.0005%.
  • Al is an element that acts as a deoxidizer. However, if added excessively, the oxide is reduced to form a coarse Al oxide, and the HAZ toughness deteriorates. Therefore, the Al amount must be suppressed to 0.050% or less.
  • the amount of Al is preferably 0.04% or less, more preferably 0.03% or less, still more preferably 0.025% or less, and particularly preferably 0.010% or less. Al is usually unavoidably contained in an amount of about 0.0005%.
  • Ti is an element that generates nitrides such as TiN and oxides containing Ti in the steel material and contributes to the improvement of HAZ toughness. In order to exert such effects, it is necessary to contain Ti by 0.005% or more.
  • the amount of Ti is preferably 0.007% or more, more preferably 0.010% or more. However, if excessively added, the base metal itself is hardened by solid solution strengthening of Ti, leading to a reduction in HAZ toughness. Therefore, Ti should be suppressed to 0.10% or less.
  • the amount of Ti is preferably 0.07% or less, more preferably 0.06% or less.
  • REM rare earth element
  • Ca are elements necessary for generating respective oxides.
  • the oxides are easily finely dispersed, and the finely dispersed oxides become the nucleus of intragranular ⁇ transformation, which contributes to the improvement of HAZ toughness.
  • REM should be contained at 0.0003% or more, preferably 0.001% or more, more preferably 0.0020% or more.
  • REM when REM is added excessively, a coarse oxide is excessively generated, so that the HAZ toughness is deteriorated.
  • solid solution REM when REM is added excessively, solid solution REM will produce
  • the amount of REM is preferably 0.010% or less, more preferably 0.007% or less.
  • REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium).
  • Y yttrium
  • Ca should be contained 0.0003% or more, preferably 0.0005% or more, more preferably 0.0008% or more, and further preferably 0.001% or more.
  • CaO is excessively generated and inclusions with a high CaO concentration are generated. Therefore, the effect of acting as an intragranular transformation nucleus of inclusions is weakened, and the HAZ toughness is deteriorated. Therefore, the amount of Ca needs to be suppressed to 0.010% or less.
  • the amount of Ca is preferably 0.009% or less, and more preferably 0.008% or less.
  • Zr is an element that generates a complex oxide containing Zr and contributes to the improvement of HAZ toughness. In order to exhibit such an action effectively, it is necessary to contain 0.0010% or more.
  • the amount of Zr is preferably 0.002% or more, more preferably 0.0023% or more. However, if Zr is added excessively, a large amount of ZrO 2 is generated, and the effect of acting as an intragranular transformation nucleus of inclusions is weakened. If Zr is added excessively, fine nitrides (ZrN) and carbides (ZrC) that cause precipitation strengthening are formed, and the toughness of the base metal itself is reduced. Therefore, the amount of Zr is suppressed to 0.050% or less.
  • the amount of Zr is preferably 0.04% or less, more preferably 0.03% or less, and still more preferably 0.01% or less.
  • N is an element that precipitates nitrides (for example, ZrN and TiN), and the nitrides prevent the austenite grains formed in the HAZ during welding and promote ferrite transformation by the pinning effect. , Contributing to the improvement of HAZ toughness. In order to exhibit such an effect effectively, it is preferable to contain N 0.003% or more. The amount of N is more preferably 0.004% or more, and still more preferably 0.005% or more. As N increases, nitrides are formed to promote the refinement of austenite grains, so that it effectively works to improve the toughness of HAZ.
  • the N amount if the N amount exceeds 0.010%, the solid solution N amount increases, the toughness of the base metal itself deteriorates, and the HAZ toughness also decreases. Therefore, the N amount needs to be suppressed to 0.010% or less.
  • the N amount is preferably 0.009% or less, more preferably 0.008% or less.
  • the steel material of the present invention contains the above elements as essential components, and the O (oxygen) amount is 0.0005 to 0.010%.
  • the amount of O (oxygen) 0.0005 to 0.010% indicates the total amount of oxygen, and is composed of O (oxygen) forming oxides and free O (oxygen) dissolved in the steel material. It means the total amount.
  • the remaining components of the steel material are iron and inevitable impurities (for example, Mg, As, Se, etc.).
  • the steel material of the present invention is still another element, [1] At least one element selected from the group consisting of Cu: 2% or less, Ni: 3.5% or less, Cr: 3% or less, and Mo: 1% or less, [2] At least one of Nb: 0.25% or less and V: 0.1% or less, [3] B: 0.005% or less, It is also effective to contain such elements.
  • the reasons for setting these ranges are as follows.
  • At least one element selected from the group consisting of Cu, Ni, Cr, and Mo is an element that contributes to increasing the strength of the steel material, Alternatively, they can be added in combination.
  • the Cu content is preferably 2% or less.
  • the amount of Cu is more preferably 1.8% or less, still more preferably 1.5% or less.
  • the amount of Cu is more preferably 0.1% or more, and still more preferably 0.2% or more.
  • the Ni content is preferably 3.5% or less.
  • the amount of Ni is more preferably 3.0% or less, still more preferably 2.5% or less.
  • the amount of Ni is more preferably 0.1% or more, and still more preferably 0.2% or more.
  • the Cr content is preferably 3% or less.
  • the amount of Cr is more preferably 2% or less, still more preferably 1% or less.
  • the amount of Cr is more preferably 0.1% or more, and still more preferably 0.15% or more.
  • the Mo amount is preferably 1% or less.
  • the amount of Mo is more preferably 0.9% or less, and still more preferably 0.80% or less.
  • the amount of Mo is more preferably 0.1% or more, and still more preferably 0.15% or more.
  • Nb and V Nb and V are both precipitated as carbonitrides, and the pinning effect of the carbonitrides prevents austenite grains from coarsening during welding and improves HAZ toughness It is an element which has the effect
  • Nb and V can be added alone or in combination.
  • the Nb content is preferably 0.25% or less.
  • the amount of Nb is more preferably 0.2% or less, still more preferably 0.15% or less. In order to effectively exhibit the effect of Nb addition, it is preferable to contain 0.002% or more.
  • the amount of Nb is more preferably 0.01% or more, and further preferably 0.02% or more.
  • the V amount is preferably 0.1% or less.
  • the amount of V is more preferably 0.09% or less, still more preferably 0.08% or less.
  • the amount of V is more preferably 0.005% or more, and still more preferably 0.01% or more.
  • B (boron) B is an element that suppresses the formation of grain boundary ferrite and improves the HAZ toughness. However, if the amount of B exceeds 0.005%, it may precipitate as BN at the austenite grain boundary, which may lead to a decrease in toughness. Therefore, the amount of B is preferably 0.005% or less. The amount of B is more preferably 0.0040% or less. In addition, in order to exhibit the effect
  • the steel material of the present invention can be used as a material for structures such as bridges, high-rise buildings, ships, etc., and can be used not only for small to medium heat input welding but also for large heat input welding with a heat input of 60 kJ / mm or more. It is possible to prevent toughness deterioration of the weld heat affected zone.
  • the steel material of the present invention is intended for a thick steel plate having a thickness of about 3.0 mm or more.
  • the steel material of the present invention may be obtained by deoxidizing molten steel and then adding Ti after adding Ti.
  • Ti ⁇ Al Al
  • the ratio can be controlled appropriately, and a steel material that satisfies the above formula (1) can be manufactured. That is, since Ti oxide has a smaller interfacial energy with molten steel than Al oxide and Zr / REM / Ca-based oxide, Ti is added before adding Al, Zr, REM, and Ca to molten steel.
  • a fine Ti oxide can be formed, and as a result, a predetermined amount of fine inclusions having an equivalent circle diameter of 0.1 to 2 ⁇ m contributing to the HAZ toughness can be generated.
  • a composite oxide containing Ti and Al can be generated, and the activity as Ti oxide can be reduced to less than 1.
  • Zr, REM, and Ca-based oxides are formed by adding Zr, REM, and Ca, which are stronger deoxidizing elements than Ti and Al.
  • a predetermined amount of Ti oxide and Al oxide can be contained in the Zr / REM / Ca oxide.
  • Ti has a weaker deoxidizing power than Al, so even if Ti is added to molten steel and Ti is not added, the previously formed Al oxide cannot be reduced.
  • the REM / Ca-based oxide cannot contain a predetermined amount of Ti oxide. Further, the Ti oxide formed at this time exists as a single oxide, and the activity as the Ti oxide is close to 1. Therefore, when Zr, REM, and Ca, which have a stronger deoxidizing power than Ti, are added in this state, the Ti oxide is reduced and the amount of Ti oxide generated is reduced, resulting in a Zr / REM / Ca oxide.
  • a fixed amount of Ti oxide cannot be contained. Therefore, when manufacturing the steel material of the present invention, it is recommended not to use Al for deoxidation of molten steel.
  • Al deoxidation is performed, Al oxide may remain in the molten steel, so that it is difficult to form a Zr / REM / Ca-based oxide containing a predetermined amount of Ti oxide.
  • the molten steel may be deoxidized by a known method. For example, after adjusting components for elements other than Al, Ti, REM, Ca, and Zr, at least one element selected from C, Si, and Mn is used. Then, deoxidization may be performed, and then Ti may be added and then Al may be added.
  • Al, REM, Ca, and Zr after adding Ti, for example, (1) After adding Ti, REM, Ca, and Zr may be added in any order after adding Al. (2) After adding Ti, after adding Al, REM, Ca, and Zr may be added simultaneously, (3) After adding Ti, Al, REM, Ca, and Zr may be added simultaneously.
  • REM may be pure La, pure Ce, pure Y, or pure Ca, pure Zr, pure Ti, and further Fe—Si.
  • Misch metal is a mixture of rare earth elements, and specifically contains about 40 to 50% of Ce and about 20 to 40% of La. However, since the misch metal often contains Ca as an impurity, when the misch metal contains Ca, the total Ca amount including this Ca amount needs to satisfy the range defined in the present invention.
  • the molten steel obtained by adjusting the components in this manner can be continuously cast according to a conventional method to form a slab, and then hot rolled or the like according to a conventional method to produce the steel material of the present invention.
  • a test steel containing the chemical components shown in Table 1 below (the balance being iron and inevitable impurities) was melted.
  • components are adjusted for elements other than Al, Ti, REM, Ca, and Zr, and deoxidation is performed using at least one element selected from C, Si, and Mn.
  • the dissolved oxygen amount of the molten steel was adjusted.
  • REM, Ca, and Zr were added after adding Al and Ti to the molten steel which adjusted the amount of dissolved oxygen.
  • Table 1 below shows the order of addition of Al and Ti.
  • the test steels shown in Table 1 below were manufactured by the same method except that the order of addition of Ti and Al was changed.
  • Ti is in the form of an Fe—Ti alloy
  • Zr is in the form of an Fe—Zr alloy
  • REM is in the form of a misch metal containing about 25% La and about 50% Ce
  • Ca is a Ni—Ca alloy.
  • the component composition of all oxides and the number density of inclusions and oxides were measured by the following procedure. That is, a sample was cut out from the cross section at t / 4 (where t is the thickness of the steel plate) of the obtained thick steel plate, and the cut sample surface was used as EPMA “JXA-8500F (device name) manufactured by JEOL Datum.
  • the component composition was quantitatively analyzed for inclusions having an equivalent circle diameter of 0.1 ⁇ m or more.
  • the observation conditions are an acceleration voltage of 20 kV, a sample current of 0.01 ⁇ A, an observation visual field area of 1 to 5 cm 2 , an analysis number of 100 or more, and the component composition at the center of the inclusion is the wavelength dispersion spectroscopy of characteristic X-rays.
  • the analysis target elements are Si, Mn, S, Al, Ti, La, Ce, Ca, Zr, and O (oxygen), and using a known substance, the relationship between the X-ray intensity and the element concentration of each element is pre-calibrated. The amount of elements contained in the inclusions was quantified from the X-ray intensity obtained from the inclusions to be analyzed and the calibration curve.
  • inclusions having an oxygen content of 5% or more were defined as oxides.
  • the composition of the oxide was calculated in terms of the X-ray intensity ratio indicating the presence of these elements and converted into a single oxide of each element.
  • the average of the mass converted as a single oxide is defined as the average composition of the oxide.
  • the average composition of the REM oxide, ZrO 2 , and CaO is shown in Table 2 below.
  • the composition was calculated.
  • the “others” shown in Table 2 below are oxides other than REM oxide, ZrO 2 , and CaO (for example, Al 2 O 3 , MnO, SiO 2, etc.).
  • the equivalent circle diameter was measured by TEM observation (observation magnification 30,000 times), and the number of inclusions having an equivalent circle diameter (particle diameter) of 0.1 to 2 ⁇ m was measured.
  • Table 2 shows values obtained by converting the number of inclusions per 1 mm 2 of the observation visual field area.
  • inclusions having an oxygen content of 5% by mass or more were used as oxides, and the equivalent circle diameter of these oxides was measured by TEM observation (observation magnification 30,000 times). The number of oxides having a particle size exceeding 3 ⁇ m was measured. Table 2 shows values obtained by converting the number of oxides per 1 mm 2 observation field area.
  • a 10 mm ⁇ 20 mm ⁇ 20 mm sample is cut out from the cross section at the position of t / 4 (where t is the thickness of the steel plate) of the obtained thick steel plate, and the electrolytic solution after electrolytic extraction has an opening of 0.1 ⁇ m or Each filter was filtered using a filter having a mesh size of 2.0 ⁇ m, and the extraction residue remaining on the filter was collected.
  • As the electrolytic solution a methanol solution of 10% acetylacetone-1% tetramethylammonium chloride was used. Electrolytic extraction was performed at a current density of 100 to 200 A / m 2 .
  • the amount of Ti and Al contained in the collected extraction residue was determined by ICP emission spectrometry, and the amount of N was determined by indophenol blue absorptiometry using an ultraviolet-visible spectrophotometer “UVmini-1240 (manufactured by Shimadzu Corporation)”.
  • UVmini-1240 ultraviolet-visible spectrophotometer
  • the impact characteristics of the sample after cooling were evaluated by taking three V-notch Charpy test pieces in the rolling direction from the sample after applying the thermal cycle and conducting an impact test according to JIS Z2242.
  • the absorbed energy (vE -40 ) at -40 ° C was measured, and the average value of three times was calculated.
  • vE- 40 of 100 J or more is regarded as acceptable (haz toughness is good).
  • Table 2 The measurement results are shown in Table 2 below.
  • Table 1 and Table 2 can be considered as follows. No. Nos. 1 to 18 and 32 are examples satisfying the conditions defined in the present invention, and fine inclusions having an equivalent circle diameter of 0.1 to 2 ⁇ m are formed so that an oxide having an equivalent circle diameter of more than 3 ⁇ m is not generated. A large amount of steel is produced, and the component composition of the fine inclusions is appropriately controlled, so that a steel material having good HAZ toughness is obtained.
  • No. Reference numerals 19 to 31, 33 are examples that do not satisfy any of the requirements defined in the present invention.
  • No. In No. 19 since the amount of Al contained in the steel material is too large, a large amount of coarse oxide having an equivalent circle diameter exceeding 3 ⁇ m is generated, and the HAZ toughness is deteriorated.
  • No. No. 20 is an example in which the amount of N contained in the steel material is excessive, and the amount of solute N contained in the steel material becomes excessive, and it is considered that the HAZ toughness is deteriorated.
  • the amount of Zr contained in the steel material is too small, so the amount of ZrO 2 is reduced, and the amount of Zr / REM / Ca oxides that are the core of intragranular ⁇ transformation is considered to be reduced. Therefore, it is considered that the HAZ toughness is deteriorated.
  • No. 29, no. 30, and no. No. 33 is an example in which the value of the above formula (1) deviates from the requirement defined in the present invention because the order of addition of Ti and Al during melting is out of the conditions recommended in the present invention. Therefore, the HAZ toughness is deteriorated.
  • No. No. 31 has a poor balance of Ti, N and Al amounts, and the composition of inclusions contained in the steel material does not satisfy the relationship of the above formula (1) and exceeds the range specified in the present invention. It is considered that the melting point of the product is increased, the inclusion does not become a liquid phase at the time of high heat input welding, the inclusion that becomes the nucleus of the intragranular ⁇ transformation is hardly formed, and the HAZ toughness is not improved.

Abstract

Provided is a steel material having excellent HAZ toughness even when high-heat-input welding with a heat input amount of 60 kJ/mm or higher is performed. This steel material: (a) includes an oxide containing Zr, REM, and Ca; (b) is one for which, of all of the inclusions, those having a circle equivalent diameter of 0.1-2 μm are present in an amount of 120 or greater in an observation field with an area of 1 mm2, and oxides with a circle equivalent diameter exceeding 3 μm are present in an amount of 5.0 or less in an observation field with an area of 1 mm2; and (c) is one for which the component composition of the inclusions contained in the steel material satisfies the relationship in formula (1). (Insol.Ti - 3.4 × Insol.N)/Insol.Al = 1.0-8 . . .(1)

Description

溶接熱影響部の靭性に優れた鋼材Steel with excellent toughness in weld heat affected zone
　本発明は、橋梁や高層建造物、船舶などに使用される鋼材に関するものであり、特に、溶接したときに熱影響を受ける部位（以下、「溶接熱影響部」または「ＨＡＺ」と呼ぶことがある。）の靭性に優れた鋼材に関する。 The present invention relates to a steel material used for a bridge, a high-rise building, a ship, and the like, and in particular, a part that is affected by heat when welded (hereinafter referred to as “welding heat affected zone” or “HAZ”). There is a steel material with excellent toughness.
　橋梁や高層建造物、船舶などに使用される鋼材に要求される特性は、近年益々厳しくなっており、とりわけ良好な靭性が求められる。これらの鋼材は、一般的に溶接して接合されることが多いが、溶接継手部のうち、特にＨＡＺは溶接時に熱影響を受けて靭性が劣化しやすいという問題がある。この靭性劣化は溶接時の入熱量が大きくなるほど顕著に現れる。その原因は溶接時の入熱量が大きくなるとＨＡＺの冷却速度が遅くなり、焼入性が低下して粗大な島状マルテンサイトを生成することにあると考えられる。従ってＨＡＺの靭性を改善するには、溶接時の入熱量を極力抑えればよいと考えられる。しかしその一方で、溶接作業効率を高めるうえでは、例えばエレクトロガス溶接、エレクトロスラグ溶接、サブマージアーク溶接などの溶接入熱量が５０ｋＪ／ｍｍ以上の大入熱溶接法の採用が望まれる。 The properties required for steel materials used in bridges, high-rise buildings, ships, etc. have become increasingly severe in recent years, and particularly good toughness is required. Generally, these steel materials are often joined by welding, but among the welded joints, particularly HAZ has a problem that the toughness is likely to deteriorate due to thermal influence during welding. This deterioration in toughness becomes more noticeable as the heat input during welding increases. The cause is considered to be that when the amount of heat input during welding increases, the cooling rate of the HAZ decreases, and the hardenability decreases and coarse island martensite is generated. Therefore, in order to improve the toughness of the HAZ, it is considered that the heat input during welding should be suppressed as much as possible. However, on the other hand, in order to increase the welding work efficiency, it is desired to employ a high heat input welding method in which the heat input of welding is 50 kJ / mm or more, such as electrogas welding, electroslag welding, submerged arc welding, and the like.
　そこで本出願人は、大入熱溶接法を採用した場合のＨＡＺ靭性劣化を抑制する鋼材を特許文献１～３に提案している。これらの鋼材は、粒内フェライト変態の核となる酸化物としてＲＥＭの酸化物とＣａＯの少なくとも一方、およびＺｒＯ2を含有しているところに特徴がある。上記酸化物は、溶鋼中では液状で存在するため鋼中に微細分散する。しかも上記酸化物は熱的に安定であり、例えば、１４００℃レベルの高温に長時間曝されても固溶して消失しないため、ＨＡＺ靭性の向上に大きく寄与する。 In view of this, the present applicants have proposed steel materials that suppress HAZ toughness deterioration when the high heat input welding method is employed in Patent Documents 1 to 3. These steel materials are characterized in that they contain at least one of a REM oxide and CaO, and ZrO 2 as an oxide that becomes the nucleus of the intragranular ferrite transformation. Since the oxide exists in a liquid state in molten steel, it is finely dispersed in the steel. In addition, the oxide is thermally stable, and, for example, it does not dissolve and disappear even when exposed to a high temperature of 1400 ° C. for a long time, and thus greatly contributes to the improvement of HAZ toughness.
　また、本出願人は、特許文献１に開示した粒内フェライト変態の核となる酸化物を利用した技術を改良し、より大きな入熱量で溶接を行ってもＨＡＺ靭性が劣化しない鋼材を提供するために研究を重ね、特許文献４の技術を提案した。特許文献４では、鋼材中の全酸化物（粒内フェライト変態の核となる酸化物に限定されず、全ての酸化物を対象とする。）の大きさと個数がＨＡＺ靭性の向上に深く関与しており、特に、円相当直径で５．０μｍ超の粗大な酸化物を５個以下に低減すれば、入熱量が概ね５０ｋＪ／ｍｍ程度の大入熱溶接を行なってもＨＡＺ靭性に優れた鋼材が得られることを開示している。 In addition, the present applicant improves the technique using the oxide that becomes the nucleus of the intragranular ferrite transformation disclosed in Patent Document 1, and provides a steel material in which the HAZ toughness does not deteriorate even when welding is performed with a larger amount of heat input. Therefore, research was repeated and the technique of Patent Document 4 was proposed. In Patent Document 4, the size and number of all oxides in steel (not limited to oxides that become the core of intragranular ferrite transformation, but all oxides) are deeply involved in improving HAZ toughness. In particular, if the number of coarse oxides with an equivalent circle diameter of more than 5.0 μm is reduced to 5 or less, the steel material has excellent HAZ toughness even if large heat input welding with a heat input of about 50 kJ / mm is performed. Is disclosed.
特開２００７－１００２１３号公報Japanese Patent Laid-Open No. 2007-100193 特開２００７－２４７００４号公報JP 2007-247004 A 特開２００７－２４７００５号公報JP 2007-247005 A 特開２００９－１９７２６７号公報JP 2009-197267 A
　上記特許文献４によれば、粗大な酸化物の個数が著しく抑えられているため、上記特許文献１の実施例に開示されたＨＡＺ靭性評価方法よりも大きな入熱量で溶接を行なってもＨＡＺ靭性を高めることができた。つまり上記特許文献１では、１４００℃の加熱温度で５秒間保持した後８００℃から５００℃までの温度を３００秒で冷却する熱サイクル（入熱条件：１４００℃×５秒、冷却時間Ｔｃ＝３００秒）を与え、－４０℃における吸収エネルギー（ｖＥ-40）を測定した。一方、上記特許文献４では、１４００℃の保持時間を３０秒間と長くした熱サイクル（入熱条件：１４００℃×３０秒、冷却時間Ｔｃ＝３００秒）を与えたときの吸収エネルギーを上記と同様にして測定しており、この場合でも良好なＨＡＺ靭性が得られたことを確認している。しかし溶接入熱量は、近年益々大きくなっているため、更に大入熱の溶接を行った場合のＨＡＺ靭性向上が求められている。 According to Patent Document 4, since the number of coarse oxides is remarkably suppressed, even if welding is performed with a larger heat input than the HAZ toughness evaluation method disclosed in the Examples of Patent Document 1, HAZ toughness is achieved. I was able to increase. That is, in Patent Document 1, a heat cycle in which a temperature from 800 ° C. to 500 ° C. is cooled in 300 seconds after being held at a heating temperature of 1400 ° C. for 5 seconds (heat input condition: 1400 ° C. × 5 seconds, cooling time Tc = 300 Second), and the absorbed energy (vE -40 ) at -40 ° C was measured. On the other hand, in the above-mentioned Patent Document 4, the absorbed energy when giving a heat cycle (heat input condition: 1400 ° C. × 30 seconds, cooling time Tc = 300 seconds) in which the holding time at 1400 ° C. is increased to 30 seconds is the same as above. It was confirmed that good HAZ toughness was obtained even in this case. However, since the amount of welding heat input has been increasing in recent years, there has been a demand for improved HAZ toughness when welding with higher heat input is performed.
　本発明は上記の様な事情に着目してなされたものであって、その目的は、入熱量が６０ｋＪ／ｍｍ以上の大入熱溶接を行なった場合であってもＨＡＺ靭性に優れた鋼材を提供することにある。 The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to produce a steel material having excellent HAZ toughness even when large heat input welding with a heat input of 60 kJ / mm or more is performed. It is to provide.
　上記課題を解決することのできた本発明に係る溶接熱影響部の靭性に優れた鋼材とは、Ｃ：０．０２～０．１５％（質量％の意味。以下成分について同じ。）、Ｓｉ：０．５％以下、Ｍｎ：２．５％以下、Ｐ：０．０３％以下、Ｓ：０．０２％以下、Ａｌ：０．０５０％以下、Ｔｉ：０．００５～０．１０％、ＲＥＭ：０．０００３～０．０１５％、Ｃａ：０．０００３～０．０１０％、Ｚｒ：０．００１０～０．０５０％、Ｎ：０．０１０％以下、Ｏ：０．０００５～０．０１０％を含有し、残部が鉄および不可避不純物からなる鋼材である。そして、
（ａ）前記鋼材は、Ｚｒ、ＲＥＭ、およびＣａを含有する酸化物を含み、
（ｂ）前記鋼材に含まれる全介在物のうち、円相当直径で０．１～２μｍの介在物が観察視野面積１ｍｍ2あたり１２０個以上、円相当直径で３μｍ超の酸化物が観察視野面積１ｍｍ2あたり５．０個以下であり、且つ
（ｃ）前記鋼材に含まれる円相当直径が０．１～２μｍの介在物の成分組成が、下記式（１）の関係を満足しているところに要旨を有している。
（Insol.Ｔｉ－３．４×Insol.Ｎ）／Insol.Ａｌ＝１．０～８　　・・・（１） The steel material excellent in the toughness of the weld heat affected zone according to the present invention that has solved the above problems is C: 0.02 to 0.15% (meaning mass%, the same applies to the following components), 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, Ti: 0.005 to 0.10%, REM : 0.0003 to 0.015%, Ca: 0.0003 to 0.010%, Zr: 0.0010 to 0.050%, N: 0.010% or less, O: 0.0005 to 0.010% And the balance is steel made of iron and inevitable impurities. And
(A) The steel material includes an oxide containing Zr, REM, and Ca,
(B) Of all the inclusions contained in the steel material, there are 120 or more inclusions with an equivalent circle diameter of 0.1 to 2 μm per 1 mm 2 of the observation field area, and an oxide with an equivalent circle diameter of more than 3 μm is the observation field area. (C) The composition of inclusions having an equivalent circle diameter of 0.1 to 2 μm contained in the steel material satisfies the relationship of the following formula (1): 5.0 or less per 1 mm 2 Has a summary.
(Insol.Ti-3.4 × Insol.N) /Insol.Al=1.0-8 (1)
　上記（ｂ）で規定している介在物の個数密度は、電子プローブＸ線マイクロ分析計（ＥＰＭＡ；Electron Probe X-ray Micro Analyzer）で観察して求められる値である。 The number density of inclusions defined in (b) above is a value obtained by observation with an electron probe X-ray microanalyzer (EPMA; Electron Probe X-ray Micro Analyzer).
　また、上記式（１）において、Ｔｉ、Ｎ、およびＡｌを元素Ｘとしたとき、Insol.Ｘは、鋼材を電解抽出した後の電解液を目開き０．１μｍまたは目開き２．０μｍのフィルターを用いて夫々濾過し、フィルター上に残った抽出残渣中の元素Ｔｉ、Ａｌ量を誘導結合プラズマ発光分析法（ＩＣＰ発光分析法）、元素Ｎ量をインドフェノール青吸光光度法によって夫々定量し、目開き０．１μｍのフィルター上に残った抽出残渣中の元素Ｘ量Insol.Ｘ0.1から目開き２．０μｍのフィルター上に残った抽出残渣中の元素Ｘ量Insol.Ｘ2.0を引いて算出した値である。 In the above formula (1), when Ti, N, and Al are elements X, Insol.X is a filter having an aperture of 0.1 μm or an aperture of 2.0 μm after electrolytic extraction of the steel material. The amount of element Ti and Al in the extraction residue remaining on the filter is respectively determined by inductively coupled plasma emission spectrometry (ICP emission analysis), and the amount of element N is determined by indophenol blue spectrophotometry. Calculation was performed by subtracting the element X amount Insol.X 2.0 in the extraction residue remaining on the filter having the aperture of 2.0 μm from the element X amount Insol.X 2.0 remaining in the extraction residue remaining on the filter having the aperture of 0.1 μm. Value.
　前記鋼材は、更に他の元素として、
［１］Ｃｕ：２％以下、Ｎｉ：３．５％以下、Ｃｒ：３％以下、およびＭｏ：１％以下よりなる群から選ばれる少なくとも１種の元素、
［２］Ｎｂ：０．２５％以下とＶ：０．１％以下の少なくとも一方、
［３］Ｂ：０．００５％以下、
等の元素を含有してもよい。 The steel material, as another element,
[1] At least one element selected from the group consisting of Cu: 2% or less, Ni: 3.5% or less, Cr: 3% or less, and Mo: 1% or less,
[2] At least one of Nb: 0.25% or less and V: 0.1% or less,
[3] B: 0.005% or less,
Etc. may be contained.
　本発明によれば、粒内α変態（αは、フェライトまたはフェライトおよびベイナイトの混合組織を意味する。以下同じ）の核となる酸化物（Ｚｒ、ＲＥＭ、およびＣａを含有する酸化物）が生成しており、しかも鋼材中に存在する介在物および酸化物の大きさと個数（即ち、粒度分布）も適切に制御されている。そのため、入熱量が６０ｋＪ／ｍｍ以上の大入熱溶接時のＨＡＺ靭性に優れた鋼材を提供できる。即ち、本発明の鋼材は、特に、ＨＡＺ靭性向上に有用な円相当直径が０．１～２μｍの微細な介在物が所定量以上存在するだけでなく、ＨＡＺ靭性向上に悪影響を及ぼす円相当直径が３μｍ超の粗大な酸化物の個数が有意に抑制されているため、ＨＡＺ靭性に優れたものとなる。しかも本発明によれば、上記微細な介在物に含まれるＴｉ酸化物とＡｌ酸化物の組成比を適切に制御しているため、上記特許文献４の実施例に開示されたＨＡＺ靭性評価方法より大きな入熱量で溶接を行なってもＨＡＺ靭性を高めることができる。 According to the present invention, an oxide (an oxide containing Zr, REM, and Ca) that forms the nucleus of the intragranular α transformation (α means a ferrite or a mixed structure of ferrite and bainite; the same applies hereinafter) is formed. In addition, the size and number of inclusions and oxides (ie, particle size distribution) present in the steel material are also appropriately controlled. Therefore, it is possible to provide a steel material excellent in HAZ toughness during large heat input welding with a heat input amount of 60 kJ / mm or more. That is, the steel material of the present invention not only has a predetermined amount or more of fine inclusions having an equivalent circle diameter of 0.1 to 2 μm useful for improving HAZ toughness, but also has an equivalent circle diameter that adversely affects the improvement of HAZ toughness. However, since the number of coarse oxides exceeding 3 μm is significantly suppressed, the HAZ toughness is excellent. Moreover, according to the present invention, since the composition ratio of Ti oxide and Al oxide contained in the fine inclusions is appropriately controlled, the HAZ toughness evaluation method disclosed in the example of Patent Document 4 is used. Even if welding is performed with a large heat input, the HAZ toughness can be increased.
　本発明者らは、上記特許文献４を提案した後も、一層高いレベルの大入熱溶接時のＨＡＺ靭性に優れた鋼材を提供するため研究を進めてきた。その結果、上記特許文献４よりも更に大入熱量の条件である「１４００℃の加熱温度で６０秒間保持した後８００℃から５００℃までの温度を４５０秒で冷却する熱サイクル」（入熱条件：１４００℃×６０秒、冷却時間Ｔｃ＝４５０秒）を与えた場合でもＨＡＺ靭性に優れた鋼材を提供するには、上記特許文献４のように円相当直径で５．０μｍ超の酸化物を５個以下に低減するだけでは不充分であり、上記特許文献４を含め従来は全く着目されていなかった円相当直径（以下、単に、粒径と略記する場合がある）が３μｍ超の酸化物の個数を低減することが極めて重要であること、および円相当直径が０．１～２μｍの微細な介在物に含まれるＴｉ酸化物とＡｌ酸化物の組成比が重要であることを見出し、本発明を完成した。 The inventors of the present invention have been conducting research to provide a steel material having higher HAZ toughness at the time of higher heat input welding even after proposing the above-mentioned Patent Document 4. As a result, “a heat cycle in which the temperature from 800 ° C. to 500 ° C. is cooled in 450 seconds after holding at the heating temperature of 1400 ° C. for 60 seconds”, which is a condition of a larger heat input than the above-mentioned Patent Document 4 (heat input conditions 1400 ° C. × 60 seconds, cooling time Tc = 450 seconds) In order to provide a steel material having excellent HAZ toughness, an oxide having an equivalent circle diameter of more than 5.0 μm as described in Patent Document 4 is used. It is not sufficient to reduce the number to 5 or less. An oxide having an equivalent circle diameter (hereinafter simply referred to as a particle size) of more than 3 μm, which has not been paid attention in the past, including Patent Document 4 above. It has been found that it is extremely important to reduce the number of Ti and that the composition ratio of Ti oxide and Al oxide contained in fine inclusions having an equivalent circle diameter of 0.1 to 2 μm is important. Completed the invention.
　このように本発明の特徴部分は、
（Ａ）ＨＡＺ靭性向上に有用な円相当直径０．１～２μｍの微細な介在物の個数を増大させる（１２０個／ｍｍ2以上）と共に、
（Ｂ）ＨＡＺ靭性向上に悪影響を及ぼす円相当直径３μｍ超の酸化物の個数を低減させ（５．０個／ｍｍ2以下）、更に、
（Ｃ）上記円相当直径が０．１～２μｍの微細な介在物に含まれるＴｉ酸化物とＡｌ酸化物の組成比を所定の範囲にする（具体的な測定手段としては、電解抽出法によって算出した値が上記式（１）を満足する）ことによって、上記特許文献４よりも一層大きな入熱量で溶接を行なってもＨＡＺ靭性を改善できたところにある。 Thus, the characteristic part of the present invention is
(A) Increasing the number of fine inclusions having an equivalent circle diameter of 0.1 to 2 μm useful for improving HAZ toughness (120 pieces / mm 2 or more),
(B) The number of oxides having an equivalent circle diameter of more than 3 μm that adversely affects the HAZ toughness improvement is reduced (5.0 pieces / mm 2 or less),
(C) The composition ratio of Ti oxide and Al oxide contained in fine inclusions having a circle-equivalent diameter of 0.1 to 2 μm is set within a predetermined range (specific measurement means is an electrolytic extraction method). When the calculated value satisfies the above formula (1)), the HAZ toughness can be improved even if welding is performed with a larger amount of heat input than that of Patent Document 4.
　即ち、上記特許文献４との関係で言えば、上記（Ａ）に加え、上記（Ｂ）および上記（Ｃ）を規定したところに本発明の特徴部分が存在する。上記（Ｃ）で規定するように、上記微細な介在物についてＴｉ酸化物とＡｌ酸化物の組成比を適切に制御すれば、この介在物の融点が低下するため、大入熱溶接時に介在物が液相化し、粒内α変態の核となる介在物が形成され易くなり、ＨＡＺ靭性が向上することが判明した。 That is, in relation to the above-mentioned Patent Document 4, in addition to the above (A), the characteristic part of the present invention exists where the above (B) and (C) are defined. As defined in (C) above, if the composition ratio of Ti oxide and Al oxide is appropriately controlled with respect to the fine inclusions, the melting point of the inclusions is lowered. It became clear that the liquid phase became easier to form inclusions serving as nuclei for intragranular α transformation, and the HAZ toughness was improved.
　なお、上記微細な介在物におけるＴｉ酸化物とＡｌ酸化物の組成比をＥＰＭＡで精度良く測定することは、後述するように困難であるため、本発明では、電解抽出法とＩＣＰ発光分析法とインドフェノール青吸光光度法を組み合わせて測定している。従って上記（Ｃ）では、目開き２．０μｍのフィルターを通過するが、目開き０．１μｍのフィルターを通過せずに残った抽出残渣に含まれるＴｉ酸化物とＡｌ酸化物の組成比を規定している。よって本発明の特徴部分は、円相当直径が０．１～２μｍの介在物の個数密度（上記（Ａ））と、この介在物に含まれるＴｉ酸化物とＡｌ酸化物の組成比（上記（Ｃ））を規定しているところにある。 In addition, since it is difficult to accurately measure the composition ratio of Ti oxide and Al oxide in the fine inclusions with EPMA as described later, in the present invention, the electrolytic extraction method, the ICP emission analysis method, Measured in combination with indophenol blue spectrophotometry. Therefore, in the above (C), the composition ratio of Ti oxide and Al oxide contained in the extraction residue that passes through the filter having a mesh size of 2.0 μm but does not pass through the filter having a mesh size of 0.1 μm is defined. is doing. Therefore, the characteristic part of the present invention is that the number density of inclusions having an equivalent circle diameter of 0.1 to 2 μm (above (A)) and the composition ratio of Ti oxide and Al oxide contained in the inclusion (above ( C)).
　また、上記特許文献４では、円相当直径で５．０μｍ超の酸化物の個数を制御しているのに対し、本発明では、上記（Ｂ）で規定するように、円相当直径で３μｍ超の酸化物の個数を制御することによって、ＨＡＺ靭性を更に向上させることができる。本発明者らの検討結果によれば、良好なＨＡＺ靭性を実現するには、上記微細な介在物に含まれるＴｉ酸化物とＡｌ酸化物の組成比を制御すれば、上記特許文献４のように、円相当直径で３μｍ超５μｍ以下の酸化物に注目して特別に制御する必要はなく、円相当直径で３μｍ超の酸化物の個数を制御すればよいことが明らかになった。 In Patent Document 4, the number of oxides having an equivalent circle diameter of more than 5.0 μm is controlled. In the present invention, the equivalent circle diameter is more than 3 μm as defined in (B) above. The HAZ toughness can be further improved by controlling the number of oxides. According to the study results of the present inventors, in order to achieve good HAZ toughness, the composition ratio of Ti oxide and Al oxide contained in the fine inclusions is controlled as in Patent Document 4 above. Further, it has been clarified that it is not necessary to pay special attention to an oxide having an equivalent circle diameter of more than 3 μm and not more than 5 μm, and the number of oxides having an equivalent circle diameter of more than 3 μm may be controlled.
　本明細書では、粒内α変態の核となる酸化物（即ち、Ｚｒ、ＲＥＭ、およびＣａを含有する酸化物）と、鋼材中に含まれるすべての酸化物を区別するため、説明の便宜上、前者を特に「Ｚｒ・ＲＥＭ・Ｃａ系酸化物」と呼び、後者を特に「全酸化物」と呼ぶ場合がある。なお、酸化物には、酸化物のみから構成される単独酸化物の他、酸化物と酸化物以外の介在物（例えば、硫化物や窒化物、炭化物、或いはこれらの複合化合物）が複合している複合酸化物も含む意味である。 In this specification, in order to distinguish the oxide (that is, an oxide containing Zr, REM, and Ca) that is the nucleus of the intragranular α-transformation from all the oxides contained in the steel material, The former is particularly referred to as “Zr / REM / Ca-based oxide”, and the latter is particularly referred to as “total oxide”. In addition to oxides composed solely of oxides, oxides include oxides and inclusions other than oxides (for example, sulfides, nitrides, carbides, or composite compounds thereof). It is also meant to include complex oxides.
　また、上記のＺｒ・ＲＥＭ・Ｃａ系酸化物を構成する必須成分（Ｚｒ、ＲＥＭ、およびＣａ）を特に「粒内α変態核生成元素」と呼ぶ場合がある。 Further, the essential components (Zr, REM, and Ca) constituting the Zr / REM / Ca-based oxide may be particularly referred to as “intragranular α-transformation nucleation elements”.
　また、本発明の鋼材には、上記の酸化物以外に硫化物や窒化物、炭化物などの非酸化物、或いはこれらの複合化合物等も含まれるが、本明細書では、鋼材中に含まれる酸化物、硫化物、窒化物、炭化物、或いはこれらの複合化合物等を総称して「全介在物」と呼ぶ。本明細書では、鋼材に含まれる全介在物のうち、円相当直径が０．１～２μｍの介在物を「微細な介在物」と呼ぶ。 Further, the steel material of the present invention includes non-oxides such as sulfides, nitrides and carbides, or composite compounds thereof in addition to the oxides described above, but in this specification, the oxidation material contained in the steel material is included. Materials, sulfides, nitrides, carbides, or composite compounds thereof are collectively referred to as “total inclusions”. In the present specification, among all the inclusions contained in the steel material, an inclusion having an equivalent circle diameter of 0.1 to 2 μm is referred to as a “fine inclusion”.
　また、本明細書では、鋼材に含まれる全酸化物のうち、円相当直径が０．１～２μｍの酸化物を「微細な酸化物」、円相当直径が３μｍ超の酸化物を「粗大な酸化物」と夫々呼び、これらを区別する場合がある。なお、上記特許文献４では、円相当直径で５μｍ超の酸化物を「粗大な酸化物」と定義していたが、本明細書では、円相当直径で３μｍ超の酸化物を「粗大な酸化物」としている。 In the present specification, among all oxides contained in steel materials, an oxide having an equivalent circle diameter of 0.1 to 2 μm is referred to as “fine oxide”, and an oxide having an equivalent circle diameter of more than 3 μm is referred to as “coarse”. These are sometimes referred to as “oxides”. In Patent Document 4, an oxide having a circle equivalent diameter of more than 5 μm is defined as “coarse oxide”. However, in this specification, an oxide having a circle equivalent diameter of more than 3 μm is defined as “coarse oxidation”. Things ".
　本明細書において「大入熱溶接のＨＡＺ靭性に優れた鋼材」とは、鋼材に対し、１４００℃で６０秒間保持した後、８００℃から５００℃までの温度を４５０秒で冷却する熱サイクル（熱履歴）を与えたとき、－４０℃における吸収エネルギー（ｖＥ-40）が１００Ｊ以上を満足するものを意味する。この熱履歴は、入熱量が６０ｋＪ／ｍｍ以上の大入熱溶接を行った場合に受ける熱履歴に相当している。上記熱履歴を特に「大入熱熱履歴」と呼ぶ場合がある。この熱サイクルによる入熱量は、上記特許文献４に記載の熱サイクルによる入熱量（５０ｋＪ／ｍｍ程度）に比べて高いものであり、その意味で本発明の「大入熱溶接」と、上記特許文献４に記載の「大入熱溶接」の入熱レベルが相違するものである。上記ｖＥ-40は大きい程良く、好ましくはｖＥ-40が１３０Ｊ以上である。 In this specification, “steel material having excellent HAZ toughness of high heat input welding” means a heat cycle in which a steel material is held at 1400 ° C. for 60 seconds and then cooled from 800 ° C. to 500 ° C. in 450 seconds. When the thermal history is given, it means that the absorbed energy (vE -40 ) at -40 ° C satisfies 100 J or more. This heat history corresponds to the heat history received when high heat input welding with a heat input of 60 kJ / mm or more is performed. The above heat history is sometimes called “large heat input heat history”. The amount of heat input by this heat cycle is higher than the amount of heat input by the heat cycle described in Patent Document 4 (about 50 kJ / mm). In that sense, the “large heat input welding” of the present invention and the above patent The heat input level of “Large heat input welding” described in Document 4 is different. The larger vE -40 is better. Preferably, vE -40 is 130 J or more.
　以下、本発明を構成する上記（ａ）～（ｃ）の要件について、詳しく説明する。 Hereinafter, the requirements (a) to (c) constituting the present invention will be described in detail.
　（ａ）Ｚｒ・ＲＥＭ・Ｃａ系酸化物について
　まず、粒内α変態の起点となるＺｒ・ＲＥＭ・Ｃａ系酸化物について説明する。上記Ｚｒ・ＲＥＭ・Ｃａ系酸化物は、Ｚｒの酸化物、ＲＥＭの酸化物、およびＣａの酸化物を全て含んでいるものを意味している。 (A) Zr / REM / Ca-based oxide First, the Zr / REM / Ca-based oxide serving as the starting point of the intragranular α transformation will be described. The Zr / REM / Ca-based oxide means an oxide containing all of an oxide of Zr, an oxide of REM, and an oxide of Ca.
　上記Ｚｒ・ＲＥＭ・Ｃａ系酸化物の一部は、粒内α変態核生成元素を単独で含有する単独酸化物として存在していても良いし、２種以上の粒内α変態核生成元素を含む複合酸化物として存在していても良い。単独酸化物の例としては、ＺｒではＺｒＯ2；ＣａではＣａＯ；ＲＥＭでは、ＲＥＭを「Ｍ」の記号で表したとき、Ｍ2Ｏ3、Ｍ3Ｏ5、ＭＯ2などが例示される。また、これらの酸化物は、互いに凝集して存在しても良いし、上記酸化物に硫化物や窒化物などの他の化合物が複合析出した形態で存在しても良い。 A part of the Zr / REM / Ca-based oxide may exist as a single oxide containing an intragranular α-transformation nucleation element alone, or two or more intragranular α-transformation nucleation elements may be present. It may exist as a complex oxide containing. Examples of the single oxide include ZrO 2 for Zr; CaO for Ca; and REM for REM represented by the symbol “M”, such as M 2 O 3 , M 3 O 5 , and MO 2 . These oxides may exist in an aggregated state, or may exist in a form in which other compounds such as sulfides and nitrides are complex-deposited on the oxides.
　上記Ｚｒ・ＲＥＭ・Ｃａ系酸化物は、Ｔｉ酸化物およびＡｌ酸化物を含有している必要がある。円相当直径が０．１～２μｍの微細なＺｒ・ＲＥＭ・Ｃａ系酸化物にＴｉ酸化物およびＡｌ酸化物を含有させることによって、粒内α変態が促進され、ＨＡＺ靭性の向上が一層高められるようになる。上記微細なＺｒ・ＲＥＭ・Ｃａ系酸化物に含まれるＴｉ酸化物とＡｌ酸化物の組成比の詳細については後述する。 The Zr / REM / Ca-based oxide needs to contain Ti oxide and Al oxide. By incorporating a Ti oxide and an Al oxide into a fine Zr / REM / Ca-based oxide having an equivalent circle diameter of 0.1 to 2 μm, the intragranular α transformation is promoted and the HAZ toughness is further improved. It becomes like this. Details of the composition ratio of the Ti oxide and the Al oxide contained in the fine Zr / REM / Ca oxide will be described later.
　上記Ｔｉ酸化物の一部は、単独酸化物（例えば、Ｔｉ2Ｏ3、Ｔｉ3Ｏ5、ＴｉＯ2）として存在していても良い。また、上記Ａｌ酸化物の一部は、単独酸化物（例えば、Ａｌ2Ｏ3）として存在していても良い。 A part of the Ti oxide may exist as a single oxide (for example, Ti 2 O 3 , Ti 3 O 5 , TiO 2 ). A part of the Al oxide may exist as a single oxide (for example, Al 2 O 3 ).
　（ｂ）全介在物の粒度分布について
　次に、本発明を特徴付ける全介在物の個数と大きさについて説明する。本発明の鋼材は、ＥＰＭＡで観察したときに、
（ｉ）円相当直径で０．１～２μｍの微細な介在物が観察視野面積１ｍｍ2あたり１２０個以上であり、
（ｉｉ）円相当直径で３μｍを超える粗大な酸化物が観察視野面積１ｍｍ2あたり５．０個以下である。 (B) Particle size distribution of all inclusions Next, the number and size of all inclusions that characterize the present invention will be described. When the steel material of the present invention is observed with EPMA,
(I) There are 120 or more fine inclusions having an equivalent circle diameter of 0.1 to 2 μm per 1 mm 2 of the observation visual field area,
(Ii) The number of coarse oxides having an equivalent circle diameter exceeding 3 μm is 5.0 or less per 1 mm 2 of the observation visual field area.
　本発明の鋼材においては、円相当直径で３μｍを超える酸化物の個数を制御しており、上記特許文献４で規定しているように、円相当直径で３μｍ超の酸化物と円相当直径で５μｍ超の酸化物を区別して制御する必要はない。本発明の鋼材では、上記微細な介在物に含まれるＴｉ酸化物とＡｌ酸化物の組成比を適切に制御しているからである。 In the steel material of the present invention, the number of oxides having a circle equivalent diameter exceeding 3 μm is controlled, and as defined in Patent Document 4 above, an oxide having a circle equivalent diameter of more than 3 μm and an equivalent circle diameter. It is not necessary to distinguish and control oxides greater than 5 μm. This is because in the steel material of the present invention, the composition ratio of Ti oxide and Al oxide contained in the fine inclusions is appropriately controlled.
　本発明の鋼材においては、上記（ｉｉ）で規定するように、上記円相当直径が３μｍ超の粗大な酸化物の個数は観察視野面積１ｍｍ2あたり５．０個以下とする必要がある。この個数は少なければ少ない程良く、好ましくは１ｍｍ2あたり３個以下、より好ましくは１ｍｍ2あたり１個以下、最も好ましくは１ｍｍ2あたり実質的に０個である。 In the steel material of the present invention, as defined in (ii) above, the number of coarse oxides having an equivalent circle diameter of more than 3 μm needs to be 5.0 or less per 1 mm 2 of the viewing field area. This number may as small as possible, preferably not more than 3 per 1 mm 2, more preferably less than 1 per 1 mm 2, substantially zero and most preferably per 1 mm 2.
　なお、上記円相当直径で３μｍを超える粗大な酸化物の個数は、鋼材の断面を、例えば、ＥＰＭＡで観察し、観察視野内に認められる介在物の成分組成を定量分析し、酸素含有量が５質量％以上の介在物を酸化物とし、該酸化物の円相当直径を、例えば、透過型電子顕微鏡（ＴＥＭ）で観察して測定して求めればよい。 The number of coarse oxides having a diameter equivalent to a circle exceeding 3 μm is determined by observing the cross section of the steel material with, for example, EPMA, quantitatively analyzing the component composition of inclusions observed in the observation field, and determining the oxygen content. The inclusion equivalent to 5% by mass or more may be used as an oxide, and the equivalent circle diameter of the oxide may be obtained by observing and measuring with, for example, a transmission electron microscope (TEM).
　一方、本発明の鋼材においては、上記（ｉ）で規定するように、上記円相当直径が０．１～２μｍの微細な介在物の個数は観察視野面積１ｍｍ2あたり１２０個以上とする必要がある。上記微細な介在物を所定量以上生成させることによって、粒内α変態の核となる酸化物が増えるため、ＨＡＺ靭性を向上させることができる。上記微細な介在物の個数は、好ましくは１ｍｍ2あたり２００個以上、より好ましくは１ｍｍ2あたり５００個以上、更に好ましくは１ｍｍ2あたり１０００個以上である。 On the other hand, in the steel material of the present invention, as defined in (i) above, the number of fine inclusions having an equivalent circle diameter of 0.1 to 2 μm needs to be 120 or more per 1 mm 2 of the viewing field area. is there. By generating a predetermined amount or more of the fine inclusions, the number of oxides that become the nucleus of the intragranular α transformation increases, so that HAZ toughness can be improved. The number of the fine inclusions is preferably 200 or more per 1 mm 2 , more preferably 500 or more per 1 mm 2, and still more preferably 1000 or more per 1 mm 2 .
　なお、上記円相当直径で０．１～２μｍの微細な介在物の個数は、鋼材の断面を、例えば、ＴＥＭ観察により測定して求めればよい。なお、本発明の鋼材では、円相当直径で０．１μｍ未満の介在物は、介在物分散によるＨＡＺ靭性向上作用に殆ど寄与しないため、上記介在物の個数には含めていない。 The number of fine inclusions having a circle equivalent diameter of 0.1 to 2 μm may be obtained by measuring the cross section of the steel material by, for example, TEM observation. In the steel material of the present invention, inclusions having an equivalent circle diameter of less than 0.1 μm hardly contribute to the HAZ toughness improving effect by inclusion dispersion, and thus are not included in the number of inclusions.
　上記「円相当直径」とは、上記介在物（酸化物の場合は酸化物の意味）の面積が等しくなる様に想定した円の直径であり、ＴＥＭ観察面上で認められるものである。 The “equivalent circle diameter” is a diameter of a circle assumed to have the same area of the inclusion (in the case of oxide, meaning of oxide), and is recognized on the TEM observation surface.
　（ｃ）微細な介在物中のＴｉ酸化物とＡｌ酸化物の組成比
　本発明の鋼材は、ＨＡＺ靭性の向上に寄与する円相当直径が０．１～２μｍの微細な介在物について、Ｔｉ酸化物とＡｌ酸化物の組成比が、所定の範囲を満足するように含有しているところに最大の特徴がある。即ち、粒内α変態の核となる微細なＺｒ・ＲＥＭ・Ｃａ系酸化物に含まれているＴｉ酸化物とＡｌ酸化物を所定の範囲内に制御すれば、大入熱溶接したときのＨＡＺにおいて、Ｚｒ・ＲＥＭ・Ｃａ系酸化物の一部が液相化し、この液状物がその後の冷却過程で粒内α変態の核として有効に作用する結晶構造となって結晶化する。そのため、粒内αと、母相となるオーステナイト（γ）との界面エネルギーが低減されると共に、粒内αとＺｒ・ＲＥＭ・Ｃａ系酸化物との界面エネルギーも一層低くなるため、粒内α変態が一段と促進される。その結果、鋼材のＨＡＺ靭性が向上する。 (C) Composition ratio of Ti oxide and Al oxide in fine inclusions The steel material of the present invention is a Ti oxide for fine inclusions having an equivalent circle diameter of 0.1 to 2 μm that contributes to the improvement of HAZ toughness. The greatest feature is that the composition ratio of the material and the Al oxide is contained so as to satisfy a predetermined range. That is, if the Ti oxide and Al oxide contained in the fine Zr, REM, and Ca-based oxides that become the nucleus of the intragranular α transformation are controlled within a predetermined range, the HAZ when high heat input welding is performed. In this case, a part of the Zr / REM / Ca-based oxide becomes a liquid phase, and this liquid material is crystallized into a crystal structure that effectively acts as a nucleus of intragranular α transformation in the subsequent cooling process. Therefore, the interfacial energy between the intragranular α and the austenite (γ) serving as the parent phase is reduced, and the interfacial energy between the intragranular α and the Zr / REM / Ca-based oxide is further reduced. Transformation is further promoted. As a result, the HAZ toughness of the steel material is improved.
　上記円相当直径が０．１～２μｍの微細な介在物のＴｉ酸化物とＡｌ酸化物の組成比は、電解抽出法とＩＣＰ発光分析法とインドフェノール青吸光光度法を組み合わせることによって測定する。具体的には、本発明の鋼材は、下記式（１）の関係を満足している必要がある。
（Insol.Ｔｉ－３．４×Insol.Ｎ）／Insol.Ａｌ＝１．０～８　　・・・（１） The composition ratio of the fine inclusion Ti oxide and Al oxide having an equivalent circle diameter of 0.1 to 2 μm is measured by combining electrolytic extraction, ICP emission analysis, and indophenol blue absorptiometry. Specifically, the steel material of this invention needs to satisfy the relationship of following formula (1).
(Insol.Ti-3.4 × Insol.N) /Insol.Al=1.0-8 (1)
　上記Insol.Ｔｉ、Insol.Ｎ、およびInsol.Ａｌは、鋼材に含まれる化合物型のＴｉ、Ｎ、Ａｌの各濃度を示しており、次の手順で算出した値である。即ち、鋼材を電解抽出し、抽出後の電解液を目開き０．１μｍまたは目開き２．０μｍのフィルターを用いて夫々濾過し、フィルター上に残った抽出残渣を夫々回収する。次に、抽出残渣に含まれるＴｉ、Ｎ、およびＡｌ量（以下、これらの元素をＸで代表する）のうち、元素ＴｉおよびＡｌ量をＩＣＰ発光分析法、元素Ｎ量をインドフェノール青吸光光度法によって夫々定量し、目開き０．１μｍのフィルター上に残った抽出残渣に含まれる元素Ｘ量をInsol.Ｘ0.1、目開き２．０μｍのフィルター上に残った抽出残渣に含まれる元素Ｘ量をInsol.Ｘ2.0とし、Insol.Ｘ0.1からInsol.Ｘ2.0を引くことによって、Insol.Ｘを算出する（下記式参照）。
Insol.Ｘ＝Insol.Ｘ0.1－Insol.Ｘ2.0 The above Insol.Ti, Insol.N, and Insol.Al indicate the concentrations of compound-type Ti, N, and Al contained in the steel material, and are values calculated by the following procedure. That is, the steel material is subjected to electrolytic extraction, and the extracted electrolytic solution is filtered using a filter having an opening of 0.1 μm or an opening of 2.0 μm, respectively, and the extraction residue remaining on the filter is recovered. Next, among the amounts of Ti, N, and Al contained in the extraction residue (hereinafter, these elements are represented by X), the amounts of element Ti and Al are the ICP emission analysis method, and the amount of element N is the indophenol blue absorbance. The amount of element X contained in the extraction residue remaining on the filter having a mesh opening of 0.1 μm was determined by the method of Insol.X 0.1 and the amount of element X contained in the extraction residue remaining on the filter having a mesh opening of 2.0 μm. Insol.X 2.0, and Insol.X 2.0 is calculated by subtracting Insol.X 2.0 from Insol.X 0.1 (see the following formula).
Insol.X = Insol.X 0.1 -Insol.X 2.0
　即ち、上記式（１）中、Insol.Ｔｉ、Insol.Ｎ、およびInsol.Ａｌは、目開き２．０μｍフィルターを通過し、目開き０．１μｍを通過しない介在物に含まれるＴｉ、Ｎ、およびＡｌ量を示している。そして本発明では、このようにして測定されたものを夫々上記円相当直径が０．１～２μｍの微細な介在物に含まれるＴｉ、Ｎ、およびＡｌ量とみなしている。 That is, in the above formula (1), Insol.Ti, Insol.N, and Insol.Al pass through a 2.0 μm aperture filter, and Ti, N, contained in inclusions that do not pass 0.1 μm aperture. And the amount of Al is shown. In the present invention, those measured in this way are regarded as the amounts of Ti, N, and Al contained in the fine inclusions having an equivalent circle diameter of 0.1 to 2 μm.
　本発明では、鋼材中の円相当直径が０．１～２μｍの微細な介在物に含まれるＴｉ、Ｎ、およびＡｌ量の関係を規定することが重要であり、こうした微細な介在物はＨＡＺ靭性の向上に有用に作用する。一方、円相当直径が２μｍを超える（特に、円相当直径が３μｍを超える）介在物は、脆性破壊の起点となり、ＨＡＺ靭性を却って劣化させるからである。 In the present invention, it is important to define the relationship between the amounts of Ti, N, and Al contained in fine inclusions having a circle-equivalent diameter of 0.1 to 2 μm in the steel material, and such fine inclusions have HAZ toughness. Useful for improving On the other hand, inclusions with an equivalent circle diameter exceeding 2 μm (particularly with an equivalent circle diameter exceeding 3 μm) become the starting point of brittle fracture and deteriorate the HAZ toughness.
　上記「Insol.Ｔｉ－３．４×Insol.Ｎ」は、上記電解抽出残渣中に、Ｔｉ酸化物として含有しているＴｉ量を意味している。 “Insol.Ti-3.4 × Insol.N” means the amount of Ti contained as a Ti oxide in the electrolytic extraction residue.
　即ち、上記Insol.Ｔｉは、鋼材中に化合物として存在している化合物型のＴｉ量を意味しており、ＴｉはＴｉ酸化物（例えば、ＴｉＯ2）、Ｔｉ窒化物（ＴｉＮ）、またはこれらの複合化合物（例えば、酸窒化物など）として存在している。なお、化合物として存在するＴｉとしては、上記の他に炭化物などが挙げられるが、目開き０．１μｍのフィルター上に残るような粒径０．１μｍ超のＴｉ炭化物は殆ど存在しないため、Insol.ＴｉにはＴｉ炭化物に由来するＴｉ量は含まれない。 That is, the above Insol.Ti means a compound-type Ti amount existing as a compound in the steel material, and Ti is a Ti oxide (for example, TiO 2 ), Ti nitride (TiN), or these It exists as a complex compound (for example, oxynitride). In addition to the above, Ti present as a compound includes carbides, but since there is almost no Ti carbide having a particle size exceeding 0.1 μm that remains on a filter having an aperture of 0.1 μm, Insol. Ti does not include the amount of Ti derived from Ti carbide.
　一方、上記Insol.Ｎは、鋼材中に化合物として存在している化合物型のＮ量を意味しており、Ｎは窒化物として存在している。窒化物としては、ＴｉＮやＺｒＮ、ＢＮ、ＡｌＮなどが挙げられるが、上記Insol.Ｎは実質的にＴｉＮを構成しているＮ量を意味している。ＺｒＮやＢＮ、ＡｌＮは、目開き０．１μｍのフィルター上に残るような大きさには殆ど成長しないため、上記Insol.ＮにはＺｒＮやＢＮ、ＡｌＮに由来するＮ量は含まれない。 On the other hand, the above Insol.N means a compound-type N amount present as a compound in the steel material, and N exists as a nitride. Examples of the nitride include TiN, ZrN, BN, and AlN. Insol.N means the amount of N that substantially constitutes TiN. Since ZrN, BN, and AlN hardly grow to a size that remains on a filter having an aperture of 0.1 μm, the above-mentioned Insol.N does not include the amount of N derived from ZrN, BN, or AlN.
　そしてＴｉの原子量は４７．８８で、Ｎの原子量は１４．０１であるため、Ｔｉの原子量とＮの原子量の比は、おおよそ３．４となる。そこで３．４×Insol.Ｎを算出することによって、ＴｉＮを形成しているＴｉ量を求めることができる。また、Insol.Ｔｉから、ＴｉＮを形成しているＴｉ量（３．４×Insol.Ｎ）を引くことによって、鋼材中にＴｉ酸化物として存在しているＴｉ量を算出できる。 Since the atomic weight of Ti is 47.88 and the atomic weight of N is 14.01, the ratio of the atomic weight of Ti to the atomic weight of N is approximately 3.4. Therefore, by calculating 3.4 × Insol.N, the amount of Ti forming TiN can be obtained. Further, by subtracting the amount of Ti forming TiN (3.4 × Insol.N) from Insol.Ti, the amount of Ti present as Ti oxide in the steel material can be calculated.
　上記Insol.Ａｌは、鋼材中に化合物として存在しているＡｌ量を意味しており、Ａｌ酸化物（Ａｌ2Ｏ3に代表されるＡｌ化合物）を構成しているＡｌ量を実質的に意味している。Ａｌは、酸化物の他、窒化物などとしても存在する可能性があるが、上述したように、目開き０．１μｍのフィルター上に残るような大きさに成長するＡｌ窒化物は殆ど無いため、Insol.ＡｌにはＡｌ窒化物由来のＡｌ量は含まれない。 The above Insol.Al means the amount of Al present as a compound in the steel material, and substantially means the amount of Al constituting the Al oxide (Al compound typified by Al 2 O 3 ). is doing. Al may exist as nitrides as well as oxides, but as described above, there is almost no Al nitride that grows in such a size that it remains on a filter with an aperture of 0.1 μm. Insol.Al does not contain Al nitride-derived Al content.
　そして上記式（１）は、目開き２．０μｍフィルターを通過し、目開き０．１μｍを通過しない抽出残渣（即ち、円相当直径で０．１～２μｍの介在物に相当）に含まれるＴｉ酸化物とＡｌ酸化物の組成比（質量基準）を示しており、ＨＡＺ靭性の向上に有効な介在物のみの組成を示している。 Then, the above formula (1) indicates that Ti contained in an extraction residue that passes through a 2.0 μm aperture filter and does not pass through a 0.1 μm aperture (ie, equivalent to an inclusion having a circle equivalent diameter of 0.1 to 2 μm). The composition ratio (mass basis) of the oxide and Al oxide is shown, and the composition of only inclusions effective for improving the HAZ toughness is shown.
　上記式（１）を規定する意義は、後述する実施例で実証している。即ち、下記表１、表２に示すＮｏ．３２と３３は、成分組成がほぼ等しい鋼材であるが、Ｎｏ．３２は、上記式（１）の値が１．０～８の範囲に制御されているため、ＨＡＺ靭性が良好である。これに対し、Ｎｏ．３３は、上記式（１）の値が１．０を下回っているため、ＨＡＺ靭性を改善できていない。 The significance of defining the above formula (1) has been demonstrated in the examples described later. That is, No. 1 shown in Tables 1 and 2 below. Nos. 32 and 33 are steel materials having almost the same composition. No. 32 has a good HAZ toughness because the value of the above formula (1) is controlled in the range of 1.0 to 8. In contrast, no. No. 33 has not improved the HAZ toughness because the value of the above formula (1) is less than 1.0.
　また、下記表１、表２に示すＮｏ．４、１６、２９を比較しても同様の考察ができる。即ち、これらは成分組成がほぼ等しい鋼材であるが、Ｎｏ．４と１６は、上記式（１）の値が１．０～８の範囲に制御されているため、ＨＡＺ靭性が良好である。これに対し、Ｎｏ．２９は、上記式（１）の値が１．０を下回っているため、ＨＡＺ靭性を改善できていない。 In addition, No. shown in Table 1 and Table 2 below. Similar considerations can be made by comparing 4, 16, and 29. That is, these are steel materials having almost the same component composition. 4 and 16 have good HAZ toughness because the value of the above formula (1) is controlled in the range of 1.0 to 8. In contrast, no. No. 29 has not improved the HAZ toughness because the value of the above formula (1) is less than 1.0.
　上記電解液としては、電気分解によって鋼材の母相（マトリックス）を溶解できる溶液を用いることができ、例えば、１０％アセチルアセトン－１％テトラメチルアンモニウムクロリドのメタノール溶液などを用いることができる。 As the electrolytic solution, a solution capable of dissolving the matrix (matrix) of the steel material by electrolysis can be used. For example, a methanol solution of 10% acetylacetone-1% tetramethylammonium chloride can be used.
　電解条件は鋼材の母相を溶解できる条件を採用すればよく、例えば、電流密度は１００～２００Ａ／ｍ2とすることが好ましい。 The electrolytic conditions may be those which can dissolve the parent phase of the steel material. For example, the current density is preferably 100 to 200 A / m 2 .
　本発明では、上述したように、電解抽出法によって鋼材に含まれる介在物を回収し、回収された介在物を目開きの異なるフィルターを用いて分離し、円相当直径が０．１～２μｍの微細な介在物の成分組成をＩＣＰ発光分析法とインドフェノール青吸光光度法にて測定している。そのため、微細な介在物に含まれるＴｉ酸化物を構成しているＴｉ量を正確に定量できる。即ち、鋼材に含まれる介在物の成分組成の分析には、従来ＥＰＭＡを用いて介在物を同定し、介在物の成分組成を定量分析することが一般的であるが、円相当直径が０．１～２μｍ程度の微細な介在物の成分組成をＥＰＭＡで分析しても、例えばＴｉ酸化物を構成しているＴｉ量と、Ｔｉ窒化物を構成しているＴｉ量とを区別して正確に定量することは困難であった。ＨＡＺ靭性の向上に有効な介在物の円相当直径が０．１～２μｍと微細であるうえ、Ｔｉ酸化物とＴｉ窒化物は、鋼材中に夫々が単独で存在していることは稀であり、通常、複合化合物として存在しているからである。従ってＥＰＭＡで分析しても、Ｔｉ酸化物とＴｉ窒化物の複合化合物からＴｉ酸化物を構成しているＴｉ量のみを正確に定量することはできない。これに対し、本発明では、電解抽出法とＩＣＰ発光分析法とインドフェノール青吸光光度法を組み合わせて介在物の成分組成を測定しているため、微細な介在物中のＴｉ酸化物とＡｌ酸化物の組成比を精度良く定量できる。 In the present invention, as described above, inclusions contained in the steel material are recovered by electrolytic extraction, and the recovered inclusions are separated using filters having different openings, and the equivalent circle diameter is 0.1 to 2 μm. The composition of fine inclusions is measured by ICP emission analysis and indophenol blue spectrophotometry. Therefore, the amount of Ti constituting the Ti oxide contained in the fine inclusions can be accurately quantified. That is, for the analysis of the component composition of inclusions contained in steel materials, it is common to identify inclusions using EPMA and quantitatively analyze the component composition of inclusions. Even if the component composition of fine inclusions of about 1 to 2 μm is analyzed by EPMA, for example, the amount of Ti constituting Ti oxide and the amount of Ti constituting Ti nitride are distinguished and accurately determined. It was difficult to do. Inclusions effective in improving HAZ toughness have a fine equivalent circle diameter of 0.1-2 μm, and Ti oxide and Ti nitride rarely exist alone in steel. This is because they usually exist as complex compounds. Therefore, even if analyzed by EPMA, it is not possible to accurately quantify only the amount of Ti constituting the Ti oxide from the composite compound of Ti oxide and Ti nitride. On the other hand, in the present invention, the composition of inclusions is measured by combining electrolytic extraction, ICP emission analysis, and indophenol blue absorptiometry, so that Ti oxide and Al oxidation in fine inclusions are measured. The composition ratio of the product can be accurately determined.
　上記式（１）の左辺値が１．０を下回ると、Ｔｉ酸化物に対してＡｌ酸化物が過剰になるため、粒内α変態能が低下し、ＨＡＺ靭性が劣化する。従って上記式（１）の左辺値は、１．０以上、好ましくは１．５以上、より好ましくは２．０以上とする。 When the value on the left side of the above formula (1) is less than 1.0, the Al oxide becomes excessive with respect to the Ti oxide, so that the intragranular α-transformation ability is lowered and the HAZ toughness is deteriorated. Therefore, the left side value of the above formula (1) is 1.0 or more, preferably 1.5 or more, more preferably 2.0 or more.
　しかし上記式（１）の左辺値が８を超えると、Ａｌ酸化物に対してＴｉ酸化物が過剰になるため、酸化物の融点が上昇し、溶接時のＨＡＺにおいて酸化物が液相化し難くなる。そのためＨＡＺ靭性を改善できない。従って上記式（１）の左辺値は８以下、好ましくは７．５以下、より好ましくは７．０以下とする。 However, when the value on the left side of the above formula (1) exceeds 8, the Ti oxide becomes excessive with respect to the Al oxide, so the melting point of the oxide rises, and the oxide is difficult to be in a liquid phase in the HAZ during welding. Become. Therefore, HAZ toughness cannot be improved. Therefore, the value on the left side of the above formula (1) is 8 or less, preferably 7.5 or less, more preferably 7.0 or less.
　（ｄ）好ましい態様
　本発明の鋼材は、鋼材に含まれる全酸化物の組成を測定して単独酸化物（合計が１００％）として質量換算したときに、平均組成で、ＺｒＯ2を５～５０％、ＲＥＭの酸化物（ＲＥＭをＭの記号で表すとＭ2Ｏ3）：５～５０％、ＣａＯ：５０％以下、を満足していることが好ましい。この組成を満足することにより酸化物が粒内フェライト変態の核として有効に作用する。各酸化物の下限値を下回ると、溶接時に粒内フェライトの生成核となる酸化物量が不足し、ＨＡＺ靭性の向上作用が発揮されにくくなる。一方、各酸化物の上限値を超えると、酸化物が粗大化し、粒内フェライトの生成核として有効に作用する微細な酸化物の個数が少なくなり、ＨＡＺ靭性向上作用が有効に発揮されにくくなる。 (D) Preferred Embodiment The steel material of the present invention has an average composition of ZrO 2 of 5 to 50 when the composition of all oxides contained in the steel material is measured and converted to mass as a single oxide (total is 100%). %, REM oxide (representing M 2 O 3 when REM is represented by the symbol M): 5 to 50%, CaO: 50% or less are preferably satisfied. By satisfying this composition, the oxide effectively acts as a nucleus of intragranular ferrite transformation. If the lower limit value of each oxide is not reached, the amount of oxides that form intragranular ferrite nuclei at the time of welding becomes insufficient, and the effect of improving HAZ toughness is hardly exhibited. On the other hand, when the upper limit value of each oxide is exceeded, the oxide becomes coarse, the number of fine oxides that effectively act as the nuclei of intragranular ferrite decreases, and the HAZ toughness improving effect is hardly exhibited effectively. .
　上記ＺｒＯ2は、より好ましくは８％以上、更に好ましくは１０％以上である。一方、より好ましい上限は４５％、更に好ましい上限は４０％である。 The ZrO 2 is more preferably 8% or more, and still more preferably 10% or more. On the other hand, a more preferable upper limit is 45%, and a more preferable upper limit is 40%.
　上記ＲＥＭの酸化物は、より好ましくは１０％以上、更に好ましくは１３％以上である。一方、より好ましい上限は４５％、更に好ましい上限は４０％である。なお、ＲＥＭの酸化物は、ＲＥＭを記号Ｍで表すと、鋼材中にＭ2Ｏ3、Ｍ3Ｏ5、ＭＯ2などの形態で存在するが、ＲＥＭの酸化物をすべてＭ2Ｏ3に換算したときの量を意味する。 The REM oxide is more preferably 10% or more, and still more preferably 13% or more. On the other hand, a more preferable upper limit is 45%, and a more preferable upper limit is 40%. In addition, when the REM is represented by the symbol M, the REM oxide is present in the steel material in the form of M 2 O 3 , M 3 O 5 , MO 2, etc., but all the REM oxide is converted to M 2 O 3 . It means the amount when converted.
　上記ＣａＯは、粒内フェライト変態の核として有効に作用するが、過剰に含まれると却って粒内フェライト変態能が劣化することがある。また、ＣａＯが過剰に含まれると鋳造時に用いるノズルの溶損を引き起こすことがある。従ってＣａＯの上限は５０％とすることが好ましく、より好ましくは４５％以下、更に好ましくは４０％以下、特に好ましくは３０％以下である。上記作用を有効に発揮させるには、ＣａＯは、３％以上含有していることが好ましく、より好ましくは５％以上、更に好ましくは１０％以上である。 The CaO effectively acts as a nucleus of intragranular ferrite transformation, but if contained excessively, the intragranular ferrite transformation ability may be deteriorated. Further, if CaO is excessively contained, the nozzle used for casting may be melted. Therefore, the upper limit of CaO is preferably 50%, more preferably 45% or less, still more preferably 40% or less, and particularly preferably 30% or less. In order to effectively exhibit the above action, CaO is preferably contained in an amount of 3% or more, more preferably 5% or more, and further preferably 10% or more.
　なお、全酸化物の組成の残りの成分は特に限定されず、本発明の鋼材中に含まれる酸化物形成元素の酸化物（例えば、ＳｉＯ2やＡｌ2Ｏ3、ＭｎＯなど）が挙げられる。 The remaining components of the total oxide composition are not particularly limited, and examples thereof include oxides of oxide-forming elements contained in the steel material of the present invention (for example, SiO 2 , Al 2 O 3 , MnO, etc.).
　鋼材に含まれる全酸化物の組成は、鋼材の表面を例えばＥＰＭＡで観察し、観察視野内に認められる酸化物を定量分析して測定する。測定条件の詳細は、後記する実施例の欄で説明する。 The composition of the total oxide contained in the steel material is measured by observing the surface of the steel material with, for example, EPMA, and quantitatively analyzing the oxide found in the observation field. Details of the measurement conditions will be described in the column of Examples described later.
　次に、本発明の鋼材（母材）における成分組成について説明する。本発明の鋼材は、基本成分として、Ｃ：０．０２～０．１５％、Ｓｉ：０．５％以下、Ｍｎ：２．５％以下、Ｐ：０．０３％以下、Ｓ：０．０２％以下、Ａｌ：０．０５０％以下、Ｔｉ：０．００５～０．１０％、ＲＥＭ：０．０００３～０．０１５％、Ｃａ：０．０００３～０．０１０％、Ｚｒ：０．００１０～０．０５０％、Ｎ：０．０１０％以下を含有している。こうした範囲を定めた理由は以下の通りである。 Next, the component composition in the steel material (base material) of the present invention will be described. The steel material of the present invention has, as basic components, C: 0.02 to 0.15%, Si: 0.5% or less, Mn: 2.5% or less, P: 0.03% or less, S: 0.02 %: Al: 0.050% or less, Ti: 0.005 to 0.10%, REM: 0.0003 to 0.015%, Ca: 0.0003 to 0.010%, Zr: 0.0010 to It contains 0.050% and N: 0.010% or less. The reasons for setting these ranges are as follows.
　Ｃは、鋼材（母材）の強度を確保するために欠くことのできない元素であり、０．０２％以上含有させる必要がある。Ｃ量は、好ましくは０．０４％以上であり、より好ましくは０．０５％以上である。しかしＣ量が０．１５％を超えると、溶接時にＨＡＺに島状マルテンサイト（ＭＡ）が多く生成してＨＡＺの靭性劣化を招くばかりでなく、溶接性にも悪影響を及ぼす。従ってＣ量は０．１５％以下、好ましくは０．１０％以下、より好ましくは０．０８％以下である。 C is an element indispensable for securing the strength of the steel material (base material), and needs to be contained by 0.02% or more. The amount of C is preferably 0.04% or more, and more preferably 0.05% or more. However, if the amount of C exceeds 0.15%, a large amount of island martensite (MA) is generated in the HAZ during welding, leading to deterioration of the toughness of the HAZ, and also adversely affecting the weldability. Accordingly, the C content is 0.15% or less, preferably 0.10% or less, and more preferably 0.08% or less.
　Ｓｉは、脱酸作用を有すると共に、固溶強化により鋼材（母材）の強度向上に寄与する元素である。こうした作用を有効に発揮させるには、Ｓｉは、０．０１％以上含有させることが好ましい。Ｓｉ量は、より好ましくは０．０２％以上、更に好ましくは０．０５％以上、特に好ましくは０．１０％以上含有させるのがよい。しかしＳｉ量が０．５％を超えると、鋼材の溶接性や靭性が劣化する。従ってＳｉ量は、０．５％以下、好ましくは０．４５％以下、より好ましくは０．４０％以下である。 Si is an element that has a deoxidizing action and contributes to improving the strength of the steel (base metal) by solid solution strengthening. In order to exhibit such an action effectively, Si is preferably contained in an amount of 0.01% or more. The Si content is more preferably 0.02% or more, still more preferably 0.05% or more, and particularly preferably 0.10% or more. However, if the amount of Si exceeds 0.5%, the weldability and toughness of the steel material deteriorate. Therefore, the Si content is 0.5% or less, preferably 0.45% or less, more preferably 0.40% or less.
　なお、特にＨＡＺ靭性を高めるには、Ｓｉは０．３０％以下とすることが推奨され、好ましくは０．０５％以下、より好ましくは０．０１％以下である。但し、Ｓｉ量を抑えるほどＨＡＺ靭性は向上するが、鋼材の強度が低下することがある。 In particular, in order to increase the HAZ toughness, Si is recommended to be 0.30% or less, preferably 0.05% or less, more preferably 0.01% or less. However, as the amount of Si is suppressed, the HAZ toughness is improved, but the strength of the steel material may be reduced.
　Ｍｎは、鋼材（母材）の強度向上に寄与する元素である。こうした効果を有効に発揮させるには、０．４％以上含有させることが好ましい。Ｍｎ量は、より好ましくは０．５０％以上、更に好ましくは０．７％以上、特に好ましくは０．８％以上である。しかしＭｎ量が２．５％を超えると、鋼材（母材）の溶接性を劣化させる。従ってＭｎ量は、２．５％以下に抑える必要がある。Ｍｎ量は、好ましくは２．３％以下、より好ましくは２．０％以下である。 Mn is an element that contributes to improving the strength of the steel (base material). In order to exhibit such an effect effectively, it is preferable to make it contain 0.4% or more. The amount of Mn is more preferably 0.50% or more, still more preferably 0.7% or more, and particularly preferably 0.8% or more. However, if the amount of Mn exceeds 2.5%, the weldability of the steel material (base material) is deteriorated. Therefore, the amount of Mn needs to be suppressed to 2.5% or less. The amount of Mn is preferably 2.3% or less, more preferably 2.0% or less.
　Ｐは、偏析し易い元素であり、特に鋼材中の結晶粒界に偏析してＨＡＺ靭性を劣化させる。従ってＰ量は０．０３％以下に抑制する必要がある。Ｐ量は、好ましくは０．０２０％以下、より好ましくは０．０１５％以下である。なお、Ｐは、通常、不可避的に０．００１％程度含有している。 P is an element that is easily segregated, and particularly segregates at the grain boundaries in the steel material to deteriorate the HAZ toughness. Therefore, the P amount needs to be suppressed to 0.03% or less. The amount of P is preferably 0.020% or less, more preferably 0.015% or less. In general, P is unavoidably contained in an amount of about 0.001%.
　Ｓは、Ｍｎと結合して硫化物（ＭｎＳ）を生成し、母材の靭性や板厚方向の延性を劣化させる有害な元素である。また、ＳがＬａやＣｅなどのＲＥＭと結合してＲＥＭの硫化物（例えば、ＬａＳやＣｅＳ）を生成すると、ＲＥＭの酸化物の生成が阻害されるため、ＨＡＺ靭性が劣化する。従ってＳ量は０．０２％以下に抑制する必要がある。Ｓ量は、好ましくは０．０１５％以下、より好ましくは０．０１０％以下、更に好ましくは０．００６％以下である。なお、Ｓは、通常、不可避的に０．０００５％程度含有している。 S is a harmful element that combines with Mn to produce sulfide (MnS) and degrades the toughness of the base metal and the ductility in the thickness direction. Further, when S is combined with REM such as La or Ce to generate REM sulfide (for example, LaS or CeS), generation of oxide of REM is inhibited, and thus HAZ toughness is deteriorated. Therefore, the S amount needs to be suppressed to 0.02% or less. The amount of S is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.006% or less. Note that S is usually unavoidably contained in an amount of about 0.0005%.
　Ａｌは、脱酸剤として作用する元素である。しかし過剰に添加すると酸化物を還元して粗大なＡｌ酸化物を形成し、ＨＡＺ靭性が劣化する。従ってＡｌ量は０．０５０％以下に抑える必要がある。Ａｌ量は、好ましくは０．０４％以下、より好ましくは０．０３％以下、更に好ましくは０．０２５％以下、特に好ましくは０．０１０％以下である。なお、Ａｌは、通常、不可避的に０．０００５％程度含有している。 Al is an element that acts as a deoxidizer. However, if added excessively, the oxide is reduced to form a coarse Al oxide, and the HAZ toughness deteriorates. Therefore, the Al amount must be suppressed to 0.050% or less. The amount of Al is preferably 0.04% or less, more preferably 0.03% or less, still more preferably 0.025% or less, and particularly preferably 0.010% or less. Al is usually unavoidably contained in an amount of about 0.0005%.
　Ｔｉは、鋼材中にＴｉＮなどの窒化物や、Ｔｉを含む酸化物を生成し、ＨＡＺ靭性の向上に寄与する元素である。こうした効果を発揮させるには、Ｔｉは０．００５％以上含有させる必要がある。Ｔｉ量は、好ましくは０．００７％以上、より好ましくは０．０１０％以上である。しかし過剰に添加するとＴｉの固溶強化によって母材自体が硬化し、ＨＡＺ靭性の低下に繋がるため、Ｔｉは０．１０％以下に抑えるべきである。Ｔｉ量は、好ましくは０．０７％以下、より好ましくは０．０６％以下である。 Ti is an element that generates nitrides such as TiN and oxides containing Ti in the steel material and contributes to the improvement of HAZ toughness. In order to exert such effects, it is necessary to contain Ti by 0.005% or more. The amount of Ti is preferably 0.007% or more, more preferably 0.010% or more. However, if excessively added, the base metal itself is hardened by solid solution strengthening of Ti, leading to a reduction in HAZ toughness. Therefore, Ti should be suppressed to 0.10% or less. The amount of Ti is preferably 0.07% or less, more preferably 0.06% or less.
　ＲＥＭ（希土類元素）とＣａは、夫々の酸化物を生成させるのに必要な元素である。これらの酸化物を含有することで、酸化物が微細分散し易くなり、この微細分散した酸化物が粒内α変態の核となるため、ＨＡＺ靭性の向上に寄与する。 REM (rare earth element) and Ca are elements necessary for generating respective oxides. By containing these oxides, the oxides are easily finely dispersed, and the finely dispersed oxides become the nucleus of intragranular α transformation, which contributes to the improvement of HAZ toughness.
　ＲＥＭは、０．０００３％以上含有させるべきであり、好ましくは０．００１％以上、より好ましくは０．００２０％以上である。しかしＲＥＭを過剰に添加すると、粗大な酸化物が過剰に生成するため、ＨＡＺ靭性が劣化する。また、ＲＥＭを過剰に添加すると、固溶ＲＥＭが生成し、これが偏析することで母材の靭性が劣化する。従ってＲＥＭ量は０．０１５％以下に抑えるべきである。ＲＥＭ量は、好ましくは０．０１０％以下、より好ましくは０．００７％以下である。 REM should be contained at 0.0003% or more, preferably 0.001% or more, more preferably 0.0020% or more. However, when REM is added excessively, a coarse oxide is excessively generated, so that the HAZ toughness is deteriorated. Moreover, when REM is added excessively, solid solution REM will produce | generate and this will segregate and the toughness of a base material will deteriorate. Therefore, the amount of REM should be suppressed to 0.015% or less. The amount of REM is preferably 0.010% or less, more preferably 0.007% or less.
　なお、本発明において、ＲＥＭとは、ランタノイド元素（ＬａからＬｕまでの１５元素）およびＳｃ（スカンジウム）とＹ（イットリウム）を含む意味である。これらの元素のなかでも、Ｌａ、ＣｅおよびＹよりなる群から選ばれる少なくとも１種の元素を含有することが好ましく、より好ましくはＬａとＣｅの少なくとも一方を含有するのがよい。 In the present invention, REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, more preferably at least one of La and Ce.
　Ｃａは、０．０００３％以上含有させるべきであり、好ましくは０．０００５％以上、より好ましくは０．０００８％以上、更に好ましくは０．００１％以上である。しかしＣａを過剰に添加すると、ＣａＯが過剰に生成して高ＣａＯ濃度の介在物が生成するため、介在物の粒内変態核として作用する効果が弱まり、ＨＡＺ靭性が却って劣化する。従ってＣａ量は、０．０１０％以下に抑える必要がある。Ｃａ量は、好ましくは０．００９％以下であり、より好ましくは０．００８％以下である。 Ca should be contained 0.0003% or more, preferably 0.0005% or more, more preferably 0.0008% or more, and further preferably 0.001% or more. However, when Ca is added excessively, CaO is excessively generated and inclusions with a high CaO concentration are generated. Therefore, the effect of acting as an intragranular transformation nucleus of inclusions is weakened, and the HAZ toughness is deteriorated. Therefore, the amount of Ca needs to be suppressed to 0.010% or less. The amount of Ca is preferably 0.009% or less, and more preferably 0.008% or less.
　Ｚｒは、Ｚｒを含む複合酸化物を生成してＨＡＺ靭性の向上に寄与する元素である。こうした作用を有効に発揮させるには、０．００１０％以上含有させる必要がある。Ｚｒ量は、好ましくは０．００２％以上、より好ましくは０．００２３％以上である。しかしＺｒを過剰に添加すると、ＺｒＯ2が多く生成するため、介在物の粒内変態核として作用する効果が弱まる。また、Ｚｒを過剰に添加すると、析出強化をもたらす微細な窒化物（ＺｒＮ）や炭化物（ＺｒＣ）が形成し、母材自体の靭性低下を招く。従ってＺｒ量は０．０５０％以下に抑える。Ｚｒ量は、好ましくは０．０４％以下、より好ましくは０．０３％以下、更に好ましくは０．０１％以下である。 Zr is an element that generates a complex oxide containing Zr and contributes to the improvement of HAZ toughness. In order to exhibit such an action effectively, it is necessary to contain 0.0010% or more. The amount of Zr is preferably 0.002% or more, more preferably 0.0023% or more. However, if Zr is added excessively, a large amount of ZrO 2 is generated, and the effect of acting as an intragranular transformation nucleus of inclusions is weakened. If Zr is added excessively, fine nitrides (ZrN) and carbides (ZrC) that cause precipitation strengthening are formed, and the toughness of the base metal itself is reduced. Therefore, the amount of Zr is suppressed to 0.050% or less. The amount of Zr is preferably 0.04% or less, more preferably 0.03% or less, and still more preferably 0.01% or less.
　Ｎは、窒化物（例えば、ＺｒＮやＴｉＮなど）を析出する元素であり、該窒化物は、ピン止め効果により、溶接時にＨＡＺに生成するオーステナイト粒の粗大化を防止してフェライト変態を促進し、ＨＡＺ靭性の向上に寄与する。こうした効果を有効に発揮させるには、Ｎを０．００３％以上含有させることが好ましい。Ｎ量は、より好ましくは０．００４％以上、更に好ましくは０．００５％以上である。Ｎは多いほど窒化物を形成してオーステナイト粒の微細化を促進するため、ＨＡＺの靭性向上に有効に作用する。しかしＮ量が０．０１０％を超えると、固溶Ｎ量が増大して母材自体の靭性が劣化し、ＨＡＺ靭性も低下する。従ってＮ量は０．０１０％以下に抑える必要がある。Ｎ量は、好ましくは０．００９％以下、より好ましくは０．００８％以下である。 N is an element that precipitates nitrides (for example, ZrN and TiN), and the nitrides prevent the austenite grains formed in the HAZ during welding and promote ferrite transformation by the pinning effect. , Contributing to the improvement of HAZ toughness. In order to exhibit such an effect effectively, it is preferable to contain N 0.003% or more. The amount of N is more preferably 0.004% or more, and still more preferably 0.005% or more. As N increases, nitrides are formed to promote the refinement of austenite grains, so that it effectively works to improve the toughness of HAZ. However, if the N amount exceeds 0.010%, the solid solution N amount increases, the toughness of the base metal itself deteriorates, and the HAZ toughness also decreases. Therefore, the N amount needs to be suppressed to 0.010% or less. The N amount is preferably 0.009% or less, more preferably 0.008% or less.
　本発明の鋼材は、上記元素を必須成分として含有するものであり、Ｏ（酸素）量は０．０００５～０．０１０％である。ここでＯ（酸素）量０．０００５～０．０１０％は、トータル酸素量を示し、酸化物を形成しているＯ（酸素）と鋼材中に固溶しているフリーなＯ（酸素）の合計量を意味している。 The steel material of the present invention contains the above elements as essential components, and the O (oxygen) amount is 0.0005 to 0.010%. Here, the amount of O (oxygen) 0.0005 to 0.010% indicates the total amount of oxygen, and is composed of O (oxygen) forming oxides and free O (oxygen) dissolved in the steel material. It means the total amount.
　上記鋼材の残部成分は、鉄および不可避不純物（例えば、ＭｇやＡｓ、Ｓｅなど）である。 The remaining components of the steel material are iron and inevitable impurities (for example, Mg, As, Se, etc.).
　本発明の鋼材は、更に他の元素として、
［１］Ｃｕ：２％以下、Ｎｉ：３．５％以下、Ｃｒ：３％以下、およびＭｏ：１％以下よりなる群から選ばれる少なくとも１種の元素、
［２］Ｎｂ：０．２５％以下とＶ：０．１％以下の少なくとも一方、
［３］Ｂ：０．００５％以下、
等の元素を含有することも有効である。こうした範囲を定めた理由は以下の通りである。 The steel material of the present invention is still another element,
[1] At least one element selected from the group consisting of Cu: 2% or less, Ni: 3.5% or less, Cr: 3% or less, and Mo: 1% or less,
[2] At least one of Nb: 0.25% or less and V: 0.1% or less,
[3] B: 0.005% or less,
It is also effective to contain such elements. The reasons for setting these ranges are as follows.
　［１］Ｃｕ、Ｎｉ、Ｃｒ、およびＭｏよりなる群から選ばれる少なくとも１種の元素
　Ｃｕ、Ｎｉ、Ｃｒ、およびＭｏは、いずれも鋼材の強度を高めるのに寄与する元素であり、夫々単独で、或いは複合して添加できる。 [1] At least one element selected from the group consisting of Cu, Ni, Cr, and Mo Each of Cu, Ni, Cr, and Mo is an element that contributes to increasing the strength of the steel material, Alternatively, they can be added in combination.
　Ｃｕ量が２％を超えると、母材の強度を著しく高め過ぎて母材の靭性を却って劣化させるため、ＨＡＺ靭性も低下することがある。従ってＣｕ量は２％以下とすることが好ましい。Ｃｕ量は、より好ましくは１．８％以下、更に好ましくは１．５％以下である。なお、Ｃｕ添加による作用を有効に発揮させるには、０．０５％以上含有させることが好ましい。Ｃｕ量は、より好ましくは０．１％以上、更に好ましくは０．２％以上である。 If the amount of Cu exceeds 2%, the strength of the base material is remarkably increased and the toughness of the base material is deteriorated, so that the HAZ toughness may be lowered. Accordingly, the Cu content is preferably 2% or less. The amount of Cu is more preferably 1.8% or less, still more preferably 1.5% or less. In addition, in order to exhibit the effect | action by Cu addition effectively, it is preferable to make it contain 0.05% or more. The amount of Cu is more preferably 0.1% or more, and still more preferably 0.2% or more.
　Ｎｉ量が３．５％を超えると、母材の強度を著しく高め過ぎて母材の靭性を却って劣化させるため、ＨＡＺ靭性も低下することがある。従ってＮｉ量は３．５％以下とすることが好ましい。Ｎｉ量は、より好ましくは３．０％以下、更に好ましくは２．５％以下である。なお、Ｎｉ添加による作用を有効に発揮させるには、０．０５％以上含有させることが好ましい。Ｎｉ量は、より好ましくは０．１％以上、更に好ましくは０．２％以上である。 When the amount of Ni exceeds 3.5%, the strength of the base material is remarkably increased and the toughness of the base material is deteriorated, so that the HAZ toughness may be lowered. Accordingly, the Ni content is preferably 3.5% or less. The amount of Ni is more preferably 3.0% or less, still more preferably 2.5% or less. In order to effectively exhibit the effect of adding Ni, it is preferable to contain 0.05% or more. The amount of Ni is more preferably 0.1% or more, and still more preferably 0.2% or more.
　Ｃｒ量が３％を超えると、母材の強度を著しく高め過ぎて母材の靭性を却って劣化させるため、ＨＡＺ靭性も低下することがある。従ってＣｒ量は３％以下とすることが好ましい。Ｃｒ量は、より好ましくは２％以下、更に好ましくは１％以下である。なお、Ｃｒ添加による作用を有効に発揮させるには、０．０５％以上含有させることが好ましい。Ｃｒ量は、より好ましくは０．１％以上、更に好ましくは０．１５％以上である。 When the Cr content exceeds 3%, the strength of the base material is remarkably increased and the toughness of the base material is deteriorated, so that the HAZ toughness may be lowered. Therefore, the Cr content is preferably 3% or less. The amount of Cr is more preferably 2% or less, still more preferably 1% or less. In order to effectively exhibit the effect of addition of Cr, it is preferable to contain 0.05% or more. The amount of Cr is more preferably 0.1% or more, and still more preferably 0.15% or more.
　Ｍｏ量が１％を超えると、母材の強度を著しく高め過ぎて母材の靭性を却って劣化させるため、ＨＡＺ靭性も低下することがある。従ってＭｏ量は１％以下とすることが好ましい。Ｍｏ量は、より好ましくは０．９％以下、更に好ましくは０．８０％以下である。なお、Ｍｏ添加による作用を有効に発揮させるには、０．０５％以上含有させることが好ましい。Ｍｏ量は、より好ましくは０．１％以上、更に好ましくは０．１５％以上である。 When the amount of Mo exceeds 1%, the strength of the base material is remarkably increased and the toughness of the base material is deteriorated, so that the HAZ toughness may be lowered. Therefore, the Mo amount is preferably 1% or less. The amount of Mo is more preferably 0.9% or less, and still more preferably 0.80% or less. In addition, in order to exhibit the effect | action by Mo addition effectively, it is preferable to make it contain 0.05% or more. The amount of Mo is more preferably 0.1% or more, and still more preferably 0.15% or more.
　［２］ＮｂとＶの少なくとも一方
　ＮｂとＶは、いずれも炭窒化物として析出し、該炭窒化物のピン止め効果により、溶接時にオーステナイト粒が粗大化するのを防止し、ＨＡＺ靭性を向上させる作用を有する元素である。ＮｂとＶは、夫々単独で、或いは複合して添加できる。 [2] At least one of Nb and V Nb and V are both precipitated as carbonitrides, and the pinning effect of the carbonitrides prevents austenite grains from coarsening during welding and improves HAZ toughness It is an element which has the effect | action to make it. Nb and V can be added alone or in combination.
　しかしＮｂ量が０．２５％を超えると、析出する炭窒化物が粗大化し、ＨＡＺ靭性を却って劣化させることがある。従ってＮｂ量は０．２５％以下とすることが好ましい。Ｎｂ量は、より好ましくは０．２％以下、更に好ましくは０．１５％以下である。なお、Ｎｂ添加による作用を有効に発揮させるには、０．００２％以上含有させることが好ましい。Ｎｂ量は、より好ましくは０．０１％以上、更に好ましくは０．０２％以上である。 However, if the Nb content exceeds 0.25%, the precipitated carbonitrides become coarse and may deteriorate the HAZ toughness. Accordingly, the Nb content is preferably 0.25% or less. The amount of Nb is more preferably 0.2% or less, still more preferably 0.15% or less. In order to effectively exhibit the effect of Nb addition, it is preferable to contain 0.002% or more. The amount of Nb is more preferably 0.01% or more, and further preferably 0.02% or more.
　Ｖ量が０．１％を超えると、上記Ｎｂと同様に、析出する炭窒化物が粗大化し、ＨＡＺ靭性を却って劣化させることがある。従ってＶ量は０．１％以下とすることが好ましい。Ｖ量は、より好ましくは０．０９％以下、更に好ましくは０．０８％以下である。なお、Ｖ添加による作用を有効に発揮させるには、０．００２％以上含有させることが好ましい。Ｖ量は、より好ましくは０．００５％以上、更に好ましくは０．０１％以上である。 When the amount of V exceeds 0.1%, the precipitated carbonitride becomes coarse like the above Nb, and may deteriorate the HAZ toughness. Therefore, the V amount is preferably 0.1% or less. The amount of V is more preferably 0.09% or less, still more preferably 0.08% or less. In order to effectively exhibit the effect of V addition, it is preferable to contain 0.002% or more. The amount of V is more preferably 0.005% or more, and still more preferably 0.01% or more.
　［３］Ｂ（ホウ素）
　Ｂは、粒界フェライトの生成を抑制してＨＡＺ靭性を向上させる元素である。しかしＢ量が０．００５％を超えると、オーステナイト粒界にＢＮとして析出し、却って靭性の低下を招くことがある。従ってＢ量は０．００５％以下が好ましい。Ｂ量は、より好ましくは０．００４０％以下である。なお、Ｂ添加による作用を有効に発揮させるには、０．００１０％以上含有させることが好ましい。Ｂ量は、より好ましくは０．００１５％以上である。 [3] B (boron)
B is an element that suppresses the formation of grain boundary ferrite and improves the HAZ toughness. However, if the amount of B exceeds 0.005%, it may precipitate as BN at the austenite grain boundary, which may lead to a decrease in toughness. Therefore, the amount of B is preferably 0.005% or less. The amount of B is more preferably 0.0040% or less. In addition, in order to exhibit the effect | action by B addition effectively, it is preferable to make it contain 0.0010% or more. The amount of B is more preferably 0.0015% or more.
　本発明の鋼材は、１４５０℃で６０秒間保持した後、８００℃から５００℃への冷却時間を４５０秒として冷却する熱履歴を与えた場合であっても、－４０℃における吸収エネルギー（ｖＥ-40）で１００Ｊ以上（特に、１３０Ｊ以上）を確保できる。そのため、本発明に係る鋼材は、例えば橋梁や高層建造物、船舶などの構造物の材料として使用でき、小～中入熱溶接はもとより、入熱量が６０ｋＪ／ｍｍ以上の大入熱溶接においても溶接熱影響部の靭性劣化を防ぐことができる。本発明の鋼材は、板厚が約３．０ｍｍ以上の厚鋼板などを対象としている。 Even when the steel material of the present invention is held at 1450 ° C. for 60 seconds and then has a heat history of cooling from 800 ° C. to 500 ° C. with a cooling time of 450 seconds, the absorbed energy at −40 ° C. (vE − 40 ) can secure 100 J or more (particularly 130 J or more). Therefore, the steel material according to the present invention can be used as a material for structures such as bridges, high-rise buildings, ships, etc., and can be used not only for small to medium heat input welding but also for large heat input welding with a heat input of 60 kJ / mm or more. It is possible to prevent toughness deterioration of the weld heat affected zone. The steel material of the present invention is intended for a thick steel plate having a thickness of about 3.0 mm or more.
　次に、本発明の鋼材を製造するにあたり、好適に採用できる製法について説明する。本発明の鋼材は、溶鋼を脱酸し、その後、Ｔｉを添加してからＡｌを添加すればよい。脱酸した溶鋼にＴｉを添加してからＡｌを添加（Ｔｉ→Ａｌ）することによって、円相当直径が０．１～２μｍ程度の微細な介在物に含まれるＴｉ酸化物とＡｌ酸化物の組成比を適切に制御でき、上記式（１）を満足する鋼材を製造できる。即ち、Ｔｉ酸化物は、Ａｌ酸化物やＺｒ・ＲＥＭ・Ｃａ系酸化物に比べて溶鋼との界面エネルギーが小さいため、溶鋼にＡｌ、Ｚｒ、ＲＥＭ、およびＣａを添加する前にＴｉを添加することによって、微細なＴｉ酸化物を形成することができ、結果的に、ＨＡＺ靭性に寄与する円相当直径が０．１～２μｍの微細な介在物を所定量生成させることができる。また、Ｔｉを添加した後に、Ａｌを添加することによって、ＴｉとＡｌを含む複合酸化物を生成させることができ、Ｔｉ酸化物としての活量を１未満に低下させることができる。そして、この複合酸化物を形成した後に、ＴｉやＡｌより強脱酸元素であるＺｒ、ＲＥＭ、およびＣａを添加することによって、Ｚｒ・ＲＥＭ・Ｃａ系酸化物が形成され、このときＴｉ酸化物やＡｌ酸化物の還元は抑制されるため、Ｚｒ・ＲＥＭ・Ｃａ系酸化物に、所定量のＴｉ酸化物とＡｌ酸化物を含有させることができる。このようにＴｉを添加してからＡｌを添加することによって、Ｔｉ、Ａｌ、Ｚｒ、ＲＥＭ、およびＣａの添加量が同じだとしても粒内α変態の核となる酸化物を多量に生成させることができる。 Next, a manufacturing method that can be suitably employed in manufacturing the steel material of the present invention will be described. The steel material of the present invention may be obtained by deoxidizing molten steel and then adding Ti after adding Ti. The composition of Ti oxide and Al oxide contained in fine inclusions with an equivalent circle diameter of about 0.1 to 2 μm by adding Ti to the deoxidized molten steel and then adding Al (Ti → Al) The ratio can be controlled appropriately, and a steel material that satisfies the above formula (1) can be manufactured. That is, since Ti oxide has a smaller interfacial energy with molten steel than Al oxide and Zr / REM / Ca-based oxide, Ti is added before adding Al, Zr, REM, and Ca to molten steel. As a result, a fine Ti oxide can be formed, and as a result, a predetermined amount of fine inclusions having an equivalent circle diameter of 0.1 to 2 μm contributing to the HAZ toughness can be generated. Moreover, by adding Al after adding Ti, a composite oxide containing Ti and Al can be generated, and the activity as Ti oxide can be reduced to less than 1. After forming this composite oxide, Zr, REM, and Ca-based oxides are formed by adding Zr, REM, and Ca, which are stronger deoxidizing elements than Ti and Al. Further, since reduction of Al oxide is suppressed, a predetermined amount of Ti oxide and Al oxide can be contained in the Zr / REM / Ca oxide. By adding Al after adding Ti in this way, a large amount of oxides that become the nucleus of intragranular α-transformation can be produced even if the addition amounts of Ti, Al, Zr, REM, and Ca are the same. Can do.
　一方、Ａｌを添加した後にＴｉを添加（Ａｌ→Ｔｉ）しても、介在物の組成を、上記式（１）を満足するように調整することはできない。ＴｉはＡｌよりも脱酸力が弱いため、溶鋼にＡｌを添加した後にＴｉを添加しても先に形成されたＡｌ酸化物を還元できないため、Ｔｉ酸化物の生成量が減少し、Ｚｒ・ＲＥＭ・Ｃａ系酸化物に所定量のＴｉ酸化物を含有させることができない。また、このとき形成されるＴｉ酸化物は、単独酸化物と存在しており、Ｔｉ酸化物としての活量は１に近くなっている。そのため、この状態でＴｉよりも脱酸力の強いＺｒ、ＲＥＭ、およびＣａを添加すると、Ｔｉ酸化物は還元されてＴｉ酸化物の生成量が減少し、Ｚｒ・ＲＥＭ・Ｃａ系酸化物に所定量のＴｉ酸化物を含有させることができない。よって本発明の鋼材を製造する際には、溶鋼の脱酸にはＡｌを用いないことが推奨される。Ａｌ脱酸を行うと、溶鋼中にＡｌ酸化物が残留することがあるため、Ｔｉ酸化物を所定量含むＺｒ・ＲＥＭ・Ｃａ系酸化物を形成することが困難となる。 On the other hand, even if Ti is added after adding Al (Al → Ti), the composition of inclusions cannot be adjusted to satisfy the above formula (1). Ti has a weaker deoxidizing power than Al, so even if Ti is added to molten steel and Ti is not added, the previously formed Al oxide cannot be reduced. The REM / Ca-based oxide cannot contain a predetermined amount of Ti oxide. Further, the Ti oxide formed at this time exists as a single oxide, and the activity as the Ti oxide is close to 1. Therefore, when Zr, REM, and Ca, which have a stronger deoxidizing power than Ti, are added in this state, the Ti oxide is reduced and the amount of Ti oxide generated is reduced, resulting in a Zr / REM / Ca oxide. A fixed amount of Ti oxide cannot be contained. Therefore, when manufacturing the steel material of the present invention, it is recommended not to use Al for deoxidation of molten steel. When Al deoxidation is performed, Al oxide may remain in the molten steel, so that it is difficult to form a Zr / REM / Ca-based oxide containing a predetermined amount of Ti oxide.
　上記溶鋼は公知の方法で脱酸すればよく、例えば、Ａｌ、Ｔｉ、ＲＥＭ、Ｃａ、およびＺｒ以外の元素について成分調整した後、Ｃ、Ｓｉ、およびＭｎから選ばれる少なくとも１種の元素を用いて脱酸し、その後、Ｔｉを添加してからＡｌを添加してもよい。 The molten steel may be deoxidized by a known method. For example, after adjusting components for elements other than Al, Ti, REM, Ca, and Zr, at least one element selected from C, Si, and Mn is used. Then, deoxidization may be performed, and then Ti may be added and then Al may be added.
　上記Ｔｉを添加した後にＡｌ、ＲＥＭ、Ｃａ、およびＺｒを添加するにあたっては、例えば、
（１）Ｔｉを添加した後に、Ａｌを添加してからＲＥＭ、Ｃａ、およびＺｒを任意の順で添加してもよいし、
（２）Ｔｉを添加した後、Ａｌを添加してからＲＥＭ、Ｃａ、およびＺｒを同時に添加してもよいし、
（３）Ｔｉを添加した後、Ａｌ、ＲＥＭ、Ｃａ、およびＺｒを同時に添加してもよい。 In adding Al, REM, Ca, and Zr after adding Ti, for example,
(1) After adding Ti, REM, Ca, and Zr may be added in any order after adding Al.
(2) After adding Ti, after adding Al, REM, Ca, and Zr may be added simultaneously,
(3) After adding Ti, Al, REM, Ca, and Zr may be added simultaneously.
　上記溶鋼へ添加するＲＥＭやＣａ、Ｚｒ、Ｔｉの形態は特に限定されず、例えば、ＲＥＭとして、純Ｌａや純Ｃｅ、純Ｙなど、或いは純Ｃａ、純Ｚｒ、純Ｔｉ、更にはＦｅ－Ｓｉ－Ｌａ合金、Ｆｅ－Ｓｉ－Ｃｅ合金、Ｆｅ－Ｓｉ－Ｃａ合金、Ｆｅ－Ｓｉ－Ｌａ－Ｃｅ合金、Ｆｅ－Ｃａ合金、Ｆｅ－Ｚｒ合金、Ｆｅ－Ｔｉ合金、Ｎｉ－Ｃａ合金などを添加すればよい。また、溶鋼へミッシュメタルを添加してもよい。ミッシュメタルとは、希土類元素の混合物であり、具体的には、Ｃｅを４０～５０％程度、Ｌａを２０～４０％程度含有している。但し、ミッシュメタルには不純物としてＣａを含むことが多いので、ミッシュメタルがＣａを含む場合は、このＣａ量も含めて全Ｃａ量が本発明で規定する範囲を満足する必要がある。 The form of REM, Ca, Zr, or Ti added to the molten steel is not particularly limited. For example, REM may be pure La, pure Ce, pure Y, or pure Ca, pure Zr, pure Ti, and further Fe—Si. Add -La alloy, Fe-Si-Ce alloy, Fe-Si-Ca alloy, Fe-Si-La-Ce alloy, Fe-Ca alloy, Fe-Zr alloy, Fe-Ti alloy, Ni-Ca alloy, etc. That's fine. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of rare earth elements, and specifically contains about 40 to 50% of Ce and about 20 to 40% of La. However, since the misch metal often contains Ca as an impurity, when the misch metal contains Ca, the total Ca amount including this Ca amount needs to satisfy the range defined in the present invention.
　こうして成分調整して得られた溶鋼は、常法に従って連続鋳造してスラブとした後、常法に従って熱間圧延等を行うことによって本発明の鋼材を製造できる。 The molten steel obtained by adjusting the components in this manner can be continuously cast according to a conventional method to form a slab, and then hot rolled or the like according to a conventional method to produce the steel material of the present invention.
　本願は、２０１２年６月１９日に出願された日本国特許出願第２０１２－１３８０４７号に基づく優先権の利益を主張するものである。２０１２年６月１９日に出願された日本国特許出願第２０１２－１３８０４７号の明細書の全内容が、本願に参考のため援用される。 This application claims the benefit of priority based on Japanese Patent Application No. 2012-138047 filed on June 19, 2012. The entire contents of Japanese Patent Application No. 2012-138047 filed on June 19, 2012 are incorporated herein by reference.
　以下、実施例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも勿論可能であり、それらはいずれも本発明の技術的範囲に包含される。 EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.
　真空溶解炉（容量１５０ｋｇ）を用い、下記表１に示す化学成分を含有する供試鋼（残部は鉄および不可避不純物）を溶製した。供試鋼を溶製するに当っては、Ａｌ、Ｔｉ、ＲＥＭ、Ｃａ、およびＺｒ以外の元素について成分調整すると共に、Ｃ、Ｓｉ、およびＭｎから選ばれる少なくとも１種の元素を用いて脱酸して溶鋼の溶存酸素量を調整した。その後、溶存酸素量を調整した溶鋼に、ＡｌとＴｉを添加した後、ＲＥＭ、Ｃａ、およびＺｒを添加した。下記表１に、ＡｌとＴｉの添加順序を示す。なお、下記表１に示した供試鋼は、ＴｉとＡｌの添加順序を変えた以外は、同じ方法で製造した。また、ＴｉはＦｅ－Ｔｉ合金の形態で、ＺｒはＦｅ－Ｚｒ合金の形態で、ＲＥＭはＬａを約２５％とＣｅを約５０％含有するミッシュメタルの形態で、ＣａはＮｉ－Ｃａ合金の形態で、夫々添加した。また、下記表１に示す供試鋼のうち、本発明で規定する要件を満足する供試鋼のトータルＯ量（酸素量）は０．０００５～０．０１０％の範囲であることを確認している。 Using a vacuum melting furnace (capacity 150 kg), a test steel containing the chemical components shown in Table 1 below (the balance being iron and inevitable impurities) was melted. In melting the test steel, components are adjusted for elements other than Al, Ti, REM, Ca, and Zr, and deoxidation is performed using at least one element selected from C, Si, and Mn. Then, the dissolved oxygen amount of the molten steel was adjusted. Then, REM, Ca, and Zr were added after adding Al and Ti to the molten steel which adjusted the amount of dissolved oxygen. Table 1 below shows the order of addition of Al and Ti. The test steels shown in Table 1 below were manufactured by the same method except that the order of addition of Ti and Al was changed. Further, Ti is in the form of an Fe—Ti alloy, Zr is in the form of an Fe—Zr alloy, REM is in the form of a misch metal containing about 25% La and about 50% Ce, and Ca is a Ni—Ca alloy. Each was added in form. In addition, among the test steels shown in Table 1 below, it was confirmed that the total O amount (oxygen amount) of the test steels satisfying the requirements specified in the present invention was in the range of 0.0005 to 0.010%. ing.
　上記元素を添加した後、１５０ｋｇのインゴットに鋳造して冷却した。得られたインゴットを加熱し、熱間圧延し、厚さが３０～８０ｍｍの厚鋼板を製造した。熱間圧延は、加熱温度を１１００℃、圧延終了温度を８８０℃として行った。 After adding the above elements, it was cast into a 150 kg ingot and cooled. The obtained ingot was heated and hot-rolled to produce a thick steel plate having a thickness of 30 to 80 mm. Hot rolling was performed at a heating temperature of 1100 ° C. and a rolling end temperature of 880 ° C.
　得られた厚鋼板について、全酸化物の成分組成、および介在物と酸化物の個数密度を次の手順で測定した。即ち、得られた厚鋼板のｔ／４（但し、ｔは鋼板の厚み）位置における横断面からサンプルを切り出し、切り出されたサンプル表面を、日本電子データム製のＥＰＭＡ「ＪＸＡ－８５００Ｆ（装置名）」を用いて観察し、円相当直径が０．１μｍ以上の介在物について成分組成を定量分析した。観察条件は、加速電圧を２０ｋＶ，試料電流を０．０１μＡ，観察視野面積を１～５ｃｍ2，分析個数を１００個以上とし、介在物の中央部での成分組成を特性Ｘ線の波長分散分光により定量分析した。分析対象元素は、Ｓｉ、Ｍｎ、Ｓ、Ａｌ、Ｔｉ、Ｌａ、Ｃｅ、Ｃａ、Ｚｒ、およびＯ（酸素）とし、既知物質を用いて各元素のＸ線強度と元素濃度の関係を予め検量線として求めておき、分析対象とする上記介在物から得られたＸ線強度と上記検量線からその介在物に含まれる元素量を定量した。 About the obtained thick steel plate, the component composition of all oxides and the number density of inclusions and oxides were measured by the following procedure. That is, a sample was cut out from the cross section at t / 4 (where t is the thickness of the steel plate) of the obtained thick steel plate, and the cut sample surface was used as EPMA “JXA-8500F (device name) manufactured by JEOL Datum. The component composition was quantitatively analyzed for inclusions having an equivalent circle diameter of 0.1 μm or more. The observation conditions are an acceleration voltage of 20 kV, a sample current of 0.01 μA, an observation visual field area of 1 to 5 cm 2 , an analysis number of 100 or more, and the component composition at the center of the inclusion is the wavelength dispersion spectroscopy of characteristic X-rays. Was quantitatively analyzed. The analysis target elements are Si, Mn, S, Al, Ti, La, Ce, Ca, Zr, and O (oxygen), and using a known substance, the relationship between the X-ray intensity and the element concentration of each element is pre-calibrated. The amount of elements contained in the inclusions was quantified from the X-ray intensity obtained from the inclusions to be analyzed and the calibration curve.
　得られた定量結果のうち酸素含量が５％以上の介在物を酸化物とした。このとき、一つの介在物から複数の元素が観測された場合には、それらの元素の存在を示すＸ線強度の比から各元素の単独酸化物に換算して酸化物の組成を算出した。本発明では、このように単独酸化物として質量換算したものの平均を酸化物の平均組成とした。酸化物のうち、ＲＥＭの酸化物、ＺｒＯ2、およびＣａＯの平均組成を下記表２に示す。なお、ＲＥＭの酸化物は、金属元素をＭで表すと、鋼材中にＭ2Ｏ3やＭ3Ｏ5，ＭＯ2の形態で存在するが、全ての酸化物をＭ2Ｏ3に換算して組成を算出した。また、下記表２に示した「その他」とは、ＲＥＭの酸化物、ＺｒＯ2、およびＣａＯ以外の酸化物（例えば、Ａｌ2Ｏ3、ＭｎＯ、ＳｉＯ2など）である。 Of the obtained quantitative results, inclusions having an oxygen content of 5% or more were defined as oxides. At this time, when a plurality of elements were observed from one inclusion, the composition of the oxide was calculated in terms of the X-ray intensity ratio indicating the presence of these elements and converted into a single oxide of each element. In the present invention, the average of the mass converted as a single oxide is defined as the average composition of the oxide. Among the oxides, the average composition of the REM oxide, ZrO 2 , and CaO is shown in Table 2 below. The REM oxide, when the metal element is represented by M, exists in the form of M 2 O 3 , M 3 O 5 , and MO 2 in the steel material, but all oxides are converted to M 2 O 3. The composition was calculated. The “others” shown in Table 2 below are oxides other than REM oxide, ZrO 2 , and CaO (for example, Al 2 O 3 , MnO, SiO 2, etc.).
　次に、定量した介在物についてＴＥＭ観察（観察倍率３０，０００倍）により円相当直径を測定し、円相当直径（粒径）が０．１～２μｍの介在物の個数を測定した。介在物の個数を観察視野面積１ｍｍ2あたりに換算した値を下記表２に示す。 Next, for the quantified inclusions, the equivalent circle diameter was measured by TEM observation (observation magnification 30,000 times), and the number of inclusions having an equivalent circle diameter (particle diameter) of 0.1 to 2 μm was measured. Table 2 below shows values obtained by converting the number of inclusions per 1 mm 2 of the observation visual field area.
　また、得られた定量結果のうち酸素含量が５質量％以上の介在物を酸化物とし、この酸化物の円相当直径をＴＥＭ観察（観察倍率３０，０００倍）により測定し、円相当直径（粒径）が３μｍを超える酸化物の個数を測定した。酸化物の個数を観察視野面積１ｍｍ2あたりに換算した値を下記表２に示す。 Further, among the obtained quantitative results, inclusions having an oxygen content of 5% by mass or more were used as oxides, and the equivalent circle diameter of these oxides was measured by TEM observation (observation magnification 30,000 times). The number of oxides having a particle size exceeding 3 μm was measured. Table 2 shows values obtained by converting the number of oxides per 1 mm 2 observation field area.
　次に、得られた厚鋼板のｔ／４（但し、ｔは鋼板の厚み）位置における横断面から１０ｍｍ×２０ｍｍ×２０ｍｍのサンプルを切り出し、電解抽出した後の電解液を目開き０．１μｍまたは目開き２．０μｍのフィルターを用いて夫々濾過し、フィルター上に残った抽出残渣を回収した。電解液としては、１０％アセチルアセトン－１％テトラメチルアンモニウムクロリドのメタノール溶液を用いた。電解抽出は、電流密度を１００～２００Ａ／ｍ2として行った。 Next, a 10 mm × 20 mm × 20 mm sample is cut out from the cross section at the position of t / 4 (where t is the thickness of the steel plate) of the obtained thick steel plate, and the electrolytic solution after electrolytic extraction has an opening of 0.1 μm or Each filter was filtered using a filter having a mesh size of 2.0 μm, and the extraction residue remaining on the filter was collected. As the electrolytic solution, a methanol solution of 10% acetylacetone-1% tetramethylammonium chloride was used. Electrolytic extraction was performed at a current density of 100 to 200 A / m 2 .
　回収した抽出残渣に含まれるＴｉ、Ａｌ量をＩＣＰ発光分析法、Ｎ量を紫外可視分光光度計「ＵＶｍｉｎｉ－１２４０（株式会社島津製作所製）」を用いてインドフェノール青吸光光度法によって夫々定量し、上述した手順で上記式（１）の左辺の値を算出した。算出結果を下記表２に示す。 The amount of Ti and Al contained in the collected extraction residue was determined by ICP emission spectrometry, and the amount of N was determined by indophenol blue absorptiometry using an ultraviolet-visible spectrophotometer “UVmini-1240 (manufactured by Shimadzu Corporation)”. The value of the left side of the above formula (1) was calculated by the procedure described above. The calculation results are shown in Table 2 below.
　次に、溶接時に熱影響を受けるＨＡＺの靭性を評価するために大入熱溶接を模擬して下記に示す溶接再現試験を行なった。溶接再現試験は、厚鋼板のｔ／４位置（但し、ｔは板厚）から切り出したサンプルが１４００℃になる様に加熱し、この温度で６０秒間保持した後、冷却する熱サイクルを与えた。冷却速度は、８００℃から５００℃への冷却時間が４５０秒となるように調整した。 Next, in order to evaluate the toughness of the HAZ that is affected by heat during welding, a high heat input welding was simulated and the following welding reproduction test was performed. In the welding reproduction test, a sample cut from the t / 4 position (where t is the plate thickness) of the thick steel plate was heated to 1400 ° C., held at this temperature for 60 seconds, and then given a heat cycle for cooling. . The cooling rate was adjusted so that the cooling time from 800 ° C. to 500 ° C. was 450 seconds.
　冷却後のサンプルの衝撃特性は、上記熱サイクルを与えた後のサンプルから圧延方向にＶノッチシャルピー試験片を３本採取し、ＪＩＳ　Ｚ２２４２に従って衝撃試験を行なって評価した。衝撃試験では、－４０℃における吸収エネルギー（ｖＥ-40）を測定し、３回の平均値を算出した。本発明では、ｖＥ-40の平均値が１００Ｊ以上のものを合格（ＨＡＺ靭性良好）とする。測定結果を下記表２に示す。 The impact characteristics of the sample after cooling were evaluated by taking three V-notch Charpy test pieces in the rolling direction from the sample after applying the thermal cycle and conducting an impact test according to JIS Z2242. In the impact test, the absorbed energy (vE -40 ) at -40 ° C was measured, and the average value of three times was calculated. In the present invention, an average value of vE- 40 of 100 J or more is regarded as acceptable (haz toughness is good). The measurement results are shown in Table 2 below.
　下記表１、表２から次のように考察できる。Ｎｏ．１～１８、３２は、本発明で規定する条件を満足する例であり、円相当直径が３μｍ超の酸化物が生成しないように、円相当直径が０．１～２μｍの微細な介在物を多く生成させており、しかも上記微細な介在物の成分組成を適切に制御しているため、ＨＡＺ靭性が良好な鋼材が得られている。 The following Table 1 and Table 2 can be considered as follows. No. Nos. 1 to 18 and 32 are examples satisfying the conditions defined in the present invention, and fine inclusions having an equivalent circle diameter of 0.1 to 2 μm are formed so that an oxide having an equivalent circle diameter of more than 3 μm is not generated. A large amount of steel is produced, and the component composition of the fine inclusions is appropriately controlled, so that a steel material having good HAZ toughness is obtained.
　一方、Ｎｏ．１９～３１、３３は、本発明で規定するいずれかの要件を外れる例である。これらのうちＮｏ．１９は、鋼材に含まれるＡｌ量が多過ぎるため、円相当直径が３μｍを超える粗大な酸化物を多く生成し、ＨＡＺ靭性が劣化している。Ｎｏ．２０は、鋼材に含まれるＮ量が多過ぎる例であり、鋼材に含まれる固溶Ｎ量が過剰となり、ＨＡＺ靭性が劣化していると考えられる。 On the other hand, No. Reference numerals 19 to 31, 33 are examples that do not satisfy any of the requirements defined in the present invention. Of these, No. In No. 19, since the amount of Al contained in the steel material is too large, a large amount of coarse oxide having an equivalent circle diameter exceeding 3 μm is generated, and the HAZ toughness is deteriorated. No. No. 20 is an example in which the amount of N contained in the steel material is excessive, and the amount of solute N contained in the steel material becomes excessive, and it is considered that the HAZ toughness is deteriorated.
　Ｎｏ．２１は、鋼材に含まれるＴｉ量が多過ぎるため、Ｔｉの固溶により母材が固溶強化されたため、結果的にＨＡＺ靭性が劣化している。Ｎｏ．２２は、鋼材に含まれるＴｉ量が少な過ぎるため、ＨＡＺ靭性が劣化している。Ｎｏ．２３は、鋼材に含まれるＺｒ量が多過ぎるため、ＺｒＯ2量が多くなり、粒内α変態の核となるＺｒ・ＲＥＭ・Ｃａ系酸化物の作用が弱まり、微細組織が得られずＨＡＺ靭性が劣化していると考えられる。Ｎｏ．２４は、鋼材に含まれるＺｒ量が少な過ぎるため、ＺｒＯ2量が少なくなり、粒内α変態の核となるＺｒ・ＲＥＭ・Ｃａ系酸化物量が少なくなっていると考えられる。そのためＨＡＺ靭性が劣化していると考えられる。 No. In No. 21, since the amount of Ti contained in the steel material is too large, the base material was solid solution strengthened by the solid solution of Ti, and as a result, the HAZ toughness is deteriorated. No. In No. 22, since the amount of Ti contained in the steel material is too small, the HAZ toughness is deteriorated. No. No. 23 has an excessive amount of Zr contained in the steel material, so the amount of ZrO 2 increases, the action of the Zr / REM / Ca-based oxide that becomes the nucleus of the intragranular α-transformation is weakened, and the microstructure is not obtained, and HAZ toughness Is considered to have deteriorated. No. In No. 24, the amount of Zr contained in the steel material is too small, so the amount of ZrO 2 is reduced, and the amount of Zr / REM / Ca oxides that are the core of intragranular α transformation is considered to be reduced. Therefore, it is considered that the HAZ toughness is deteriorated.
　Ｎｏ．２５は、鋼材に含まれるＲＥＭ量が多いため、ＲＥＭの酸化物量が多くなり、またＲＥＭの酸化物が粗大化し、円相当直径が３μｍを超える粗大な酸化物が過剰に生成したため、ＨＡＺ靭性向上作用が発揮されていないと考えられる。Ｎｏ．２６は、鋼材に含まれるＲＥＭ量が少な過ぎるため、ＲＥＭの酸化物量が少なくなり、粒内α変態の核となるＺｒ・ＲＥＭ・Ｃａ系酸化物量が少なくなっていると考えられる。そのためＨＡＺ靭性が劣化していると考えられる。Ｎｏ．２７は、鋼材に含まれるＣａ量が多過ぎるため、ＣａＯ量が多くなり、粒内α変態の核となるＺｒ・ＲＥＭ・Ｃａ系酸化物の作用が弱まり、微細組織が得られずＨＡＺ靭性が劣化していると考えられる。Ｎｏ．２８は、鋼材に含まれるＣａ量が少な過ぎるため、ＣａＯが生成せず、粒内α変態の核となるＺｒ・ＲＥＭ・Ｃａ系酸化物量が少なくなっていると考えられる。そのためＨＡＺ靭性が劣化していると考えられる。 No. 25, because the amount of REM contained in the steel material is large, the amount of REM oxide is increased, and the oxide of REM is coarsened, resulting in excessive generation of coarse oxide with an equivalent circle diameter exceeding 3 μm, thus improving HAZ toughness. It is thought that the effect is not exerted. No. In No. 26, the amount of REM contained in the steel material is too small, so the amount of REM oxide is reduced, and the amount of Zr, REM, and Ca-based oxides that are the core of intragranular α transformation is considered to be reduced. Therefore, it is considered that the HAZ toughness is deteriorated. No. 27, since the amount of Ca contained in the steel material is too large, the amount of CaO is increased, the action of the Zr / REM / Ca-based oxide serving as the nucleus of the intragranular α transformation is weakened, and a microstructure cannot be obtained, resulting in HAZ toughness. It is thought that it has deteriorated. No. In No. 28, since the amount of Ca contained in the steel material is too small, CaO is not generated, and it is considered that the amount of Zr / REM / Ca-based oxide serving as the nucleus of intragranular α transformation is reduced. Therefore, it is considered that the HAZ toughness is deteriorated.
　Ｎｏ．２９、Ｎｏ．３０、およびＮｏ．３３は、溶製時におけるＴｉとＡｌの添加順序が本発明で推奨する条件から外れているため、上記式（１）の値が本発明で規定する要件から外れている例である。そのためＨＡＺ靭性が劣化している。Ｎｏ．３１は、Ｔｉ、Ｎ、Ａｌ量のバランスが悪く、鋼材に含まれる介在物の成分組成が、上記式（１）の関係を満足せず、本発明で規定する範囲を超えているため、介在物の融点が上昇し、大入熱溶接時に介在物が液相化せず、粒内α変態の核となる介在物が形成され難くなり、ＨＡＺ靭性が向上していないと考えられる。 No. 29, no. 30, and no. No. 33 is an example in which the value of the above formula (1) deviates from the requirement defined in the present invention because the order of addition of Ti and Al during melting is out of the conditions recommended in the present invention. Therefore, the HAZ toughness is deteriorated. No. No. 31 has a poor balance of Ti, N and Al amounts, and the composition of inclusions contained in the steel material does not satisfy the relationship of the above formula (1) and exceeds the range specified in the present invention. It is considered that the melting point of the product is increased, the inclusion does not become a liquid phase at the time of high heat input welding, the inclusion that becomes the nucleus of the intragranular α transformation is hardly formed, and the HAZ toughness is not improved.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002

Claims (5)

  1. 　Ｃ　：０．０２～０．１５％（質量％の意味。以下成分について同じ。）、
    　Ｓｉ：０．５％以下、
    　Ｍｎ：２．５％以下、
    　Ｐ　：０．０３％以下、
    　Ｓ　：０．０２％以下、
    　Ａｌ：０．０５０％以下、
    　Ｔｉ：０．００５～０．１０％、
    　ＲＥＭ：０．０００３～０．０１５％、
    　Ｃａ：０．０００３～０．０１０％、
    　Ｚｒ：０．００１０～０．０５０％、
    　Ｎ　：０．０１０％以下、
    　Ｏ　：０．０００５～０．０１０％を含有し、
    　残部が鉄および不可避不純物からなる鋼材であり、
    　（ａ）前記鋼材は、Ｚｒ、ＲＥＭ、およびＣａを含有する酸化物を含み、
    　（ｂ）前記鋼材に含まれる全介在物のうち、
    　円相当直径で０．１～２μｍの介在物が観察視野面積１ｍｍ2あたり１２０個以上、
    　円相当直径で３μｍ超の酸化物が観察視野面積１ｍｍ2あたり５．０個以下であり、且つ
    　（ｃ）前記鋼材に含まれる円相当直径が０．１～２μｍの介在物の成分組成が、下記式（１）の関係を満足することを特徴とする溶接熱影響部の靭性に優れた鋼材。
    　（Insol.Ｔｉ－３．４×Insol.Ｎ）／Insol.Ａｌ＝１．０～８　　・・・（１）     C: 0.02 to 0.15% (meaning mass%; the same applies to the following components),
    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,
    Ti: 0.005 to 0.10%,
    REM: 0.0003 to 0.015%,
    Ca: 0.0003 to 0.010%,
    Zr: 0.0010 to 0.050%,
    N: 0.010% or less,
    O 2: contains 0.0005 to 0.010%,
    The balance is steel consisting of iron and inevitable impurities,
    (A) The steel material includes an oxide containing Zr, REM, and Ca,
    (B) Of all the inclusions contained in the steel material,
    More than 120 inclusions with an equivalent circle diameter of 0.1 to 2 μm per 1 mm 2 of the viewing field area,
    The number of oxides having an equivalent circle diameter of more than 3 μm is 5.0 or less per 1 mm 2 of the observation visual field area, and (c) the composition of inclusions having an equivalent circle diameter of 0.1 to 2 μm included in the steel material is A steel material excellent in the toughness of the weld heat affected zone, characterized by satisfying the relationship of the following formula (1).
    (Insol.Ti-3.4 × Insol.N) /Insol.Al=1.0-8 (1)
  2. 　前記鋼材が、更に他の元素として、
    　Ｃｕ：２％以下、
    　Ｎｉ：３．５％以下、
    　Ｃｒ：３％以下、および
    　Ｍｏ：１％以下よりなる群から選ばれる少なくとも１種の元素を含有する請求項１に記載の鋼材。     The steel material is still another element,
    Cu: 2% or less,
    Ni: 3.5% or less,
    The steel material according to claim 1, containing at least one element selected from the group consisting of Cr: 3% or less and Mo: 1% or less.
  3. 　前記鋼材が、更に他の元素として、
    　Ｎｂ：０．２５％以下と
    　Ｖ　：０．１％以下の少なくとも一方を含有する請求項１または２に記載の鋼材。     The steel material is still another element,
    The steel material according to claim 1 or 2, containing at least one of Nb: 0.25% or less and V: 0.1% or less.
  4. 　前記鋼材が、更に他の元素として、
    　Ｂ：０．００５％以下を含有する請求項１または２に記載の鋼材。     The steel material is still another element,
    B: The steel material of Claim 1 or 2 containing 0.005% or less.
  5. 　前記鋼材が、更に他の元素として、
    　Ｂ：０．００５％以下を含有する請求項３に記載の鋼材。     The steel material is still another element,
    B: The steel material of Claim 3 containing 0.005% or less.
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