EP3239333A1 - Stahlplatte mit hervorragender zähigkeit und resistenz gegen wasserstoffinduzierte rissbildung und stahlrohr für ein leitungsrohr - Google Patents

Stahlplatte mit hervorragender zähigkeit und resistenz gegen wasserstoffinduzierte rissbildung und stahlrohr für ein leitungsrohr Download PDF

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EP3239333A1
EP3239333A1 EP15873096.0A EP15873096A EP3239333A1 EP 3239333 A1 EP3239333 A1 EP 3239333A1 EP 15873096 A EP15873096 A EP 15873096A EP 3239333 A1 EP3239333 A1 EP 3239333A1
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concentration
slab
steel plate
steel
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English (en)
French (fr)
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EP3239333A4 (de
Inventor
Kiichiro TASHIRO
Taku Kato
Haruya KAWANO
Yuichi OKA
Shinsuke Sato
Sei Kimura
Takashi Miyake
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Kobe Steel Ltd
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Kobe Steel Ltd
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Priority claimed from PCT/JP2015/085871 external-priority patent/WO2016104528A1/ja
Publication of EP3239333A1 publication Critical patent/EP3239333A1/de
Publication of EP3239333A4 publication Critical patent/EP3239333A4/de
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes

Definitions

  • the present invention relates to a steel plate that has excellent hydrogen-induced cracking resistance and toughness, and is suitable for use in line pipes for transportation and tanks for storage of natural gas and crude oil, and to a steel pipe for line pipes with excellent hydrogen-induced cracking resistance and toughness, obtained by using the steel plate.
  • HIC Hydrogen-induced cracking
  • HIC is also known to have a tendency to occur in segregation zones, including a center segregation and internal cracks of a cast strip, particularly, at an inclusion, such as MnS, as a starting point. For this reason, some techniques for enhancing HIC resistance have been proposed.
  • Patent Document 1 discloses that a steel material has improved HIC resistance by suppressing segregation degrees of Mn, Nb, and Ti at the center in the thickness direction of a steel plate.
  • Patent Document 2 discloses a method for suppressing HIC that would occur in MnS or a Ca-based acid sulfide as a starting point, by using a parameter formula that includes the contents of Ca, O, and S.
  • a steel plate is subjected to melting, casting, and hot-rolling, and then it undergoes an HIC test before being dispatched as a product.
  • the above-mentioned steel plate cannot be dispatched as a product with excellent hydrogen-induced cracking resistance. Because of this, the steel plate needs to be manufactured again, that is, melted again to produce a product, and then the product needs to undergo the HIC test again. This increases the manufacturing time period and might possibly result in missing the deadline or the like.
  • HIC resistance can be evaluated at the stage of a cast strip after the casting without performing the HIC test after hot rolling, the manufacturing time period can be significantly shortened.
  • HIC occurs at segregation zones (center segregation, internal cracks) or inclusions, such as MnS, as a starting point.
  • MnS inclusions
  • a long procedure A-1 from casting to dispatching is carried out in the following way.
  • the steps of "Sample Preparation (for HIC test) ⁇ HIC Test" in performing the HIC test can be omitted as illustrated in a procedure B-1, so that products can be dispatched at an early stage.
  • Procedure A-1 Casting ⁇ Rolling ⁇ Sample Preparation (for HIC test) ⁇ HIC Test ⁇ Dispatching
  • Procedure B-1 Casting ⁇ Evaluation of HIC Resistance ⁇ Rolling ⁇ Dispatching
  • Patent Document 3 discloses a method in which internal cracks are evaluated at the stage of the cast strip. In this method, the possibility of a hot charge rolling (HCR) operation is determined based on the evaluation result of internal cracks.
  • HCR hot charge rolling
  • Patent Documents 4 to 8 disclose methods for evaluating the quality of a cast strip before rolling, although they are not intended to evaluate CaO inclusions.
  • techniques mentioned in Patent Documents 4 to 7 evaluate the quality of a cast strip based on the content of inclusions or the content of elements in the cast strip or molten steel in a tundish or the like.
  • the quality of the cast strip is evaluated from an analysis result of the molten steel in the tundish (primary determination). If the determination accuracy does not meet the predetermined accuracy, the quality of the cast strip is evaluated from an analysis result of a cast strip sample (secondary determination).
  • Patent Documents 3 to 8 Although the techniques mentioned in Patent Documents 3 to 8 are not intended to evaluate CaO inclusions as mentioned above, an evaluation method for CaO inclusions is considered to include evaluation of the content of inclusions or content of elements and the like in the cast strip or molten steel in the tundish, like Patent Documents 3 to 8.
  • the CaO inclusions are evaluated from the content of inclusions, the content of elements, or the like in the molten steel in the tundish. However, CaO inclusions are aggregated and accumulated after being charged into a mold. Thus, even though no CaO accumulation zone is evaluated to be present based on the CaO content or Ca concentration in the molten steel in the tundish, CaO inclusions can be aggregated thereafter, causing the HIC.
  • the present invention has been made in view of the foregoing circumstance, and it is an object of the present invention to achieve a steel plate and a steel pipe that have excellent hydrogen-induced cracking resistance and toughness, and further to achieve a steel plate and a steel pipe that enable the evaluation of the HIC resistance by the internal quality of a cast strip without performing an HIC test.
  • a steel plate having excellent hydrogen-induced cracking resistance and toughness according to the present invention that can solve the above-mentioned problem includes, in percent by mass:
  • the threshold value Ca drop ⁇ may be a value previously determined by a method including following (i) to (iii):
  • the slab cast on the same casting conditions as the above-mentioned slab may be the slab in which the decrease in the amount of Ca is measured.
  • the Ca concentration in the slab may be a minimum Ca concentration of two or more Ca concentrations obtained by examining the Ca concentration at two or more different positions in the thickness direction of the slab.
  • the threshold value Ca drop ⁇ may be 4 ppm (ppm by mass).
  • the steel plate may further include one or more of the elements (A) and (B) below as another element:
  • the steel plate is suitable use in line pipes and pressure containers.
  • the invention also includes a steel pipe for a line pipes, formed of the steel plate.
  • the invention can provide the steel plate and steel pipe that surely have the excellent hydrogen-induced cracking resistance and toughness. Further, the present invention can provide the steel plate and steel pipe in which the HIC resistance can be evaluated by the internal quality of a cast strip without performing the HIC test.
  • These steel plates are suitable for use in line pipes for transportation and pressure containers, such as tanks for storage, of natural gas and crude oil, and the like.
  • the inventors have diligently studied to solve the foregoing problems.
  • the NACE test is a test that involves preparing a solution at pH2.7 that contains 5% sodium chloride (NaCl) solution + 0.5% acetic acid, followed by saturating the solution with hydrogen sulfide gas, then exposing a test specimen to the solution for 96 hours, and finally evaluating the occurrence of HIC in the test specimen.
  • the inventors have performed the Charpy impact test according to ASTM A370 for examining the Charpy impact properties after the HIC test, on a surface layer part of a steel plate (for example, see CAMP-ISIJ Vol. 24(2011)-P671 ), which is known to have its hydrogen concentration particularly increased during the HIC test.
  • the result shows that there are variations in the Charpy impact absorbed energy value.
  • the Ar content in the steel should be set at 0.50 ⁇ L (microliters)/cm 3 or less.
  • the Ar content is preferably 0. 30 ⁇ L/cm 3 or less, and more preferably 0.25 ⁇ L/cm 3 or less.
  • the inventors have focused on the tendency for HIC to occur at a MnS inclusion as a starting point.
  • a steel to contain a rare earth element or Zr, which has a desulfurization effect, the formation of MnS can be suppressed, and the hydrogen-induced cracking resistance can be improved.
  • an appropriate content of such an element is found to efficiently exhibit its desulfurization effect as mentioned later.
  • the inventors have focused on the tendency for HIC to occur at a CaO accumulation zone generated as the starting point during producing a cast strip. Consequently, attention is paid to the "decrease in the amount of Ca that is obtained by subtracting the Ca concentration in the slab from the Ca concentration in the molten steel in the tundish", and which can evaluate the presence or absence of the CaO accumulation zones. It is found that if the decrease in the amount of Ca at the stage of the slab is restricted to a predetermined threshold value or less, a steel plate with higher hydrogen-induced cracking resistance can be obtained, so that products can be dispatched at an early stage. This matter will be described in detail below.
  • the composition of the steel needs to be controlled. Furthermore, to ensure the high strength, excellent weldability, and the like, which are other properties required as, for example, the steel for line pipes, the composition of the steel plate needs to be as follows. The reasons for specifying the contents of the respective components, including the aforesaid REM and Zr, will be described below.
  • Carbon (C) is an element essential to ensure the strength of a base metal and a weld bead.
  • the C content needs to be 0.02% or more.
  • the C content is preferably 0.03% or more, and more preferably 0.05% or more.
  • an extremely high C content degrades the heat-affected zone (HAZ) toughness and the weldability of the steel. Any excessive C content is more likely to form NbC or island-shaped martensite, which possibly becomes as the starting point of HIC or a fracture propagation route.
  • the C content needs to be 0.15% or less.
  • the C content is preferably 0.12% or less, and more preferably 0.10% or less.
  • Silicon (Si) has a deoxidation function and is effective in improving the strength of a base metal and a weld bead. To exhibit these effects, the Si content is set at 0.02% or more.
  • the Si content is preferably 0.05% or more, and more preferably 0.15% or more.
  • an extremely high Si content degrades the weldability and toughness of the steel. Any excessive Si content forms island-shaped martensite to generate and propagate HIC. Accordingly, the Si content needs to be suppressed to 0.50% or less.
  • the Si content is preferably 0.45% or less, and more preferably 0.35% or less.
  • Manganese (Mn) is an element that is effective in improving the strength of a base metal and a weld bead.
  • the Mn content is set at 0.6% or more.
  • the Mn content is preferably 0.8% or more, and more preferably 1.0% or more.
  • an extremely high Mn content forms MnS, degrading not only the hydrogen-induced cracking resistance, but also the HAZ toughness and weldability.
  • the upper limit of Mn content is set at 2.0%.
  • the Mn content is preferably 1.8% or less, more preferably 1.5% or less, and still more preferably 1.2% or less.
  • Phosphorus (P) is an element inevitably contained in steel.
  • the P content exceeds 0.030%, the roughness of a base metal and a HAZ are significantly degraded, and the hydrogen-induced cracking resistance of the steel is also degraded.
  • the P content is restricted to 0.030% or less.
  • the P content is preferably 0.020% or less, and more preferably 0.010% or less.
  • S Sulfur
  • MnS MnS
  • the upper limit of S content is 0.003%.
  • the S content is preferably 0.002% or less, more preferably 0.0015% or less, and still more preferably 0.0010% or less.
  • the S content is desirably low from the viewpoint of improving the hydrogen-induced cracking resistance.
  • Aluminum (Al) is a strong deoxidizing element.
  • the Al content needs to be 0.010% or more.
  • the Al content is preferably 0.020% or more, and more preferably 0.030% or more.
  • the Al content needs to be 0.08% or less.
  • the Al content is preferably 0.06% or less, and more preferably 0.05% or less.
  • Ca serves to control the form of a sulfide and has an effect of suppressing the formation of MnS by forming CaS. To obtain this effect, the Ca content needs to be 0.0003% or more.
  • the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more.
  • the upper limit of Ca content is set at 0.0060%.
  • the Ca content is preferably 0.0045% or less, more preferably 0.0035% or less, and still more preferably 0.0025% or less.
  • N Nitrogen
  • the N content needs to be 0.001% or more.
  • the N content is preferably 0.003% or more, and more preferably 0.0040% or more.
  • An extremely high N content degrades the toughness of the HAZ by the presence of the solid-solute N.
  • the N content needs to be 0.01% or less.
  • the N content is preferably 0.008% or less, and more preferably 0.0060% or less.
  • An oxygen (O) content is desirably low from the viewpoint of improving the cleanliness of a steel.
  • An extremely high O content degrades the toughness of the steel, and additionally causes HIC at an oxide as a starting point, thereby degrading the hydrogen-induced cracking resistance.
  • the O content needs to be 0.0045% or less, and is preferably 0.0030% or less, and more preferably 0.0020% or less.
  • the sulfide-based inclusion in the steel has its form controlled as CaS by adding Ca, thereby rendering S harmless for the HIC resistance.
  • the Ca/S needs to be set at 2.0 or more.
  • the Ca/S is preferably 2.5 or more, and more preferably 3.0 or more. Note that the upper limit of Ca/S is approximately 17 based on the Ca content and S content specified by the present invention.
  • a Ca content (Ca - 1.25S) that is obtained by subtracting a content in Ca present as a sulfide (CaS) in the steel from the total Ca content in the steel must not be excessive relative to the O content.
  • CaO is more likely to be formed as an oxide-based inclusion, which makes it easier for aggregates of the CaO (coarse Ca-based inclusions) to be formed in a larger amount at a superficial layer of a steel plate.
  • the (Ca - 1.25S)/O needs to be 1.80 or less in order to obtain the excellent HIC resistance.
  • (Ca - 1.25S)/O is preferably 1.40 or less, more preferably 1.30 or less, still more preferably 1.20 or less, and particularly preferably 1.00 or less.
  • the lower limit of (Ca - 1.25S)/O is approximately 0.1.
  • a rare earth metal is an element that is effective in enhancing the hydrogen-induced cracking resistance by suppressing the formation of MnS through the desulfurization effect as mentioned above.
  • the REM content is preferably 0.0002% or more.
  • the REM content is more preferably 0.0005% or more, and still more preferably 0.0010% or more.
  • the upper limit of the REM content needs to be 0.02%.
  • the REM content is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.0050% or less.
  • REM means lanthanoid elements (15 elements from La to Lu), scandium (Sc), and yttrium (Y).
  • Zirconium is an element that serves to form an oxide and disperse it finely in steel, while improving the HIC resistance by the desulfurization effect, thereby contributing to improving the HAZ toughness.
  • the Zr content is preferably set at 0.0003% or more.
  • the Zr content is more preferably 0.0005% or more, still more preferably 0.0010% or more, and yet more preferably 0.0015% or more.
  • the addition of an excessive content of Zr forms coarse inclusions to degrade the hydrogen-induced cracking resistance of a steel plate and the toughness of a base metal.
  • the Zr content needs to be 0.010% or less.
  • the Zr content is preferably 0.0070% or less, more preferably 0.0050% or less, and still more preferably 0.0030% or less.
  • the steel (steel plate, steel pipe) in the present invention have been mentioned above, with the balance being iron and inevitable impurities.
  • the steel further includes:
  • B Boron
  • B enhances the hardenability of a steel and the strength of a base metal and a weld bead. Furthermore, B is bonded to N to precipitate BN while the heated HAZ zone is cooled in welding, thus promoting ferrite transformation from the inside of an austenite grain. In this way, B improves the HAZ toughness.
  • the B content is preferably 0.0002% or more.
  • the B content is more preferably 0.0005% or more, and still more preferably 0.0010% or more.
  • any excessive B content degrades the toughness of a base metal and a HAZ zone, thus leading to degradation in the weldability.
  • the B content is preferably 0.005% or less.
  • the B content is more preferably 0.004% or less, and still more preferably 0.0030% or less.
  • Vanadium (V) is an element effective in improving the strength of steel. To obtain this effect, the V content is preferably 0.003% or more, and more preferably 0.010% or more. On the other hand, when the V content exceeds 0.1%, the weldability and the toughness of a base metal would be degraded. Thus, the V content is preferably 0.1% or less, and more preferably 0.08% or less.
  • Copper (Cu) is an element effective in improving the hardenability of steel.
  • the Cu content is preferably 0.01% or more.
  • the Cu content is more preferably 0.05% or more, and still more preferably 0.10% or more.
  • the toughness of steel is degraded.
  • the Cu content is preferably 1.5% or less.
  • the Cu content is more preferably 1.0% or less, and still more preferably 0.50% or less.
  • Nickel (Ni) is an element effective in improving the strength and toughness of a base metal and a weld bead. To obtain these effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more, and still more preferably 0.10% or more. However, an extremely high Ni content leads to an excessively expensive steel for a structure. From the economical aspect, the Ni content is preferably 1.5% or less. The Ni content is more preferably 1.0% or less, and still more preferably 0.50% or less.
  • Chromium (Cr) is an element effective in improving the strength of steel.
  • the Cr content is preferably 0.01% or more.
  • the Cr content is more preferably 0.05% or more, and still more preferably 0.10% or more.
  • the Cr content is preferably 1.5% or less.
  • the Cr content is more preferably 1.0% or less, and still more preferably 0.50% or less.
  • Molybdenum (Mo) is an element effective in improving the strength and toughness of a base metal.
  • the Mo content is preferably 0.01% or more.
  • the Mo content is more preferably 0.05% or more, and still more preferably 0.10% or more.
  • the Mo content is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.50% or less.
  • Niobium (Nb) is an element effective in enhancing the strength of steel and the toughness of a base metal without degrading its weldability. To obtain this effect, the Nb content is preferably 0.002% or more. The Nb content is more preferably 0.010% or more, and still more preferably 0.020% or more. However, when the Nb content exceeds 0.06%, the toughness of the base metal and HAZ is degraded. Thus, in the present invention, the upper limit of Nb content is preferably set at 0.06%. The Nb content is more preferably 0.050% or less, still more preferably 0.040% or less, and yet more preferably 0.030% or less.
  • Titanium (Ti) precipitates as TiN in steel, thereby preventing austenite grains in a HAZ zone from being coarsened during welding and thereby promoting the ferrite transformation.
  • Ti is an element that is effective in improving the toughness of the HAZ zone.
  • Ti exhibits the desulfurization effect, and thus is an element that is effective in improving the HIC resistance.
  • the Ti content is preferably 0.003% or more.
  • the Ti content is more preferably 0.005% or more, and still more preferably 0.010% or more.
  • any excessive Ti content leads to an increase in the amount of solid-solute Ti and precipitated TiC, thus degrading the toughnesses of a base metal and a HAZ zone.
  • the Ti content is preferably 0.03% or less, and more preferably 0.02% or less.
  • Magnesium (Mg) is an element that is effective in improving the toughness of steel through refinement of crystal grains, and also effective in improving the HIC resistance because of its desulfurization effect. To obtain these effects, the Mg content is preferably 0.0003% or more. The Mg content is more preferably 0.001% or more. On the other hand, an excessive Mg content saturates its effect. Thus, the upper limit of the Mg content is preferably 0.01%. The Mg content is more preferably 0.005% or less.
  • the steel plate of the present invention is a steel plate having high hydrogen-induced cracking resistance in which a decrease in an amount of Ca obtained by subtracting a Ca concentration in a slab from a Ca concentration in a molten steel in a tundish is a threshold value Ca drop ⁇ or less.
  • threshold value Ca drop ⁇ as used herein means the maximum decrease in the amount of Ca previously determined and which avoids the occurrence of hydrogen-induced cracking in a steel plate obtained by rolling the slab.
  • the decrease in the amount of Ca obtained by subtracting the Ca concentration in the slab from the Ca concentration in the molten-steel in the tundish is set to be the predetermined threshold value or less as mentioned above, thereby making it possible to produce the steel plate having the high hydrogen-induced cracking resistance and to dispatch products at an early stage, which will be mentioned later.
  • the reason for setting the decrease in the amount of Ca as an evaluation index will be described below.
  • the inventors have focused on MnS inclusions and progressed their studies regarding addition of Ca to a molten steel during a secondary refinement to suppress the formation of MnS.
  • CaO-Al 2 O 3 inclusions are formed in the molten steel.
  • the CaO-Al 2 O 3 has good wettability to the molten steel, and thus is not aggregated in the molten steel and remains fine without adversely affecting the HIC resistance.
  • an added amount of Ca to the molten steel is not appropriate, for example, when adding Ca in an excessive amount that exceeds a predetermined amount required to suppress MnS formation and modify Al 2 O 3 , pure CaO inclusions are also formed in the molten steel, in addition to CaO-Al 2 O 3 inclusions.
  • the pure CaO inclusion has inferior wettability to the molten steel and thereby is more likely to be aggregated in the molten steel.
  • the aggregated CaO becomes each coarse inclusion, inducing HIC.
  • the coarsened CaO inclusion has a smaller density than the molten steel, and thus most of CaO inclusions are allowed to float and then separated. However, as shown in Fig. 1 , parts of CaO inclusions receive a buoyant force while falling down deeply into a cast strip along the flow of the molten steel within a mold, and then trapped in a solidification shell to form a CaO accumulation zone.
  • the CaO accumulation zone acts as the starting point of HIC.
  • an appropriate added amount of Ca to the molten steel can be determined in advance, the occurrence of HIC due to the CaO inclusions can be suppressed.
  • the added amount of Ca is set at an amount enough to suppress the formation of MnS. Consequently, the added amount of Ca tends to become excessive, and thereby a CaO accumulation zone is more likely to be formed.
  • an accumulation degree of CaO inclusions can be identified by analyzing the Ca concentration in that position. Furthermore, whether the CaO accumulation zone occurs in a cast strip can also be presumed from the accumulation degree of CaO.
  • the positions where the CaO accumulation zones occur differ in the thickness direction of a cast strip, depending on casting conditions (casting speed, angle of a discharge port of an immersion nozzle, and the like).
  • casting conditions casting speed, angle of a discharge port of an immersion nozzle, and the like.
  • three slabs (A to C) with different casting conditions casting speed and angle of the discharge port of the immersion nozzle) differ from one another in the position (e.g., positions a to c) at the high Ca concentration where the accumulation zone occurs.
  • the positions of the CaO accumulation zones cannot be predicted.
  • the inventors have changed their viewpoints on examination positions for the Ca concentration and focused on the position with a low Ca concentration. It is considered that when a CaO accumulation zone occurs, the Ca concentration at the CaO accumulation zone becomes high, while in a position where no CaO accumulation zone occurs, the Ca concentration becomes relatively low. Taking this into account, the inventors have examined the relationship between the "Ca concentration in an arbitrary position in the thickness direction of a slab” and the "Ca concentration in a molten steel in a tundish" when CaO accumulation zones occur.
  • the "Ca concentration in the slab” is relatively low, and thereby a "value obtained by subtracting the 'Ca concentration in the slab' from the 'Ca concentration in the molten steel in the tundish'", that is, "a decrease in an amount of the Ca concentration from the tundish to the slab" is found to become large.
  • the HIC resistance is evaluated by using a value obtained by subtracting the "Ca concentration in the slab” from the "Ca concentration in the molten steel in the tundish” (hereinafter referred to as the "decrease in the amount of Ca"), which is associated with the presence or absence of the CaO accumulation zones.
  • a threshold value Ca drop ⁇ of the decrease in the amount of Ca i.e., the maximum decrease in the amount of Ca that avoids the occurrence of HIC in a steel plate obtained by rolling a slab in order to determine whether the obtained steel plate has excellent HIC resistance or not.
  • the threshold value Ca drop ⁇ is determined previously, but a method for determination thereof is not particularly limited to the following method.
  • a method for previously determining the threshold value Ca drop ⁇ will include the following processes (i) to (iii).
  • molten steel is taken out of the tundish, and its Ca concentration (Ca TD1 ) is analyzed.
  • the molten steel in the tundish is constantly supplied from a ladle, so that the Ca concentration (Ca TD1 ) remains constant even after taking out the molten steel.
  • a Ca concentration (Ca S1 ) in a slab is examined.
  • a sample is taken out of a region R4 (hereinafter referred to as a "reference-side region R4") ranging from the reference-side surface of the slab to D/2 in the thickness direction thereof, and a Ca concentration Ca S1 in the region R4 is analyzed.
  • the "reference-side region R4" as shown in Fig. 3(a) , is in a range from D/2 to D in the thickness direction of the slab oriented from an opposite-reference-side surface thereof.
  • the density of the CaO inclusion is smaller than that of the molten steel, so that the CaO inclusion in the molten steel float while receiving the buoyant force due to a difference in density between the CaO inclusions and molten steel.
  • a continuous casting machine provided with a curved portion and a horizontal portion, as illustrated in Fig. 1 , after CaO inclusions float, they will be trapped in a solidification shell on the opposite-reference-side, whereby a CaO accumulation zone occurs on the opposite-reference-side of the slab, but does not occur on the reference side thereof.
  • the Ca concentration Ca S1 is examined within the "range from the reference-side surface to D/2 in the thickness direction (reference-side region R4)" where no CaO accumulation zone occurs, that is, a range of -0.50D from the center in the slab thickness D toward the reference-side surface in examples to be mentioned later. Based on the Ca concentration Ca S1 in the reference-side region R4, the "decrease in the amount of Ca" in the position where no CaO accumulation zone occurs can be calculated to precisely evaluate the presence or absence of the CaO accumulation zones.
  • Ca drop1 Ca TD 1 ⁇ Ca S 1
  • a slab obtained through casting on the same casting conditions as the slab in which the Ca concentration Ca S1 is measured is hot-rolled to produce a steel plate for measurement of a threshold value.
  • the rolling is performed on the following conditions. Specifically, after heating the slab to a temperature of 1050 to 1250°C, the hot-rolling is performed on the slab through two or more passes. In each pass, a surface temperature of the steel plate becomes 900°C or higher, a cumulative rolling reduction is 40% or more at an average steel plate temperature of 1,000°C or higher, which is determined by calculation to be mentioned below, and a rolling reduction per pass is 10% or more.
  • a cumulative rolling reduction at 700°C or higher and lower than 900°C is 20% or more, and that a rolling-end temperature is 700°C or higher and lower than 900°C.
  • water-cooling on the steel plate is started from a temperature of 650°C or higher and stopped at a temperature of 350 to 600°C. Further, subsequently, the steel plate is air-cooled to the room temperature.
  • the average steel plate temperature is determined in the following way.
  • the temperature at an arbitrary position of the steel plate in the thickness direction is determined by using an appropriate calculation method, such as a finite difference method. Then, the average steel plate temperature is defined as the average of the determined temperatures of the steel strip in a range from the front to back surface thereof.
  • HIC test is performed on the steel plate to check the presence or absence of HIC occurrence.
  • the HIC test is performed by a method specified by the National Association of Corrosion and Engineer (NACE) standard TM0284-2003, as mentioned in examples below.
  • NACE National Association of Corrosion and Engineer
  • a region to be subjected to the HIC test is a region R41 excluding the vicinity of the center in the thickness direction of the product region R40 and corresponding to the opposite-reference-side region.
  • the coarsened CaO accumulation zones are more likely to be formed on the opposite-reference-side of the slab, and thereby HIC due to the CaO tends to occur in the region corresponding to the vicinity of the opposite-reference-side surface.
  • HIC due to the segregation tends to occur at the center in the thickness direction, so that the HIC due to the CaO cannot be evaluated at the center. For this reason, the occurrence of HIC is examined in the region R41 excluding the vicinity of the center in the thickness direction of the slab.
  • a threshold value Ca drop ⁇ of the decrease in the amount of Ca that avoids the occurrence of HIC is determined based on the "decrease Ca drop1 in Ca” and "the result of the HIC test".
  • the "threshold value Ca drop ⁇ " is defined as the maximum decrease in the amount of Ca that never causes the HIC at all.
  • the measurement results and test results of a plurality of slabs are used to obtain the threshold value with higher accuracy, which can suppress the misjudgment of the presence or absence of HIC occurrence.
  • a second embodiment that differs from the first embodiment in the calculation method for a decrease in an amount of Ca will be described below with reference to Fig. 4 .
  • the same components as those in the above-mentioned first embodiment will be briefly described. Also in Fig. 4 , the same components as those in the above-mentioned first embodiment are denoted by the same reference characters, and the description thereof will be omitted as appropriate.
  • a Ca concentration (Ca TD1 ) in the molten steel in the tundish is examined.
  • Fig. 4 samples are taken out of two or more different examination positions in the thickness direction of each of slabs obtained through casting in the same charge, and a Ca concentration of each sample is analyzed.
  • the minimum Ca concentration (Ca min1 ) is selected from the two or more Ca concentrations obtained (Ca S1 , Ca S2 , ).
  • Ca drop11 Ca TD 1 ⁇ Ca min 1
  • an examination position of the Ca concentration is set at one site within the entire range in the thickness direction of the slab. If the examination position corresponds to an accumulation zone, an extremely high Ca concentration is detected. The decrease in the amount of Ca calculated from such a high Ca concentration is small, which might lead to the determination that no CaO accumulation zone occurs, resulting in the evaluation of no HIC occurring. However, in practice, some accumulation zones are generated, which can also be thought to cause HIC.
  • the Ca concentration in the slab is examined at different two or more positions of the slab in its thickness direction.
  • the CaO accumulation zone is present in a specific position in the thickness direction that is determined depending on the casting conditions. By changing the examination position in the thickness direction, a position where the CaO accumulation zone does not occur can also be covered by the examination.
  • the two or more Ca concentrations include not only the Ca concentration in the accumulation zone, but also the Ca concentration where no accumulation zone occurs.
  • the minimum Ca concentration (Ca min1 ) is selected from these concentrations, so that the Ca concentration in the position where no accumulation zone occurs can be selected. Based on this concentration, the decrease in the amount of Ca in the position where no CaO accumulation zone occurs can be calculated, thereby making it possible to precisely evaluate the presence or absence of the CaO accumulation zones.
  • the formation mechanism of the CaO accumulation zone is the same as that of each of a CaO inclusion and an Al 2 O 3 inclusion.
  • the thickness of the accumulation zone of Al 2 O 3 inclusions is reported to be 10 mm (see reference: ISIJ International, Vol. 43(2003), No. 10, p.1548-1555 ). From this report, the thickness of the accumulation zone of the CaO inclusion can also be estimated to be 10 mm. As such, as shown in Fig. 4 , when respective examination positions for the Ca concentration are spaced apart from each other by more than 10 mm in the thickness direction, even if one of the examination positions is in the accumulation zone, the other examination positions are located where no accumulation zone occurs.
  • two or more examination positions are preferably spaced apart from each other by more than 10 mm in the thickness direction.
  • Fig. 4 shows two examination positions, a distance I between the two examination positions being more than 10 mm in the thickness direction (distance I in the thickness direction between two examination positions > 10 mm).
  • the CaO accumulation zone occurs widely in the thickness direction.
  • the Ca concentration examination position is preferably set at a region R3 with a width W-D that is mainly cooled only from the wide surface side, i.e., that excludes the regions ranging from both ends to D/2 in the width direction.
  • a slab obtained through casting on the same casting conditions as the slab in which the Ca concentration Ca S1 or the like is measured as mentioned above is hot-rolled to produce a steel plate for measurement of a threshold value.
  • the HIC test is performed on the steel plate to check the presence or absence of HIC occurrence in the "region R41 corresponding to the vicinity of the opposite-reference-side surface".
  • the HIC test is performed by a method specified by the NACE standard TM0284-2003, as mentioned in examples below.
  • a threshold value Ca drop ⁇ of the decrease in the amount of Ca that avoids the occurrence of HIC is determined based on the "decrease Ca drop11 in Ca” and the "result of the HIC test".
  • the "threshold value Ca drop ⁇ " is defined as the maximum decrease in the amount of Ca that never causes the HIC at all.
  • a Ca concentration Ca TD11 in a molten steel of the charge as a determination target in the tundish is examined.
  • the Ca concentration is examined at different two or more positions in the thickness direction of the slab cast in the same charge.
  • the minimum Ca concentration (Ca min11 ) is selected from two or more Ca concentrations (Ca S11 , Ca S12 , ).
  • the two or more examination positions are preferably spaced apart from each other by more than 10 mm in the thickness direction.
  • Ca drop Ca TD 11 ⁇ Ca min 11
  • the Ca drop as the determination target is compared with the threshold value Ca drop ⁇ .
  • the obtained steel plate is determined to have excellent HIC resistance.
  • the obtained steel plate is determined to be inferior in the HIC resistance.
  • the examination position (examined surface) of the slab is preferably a stationary part, but may be a non-stationary part.
  • non-stationary part as used herein means a part that is cast when the casting condition is varied, for example, a part that is cast at an initial stage of casting, such as when the casting speed increases, or a part that is cast at the end of casting, such as when the casting speed decreases.
  • a part adjacent to the region subjected to the HIC test is preferably examined. Such a part exhibits substantially the same HIC resistance as that in the HIC test result and can be evaluated more precisely.
  • the steel plate in the present invention is a steel plate in which the "decrease Ca drop in Ca” is calculated by subtracting the Ca concentration in the slab from the Ca concentration in the tundish at a stage of the slab before rolling, and the "decrease Ca drop in Ca” satisfies the following formula: Ca drop ⁇ threshold value Ca drop ⁇ . It is considered that the steel plate in the present invention satisfies the relationship of Ca drop ⁇ threshold value Ca drop ⁇ as mentioned above, and that no CaO accumulation zone is generated in the slab, so that no HIC occurs.
  • the "decrease in the amount of Ca concentration from the tundish to the slab” is used for evaluation of the HIC resistance.
  • This can precisely evaluate the internal quality (accumulation degree of CaO inclusions) of the cast strip.
  • the HIC resistance can be evaluated at the stage of the cast strip. Consequently, the HIC test that would require several weeks can be omitted, thereby significantly shortening a time period from the manufacture to dispatching.
  • a method for manufacturing a steel sheet in the present invention is not particularly limited as long as the method can produce the steel plate in which an Ar gas content in the steel satisfies the above-mentioned specified content.
  • the following method is recommended as a method for easily obtaining the specified steel sheet.
  • the number density of inclusions having a major axis of 3 ⁇ m or more and a melting point of 1550°C or higher is preferably 3 inclusions/cm 2 or more in a mold.
  • the inclusions with a melting point of 1550°C or higher exist as solids in a mold and thus have bad wettability to the molten steel, whereby the inclusions are aggregated together while trapping therein Ar gas.
  • the inclusions have their volumes expanded and thereby can easily float.
  • the relatively coarse inclusions having the major axis of 3 ⁇ m or more are in contact with each other in the mold to be more coarsened while trapping therein Ar bubbles, so that the flotation separation of the Ar bubbles in the mold can be promoted. Consequently, the Ar gas content in the steel can be decreased.
  • Ar gas tends to remain in the steel.
  • the flotation separation of Ar gas with the inclusions is effective.
  • inclusions having the melting point of 1550°C or higher include Al 2 O 3 , CaO, and a complex inclusion thereof.
  • the quantitative analysis of the inclusion is performed by an energy-dispersive X-ray spectroscopy (EDX) or the like.
  • An artificial inclusion that imitates the composition of the analyzed inclusion is made.
  • the temperature at which the artificial inclusion starts to melt is measured with a laser microscopy or the like, whereby a melting point of the artificial inclusion can be recognized. More simply, by using the fact that a liquid inclusion in the mold is observed in a spherical shape after being solidified, an inclusion having an aspect ratio of 1.3 or more may be deemed as an inclusion having the melting point of 1550°C or higher.
  • the number density of the inclusions is more preferably 5 inclusions/cm 2 or more, and still more preferably 10 inclusions/cm 2 or more. Any excessive number density of the inclusions degrades the toughness of a base metal and a HAZ zone. Thus, the upper limit of the number density of the inclusions is approximately 100 inclusions/cm 2 .
  • Specific means for achieving the above-mentioned number density of the inclusions is a method that involves, for example, setting a reflux time in RH at 45 minutes or less in a refining step, adding Ca to the steel in the RH and maintaining the steel until 15 minutes or more has elapsed, and thereafter performing the following step(s):
  • Another means for decreasing an Ar gas content in the steel also includes restricting and/or stopping the use of Ar gas in an injection nozzle, the RH, and/or the tundish.
  • it is effective to blow Ar into the injection nozzle from a position located higher by 50 mm or more than an upper part of a discharge port of the injection nozzle.
  • stopping of the use of Ar in the injection nozzle is not recommended.
  • an Ar blowing amount (flow rate) into the injection nozzle is recommended to be preferably 9.0 L(liters)/t(ton) or less (more preferably 6.0 L/t or less).
  • the gas used for blowing into the injection nozzle is also considered to change from Ar gas to nitrogen gas.
  • nitrogen gas the N content in a steel plate cannot be controlled, easily degrading the toughness of the steel plate, which is not preferable.
  • a step to be executed after casting in the way mentioned above is not particularly limited.
  • hot-rolling can be performed by a normal method, thereby manufacturing a steel plate.
  • the steel plate can be used to manufacture a steel pipe for line pipes by a general method.
  • a steel pipe for line pipes obtained by using the steel plate according to the present invention also has excellent HIC resistance and toughness.
  • Tables 1-1 to 4 and Figs. 6 and 7 show the experimental conditions and results for determining the threshold value.
  • Slabs each having a slab thickness D of 280 mm and a slab width W of 2100 mm, were obtained by continuous casting.
  • the casting conditions in the first embodiment are shown in Tables 1-1 and 1-2, and the casting conditions in the second embodiment are shown in Tables 2-1 and 2-2.
  • 25 charges for each were manufactured to obtain each of a steel plate of API (The American Petroleum Institute) X65 grade and a steel plate of API X70 grade.
  • the concentrations of C, Mn, Nb, P, and Ca were measured by an emission spectroscopy.
  • the S concentration was very low and thus was difficult to measure by the emission spectroscopy. Then, the S concentration was measured by using a combustion-infrared absorption method.
  • Specific Water Content whole secondary cooling water amount per unit from directly under the mold to a final roll of a continuous casting machine L / min . / mass of a cast strip production per unit time kg / min .
  • the casting speed is a drawing speed of the cast strip [m/min.], and was calculated from the diameter (circumferential length) and the rotational speed (the number of revolutions per unit time) of a roll (major roll) in contact with the cast strip.
  • Tables 1-1 and 1-2 show examination positions and the Ca concentrations Ca S1 when examining the Ca concentrations in the reference-side regions R4 of the slabs.
  • the hot-rolling was performed on the slab through two or more passes, in each of which a surface temperature of the steel plate was set at 900°C or higher, a cumulative rolling reduction was 40% or more at an average steel plate temperature of 1,000°C or higher, which was determined by the calculation, and a rolling reduction per pass was 10% or more. Further, another hot-rolling was performed such that the cumulative rolling reduction at a temperature of 700°C or higher and lower than 900°C was 20% or more, and that the surface temperature at the end of the rolling was 850°C. Thereafter, cooling of the rolled steel plate was started at a cooling start surface temperature of 950°C and an average cooling rate of 10°C/s.
  • each steel plate having a size of 9 to 50 mm in thickness ⁇ 2000 to 3500 mm in width ⁇ 12000 to 35000 mm in length.
  • Fig. 6 shows the result of determination of the threshold value in the first embodiment, specifically, showing the relationship among the "Ca concentration Ca TD1 in the molten steel in the tundish” examined in the process (2), the "Ca concentration Ca S1 in the slab” shown in each of Tables 1-1 and 1-2, and the HIC test results thereof.
  • Fig. 7 shows the result of determination of the threshold value in the second embodiment, specifically, showing the relationship among the "Ca concentration Ca TD1 in the molten steel in the tundish” examined in the process (2), the minimum Ca concentration Ca min1 in the slab shown in each of Tables 3-1, 3-2, and 4, and the HIC test results thereof.
  • the decrease in the amount of Ca satisfies a formula below: decrease in the amount of Ca ⁇ 4 ppm.
  • the "threshold value of the decrease in the amount of Ca” is determined based on all products, regardless of the strength grade thereof. This is because the easiness of occurrence of HIC due to coarse CaO is not related to the strength grade of products.
  • the HIC resistance of each slab as the determination target having the composition shown in Table 5 was evaluated using the threshold value.
  • the steel with the composition shown in Table 5 was melted and subjected to continuous casting, thereby producing a slab having the slab thickness D of 280 mm and the slab width W of 2100 mm.
  • the Ca concentration Ca TD11 in the molten steel in the tundish of the charge as the determination target was examined, and the minimum Ca concentration (Ca min11 ) in the slab as the determination target was determined, whereby the decrease Ca drop in Ca of the slab as the determination target was calculated as mentioned above.
  • the threshold value Ca drop ⁇ 4 ppm, which was determined in the first and second embodiments as mentioned in the process (5), was used to determine the evaluation of the HIC resistance. Specifically, when the decrease Ca drop in Ca of the slab as the determination target was 4 ppm or less, the HIC due to CaO did not occur, that is, the HIC resistance was rated as OK. On the other hand, when the decrease Ca drop in Ca was more than 4 ppm, the HIC due to the CaO occurred, that is, the HIC resistance was rated as NG. These results are shown in Table 6.
  • the steel with the composition shown in Table 5 was melted and subjected to continuous casting, thereby producing a steel strip (slab).
  • the casting was performed such that the number of inclusions having a major axis of 3 ⁇ m or more was 3 inclusions/cm 2 or more in the mold.
  • a reflux time in the RH was set at 5 minutes or more and 45 minutes or less, Ca was added after the RH reflux, and then the steel was maintained for 15 minutes or more and 45 minutes or less.
  • a molten steel was filled in the tundish that was left for 30 minutes or more and 60 minutes or less after the end of casting of the previous charge. Then, 0.04 kg/ton or more (upper limit of approximately 0.50 kg/ton) of Al metal was added to the molten steel in the tundish, which was subsequently cast. Note that the measurement of the number density of inclusions was performed on samples that were taken out of the mold ten minutes after the casting.
  • each slab was processed by either of two types of hot-rolling and cooling methods, denoted as "TMCP” or “QT” in a “hot-rolling and cooling method” column shown in Table 6. Consequently, steel plates (each having 9 to 90 mm in thickness ⁇ 2000 to 3500 mm in width ⁇ 12000 to 35000 mm in length) with various compositions were obtained.
  • TMCP hot-rolling and cooling methods
  • TMCP was a method that involved: hot-rolling through two or more passes, in each of which a surface temperature of the steel plate was set at 900°C or higher, a cumulative rolling reduction was 40% or more at an average steel plate temperature of 1000°C or higher, determined by the calculation, and a rolling reduction per pass was 10% or more.
  • the TMCP also involved: another hot-rolling such that a cumulative rolling reduction was 20% or more at a temperature of 700°C or higher and lower than 900°C such that that the surface temperature at the end of the rolling was 850°C.
  • the "TMCP” method further involved: starting to cool the rolled steel plate from a cooling start surface temperature of 950°C at an average cooling rate of 10°C/s and then stopping the cooling at a temperature of 350 to 600°C, followed by air-cooling to the room temperature.
  • the above-mentioned "QT” was a method that involved: hot-rolling such that the surface temperature at the end of the rolling was 850°C or higher, followed by air-cooling to the room temperature; quenching by reheating the rolled steel plate to a temperature of 850°C or higher and 950°C or lower; and tempering the steel plate at 600 to 700°C.
  • test specimen with a size of a product thickness ⁇ 15 mm ⁇ 15 mm, was cut out of the surface of each steel, and introduced in a vacuum chamber with a degree of vacuum set at 2 ⁇ 10 -5 Torr or less. Subsequently, the test specimen was perforated from the surface of the steel plate to the depth of 5 mm under the surface by using the G-straight drill, manufactured by MITSUBISHI MATERIALS Corporation (product ID GSDD3000, diameter D1 : 3.0 mm, groove length L3: 32 mm, entire length: 71 mm, and blade diameter: 3.0 mm), whereby gas components of the steel were extracted.
  • G-straight drill manufactured by MITSUBISHI MATERIALS Corporation
  • the gas components were subjected to quantitative analysis using a quadrupole mass spectrometer, manufactured by ANELVA Corporation (M-101QA-TDM model) (a measurement range of atomic mass numbers of 1 to 100 amu). Then, the ratio of an Ar content ( ⁇ L/cm 3 ) to the volume of a part of the steel perforated by the drill mentioned above was determined. This measurement was performed at arbitrary ten positions of the steel plate of each sample, and the "Ar gas content in the steel" was defined as the maximum value among those in the ten positions.
  • the HIC test was performed using the steel plates.
  • the HIC test was performed according to the method specified by the NACE standard TM0284-2003.
  • Tables 5 and 6 show the following. Steels Nos. 1 to 4, 6, 7, and 10 to 12 satisfied the specified compositions and suppressed the decrease in the amount of Ca of each slab to the threshold value or less, thereby producing the steel plates of the present invention with excellent HIC resistance. Each of these steel plates had the Ar gas content in the steel suppressed within a specified range, and thus stably obtained the excellent toughness while having the excellent HIC resistance.
  • the steel type No. 9 was an example in which the chemical composition of the steel plate deviated from the composition range specified by the present invention. That is, in the steel plate made of the steel type No. 9, the contents of REM and Zr were 0%, and the value (Ca - 1.25S) /O deviated from the specified value, resulting in inferior HIC resistance and large variations in the toughness of the steel plate.
  • the steels Nos. 8 and 13 were examples in which the decrease in the amount of Ca of each slab was restricted to be lower than the threshold value, but the chemical composition of the steel plate deviated from the composition range specified by the present invention. That is, in the steel type No. 8, the contents of REM and Zr were 0%, and the value (Ca/S) deviated from the specified value, resulting in inferior HIC resistance. In steel type No. 13, the value (Ca/S) deviated from the specified value, resulting in inferior HIC resistance. In steel type No. 5, the Ar gas content in the steel was excessive, resulting in large variations in the toughness of the steel plate.
  • a time period required from starting of casting to completion of a production of the steel plate that is, a time period until dispatching the steel plate with the sour resistance (casting ⁇ rolling ⁇ dispatching) was 19 days.
  • a time period required from starting of casting to dispatching (casting ⁇ rolling ⁇ HIC test ⁇ dispatching) was 28 days, which was a long duration.
  • the HIC test after the rolling was able to be omitted, which could significantly shorten the time period from starting of the casting to dispatching, e.g., from 28 days to 19 days.
  • the HIC resistance can be evaluated at the stage of the slab as the cast strip without performing the HIC test after the rolling, thereby making it possible to significantly shorten the manufacturing lead time.
  • the HIC test is used for both the determination of the threshold value for evaluating the HIC resistance of a slab and the confirmation of HIC.
  • the determination method of the present invention has high accuracy.

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EP15873096.0A 2014-12-26 2015-12-22 Stahlplatte mit hervorragender zähigkeit und resistenz gegen wasserstoffinduzierte rissbildung und stahlrohr für ein leitungsrohr Withdrawn EP3239333A4 (de)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN113166896A (zh) * 2018-11-30 2021-07-23 株式会社Posco 抗氢致开裂性优异的压力容器用钢材及其制备方法

Families Citing this family (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2016125139A (ja) * 2014-12-26 2016-07-11 株式会社神戸製鋼所 耐水素誘起割れ性に優れた鋼板およびラインパイプ用鋼管
JP6869151B2 (ja) * 2016-11-16 2021-05-12 株式会社神戸製鋼所 鋼板およびラインパイプ用鋼管並びにその製造方法
WO2018092605A1 (ja) 2016-11-16 2018-05-24 株式会社神戸製鋼所 鋼板およびラインパイプ用鋼管並びにその製造方法
CN109628820A (zh) * 2019-01-10 2019-04-16 石钢京诚装备技术有限公司 一种低磷、低硫抗硫管线钢连铸圆坯的生产方法
CN113046653B (zh) * 2021-02-01 2022-03-01 南京钢铁股份有限公司 一种大线能量焊接性能优异的管线钢及其制造方法
CN115852242B (zh) * 2021-09-24 2024-03-08 宝山钢铁股份有限公司 一种高温高压耐氢腐蚀厚钢板及其制造方法
CN115652183A (zh) * 2022-11-08 2023-01-31 湖南华菱涟源钢铁有限公司 合金结构钢的制备方法和合金结构钢

Family Cites Families (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0941083A (ja) * 1995-07-28 1997-02-10 Nkk Corp 耐ｈｉｃ及び耐ｓｓｃｃ特性に優れた電縫管及びその製造方法
JP5540982B2 (ja) * 2010-08-09 2014-07-02 新日鐵住金株式会社 耐水素誘起割れ性に優れた鋼材
JP5853661B2 (ja) * 2011-12-15 2016-02-09 Ｊｆｅスチール株式会社 高強度耐サワーラインパイプ用鋼板、その素材及び高強度耐サワーラインパイプ用鋼板の製造方法
JP5910195B2 (ja) * 2012-03-14 2016-04-27 Ｊｆｅスチール株式会社 鋼材の耐ｈｉｃ性の評価方法およびそれを利用したラインパイプ用高強度厚鋼板の製造方法
JP2014077642A (ja) * 2012-10-09 2014-05-01 Jfe Steel Corp 鋼材のｈｉｃ感受性の評価方法およびそれを用いた耐ｈｉｃ性に優れたラインパイプ用高強度厚鋼板の製造方法
JP2014173892A (ja) * 2013-03-06 2014-09-22 Kobe Steel Ltd スラブにおける異なる板厚位置でのＣａ濃度分析結果を用いた耐サワー鋼スラブの品質判定方法
JP2014173893A (ja) * 2013-03-06 2014-09-22 Kobe Steel Ltd スラブにおける特定範囲の板厚位置でのＣａ濃度分析結果を用いた耐サワー鋼スラブの品質判定方法
JP6169025B2 (ja) * 2013-03-29 2017-07-26 株式会社神戸製鋼所 耐水素誘起割れ性と靭性に優れた鋼板およびラインパイプ用鋼管
JP6165088B2 (ja) * 2013-03-29 2017-07-19 株式会社神戸製鋼所 耐水素誘起割れ性と溶接熱影響部の靭性に優れた鋼板およびラインパイプ用鋼管
JP6316548B2 (ja) * 2013-07-01 2018-04-25 株式会社神戸製鋼所 耐水素誘起割れ性と靭性に優れた鋼板およびラインパイプ用鋼管
JP6584912B2 (ja) * 2014-12-26 2019-10-02 株式会社神戸製鋼所 耐水素誘起割れ性に優れた鋼板およびラインパイプ用鋼管
JP2016125137A (ja) * 2014-12-26 2016-07-11 株式会社神戸製鋼所 耐水素誘起割れ性に優れた鋼板およびラインパイプ用鋼管
JP2016125139A (ja) * 2014-12-26 2016-07-11 株式会社神戸製鋼所 耐水素誘起割れ性に優れた鋼板およびラインパイプ用鋼管

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN113166896A (zh) * 2018-11-30 2021-07-23 株式会社Posco 抗氢致开裂性优异的压力容器用钢材及其制备方法

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