EP3239319A1 - Stahlplatte mit ausgezeichnetem widerstand gegenüber wasserstoffinduzierter rissbildung und stahlrohr für leitungsrohr - Google Patents

Stahlplatte mit ausgezeichnetem widerstand gegenüber wasserstoffinduzierter rissbildung und stahlrohr für leitungsrohr Download PDF

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EP3239319A1
EP3239319A1 EP15873094.5A EP15873094A EP3239319A1 EP 3239319 A1 EP3239319 A1 EP 3239319A1 EP 15873094 A EP15873094 A EP 15873094A EP 3239319 A1 EP3239319 A1 EP 3239319A1
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concentration
slab
steel plate
steel
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EP15873094.5A
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English (en)
French (fr)
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EP3239319A4 (de
Inventor
Kiichiro TASHIRO
Taku Kato
Yuichi OKA
Shinsuke Sato
Haruya KAWANO
Takashi Miyake
Sei Kimura
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Kobe Steel Ltd
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Kobe Steel Ltd
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Priority claimed from PCT/JP2015/085869 external-priority patent/WO2016104526A1/ja
Publication of EP3239319A1 publication Critical patent/EP3239319A1/de
Publication of EP3239319A4 publication Critical patent/EP3239319A4/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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys 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
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • 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
    • 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
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • 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
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips

Definitions

  • the present invention relates to a steel plate that has excellent hydrogen-induced cracking resistance and is suitable for use in line pipes for transportation and tanks for storage of natural gas, crude oil, and the like.
  • the present invention also relates to a steel pipe for line pipes with excellent hydrogen-induced cracking resistance, 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 of a cast strip and internal cracks, particularly, at an inclusion, such as MnS, as a starting point. For this reason, some techniques for enhancing HIC resistance have been hitherto 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.
  • the conventional method will perform the following procedure A-2, where it takes a long time to perform steps from the casting to re-melting.
  • the steps of "Rolling ⁇ Sample Preparation (for the HIC Test) ⁇ HIC Test" in the procedure A-2 below can be omitted, which enables a quick start of re-melting.
  • 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 assessment result on internal cracks.
  • HCR hot charge rolling
  • Patent Documents 4 to 8 disclose a method for evaluating the quality of a cast strip before rolling, which is not performed to evaluate CaO inclusions.
  • the techniques mentioned in Patent Documents 4 to 7 evaluate the quality of a cast strip, which is based on the content of inclusions or the content of elements in the cast strip or a molten steel in a tundish or-the like.
  • the quality of the cast strip is evaluated (primary determination) from an analytical result of the molten steel in the tundish. If the determination accuracy does not meet the desired accuracy, the quality of the cast strip is evaluated from an analytical 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 elements and the like in the cast strip or the molten steel in the tundish, like Patent Documents 3 to 8.
  • the CaO inclusions are evaluated from the amount of inclusions or elements 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 further to achieve a steel plate and a steel pipe with HIC resistance that can be evaluated by the quality of an internal structure of a cast strip without executing an HIC test.
  • a steel plate having excellent hydrogen-induced cracking resistance 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 casted 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 one of two or more Ca concentrations obtained by examining the Ca concentration at two or more different sites 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 pipe formed of the steel plate.
  • the invention can provide the steel plate and steel pipe that surely have the excellent hydrogen-induced cracking resistance. Further, the present invention can provide the steel plate and steel pipe in-which the HIC resistance can be evaluated by the quality of the internal structure of the cast strip without executing an HIC test.
  • These steel plates are suitable for use in line pipes for transportation of natural gas and crude oil, pressure containers, such as the storage tank, and the like.
  • the inventors have intensively studied to solve the foregoing problems.
  • 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 in such an element is found to efficiently exhibit the desulfurization effect as mentioned later.
  • the inventors have focused on the tendency for HIC to occur at a CaO accumulation zone generated during producing a cast strip. Consequently, attention is paid to 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" that can evaluate the presence or absence of the CaO accumulation zone. 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 below.
  • the component 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 component composition of the steel plate needs to be as follows. The reasons for specifying the contents of the respective components, including the aforesaid rare earth elements 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 plate. 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 to significantly degrade the hydrogen-induced cracking resistance.when contained in a large amount.
  • 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 desirable 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 0.005% or less, more preferably 0.0035% or less, and still more preferably 0.0025% or less.
  • N Nitrogen
  • the N content needs to bye 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, 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.
  • Ca-based oxysulfide To avoid the occurrence of HIC due to a Ca-based oxysulfide, it is effective to suppress, especially, CaO that is the most likely to form aggregates among Ca-based inclusions. For this reason, 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. When the Ca content (Ca-1.25S) is 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 even more preferably 0.0050% or less.
  • REM means lanthanoid elements (15 elements from La to Lu), scandium (Sc), and yttrium (Y).
  • Zirconium (Zr) 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, more preferably 0.0005% or more, still more preferably 0.0010% or more, and still more preferably 0.0015% or more.
  • any excessive Zr content forms coarse inclusions to degrade the hydrogen-induced cracking resistance and the toughness of the 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 needs to be 0.01% or more.
  • the Ni content is more preferably 0.05% or more, and still more preferably 0.10% or more.
  • 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 further 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 still 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 with 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.
  • the threshold value Ca drop ⁇ means the maximum decrease in the amount of Ca previously determined and which does not cause 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 as will be mentioned later.
  • the reason for setting the above-mentioned 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 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. CaO as the aggregate becomes a coarse inclusion, causing HIC.
  • the coarsen CaO inclusion has a smaller density than the molten steel, and 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 serves as the starting point of HIC.
  • 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 position of the CaO accumulation zone cannot be predicted.
  • the inventors have changed their viewpoints about examination positions for the Ca concentration and focused on the position with a low Ca concentration.
  • the Ca concentration at the CaO accumulation zone becomes high, while in a position where no CaO accumulation zone occurs, the CaO concentration becomes relatively low.
  • the inventors have examined the relationship between the "Ca concentration in any 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 present invention uses 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 relates to the presence or absence of the CaO accumulation zone, to thereby evaluate the HIC resistance.
  • a threshold value Ca drop ⁇ of the decrease in the amount of Ca i.e., the maximum decrease in the amount of Ca that does not cause 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 in advance, but a method for determination thereof is not particularly limited to the following method.
  • An example of the method for determining the threshold value Ca drop ⁇ in advance 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 CaO inclusions is smaller than that of the molten steel, so that the CaO inclusions in the molten steel floats while receiving buoyant force due to a difference in the 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 zone.
  • 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 as mentioned above 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.
  • the temperature at any 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 slab 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.
  • An example of the HIC test is 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 opposite-reference-side region.
  • 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 41 except for the vicinity of the center in the thickness direction.
  • a threshold value Ca drop ⁇ for the decrease in the amount of Ca that does not cause 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 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 a molten steel in the tundish is examined.
  • samples are taken out of two or more sites 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 leads to the determination that no CaO accumulation zone occurs with no HIC occurring. However, in practice, some accumulation zones are generated, which can cause HIC.
  • the Ca concentration in the slab is examined at different two or more sites of the slab in its thickness direction.
  • the CaO accumulation zone is present in a specific position in the thickness direction that depends 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. However, 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 to precisely evaluate the presence or absence of the CaO accumulation zone.
  • 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 position 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 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".
  • An example of the HIC test is a method specified by the NACE standard TM0284-2003, as mentioned in examples below.
  • a threshold value Ca drop ⁇ for the decrease in the amount of Ca that does not cause 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 sites in the thickness direction of the slab casted 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 casted when the casting condition is varied, for example, a part casted at an initial stage of casting, such as when the casting speed increases, or a part casted 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 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 the above-mentioned Ca drop ⁇ threshold value Ca drop ⁇ , and that no CaO accumulation zone is generated in the slab, resulting in no occurrence of HIC.
  • 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 quality of the internal structure (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.
  • 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 thickness D of 280 mm and a 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 cast 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 time from directly under the mold to a final roll of a continuous casting machine [1/min.]/(weight of cast strip production per unit time [kg/min.])
  • the casting speed is a drawing speed of the cast strip [m/min.], and 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 1000°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 less 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 the 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 a 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 "threshold value of the decrease in the amount of Ca” is determined based on all products, regardless of the strength grade. This is because the easiness of occurrence of HIC due to coarse CaO is not related to the strength grade of products.
  • the steel with the component composition shown in Table 5 was melted and subjected to continuous casting, thereby producing a slab having a thickness D of 280mm and a 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 section (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 occurred due to the CaO, that is, the HIC resistance was rated as NG. These results are shown in Table 6.
  • 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 x 2000 to 3500 mm in width x 12000 to 35000 mm in length) with various component compositions were produced.
  • TMCP hot-rolling and cooling methods
  • the "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; and then 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 and 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 "QT” was a method that involved: hot-rolling, 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.
  • Tables 5 and 6 show the following steel types Nos. 1 to 7 and 10 to 16 satisfied the specified component composition 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.
  • 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 conducting 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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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
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
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
JP6211296B2 (ja) * 2013-04-30 2017-10-11 株式会社神戸製鋼所 耐サワー性とｈａｚ靭性に優れた鋼板
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

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111220614A (zh) * 2018-11-27 2020-06-02 宝山钢铁股份有限公司 一种快速评估钢水质量的方法

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