Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • 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
• • 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
  • 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
  • C21D9/14—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes wear-resistant or pressure-resistant pipes
• • 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
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—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
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the present invention relates to a steel plate with excellent sour resistance, HAZ toughness and HAZ hardness which is suitable as a structural component for energy field such as a line pipe and a marine structure, and a steel pipe for a line pipe manufactured using the steel plate.
• Steel for a line pipe is one of the steel material for energy field and is used for transportation of oil and natural gas (LNG), and not only the mechanical properties (strength, toughness) as a structural component but also corrosion resistance to oil and natural gas passing through the pipe is required for the steel.
• LNG oil and natural gas
• HIC resistance hydrogen-induced cracking resistance
• the steel plate of the steel for a line pipe and the like is used as a welded structural component.
• the weakest part in terms of the material of the welded structure is the heat-affected zone (HAZ) in the vicinity of the weld metal, it is required to secure the toughness of the portion, and, on the other hand, from the viewpoint of the welding efficiency, there is a constant demand of improving weldability. Namely, both of securing the HAZ toughness and weldability have been required.
• Patent Literature 1 As a related art that achieved these sour resistance and securing HAZ toughness, Patent Literature 1 and the like can be cited.
• Patent Literature 2 As one for improving the average value and minimum value of the HAZ toughness even in a large heat input welding condition by introducing inclusions and controlling the composition of inclusions, Patent Literature 2 and the like can be cited.
• line pipes are manufactured by bending a thick steel plate for a line pipe into a tubular shape and welding both edges.
• the line pipes thus manufactured are joined by welding the pipes each other, and are used as an actual oil transportation line.
• the T-cross weld joint that receives two kinds of thermal histories of seam welding in working a thick steel plate into a pipe and girth welding in joining pipes with each other is subjected to complicated thermal histories such as rapid heating and rapid cooling, the strength (hardness) increases and a cracking called sulfide stress corrosion cracking (SSCC) is liable to occur in the HAZ. Therefore, in the steel for a line pipe, it is required that the SSCC resistance of the T-cross weld joint is also secured in addition to the HIC resistance (sour resistance) of the base plate described above.
• HIC resistance strip resistance
• Patent Literature 3 As related arts taking improvement of the SSCC resistance into consideration, technologies described in Patent Literature 3 and Patent Literature 4 can be cited.
• the technology described in Patent Literature 3 is a technology of utilizing precipitation strengthening by fine Nb and V carbonitride and achieving high strength of 56 kgf/mm 2 or more of the tensile strength.
• the HIC resistance of the base plate is not taken into consideration, and only the HAZ of seam welding is taken into consideration with respect to the SSCC resistance.
• the immersion time into a solution that simulates the sour environment is 21 days which is not a sufficiently severe test condition.
• Patent Literature 4 such a composition as suppressing increase of the hardness which is deemed to deteriorate the SSCC resistance of the T-cross weld joint is shown.
• the SSCC resistance itself is not evaluated, and the HIC resistance of the base plate also is not taken into consideration.
• the object (assignment) of the present invention is to provide a steel plate satisfying three of the sour resistance, HAZ toughness and reduction of the dispersion of the HAZ hardness simultaneously, and suitable to a steel material for energy field such as a line pipe use having high yield strength and tensile strength, and a steel plate for a line pipe manufactured using the steel plate.
• the present invention provides a steel plate with excellent sour resistance, HAZ toughness and HAZ hardness including, in terms of mass%:
• the steel plate may further include at least one element selected from a group consisting of:
• Zr amount is 1-40%
• REM amount is 5-50%
• Al amount is 3-30%
• Ca amount is 5-60%
• S amount is over 0% and less than 20%.
• the steel plate may further include at least one element selected from a group consisting of:
• the steel plate may be for use of a line pipe.
• the present invention provides a steel pipe for a line pipe manufactured using the steel plate described above.
• a steel plate that is excellent in the sour resistance such as the hydrogen-induced cracking resistance, has the excellent HAZ toughness and HAZ hardness even in a large heat input welding condition, simultaneously satisfies three of the sour resistance, HAZ toughness and reduction of the dispersion of the HAZ hardness, and can be adaptable advantageously as a steel material for energy field such as a line pipe use and a marine structure use having a high functional property of high yield strength and tensile strength, and a steel pipe for a line pipe manufactured using the steel plate.
• the present inventors repeated intensive researches and studies from the viewpoint of controlling the inclusions in steel in addition to the componential composition of the steel that became the basis in exerting the property of a steel plate. As a result, it was found out that an excellent steel plate was obtained which simultaneously satisfied all properties of the sour resistance, HAZ toughness and HAZ hardness described above by holding the coarse inclusions with 1 ⁇ m or more width in a predetermined componential composition, and the present invention has been completed based on the knowledge.
• the sour resistance of steel could be improved and secured by convertingly preparing the coarse inclusions of 1 ⁇ m or more which became a cause of this hydrogen-induced cracking from inclusions having a larger coefficient of thermal expansion than that of steel into inclusions having a smaller coefficient of thermal expansion than that of steel. Also, as the inclusions having a smaller coefficient of thermal expansion than that of steel, in concrete terms, oxides of Zr, Al, REM and the like are effective.
• the intragranular acicular ⁇ originated from the inclusions came to grow actively even in the T-cross weld joint, and the SSCC resistance of the T-cross weld joint was also improved by the formation of the fine microstructure.
• % which is an expression unit of the composition means mass%.
• the percentage (mass%) based on the mass is same with the percentage (wt%) based on the weight.
• X% or less may be expressed as "over 0% and X% or less”.
• C is an indispensable element for securing the quenchability of the HAZ part, and should be contained by 0.02% or more.
• C amount is preferably 0.03% or more, and more preferably 0.05% or more.
• C amount should be 0.20% or less.
• C amount is preferably 0.15% or less, and more preferably 0.12% or less.
• Si is effective in deoxidation. In order to secure such an effect, Si amount is made 0.02% or more. Si amount is preferably 0.05% or more, and more preferably 0.15% or more. However, when Si amount is excessive, island martensite is liable to be formed, and the HAZ toughness deteriorates. Therefore, Si amount should be suppressed to 0.50% or less. Si amount is preferably 0.45% or less, and more preferably 0.35% or less.
• Mn is an element effective in securing the quenchability of the HAZ part, and is contained by 0.6% or more in the present invention.
• Mn amount is preferably 0.8% or more, and more preferably 1.0% or more.
• the upper limit of Mn amount is made 2.0%.
• Mn amount is preferably 1.8% or less, and more preferably 1.6% or less.
• P is an element inevitably included in steel material.
• P amount exceeds 0.030%, deterioration of the HAZ is extreme, and the hydrogen-induced cracking resistance also deteriorates. Therefore, in the present invention, P amount is suppressed to 0.030% or less.
• P amount is preferably 0.020% or less, and more preferably 0.010% or less.
• S amount is preferably 0.003% or less, more preferably 0.0025% or less, and still more preferably 0.0020% or less.
• S amount is preferable to be as little as possible.
• the lower limit of S amount is approximately 0.0001%.
• Al is an element effective in reducing the voids against the matrix phase of steel by reducing the coefficient of thermal expansion of inclusions, and securing the sour resistance. Also, Al is effective in lowering the melting point of the inclusions to increase the forming rate of the intragranular acicular ⁇ , securing the HAZ toughness, and reducing the hardness slope from coarse grains to fine grains. In order to exert the effect, Al should be made 0.010% or more. Al amount is preferably 0.020% or more, and more preferably 0.030% or more.
• Al amount should be 0.08% or less.
• Al amount is preferably 0.06% or less, and more preferably 0.05% or less.
• N is an element precipitating as TiN in the steel microstructure, suppressing coarsening of the austenitic grain of the HAZ part, promoting the ferritic transformation, and improving the toughness of the HAZ part.
• N should be contained by 0.001% or more.
• N amount is preferably 0.003% or more, and more preferably 0.0040% or more.
• N amount is preferably 0.008% or less, and more preferably 0.0060% or less.
• Nb is an element effective in increasing the strength without deteriorating the weldability. In order to secure this effect, Nb amount should be 0.002% or more. Nb amount is preferably 0.010% or more, and more preferably 0.020% or more. However, when Nb amount becomes 0.05% or more, the toughness of HAZ deteriorates. Therefore, in the present invention, Nb amount is made less than 0.05%. Nb amount is preferably 0.040% or less, and more preferably 0.030% or less.
• O oxygen
• O amount should be 0.0040% or less, is preferably 0.0030% or less, and more preferably 0.0020% or less.
• REM Radar Earth Metal
• REM is effective in reducing the voids against the matrix phase of steel by reducing the coefficient of thermal expansion of inclusions, and securing the sour resistance.
• REM is effective in lowering the melting point of the inclusions to increase the forming rate of the intragranular acicular ⁇ , securing the HAZ toughness, and reducing the hardness slope from coarse grains to fine grains.
• REM should be contained by 0.0002% or more.
• REM amount is preferably 0.0005% or more, and more preferably 0.0010% or more.
• the upper limit of REM amount is made 0.05%. From the viewpoint of suppressing blockage of the immersion nozzle in casting and improving the productivity, REM amount is preferably 0.03% or less, more preferably 0.010% or less, and still more preferably 0.0050% or less.
• the REM means the lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).
• Zr is effective in reducing the voids against the matrix phase of steel by reducing the coefficient of thermal expansion of inclusions, and securing the sour resistance. Also, Zr is effective in lowering the melting point of the inclusions to increase the forming rate of the intragranular acicular ⁇ , securing the HAZ toughness, and reducing the hardness slope from coarse grains to fine grains.
• Zr amount should be 0.0003% or more. Zr amount is preferably 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0015% or more.
• Zr amount should be 0.020% or less.
• Zr amount is preferably 0.010% or less, more preferably 0.0070% or less, and still more preferably 0.0050% or less.
• the componential composition of the steel of the steel plate of the present invention is as described above, and the remainder is iron and inevitable impurities. Also, by further containing at least one element selected from a group consisting of Ca, Mg, Ti, B, V, Cu, Ni, Cr, and Mo of the amount described below in addition to the elements described above, the HAZ toughness can be improved, the strength can be improved, and so on. Below, these elements will be explained.
• Ca has an action of forming CaS and finely dispersing sulfides.
• Ca amount should be 0.0003% or more.
• Ca amount is preferably 0.0005% or more, and more preferably 0.0010% or more.
• the upper limit of Ca amount is made 0.0060%.
• Ca amount is preferably 0.0050% or less, and more preferably 0.0040% or less.
• Mg has an action of forming MgS and finely dispersing sulfides. In order to secure this effect, it is preferable to contain Mg by 0.0003% or more. Mg amount is more preferably 0.001% or more. On the other hand, even when Mg is contained so as to exceed 0.005%, the effect saturates, and therefore the upper limit of Mg amount is preferably 0.005%. Mg amount is more preferably 0.0030% or less.
• Ti is an element required for improving the toughness of the HAZ part because Ti prevents coarsening of the austenitic grains and promotes ferritic transformation in the HAZ part at the time of welding by precipitating as TiN in steel. In order to secure such effects, it is preferable to contain Ti by 0.003% or more. Ti amount is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, when Ti content becomes excessive, the amount of solute Ti and the number of TiC precipitates increase, the HAZ toughness deteriorates, and therefore 0.03% or less is preferable. Ti amount is more preferably 0.02% or less.
• B enhances the quenchability, and therefore improves the HAZ toughness.
• B amount is more preferably 0.0005% or more, and still more preferably 0.0010% or more.
• B content is preferably 0.005% or less.
• B amount is more preferably 0.004% or less, and still more preferably 0.003% or less.
• V is an element effective in improving the strength, and, in order to secure this effect, it is preferable to contain V by 0.003% or more.
• V amount is more preferably 0.010% or more.
• V content exceeds 0.1%, the weldability deteriorates. Therefore, V amount is preferably 0.1% or less, and more preferably 0.08% or less.
• Cu is an element effective in improving the quenchability and increasing the strength. In order to secure these effects, it is preferable to contain Cu by 0.01% or more. Cu amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, because the strength is increased excessively and the toughness deteriorates when Cu content exceeds 1.5%, 1.5% or less is preferable. Cu amount is more preferably 1.0% or less, and still more preferably 0.50% or less.
• Ni is an element effective in improving the strength of the base plate and the HAZ toughness. In order to secure the effect, it is preferable to make Ni amount 0.01% or more. Ni amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when Ni is contained excessively, the cost increases extremely as a structural steel material, and therefore it is preferable to make Ni amount 1.5% or less from the economical viewpoint. Ni amount is more preferably 1.0% or less, and still more preferably 0.50% or less.
• Cr is an element effective in improving the strength, and, in order to secure this effect, it is preferable to contain Cr by 0.01% or more. Cr amount is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when Cr amount exceeds 1.5%, the HAZ toughness deteriorates. Therefore it is preferable to make Cr amount 1.5% or less. Cr amount is more preferably 1.0% or less, and still more preferably 0.50% or less.
• Mo is an element effective in improving the strength of the base plate. In order to secure the effect, it is preferable to make Mo amount 0.01% or more. Mo amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when Mo amount exceeds 1.5%, the HAZ toughness and weldability deteriorate. Therefore, Mo amount is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.50% or less.
• This item is to determine the Nb-Di balance namely the relation between the Nb amount and the Di value (quenchability: Expression 2), and above Expression 1 should be satisfied. All of the compositions within [ ] are in mass%. Also, above Expression 2 on the quenchability Di value is one that is described as the Grossmann Expression (Trans. Metall. Soc. AIME, 150 (1942), p.227).
• Nb-Di balance controls the steel composition so as to satisfy above Expression 1, higher strength (yield strength, tensile strength) of the base plate can be secured by accelerated cooling, and a steel plate having smaller slope of the HAZ hardness and excellent in the hydrogen-induced cracking resistance can be obtained.
• Nb-Di balance does not particularly become an issue with respect to the properties specified in the present invention, from the viewpoint of the tolerance in controlling the property of the base plate, preferable range of the Nb-Di balance is [200 ⁇ 10,000 ⁇ [Nb]+31 ⁇ Di ⁇ 300].
• Zr oxide has smaller coefficient of thermal expansion than that of the steel, when Zr amount in inclusions is secured, the voids against the surrounding matrix phase of steel can be reduced, which can function effectively in securing the sour resistance. Also, Zr oxide is effective in lowering the melting point of inclusions, increasing the intragranular forming rate, improving the HAZ toughness, and reducing the dispersion of the HAZ hardness. In order to exert such effects, Zr amount in inclusions is made 1-40%. When Zr amount is less than 1%, the sour resistance and/or HAZ toughness and reduction of the dispersion of the HAZ hardness become insufficient.
• REM oxide has smaller coefficient of thermal expansion than that of the steel, when REM amount in inclusions is secured, the voids against the surrounding matrix phase of steel can be reduced and S can be fixed and finely dispersed, which can function effectively in securing the sour resistance. Also, REM oxide is effective in lowering the melting point of inclusions, increasing the intragranular forming rate, improving the HAZ toughness, and reducing the dispersion of the HAZ hardness. In order to exert such effects, REM amount in inclusions is made 5-50%. When REM amount is less than 5%, the sour resistance and/or HAZ toughness and reduction of the dispersion of the HAZ hardness become insufficient.
• Al oxide has smaller coefficient of thermal expansion than that of the steel, when Al amount in inclusions is secured, the voids against the surrounding matrix phase of steel can be reduced, which can function effectively in securing the sour resistance. Also, Al oxide is effective in lowering the melting point of inclusions, increasing the intragranular forming rate, improving the HAZ toughness, and reducing the dispersion of the HAZ hardness. In order to exert such effects, Al amount in inclusions is made 3-30%. When Al amount is less than 3%, the sour resistance and/or HAZ toughness and reduction of the dispersion of the HAZ hardness become insufficient.
• the steel plate of the present invention contains Ca
• the Ca amount in inclusions a predetermined range
• intragranular acicular ⁇ originated from inclusions comes to be formed vigorously even in the T-cross weld joint, and the SSCC resistance of the T-cross weld joint is improved by the formation of the fine microstructure.
• Ca amount in inclusions is made 5-60%.
• Ca amount is less than 5% or exceeds 60%, the SSCC resistance of the T-cross weld joint cannot be improved.
• the Fe concentration in the slag is made 0.1% or more.
• the Fe concentration in the slag is preferably 0.5% or more, and more preferably 1.0% or more.
• the Fe concentration in the slag exceeds 10%, oxides are formed excessively, the oxides not only become the origin of the hydrogen-induced cracking but also deteriorate the toughness of the base plate and the HAZ. Therefore, the Fe concentration in the slag is made 10% or less.
• the Fe concentration in the slag is preferably 8% or less, and more preferably 5% or less.
• CaS can be prevented from being formed excessively when Ca is added after adding REM, the composition of inclusions can be prevented from deviating from a predetermined range, and thereby the HIC resistance and the SSCC resistance can be secured.
• the dissolved oxygen concentration Of of the molten steel is made 10 or less in terms of the ratio relative to the S concentration of the molten steel (Of/S).
• the REM forms oxides at the same time of forming the sulfides thereof.
• Of/S is made 10 or less as described above.
• Of/S is preferably 5 or less, more preferably 3.5 or less, and still more preferably 2.0 or less.
• the lower limit value of Of/S is approximately 0.1.
• Al is added first, and (Zr, REM) are added then.
• (Zr, REM) are added then.
• the adding order should be made Al ⁇ (Zr, REM).
• the desulfurizing capacity of REM and Ca is compared, the desulfurizing power of REM is weaker than that of Ca, therefore, if Ca is added before adding REM, a large amount of CaS is formed, the composition of the inclusions deviates from the predetermined range, and thereby the sour resistance is deteriorated.
• the adding order of Al, Zr, REM and Ca should be Al ⁇ (Zr, REM) ⁇ Ca.
• the time of adding REM and the time of adding Ca should be apart from each other by 4 min or more.
• the time after adding REM until adding Ca is preferably 5 min or more, and more preferably 8 min or more. Further, from the viewpoint of the productivity, the upper limit of the time after adding REM until adding Ca becomes approximately 60 min.
• the deoxidizing capacity of Zr, REM and Ca when the deoxidizing capacity of Zr, REM and Ca is compared, it is considered in general that the deoxidizing power of Ca is strongest and the deoxidizing power is in the order of Ca>REM>Zr, and Zr is lowest. Therefore, in order to contain Zr in the inclusions (namely to form ZrO 2 as the oxide-based inclusions), Zr should be added prior to adding Ca and REM whose deoxidizing power is stronger than that of Zr. Therefore, the adding order of Al, Zr, REM and Ca should be Al ⁇ Zr ⁇ REM ⁇ Ca. However, because the deoxidizing capacity of REM is smaller compared to Ca, even if REM is added simultaneously with Zr, Zr can be contained in the inclusions, and therefore the adding order of them may also be Al ⁇ (Zr, REM) ⁇ Ca.
• the steel plate having each desired element amount only has to be obtained, and such a method can be cited for example to add Zr so as to become 3-100 ppm in terms of the concentration in the molten steel, to add REM thereafter or simultaneously so as to become 2-500 ppm in terms of the concentration in the molten steel, and, after 4 min or more elapses thereafter, to add Ca so as to become 3-60 ppm in terms of the concentration in the molten steel.
• the time after adding Ca until completion of solidification is made 200 min or less, preferably 180 min or less, and more preferably 160 min or less.
• the lower limit of the above time is approximately 4 min from the viewpoint of homogenizing Ca.
• the cooling time is important to make the cooling time of 1,300°C-1,200°C at the time of casting 270-460 s.
• the cooling time exceeds the upper limit, complex formation of the secondary inclusions mainly of the sulfide-basis over the inclusions is promoted, the composition of the inclusions deviates from the predetermined range, and the difference of the HAZ hardness thereby deviates from a predetermined range.
• the cooling time becomes less than the lower limit thereof, the cooling load significantly increases, which is not preferable practically.
• the steel plate thin steel plate
• the steel pipe for a line pipe can be manufactured by a method generally executed.
• the steps of rolling and onward are not particularly limited, it is preferable for example to heat a casted slab to 1,100°C or above, to execute hot rolling with the compression reduction of 40% or more at the recrystallization temperature, and to cool it (accelerated cooling) from 780°C with the cooling rate of 10-20°C/s. The conditioning thereafter is not necessary.
• the molten steel melted by an ordinary method using a 240 t converter was subjected to processing (molten steel processing) such as desulfurizing, deoxidizing, composition regulating, and inclusions controlling using an LF furnace, various kinds of molten steel having the steel composition and the composition of the inclusions in steel shown in Tables 1, 2, 9, 10 (invention examples) and Tables 3, 4, 9, 10 (comparative examples) were made slabs by the continuous casting method, the slabs were subjected to accelerated cooling after hot rolling, and steel plates (thick steel plates) with 40 mm thickness and 3,500 mm width were manufactured. Further, in Tables 2, 10 (invention examples) and Tables 4, 10 (comparative examples), the composition of the coarse inclusions in steel is also shown.
• Tables 5, 11 (invention examples) and Tables 6, 11 (comparative examples) show the main process conditions in the molten steel processing, continuous casting, and accelerated cooling described above.
• Tables 7, 12 (invention examples) and Tables 8, 12 (comparative examples) show the various properties of each steel plate thus obtained.
• the cross section in the plate thickness direction of the as-rolled material was observed focusing the plate thickness center part using EPMA-8705 made by Shimadzu Corporation.
• three cross sections were observed with 400 observation magnification and approximately 50 mm 2 field of view (7 mm in the plate thickness direction and 7 mm in the plate width direction so that the plate thickness center part became the center of the field of view), and the componential composition at the inclusion center part was quantitatively analyzed by wave length dispersion spectrometry of the characteristic X-ray for the inclusions with 1 ⁇ m or more width.
• the elements of the analyzing object were made Al, Mn, Si, Mg, Ca, Ti, Zr, S, REM (La, Ce, Nd, Dy, Y), and Nb.
• the relation between the X-ray strength and the element concentration of each element was obtained beforehand as the analytical curve using known matters, and the element concentration of the inclusions was determined then from the X-ray strength obtained from the inclusions and the analytical curve described above.
• No. 4 specimen of JIS Z 2241 was taken in parallel with C direction from the t/4 position (t: plate thickness) of each steel plate, the tensile test was executed by the method described in JIS Z 2241, and the tensile strength TS and the yield strength YS were measured.
• those with 415 MPa or more of YS and 520 MPa or more of TS were evaluated to be excellent (passed) in the base plate strength, and those with less than 415 MPa ofYS and less than 520 MPa of TS were evaluated to be inferior (failed) in the base plate strength.
• test and evaluation were executed according to the method defined in NACE Standard TM 0284-2003.
• the specimen was immersed for 96 hours in the 25°C (0.5% NaCl+0.5% acetic acid) aqueous solution saturated with 1 atm hydrogen sulfide.
• each specimen was cut at 10 mm pitch in the longitudinal direction, the cut surface was polished, all cross sections were thereafter observed with 100 magnifications using an optical microscope, the number of piece of the cracking with 200 ⁇ m or more of the cracking length of HIC and the number of piece of the cracking with 1 mm or more were measured respectively
• those without the cracking with 1 mm or more of the cracking length of HIC described above were evaluated to be excellent (passed) in the HIC resistance, and the case in which one or more of the cracking with 1 mm or more existed was evaluated to be inferior (failed) in the HIC resistance.
• the specimen was applied with the deflection equivalent to 332 MPa and 374 MPa of the load stress and was immersed for 720 hours in the NACE solution A (5 mass% NaCl-0.5 mass% CH 3 COOH) saturated with 1 atm hydrogen sulfide, and those in which the cracking did not occur thereafter on the surface of the specimen were evaluated to have passed.
• NACE solution A (5 mass% NaCl-0.5 mass% CH 3 COOH) saturated with 1 atm hydrogen sulfide

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