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
• • 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • 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
  • 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 thick steel plate for a tank, excellent in toughness of the weld heat affected zone (Heat Affected Zone; hereinafter, sometimes simply referred to as HAZ).
• HAZ weld heat affected zone
• thermo-mechanical control process (hereinafter, sometimes simply referred to as TMCP) utilizing a rolling-accelerated cooling system to a thick steel plate for tanks used in the fields of an energy storage facility, a chemical plant, a power generation facility, a nuclear reactor pressure vessel, etc.
• TMCP thermo-mechanical control process
• SA-537 steel manufactured through a heat treatment such as annealing, quenching and tempering
• TMCP-type ASTM SA-841 manufactured by omitting the heat treatment above has been recently standardized.
• Patent Document 1 discloses a technique where in an oxide having a particle size of 0.2 to 5.0 ⁇ m and exerting an effect as an intragranular ferrite transformation nucleus, the proportions of Ti, Mg and Al constituting the oxide are specified so as to achieve excellent HAZ toughness particularly when the heat input is 200 kJ/cm or more, e.g., even under an ultra-large heat-input of about 1,500 kJ/cm.
• the toughness at 0°C under heat inputs of 200 kJ/cm and 920 kJ/cm is evaluated.
• Patent Document 2 discloses a technique where based on the finding that an Mn sulfide containing from 1 to 49 at% of Ca and having a particle size of 0.1 to 10 ⁇ m significantly promotes generation of intragranular ferrite transformation, the number density of the Mn sulfide is specified to enhance the HAZ toughness in ultra-large heat-input welding.
• the toughness at 0°C under a heat input of 20 kJ/cm is evaluated.
• Patent Document 3 discloses the finding that a finely dispersed particle produced by combining an oxide, etc. of Mn and formed among dendrite secondary arms refined by the addition of REM contributes to preventing an austenite grain from coarsening even in the HAZ part of ultra-large heat-input welding of 300 kJ/cm or more and the HAZ toughness is enhanced.
• the toughness at 40°C under a heat input of 300 to 1,200 kJ/cm is evaluated.
• Patent Document 4 discloses a technique where the average composition, particle size and dispersion degree of an Al-Mn oxide as an austenite pinning particle in HAZ are controlled and the toughness of large heat-input HAZ part in excess of 200 kJ/cm is thereby increased. In Examples of Patent Document 4, the toughness at 0°C under a heat input of 200 kJ/cm is evaluated.
• Patent Document 5 discloses a technique of controlling the composition of an oxide to an REM-Zr-based composite oxide containing REM and Zr.
• Patent Document 5 is a technique utilizing an REM-Zr-based composite oxide and in this point, differs from Patent Documents 1 to 4 utilizing an oxide other than that.
• Zr is not contained in the steel or REM and Zr are not contained in the steel, and thus, the composite oxide above is therefore not formed.
• Patent Document 5 discloses a technique where production of intragranular ferrite is ensured by appropriately controlling the composition of an REM-Zr-based composite oxide and where production of coarse Ti nitride that has been conventionally crystallized from an oxide as a starting point in molten steel is suppressed and production of coarse grain boundary ferrite is suppressed by adding a predetermined amount of B.
• the HAZ toughness at -40°C when welded with a heat input of 50 kJ/mm or 60 kJ/mm is evaluated.
• Patent Documents 1 to 5 relate to techniques for enhancing the HAZ toughness in the t/4 part at temperatures of down to -40°C when performing large heat-input welding where the heat input amount is approximately from several tens to hundred and tens of kJ/mm.
• the thick steel plate for tanks is required to have good HAZ toughness also when the heat input amount is lower than the above value(s), for example, when small heat-input welding of approximately from 5 to 25 kJ/mm is performed.
• Patent Document 5 is a technique of adding B as an essential component, where the N amount and the Ti amount must be controlled between the deoxidation/desulfurization process and casting process so as to control the B amount. Furthermore, the cleanliness of the steel needs to be maintained by setting the addition amount of Al to be relatively low. As a result, a large load is imposed on the steelmaking step such as deoxidation/desulfurization, or the tolerance for the steelmaking control conditions is low. Thus, there is room for improvement toward mass production.
• the present invention has been made by taking into account these circumstances, and an object thereof is to provide a thick steel plate for a tank, capable of being manufactured under a condition with a relatively low load on the steelmaking step, such as deoxidation, desulfurization and nitrogen control, when, for example, small heat-input welding of approximately from 5 to 25 kJ/mm is performed, and having excellent HAZ toughness at a very low temperature such as -51°C.
• the steel plate further comprises V: more than 0% and 0.1 % or less.
• the steel plate further comprises at least one element selected from the group consisting of Cu: more than 0% and 0.50% or less; Ni: more than 0% and 0.85% or less; Cr: more than 0% and 0.30% or less; and Mo: more than 0% and 0.5% or less.
• a thick steel plate for a tank excellent in HAZ toughness at a very low temperature, for example, at -51 °C, can be provided.
• a high-strength thick steel plate being excellent in the cryogenic HAZ toughness as described above and having a tensile strength of 485 MPa or more can be efficiently obtained without a load on the steelmaking step such as deoxidation, desulfurization and nitrogen control and therefore, the productivity, etc. are also very excellent.
• Patent Document 5 relates to a technique for enhancing the HAZ toughness in large heat-input welding (for example, 60 kJ/mm), and the prior austenite grain size is likely to be coarsened there.
• Patent Document 5 for example, B is added so as to suppress production of coarse grain boundary ferrite and improve the HAZ toughness.
• the present invention relates to a technique for enhancing the HAZ toughness in small heat-input welding where the heat input amount is small (for example, 20 kJ/mm) as compared with Patent Document 5, and the prior austenite grain size is less coarsened than in the case of Patent Document 5. Accordingly, the present invention is based on the premise that B as an indispensable element (an element for suppressing production of coarse grain boundary ferrite and improving HAZ toughness) in Patent Document 5 is not added.
• the present inventors have made studies to provide a technique for enhancing the HAZ toughness in small heat-input welding, where the number density of coarse TiN can be reduced, similarly to the case of Patent Document 5, by utilizing an REM-Zr-based composite oxide and the number density of fine TiN can be increased without addition of B, unlike the case of Patent Document 5.
• the evaluation of HAZ toughness is performed at a lower temperature of -51°C than the case of Patent Document above, and in this case, an adverse effect of a coarse inclusion such as Ti-containing nitrogen on the HAZ toughness is likely to emerge.
• control is performed as in the following (a) to (c)
• the desired object can be attained, and the present invention has been accomplished based on this finding.
• both the composition of REM-Zr-based composite oxide and the number density of Ti-containing nitrides are a value in the central part (t/2 part) of the thickness t of the steel plate.
• coarse TiN, etc. are readily formed in the central part of a steel plate and therefore, in many related arts including the above-described Patent Documents, the oxide composition, number density of Ti-containing nitrides, and the like are evaluated in the t/4 part of the steel plate.
• the present invention is very useful in the point of not containing B and in that the oxide composition and the number density of Ti-containing nitrides in the central part of steel plate, where these have been difficult to control, can be controlled.
• the "excellent cryogenic HAZ toughness" means that when a Charpy impact test is performed by the method described in Examples later and the absorption energy at -51 °C is measured, the minimum value is 48 J or more. In the following, this is sometimes simply referred as excellent HAZ toughness.
• the thick steel plate means a steel plate in which the thickness is roughly 6 mm or more and is preferably 150 mm or less.
• Ti-containing nitrides having a major axis length of 2 ⁇ m or more are sometimes simply referred to as “coarse Ti-containing nitrides”
• Ti-containing nitrides having an equivalent-circle diameter of 20 nm or more and 500 nm or less are sometimes simply referred to as "fine Ti-containing nitrides”.
• the thick steel plate for tanks in the present invention contains C: from 0.02 to 0.15%, Si: from 0.05 to 0.5%, Mn: from 0.6 to 2.0%, P: more than 0% and 0.030% or less, S: more than 0% and 0.025% or less, Al: from 0.02 to 0.07%, Nb: 0.005% or more and less than 0.050%, Ti: from 0.003 to 0.03%, N: from 0.0020 to 0.010%, O: more than 0% and 0.0040% or less, REM: from 0.0002 to 0.050%, Zr: from 0.0003 to 0.020%, and Ca: from 0.0003 to 0.0060%, with the remainder being iron and inevitable impurities, and [Ti] ⁇ [N] ⁇ 0.000085 is satisfied, wherein [Ti] is the Ti content and [N] is the N content.
• C is an indispensable element for ensuring the strength by enhancing the hardenability, and for this purpose, as the lower limit of the C amount, it is set to be 0.02% or more.
• the amount is preferably 0.03% or more, more preferably 0.05% or more.
• the upper limit of the C amount it is set to be 0.15% or less.
• the amount is preferably 0.12% or less, more preferably 0.10% or less.
• Si from 0.05 to 0.5%
• Si is an element contributing to reduction of the load in the steelmaking (deoxidation, desulfurization, nitrogen control) step and being useful for increasing the number density of the fine Ti-containing nitrides.
• the lower limit of the Si amount it is set to be 0.05% or more.
• the amount is preferably 0.08% or more, more preferably 0.15% or more.
• the Si amount is set to be 0.5% or less.
• the Si amount is preferably 0.45% or less, more preferably 0.35% or less.
• Mn is an element effective to ensure the strength by enhancing the hardenability, and as the lower limit of the Mn amount, it is therefore set to be 0.6% or more.
• the Mn amount is preferably 0.8% or more, more preferably 1.0% or more.
• the upper limit of the Mn amount it is set to be 2.0% or less.
• the amount is preferably 1.8% or less, more preferably 1.6% or less.
• the P is an element inevitably contained in the steel. If the upper limit of the P amount exceeds 0.030%, the HAZ toughness is significantly deteriorated. In addition, in the case of applying the steel to usage such as nuclear reactor pressure vessel, there is a concern about embrittlement due to neutron irradiation. Accordingly, as the upper limit of the P amount, it is set to be 0.030% or less. The amount is preferably 0.020% or less, more preferably 0.010% or less. From the viewpoint of ensuring the HAZ toughness, the P amount is preferably smaller, but it is industrially difficult to control the amount to 0%.
• the upper limit of the S amount it is set to be 0.025% or less.
• the amount is preferably 0.020% or less, more preferably 0.015% or less, still more preferably 0.010% or less. From the viewpoint of ensuring the HAZ toughness, the S amount is preferably smaller, but it is industrially difficult to control the amount to less than 0.0001%, and the lower limit of the S amount is therefore about 0.0001%.
• Al is an element, when added in an appropriate amount, contributing to reduction of the load in the steelmaking step (deoxidation, desulfurization, nitrogen control).
• Al is an element contributing to lowering the melting point of a nonmetallic particle that works out to a nucleation site of coarse Ti-containing nitride, thereby suppressing the formation of coarse Ti-containing nitride and ensuring HAZ toughness.
• Al is an element effective to complete the deoxidation step ⁇ desulfurization step in the steelmaking step within a predetermined time. In order to effectively bring out these effects, as the lower limit of the Al amount, it is set to be 0.02% or more. The amount is preferably 0.023% or more, more preferably 0.025% or more.
• the upper limit of the Al amount it is set to be 0.07% or less.
• the Al amount is preferably 0.06% or less, more preferably 0.05% or less.
• Nb 0.005% or more and less than 0.050%
• Nb is an element effective to enhance the base plate strength without deteriorating the weldability when manufacturing the base plate by applying a heat treatment-omitted type TMCP.
• the lower limit of the Nb amount it is set to be 0.005% or more.
• the Nb amount is preferably 0.010% or more, more preferably 0.020% or more. However, if the Nb amount is 0.050% or more, the HAZ toughness deteriorates. Accordingly, in the present invention, as the upper limit of the Nb amount, it is set to be less than 0.050%.
• the Nb amount is preferably 0.040% or less, more preferably 0.030% or less.
• Ti is an element essential to ensure the number density of the fine Ti-containing nitrides and obtain excellent HAZ toughness.
• the lower limit of the Ti amount it is set to be 0.003% or more.
• the amount is preferably 0.005% or more, more preferably 0.010% or more.
• the upper limit of the Ti amount it is set to be 0.03% or less.
• the amount is preferably 0.025% or less, more preferably 0.020% or less.
• N is an element essential to ensure the number density of the fine Ti-containing nitrides and obtain excellent HAZ toughness.
• the lower limit of the N amount it is set to be 0.0020% or more.
• the amount is preferably 0.003% or more, more preferably 0.0040% or more.
• the upper limit of the N amount it is set to be 0.010% or less.
• the amount is preferably 0.0095% or less, more preferably 0.0085% or less.
• the amount of O is preferably small from the viewpoint of enhancing cleanliness. In addition, if a large amount of O is contained, the HAZ toughness deteriorates. Considering these, as the upper limit of the O amount, it is set to be 0.0040% or less. The amount is preferably 0.0035% or less, more preferably 0.0030% or less. Although the O amount is preferably small, it is industrially difficult to control the amount to 0%.
• REM is an element contributing to lowering the melting point of a nonmetallic particle that becomes a nucleation site of coarse Ti-containing nitride, thereby suppressing the formation of coarse Ti-containing nitride, and consequently enhancing the HAZ toughness.
• the lower limit of the REM amount it is set to be 0.0002% or more.
• the REM amount is preferably 0.0005% or more, more preferably 0.0010% or more.
• the upper limit of the REM amount is set to be 0.050%.
• the upper limit of the REM amount it is preferably 0.03% or less, more preferably 0.010% or less, still more preferably 0.0050% or less.
• REM means lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).
• Zr is an element contributing to lowering the melting point of a nonmetallic particle that becomes a nucleation site of coarse Ti-containing nitride, thereby suppressing the formation of coarse Ti-containing nitride, and consequently enhancing the HAZ toughness.
• the lower limit of the Zr amount it is set to be 0.0003% or more.
• the Zr amount is preferably 0.0005% or more, more preferably 0.0010% or more, still more preferably 0.0015% or more.
• the upper limit of the Zr amount it is set to be 0.020% or less.
• the Zr amount is preferably 0.010% or less, more preferably 0.0070% or less, still more preferably 0.0050% or less.
• Ca is an element, when added in an appropriate amount, contributing to lowering the melting point of a nonmetallic particle that becomes a nucleation site of coarse Ti-containing nitride, thereby suppressing the formation of coarse Ti-containing nitride, and consequently enhancing the HAZ toughness.
• the lower limit of the Ca amount it is set to be 0.0003% or more.
• the amount is preferably 0.0005% or more, more preferably 0.0010% or more.
• the upper limit of the Ca amount is set to be 0.0060%.
• the Ca amount is preferably 0.0050% or less, more preferably 0.0040% or less.
• the Ti-N balance represented by the formula above is related to the driving force for forming Ti-containing nitride and is determined, as a parameter effective to ensure the number density of the fine Ti-containing nitrides, by the present inventors through a number of fundamental experiments. If the Ti-N balance is less than 0.000085, the number density of the fine Ti-containing nitrides contributing to enhancement of the HAZ toughness is not obtained.
• the Ti-N balance is preferably large and is preferably 0.000090 or more, more preferably 0.000095 or more.
• the upper limit is not particularly limited from the viewpoint of enhancing the HAZ toughness but is determined in relation to the upper limit of each of the Ti content and N content above.
• the above-described components are basic components, and the remainder is iron and inevitable impurities.
• V more than 0% and 0.1 % or less
• V is an element effective to enhance the strength.
• the lower limit of the V amount it is preferably set to be 0.003% or more.
• the amount is more preferably 0.010% or more.
• the upper limit of the V amount it is preferably set to be 0.1% or less, and the amount is more preferably 0.08% or less.
• Cu, Ni, Cr and Mo are elements contributing mainly to enhancement of the base plate strength.
• one of these elements may be used alone, or two or more thereof may be used in combination.
• the elements above are preferably controlled as follows.
• Cu is an element effective to increase the strength by enhancing the hardenability.
• the lower limit of the Cu amount it is preferably set to be 0.01% or more.
• the amount is more preferably 0.05% or more, still more preferably 0.10% or more.
• the upper limit of the Cu amount it is preferably set to be 0.50% or less.
• Ni is an element effective to enhance the base plate strength and the HAZ toughness.
• the lower limit of the Ni amount it is preferably set to be 0.01% or more.
• the amount is more preferably 0.05% or more, still more preferably 0.10% or more.
• the upper limit of the Ni amount it is preferably set to be 0.85% or less.
• Cr is an element effective to enhance the strength.
• the lower limit of the Cr amount it is preferably set to be 0.01% or more.
• the amount is more preferably 0.05% or more, still more preferably 0.10% or more.
• the upper limit of the Cr amount it is preferably set to be 0.30% or less.
• Mo is an element effective to enhance the base plate strength.
• the lower limit of the Mo amount it is preferably set to be 0.01% or more.
• the amount is more preferably 0.05% or more, still more preferably 0.10% or more.
• the upper limit of the Mo amount it is preferably set to be 0.5% or less.
• the steel plate of the present invention does not positively contain Mg and B and the upper limit of the content thereof is reduced to about an inevitable impurity level.
• the upper limit of the Mg amount it is preferably set to be 0.0005% or less. Because, Mg has high affinity for O (oxygen), and the adding order or adding method thereof is very difficult to be controlled for obtaining a nonmetallic particle having a low-melting-point composition contributing to the reduction of coarse Ti-containing nitride.
• B is not contained. Specifically, as the upper limit of the B amount, it is preferably set to be 0.0005% or less.
• B is considered to be an element segregating in the prior ⁇ grain boundary during thermal cycle of welding to retard ferrite transformation and suppress formation of coarse grain boundary ferrite, thereby contributing to enhancement of the HAZ toughness.
• such an effect of the addition of B is exerted in a region where the heat input amount is larger than that in the present invention and the prior ⁇ grain size is larger than that in the present invention (roughly on the order of 30 to 110 kJ/mm).
• the heat input amount is as small as approximately from 5 to 25 kJ/mm, and the prior ⁇ grain size is relatively fine as well.
• the hardenability is reduced, and the solute B concentration per unit area of the grain boundary decreases, and as a result, a ferrite microstructure is thoroughly developed.
• the addition of B makes a small contribution to the HAZ toughness-increasing effect, but conversely, the addition of B may make the rolling conditions difficult, and it is therefore preferable not to positively add this element.
• the average composition of nonmetallic particles having an equivalent-circle diameter of 1 ⁇ m or more satisfies, in mass%, 20% ⁇ Al 2 O 3 ⁇ 40%, 5% ⁇ TiO 2 ⁇ 20%, 5% ⁇ oxide of REM ⁇ 45%, 5% ⁇ ZrO 2 ⁇ 60%, and 5% ⁇ CaO ⁇ 40%.
• a nonmetallic particle such as oxide formed in the molten steel is in general more likely to become a formation site of Ti-containing nitride in the solidification process than the molten steel in the periphery, and as a result, coarse Ti-containing nitride of ⁇ m order is formed to reduce the HAZ toughness.
• the reason for limiting the size of the target nonmetallic particle to an equivalent circle diameter of 1 ⁇ m or more is that a nonmetallic particle with a size on this level becomes a principal formation site of coarse Ti-containing nitride causing reduction in the HAZ toughness.
• the equivalent-circle diameter as used herein means a diameter when the nonmetallic particle such as Al 2 O 3 dispersed in the steel is converted to a circle having the same area.
• nonmetallic particle means a particle usually present in a hot-rolled plate used for the thick steel plate of the present invention and is not limited to an oxide (including both a single oxide and a composite oxide) but encompasses particles of nitride, sulfide, etc. More specifically, the average composition above is not an average composition of oxide dispersed in the steel but means an average composition covering all nonmetallic particles of nitride, sulfide, etc. in addition to the oxide.
• the average composition of the above-described specific oxides i.e., Al 2 O 3 , TiO 2 , oxide of REM, ZrO 2 and CaO, satisfies the range above, and as long as the range above is satisfied, the average composition of other oxides is not particularly limited.
• the average composition of sulfide, etc. except for those oxides is not particularly limited as well. This is because these particles have no great effect on the HAZ toughness.
• the thick steel plate for tanks of the present invention satisfies the requirement that the density of the coarse Ti-containing nitrides having a major axis length of 2 ⁇ m or more is 0.8 pieces or less per 1 mm 2 and the density of the fine Ti-containing nitrides having an equivalent-circle diameter of 20 nm or more and 500 nm or less is 1.0 ⁇ 10 5 pieces or more per 1 mm 2 .
• the major axis length as used herein means the length of a long side when the Ti-containing nitride is regarded as a rectangle. Because, the Ti-containing nitrides in the cross-section observed are generally present in the form of a rectangle.
• the equivalent-circle diameter means a diameter when the Ti-containing nitride is converted to a circle having the same area.
• the number density of coarse Ti-containing nitrides is set to be 0.8 pieces or less per 1 mm 2 , whereby the minimum value of HAZ toughness at -51°C can be ensured at the specific level or more.
• the number density of coarse Ti-containing nitrides is set to be 0.8 pieces or less per 1 mm 2 .
• the number density of coarse Ti-containing nitrides is preferably as smaller as possible and is preferably 0.7 pieces or less, more preferably 0.5 pieces or less, most preferably 0, per 1 mm 2 .
• the number density of the fine Ti-containing nitrides is set to be 1.0 ⁇ 10 5 pieces or more per 1 mm 2 .
• the number density of the fine Ti-containing nitrides is further controlled, whereby for the first time, the minimum value of HAZ toughness at -51°C can be ensured at a predetermined level or more. This is a finding arose from fundamental experiments conducted by the present inventors.
• the number density of the fine Ti-containing nitrides is controlled as above.
• the number density of the fine Ti-containing nitrides is preferably as larger as possible and is preferably 1.5 ⁇ 10 5 pieces or more, more preferably 2.0 ⁇ 10 5 pieces or more, per 1 mm 2 .
• the upper limit thereof is not particularly limited in relation to the above-described action but is substantially considered to be roughly about 1.0 ⁇ 10 7 pieces or less per 1 mm 2 .
• the reason why the sizes of fine Ti-containing nitride and coarse Ti-containing nitride are set as above is because it has been found from experiments that those having sizes in the ranges above are particularly effective in ensuring the maximum value of HAZ toughness.
• the Ti-containing nitride as used in the present invention is intended to encompass all compounds as long as they are a nitride containing Ti. Typical examples thereof include TiN, but it is of course not limited thereto. For example, it may be a compound containing C, such as carbonitride of Ti, or may be a compound containing a nitride-forming element other than Ti, such as nitride of TiNb.
• the method for manufacturing the thick steel plate for tanks is described below.
• the present invention is characterized particularly in that the components in the steel are appropriately controlled and the average composition of nonmetallic particles and the number density of Ti-containing nitrides are appropriately controlled, and the preferable method in the steelmaking step and the subsequent rolling step are as follows.
• the dissolved oxygen amount in the molten steel is controlled to the range of, in mass%, from 0.002 to 0.01% by deoxidation using Mn, Si and Al and thereafter, respective elements are added in the order of Ti ⁇ (REM, Zr) ⁇ Ca.
• deoxidation is performed using Mn, Si and Al.
• These elements are an element contributing to load reduction in the steelmaking step of deoxidation ⁇ desulfurization ⁇ nitrogen control, more specifically, an element facilitating controlling the concentration of each trace element (oxygen, sulfur, nitrogen, and Ti in the molten steel) during casting to the target control range.
• the control tolerance at the control point of the steelmaking step is narrowed, and it becomes difficult for each concentration of oxygen amount, sulfur amount, nitrogen amount and Ti amount in the molten steel during casting to be controlled to the target control range.
• the target control range is specifically 40 ppm or less for oxygen, 25 ppm or less for sulfur, target value ⁇ 20 ppm or less for nitrogen, and target value ⁇ 30 ppm or less for Ti.
• the target value of Ti is set to be 0.015%
• the dissolved oxygen amount in the molten steel before adding Ti, REM, Zr and Ca is controlled to the range of 0.002 to 0.01%. If the dissolved oxygen amount is less than 0.002%, a required amount of the average composition of an oxide-based inclusion having an appropriate composition cannot be ensured. Consequently, coarse TiN may be crystallized or become an origin of occurrence of intragranular ferrite transformation (Inter Granular Bainite, IGB). On the other hand, if the dissolved oxygen amount exceeds 0.01%, the HAZ toughness deteriorates. As the lower limit of the dissolved oxygen amount, it is preferably 0.0025% or more, more preferably 0.003% or more. As the upper limit of the dissolved oxygen amount, it is preferably 0.009% or less, more preferably 0.008% or less.
• respective elements are added in the order of Al ⁇ Ti ⁇ (REM, Zr) ⁇ Ca. If respective elements are added in the order other than the adding order above, a required number of oxide-based inclusions having an appropriate composition cannot be ensured. In particular, since Ca has very strong deoxidizing power, if Ca is added in advance of Ti or Al, oxygen to be bonded to Ti or Al is entirely removed, and the desired average composition of TiO 2 or Al 2 O 3 cannot be ensured.
• the (REM, Zr) means that the adding order of REM and Zr is not particularly limited. That is, as long as the addition thereof is conducted after the addition of Ti and before the addition of Ca, it may be either in the order of REM ⁇ Zr or in the order of Zr ⁇ REM. Alternatively, REM and Zr may be added simultaneously.
• the total time from Ti addition to Ca addition is preferably controlled to roughly the range of 3 to 20 minutes. If the total time falls below 3 minutes, the reaction becomes excessive, and the Ca concentration in the inclusion rises too high. On the other hand, if the total time exceeds 20 minutes, the inclusion is coarsened to adversely affect the toughness.
• the molten steel is solidified.
• the solidification method is not particularly limited, but in particular, the cooling time in the temperature range of 1,500 to 1,450°C during casting is preferably controlled to within 300 seconds. If the cooling time in the temperature range above exceeds 300 seconds, the crystallized TiN is coarsened to adversely affect the toughness and in addition, the productivity decreases.
• TMCP normal TMCP
• the following method is preferably used.
• Heating condition before rolling for example, from 950 to 1,200°C (preferably from 1,050 to 1,150°C), finish rolling temperature: for example, from 680 to 700°C.
• Rolling reduction in two-phase region for example, 30% or more.
• Average cooling rate in the temperature region after finishing rolling temperature to 530°C: 2°C/sec or more.
• the average cooling rate in the temperature region above is less than 2°C/sec, the strength is insufficient.
• the average cooling rate is preferably 3°C/sec or more.
• the upper limit thereof is not particularly limited, but considering the productivity, etc. in the actual operation level, the average cooling rate is roughly 30°C/sec or less.
• the steel having the composition shown in Table 1 was melted in a vacuum melting furnace, and 240 ton of slab was obtained using the obtained molten steel. More specifically, during melting, the dissolved oxygen amount in the molten steel was controlled to the range of, in mass%, from 0.002 to 0.01 % by deoxidation using Mn, Si and Al. Thereafter, respective elements were added in the order of Ti ⁇ (REM, Zr) ⁇ Ca while controlling the time from Ti addition to Ca addition to become from 3 to 20 minutes, and the cooling time in the temperature range of 1,500 to 1,450°C during casting was set to be within 300 seconds.
• REM was added in the form of misch metal containing, in mass%, about 50% of Ce and about 25% of La.
• "-" means that the corresponding element was not added.
• the oxygen concentration, sulfur concentration, nitrogen concentration and Ti concentration in the molten steel were measured.
• those where all concentrations could be controlled to fall within the target control range were judged as "pass”, and those where the concentration of at least one element deviated from the target control range were judged as "fail”.
• TMCP hot-rolled plate having a thickness of 80 mm.
• the thus-obtained steel plate was designated as "TMCP" in the column of Base plate Production Method of Table 2. Heating condition before rolling: 1,100°C Rolling reduction in two-phase region: 30% Average cooling rate from finish rolling temperature to 530°C: from 2 to 8°C/sec
• the "as-air cooled" in the column of Base plate Production Method of Table 2 indicates that the slab was cooled at a lower rate (0.6°C/sec) than the above-described average cooling rate.
• nonmetallic particles were observed using EPMA-8705 manufactured by Shimadzu Corporation around the thickness central part (t/2).
• the cross-section was observed at an observation magnification of 400 times and an observation visual field of about 50 mm 2 (7 mm in sheet thickness direction and 7 mm in sheet width direction by arranging the thickness central part to become the center of the observation visual field), and the component composition in the central part of inclusion was quantitatively determined by characteristic X-ray wavelength dispersion spectroscopy for inclusions having an equivalent-circle diameter of 1 ⁇ m or more.
• the analysis target elements were Al, Mn, Si, Mg, Ca, Ti, Zr, S, REM (La, Ce, Nd, Dy, Y), and Nb.
• the relationship between the X-ray intensity of each element and the element concentration was previously determined as the calibration curve by using a known substance, and from the X-ray intensity obtained from the inclusion and the calibration curve above, the element concentration of the inclusion was quantitatively determined.
• the same operation was performed on a total of three cross-sections, and the average value thereof was defined as the average composition of nonmetallic particles having an equivalent-circle diameter of 1 ⁇ m or more.
• an inclusion having an oxygen content of 5% or more was taken as an oxide, and the concentrations converted to mass as a single oxide were averaged and defined as the average composition of oxides.
• the average composition of all oxides is shown in the Table below.
• M a metal element
• the oxide of REM is present in the form of M 2 O 3 , M 3 O 5 or MO 2 in the steel material, but the composition was measured by converting all oxides to M 2 O 3 .
• the composition of oxides was calculated by converting each element to its single oxide from the ratio of X-ray intensities indicating the presence of those elements.
• a specimen was cut out from the position at a depth of t/2 (t: plate thickness) from the surface of the hot-rolled plate, such that the axial center of the specimen passes through the position of t/2. Thereafter, a Transmission Electron Microscope (TEM) replica specimen was prepared from a cross-section parallel to the rolling direction and the plate thickness direction, and its cross-section was observed using TEM. The observation conditions were magnification: 150,000 times and observation visual field: 0.66 ⁇ m ⁇ 0.78 ⁇ m, and three or more visual fields were observed.
• a particle containing Ti and N was distinguished by an energy dispersive X-ray detector (Energy Dispersive X-ray, EDX), and the particle was defined as Ti-containing nitride.
• the area of the Ti-containing nitride in the observation visual field was observed and converted to an equivalent-circle diameter, and the number of Ti-containing nitride particles of 500 nm or less was counted and converted to the number density per 1 mm 2 .
• particles having an equivalent-circle diameter of less than 20 nm were excluded because of insufficient reliability of EDX.
• the number density per 1 mm 2 was measured similarly in all observation visual fields, and the average thereof was determined.
• a specimen was cut out from the position at a depth of t/2 (t: plate thickness) from the surface of the hot-rolled plate, such that the axial center of the specimen passes through the position of t/2. Thereafter, a cross-section parallel to the rolling direction and the plate thickness direction was photographed using an optical microscope at a magnification of 200 times in 20 visual fields, and the number of coarse Ti-containing nitrides was counted and converted to the number density per 1 mm 2 , whereby the number density was determined. The area of the measured image was 0.148 mm 2 per visual field and 2.96 mm 2 per sample.
• the Ti-containing nitride was identified based on the shape and color, and an inclusion having an angular shape and a bright orange color was regarded as Ti-containing nitride.
• the major axis of the Ti-containing nitride was measured using an analysis software. Here, coarse Ti-containing nitride is often crystallized from an oxide as a starting point, but if crystallized, the oxide in the inside was excluded from the measurement target of the major axis.
• a JIS Z2241 No. 4 specimen was sampled in parallel to the C-direction from the position at a depth of t/4 from the surface of the hot-rolled plate and subjected to a tensile test by the method described in JIS Z2241 to measure the tensile strength TS and the yield strength YS. Since the tensile test is little affected by the plate thickness direction and the value in the t/4 value is considered to have substantially the same meaning as the value in the t/2 part, in this Example, the tensile test was performed in the t/4 part.
• a welded joint specimen having a plate thickness of 40 mm was sampled by reducing the hot-rolled plate (thickness: 80 mm) from both surfaces and then subjected to electro-gas arc welding under the conditions of a groove angle of 25°, a groove width (root gap) of 6 mm, and a heat input amount of 25 kJ/mm to obtain a welded joint.
• a region near the Fusion Line also called a weld line or a weld fusion zone; in the Table, denoted by FL
• FL weld line
• the rolled steel material of the present invention is useful for tanks to be used in the fields of an energy storage facility, a chemical plant, a power generation facility, a nuclear reactor pressure vessel, etc.

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• 2014
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