WO2021156925A1 - 厚鋼板 - Google Patents

厚鋼板 Download PDF

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Publication number
WO2021156925A1
WO2021156925A1 PCT/JP2020/003986 JP2020003986W WO2021156925A1 WO 2021156925 A1 WO2021156925 A1 WO 2021156925A1 JP 2020003986 W JP2020003986 W JP 2020003986W WO 2021156925 A1 WO2021156925 A1 WO 2021156925A1
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content
thick steel
toughness
steel sheet
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PCT/JP2020/003986
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English (en)
French (fr)
Japanese (ja)
Inventor
仁秀 吉村
崇之 加賀谷
洋志 熊谷
史寿 高峰
仁志 古谷
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Nippon Steel Corp
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Nippon Steel Corp
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Priority to JP2021575118A priority Critical patent/JP7315874B2/ja
Priority to PCT/JP2020/003986 priority patent/WO2021156925A1/ja
Publication of WO2021156925A1 publication Critical patent/WO2021156925A1/ja
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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
    • 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
    • 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 sheet, more particularly a high-strength thick steel sheet having a plate thickness of 70 mm or more, and has low temperature toughness useful for use in structures such as bridges, buildings, shipbuilding and pressure vessels. Regarding excellent thick steel sheets.
  • Patent Document 1 in terms of mass%, C: 0.03 to 0.20%, Si: 0.05 to 0.60%, Mn: 0.3 to 2.0%, P: 0.015%.
  • a steel composition containing S: 0.003% or less, Al: 0.07% or less, N: 0.01% or less, and the balance consisting of Fe and unavoidable impurities, and a polygonal ferrite phase and an upper bainite phase.
  • the area fraction of the polygonal ferrite phase is 10 to 45%, the average crystal grain size is 18 ⁇ m or less, the standard deviation of the crystal grain size is 8 ⁇ m or less, and the area of island martensite in the upper bainite phase.
  • Patent Document 1 describes that according to the above configuration, a thick steel sheet having a tensile strength of 590 MPa or more and excellent low temperature toughness and total elongation can be obtained with a plate thickness of 12 mm or more.
  • Patent Document 2 a steel having a predetermined chemical composition is heated and then rolled to finish rolling in a temperature range of Ar 3 points or more, and after rolling, the plate thickness is changed from a state where the average plate thickness temperature is Ar 3 points or more. Cooling is performed at an average cooling rate of 2 ° C./sec or higher until the average temperature is 500 ° C. or lower, and then tempering is performed in a temperature range where the maximum temperature reached is 500 ° C. or higher and the heating rate is 500 ° C. or higher.
  • a method for producing high-strength steel is described, characterized in that the temperature is 5 ° C./sec or higher and the tempering parameter TP satisfies a predetermined condition.
  • post-welding heat treatment is generally performed to remove or alleviate residual stress generated by welding.
  • PWHT post-welding heat treatment
  • the strength and low temperature toughness of the steel sheet after PWHT have been improved. It is also required to let them do it.
  • brittle fracture due to a decrease in low temperature toughness is a fracture form that should be avoided because the entire structure can be collapsed instantly.
  • it is generally difficult to improve the low temperature toughness after PWHT because the low temperature toughness tends to decrease due to the concentration of impurity elements and the coarsening of alloy carbides after PWHT.
  • the present invention has been made in view of such circumstances, and an object of the present invention is to improve low temperature toughness after PWHT in thick steel sheets, more specifically, high-strength thick steel sheets.
  • the present inventors examined the chemical composition and manufacturing conditions of the thick steel sheet in order to achieve the above object. As a result, the present inventors make the heat treatment before hot rolling, the hot rolling, and / or the cooling rate after hot rolling appropriate, while keeping the chemical composition of the thick steel sheet within a predetermined range. As a result, it has been found that the crystal grains can be refined in a high-strength thick steel sheet to improve the low-temperature toughness after PWHT, and the present invention has been completed.
  • the thick steel sheets that have achieved the above objectives are as follows.
  • C 0.050 to 0.130%, Si: 0.100 to 0.600%, Mn: 1.100 to 1.800%, P: 0.0200% or less, S: 0.0100% or less, Mo: 0.050 to 0.500%, V: 0.005 to 0.100%, Nb: 0 to 0.100%, Al: 0.001 to 0.120% (However, in the case of [Al] ⁇ [N] ⁇ 3.2 ⁇ 10 -4 , Al: 0.001 to 0.080%, where [Al] and [N] is the content (mass%) of Al and N, respectively), B: 0 to 0.0030%, N: 0.0100% or less, O: 0.0100% or less, Cu: 0 to 0.500%, Ni: 0 to 0.800%, Cr: 0 to 0.50%, W: 0 to 0.50%, Ti: 0 to 0.100% (however, when the B content is 0.0003% or more, Ti: 0.00
  • It contains a structure in which the coarse particle size of the crystal grains surrounded by the boundary having an orientation difference of 15 ° or more is 45 ⁇ m or less and the average particle size of the crystal grains is 25 ⁇ m or less. It has a tensile strength of 580 to 730 MPa and has a tensile strength of 580 to 730 MPa.
  • a thick steel plate having a plate thickness of 70 mm or more.
  • the chemical composition is mass%.
  • the chemical composition is mass%.
  • the chemical composition is mass%.
  • the chemical composition is mass%.
  • the chemical composition is mass%.
  • the chemical composition is mass%. Nb: 0.005 to 0.100%, Cu: 0.050 to 0.500%, Ni: 0.100 to 0.800%, Cr: 0.05 to 0.50%, W: 0.05 to 0.50%, Ti: 0.005 to 0.100%, Sn: 0.005 to 0.050%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0100%
  • the chemical composition is mass%.
  • the chemical composition is mass%.
  • the low temperature toughness after PWHT can be remarkably improved in a high-strength thick steel sheet.
  • the thick steel sheet of the present invention is a high-strength thick steel sheet, more specifically, a tensile strength of 580 to 730 MPa, particularly 580 to 580 to 15 hours when heat treatment is performed at 650 ° C. ⁇ 15 hours, which corresponds to post-welding heat treatment (PWHT).
  • PWHT post-welding heat treatment
  • crystal grains are refined to improve low temperature toughness after PWHT.
  • the crystal grain refers to a region surrounded by a boundary in which the orientation difference of adjacent grains is 15 ° or more when the crystal orientation is measured by electron backscatter diffraction (EBSD). Is.
  • the coarse particle size and the average particle size refer to the particle size calculated based on the circle-equivalent diameter of each crystal grain measured by the EBSD. ..
  • the thick steel plate of the present invention can be realized by the specific embodiments shown below. Hereinafter, specific embodiments 1 to 4 for realizing the thick steel plate of the present invention will be described in more detail, but these explanations are intended to merely illustrate preferred embodiments of the present invention. The invention is not intended to be limited to such particular embodiments.
  • the thick steel plate according to the first embodiment of the present invention is based on mass%.
  • C 0.050 to 0.130%, Si: 0.100 to 0.600%, Mn: 1.100 to 1.800%, P: 0.0200% or less, S: 0.0100% or less, Mo: 0.050 to 0.500%, V: 0.005 to 0.100%, Nb: 0.005 to 0.100%, Al: 0.001 to 0.080%, B: Less than 0.0003%, N: 0.0100% or less, O: 0.0100% or less, Cu: 0 to 0.500%, Ni: 0 to 0.800%, Cr: 0 to 0.50%, W: 0 to 0.50%, Ti: 0 to 0.100%, Sn: 0 to 0.050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0100%, and balance: containing a chemical composition consisting of Fe and impurities.
  • It contains a structure in which the coarse particle size of the crystal grains surrounded by the boundary having an orientation difference of 15 ° or more is 45 ⁇ m or less and the average particle size of the crystal grains is 25 ⁇ m or less. It has a tensile strength of 580 to 730 MPa and has a tensile strength of 580 to 730 MPa. It is characterized by having a plate thickness of 70 mm or more.
  • NbCN as pinning particles in relation to the miniaturization of crystal grains (NbCN is NbC, NbN, NbC in which a part of C is replaced with N, and / Or, the use of NbN in which a part of N is replaced with C) was focused on and examined.
  • Nb generally has the property of segregating in the positive segregation portion
  • the formed NbCN does not effectively function as pinning particles
  • the negative segregation portion is coarse.
  • the present inventors set the Nb content in the range of 0.005 to 0.100% and perform homogenization heat treatment before hot rolling under the conditions corresponding to the Nb content. It was found that the formation of coarse particles, particularly the formation of coarse particles in the negative segregation portion, can be suppressed by diffusing the Nb segregation and effectively functioning the formed NbCN as pinning particles.
  • the present inventors determine the product S of the time spent at a temperature T ° C. or higher determined by the following formula 1 during the homogenizing heat treatment and the average temperature (° C.) at the residence time (h).
  • T ° C. or higher determined by the following formula 1 during the homogenizing heat treatment and the average temperature (° C.) at the residence time (h).
  • the temperature within the range of 20000 to 100,000 (° C. h)
  • Carbon (C) is an element necessary for ensuring the strength of the base material, and is also an element constituting NbCN, which is a pinning particle.
  • the C content is set to 0.050% or more.
  • the C content may be 0.060% or more, 0.070% or more, or 0.080% or more.
  • the toughness of the weld heat-affected zone (HAZ), especially the HAZ near the melt line (FL) may be significantly deteriorated in addition to the base metal, and the strength tends to be excessive. There is also. Therefore, the C content is set to 0.130% or less.
  • the C content may be 0.120% or less, 0.110% or less, or 0.100% or less.
  • Si is a deoxidizing element and is an element that also contributes to the improvement of strength. In order to obtain these effects sufficiently, the Si content is set to 0.100% or more. The Si content may be 0.150% or more, 0.200% or more, or 0.250% or more. On the other hand, if Si is excessively contained, island-shaped martensite may be formed and the toughness may be lowered. Therefore, the Si content is set to 0.600% or less. The Si content may be 0.500% or less, 0.400% or less, or 0.350% or less.
  • Mn Manganese (Mn) is a deoxidizing element and is also an element that improves hardenability.
  • the Mn content shall be 1.100% or more.
  • the Mn content may be 1.200% or more, 1.250% or more, or 1.350% or more.
  • the Mn content is set to 1.800% or less.
  • the Mn content may be 1.700% or less, 1.650% or less, or 1.600% or less.
  • Phosphorus (P) is an impurity that segregates at grain boundaries and reduces toughness. Therefore, the P content is set to 0.0200% or less.
  • the P content is preferably 0.0150% or less, more preferably 0.0100% or less, and most preferably 0.0080% or less. Since the smaller the P content is, the more preferable it is, the lower limit is not particularly specified. However, from the viewpoint of manufacturing cost, the P content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • S 0.0100% or less
  • Sulfur (S) is an impurity that promotes central segregation and may cause the formation of stretched MnS, which is the starting point of brittle fracture. Therefore, the S content is set to 0.0100% or less.
  • the S content is preferably 0.0080%, more preferably 0.0060%, and most preferably 0.0050% or less. Since the smaller the S content is, the more preferable it is, the lower limit is not particularly specified. However, from the viewpoint of manufacturing cost, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • Molybdenum is an element that improves the strength and toughness of the base metal. In order to obtain such an effect sufficiently, the Mo content is set to 0.050% or more. The Mo content may be 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if Mo is excessively contained, the strength of the base metal may be excessively increased and the toughness may be impaired. Therefore, the Mo content is set to 0.500% or less. The Mo content may be 0.450% or less, 0.400% or less, or 0.350% or less.
  • Vanadium (V) is an element that precipitates carbonitride in the tempering step and the post-welding heat treatment step, and contributes to the improvement of the strength of the base metal.
  • the V content is set to 0.005% or more.
  • the V content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • the V content is set to 0.100% or less.
  • the V content may be 0.080% or less, 0.060% or less, or 0.040% or less.
  • Niobium is an element that refines the structure by the pinning effect in the form of NbCN and improves low temperature toughness. In order to obtain such an effect, the Nb content needs to be 0.005% or more.
  • the Nb content is preferably 0.010% or more, more preferably 0.015% or more, and most preferably 0.020% or more.
  • the Nb content is set to 0.100% or less.
  • the Nb content may be 0.080% or less, 0.070% or less, or 0.050% or less.
  • Aluminum (Al) is a deoxidizing element and is also an element that suppresses the formation of cementite. Further, Al contributes to fine particle formation as pinning particles AlN. In order to obtain at least one of these effects, the Al content is set to 0.001% or more. The Al content may be 0.015% or more, 0.020% or more, or 0.025% or more. On the other hand, if Al is excessively contained, the amount of inclusions increases, which may lead to a decrease in toughness. Therefore, the Al content is set to 0.080% or less. The Al content may be 0.070% or less, 0.060% or less, or 0.050% or less.
  • B is an element that segregates at grain boundaries to improve hardenability. However, if B is contained excessively, the strength may become too high, or the formation of upper bainite may be promoted, resulting in a decrease in toughness. Therefore, the B content is set to less than 0.0003%.
  • the B content may be 0.0002% or less, 0.0001% or less, or 0%.
  • N Nitrogen (N) is an element that forms a nitride, and if it is contained in an excessive amount, a coarse nitride is formed, which causes a decrease in toughness. Therefore, the N content is set to 0.0100% or less.
  • the N content is preferably 0.0080% or less, more preferably 0.0060% or less, and most preferably 0.0050% or less.
  • NbCN which is a pinning particle, may not be sufficiently formed. Therefore, the N content is preferably 0.0003% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • Oxygen (O) is an impurity, so it should be 0.0100% or less.
  • the O content is preferably 0.0060% or less, more preferably 0.0040% or less, and most preferably 0.0030% or less. It is preferable to reduce O as much as possible, but from the viewpoint of deoxidation cost, the O content may be 0.0001% or more, 0.0002% or more, or 0.0003% or more.
  • the thick steel plate may contain one or more of the following optional elements, if necessary. Hereinafter, these optional elements will be described in detail.
  • Copper (Cu) is an element that contributes to the increase in strength.
  • the Cu content may be 0%, but in order to obtain such an effect, the Cu content is preferably 0.050% or more.
  • the Cu content may be 0.150% or more, 0.200% or more, or 0.250% or more.
  • the toughness of the base metal may decrease. Therefore, the Cu content is set to 0.500% or less.
  • the Cu content may be 0.450% or less, 0.400% or less, or 0.350% or less.
  • Nickel (Ni) is an effective element for ensuring toughness.
  • the Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.100% or more.
  • the Ni content may be 0.200% or more, 0.250% or more, or 0.300% or more.
  • the Ni content is set to 0.800% or less.
  • the Ni content may be 0.700% or less, 0.650% or less, or 0.600% or less.
  • Chromium (Cr) is an element that contributes to the improvement of carbon dioxide corrosion resistance and hardenability and affects the strength.
  • the Cr content may be 0%, but in order to obtain these effects, the Cr content is preferably 0.05% or more.
  • the Cr content may be 0.10% or more, 0.15% or more, or 0.20% or more.
  • the toughness of HAZ may decrease. Therefore, the Cr content is set to 0.50% or less.
  • the Cr content may be 0.45% or less, 0.40% or less, or 0.35% or less.
  • Tungsten is an element that contributes to the improvement of corrosion resistance and affects the strength.
  • the W content may be 0%, but in order to obtain these effects, the W content is preferably 0.05% or more.
  • the W content may be 0.10% or more, 0.15% or more, or 0.20% or more.
  • the toughness of HAZ may decrease. Therefore, the W content is set to 0.50% or less.
  • the W content may be 0.45% or less, 0.40% or less, or 0.35% or less.
  • Ti 0 to 0.100%
  • Ti titanium
  • the Ti content may be 0%, but in order to obtain the above effects, the Ti content is preferably 0.005% or more.
  • the Ti content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • Ti oxide and Ti-Al oxide may be formed to reduce the dispersion density, and the effect of refining the structure of the weld heat affected zone of small heat input may be reduced. .. Therefore, the Ti content is set to 0.100% or less.
  • the Ti content may be 0.080% or less, 0.060% or less, or 0.050% or less.
  • Tin (Sn) is an element that affects strength.
  • the Sn content may be 0%, but in order to obtain this effect, the Sn content is preferably 0.005% or more.
  • the Sn content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • the toughness may decrease. Therefore, the Sn content is set to 0.050% or less.
  • the Sn content may be 0.045% or less, 0.040% or less, or 0.035% or less.
  • Ca is an element that controls the morphology of oxides and sulfides.
  • the Ca content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive Ca content may saturate the above effects and impair toughness due to the formation of inclusions. Therefore, the Ca content is set to 0.0050% or less.
  • the Ca content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.
  • Mg Magnesium (Mg) is an element that controls the morphology of oxides and sulfides.
  • the Mg content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the Mg content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive content of Mg saturates the above effects and may impair toughness due to the formation of inclusions. Therefore, the Mg content is set to 0.0050% or less.
  • the Mg content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.
  • Rare earth metals are elements that control the morphology of oxides and sulfides.
  • the REM content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the REM content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive content of REM saturates the above effects and may impair toughness due to the formation of inclusions. Therefore, the REM content is set to 0.0100% or less.
  • the REM content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
  • the REMs in the present specification are lutetium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to 71, which are lanthanoids. It is one or more elements selected from the group consisting of lutetium (Lu), and the REM content is the total content of these elements.
  • the balance other than the above elements is Fe and impurities.
  • Impurities are components that are mixed in by various factors in the manufacturing process, including raw materials such as ore and scrap, when thick steel sheets are industrially manufactured.
  • Carbon equivalent (Ceq) 0.370 to 0.600 (common in embodiments 1 to 4)
  • Carbon equivalent (Ceq) is an indicator of hardenability.
  • Ceq is calculated by the following equation 2.
  • Ceq [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ...
  • Equation 2 In the formula, [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] are the contents (mass%) of each element, and are 0 when no element is contained. Is. In the present embodiment, it is possible to achieve an appropriate tensile strength by setting the chemical composition within the range described above. Therefore, the Ceq of the thick steel plate is not particularly limited, but is generally 0.370 to 0.600. For example, Ceq may be 0.400 or higher or 0.450 or higher, and / or 0.550 or lower or 0.500 or lower.
  • the coarse grain size of the crystal grains surrounded by the boundary having an orientation difference of 15 ° or more is set to 45 ⁇ m or less.
  • the coarse particle size of the crystal grains is preferably 40 ⁇ m or less, more preferably 35 ⁇ m or less, and most preferably 30 ⁇ m or less. Since the smaller the coarse particle size of the crystal grains is, the more preferable it is, the lower limit is not particularly specified. However, in general, the coarse particle size of the crystal grains may be 5 ⁇ m or more or 8 ⁇ m or more.
  • the coarse particle size of the crystal grains is determined as follows. First, the L cross section (cross section parallel to the rolling direction and the plate thickness direction of the thick steel plate) at the plate thickness 1/4 position of the thick steel plate is mirror-polished, and then an arbitrary 1.0 mm by electron backscatter diffraction method (EBSD). The crystal orientation of a region of ⁇ 0.4 mm is measured at one location, the region where the orientation difference of adjacent grains is 15 ° or more is defined as one crystal grain, and the particle size of each crystal grain is calculated as the equivalent diameter of a circle. do. Ten of these crystal grains having a large circle-equivalent diameter are selected, and the average value of the circle-equivalent diameters is determined as the "coarse grain size of the crystal grains".
  • EBSD electron backscatter diffraction method
  • the average particle size of the crystal grains is preferably 20 ⁇ m or less, more preferably 15 ⁇ m or less, and most preferably 10 ⁇ m or less. Since the smaller the average particle size of the crystal grains is, the more preferable it is, the lower limit is not particularly specified. However, in general, the average particle size of the crystal grains may be 1 ⁇ m or more or 3 ⁇ m or more.
  • the average particle size of the crystal grains is determined as follows. First, as in the case of coarse particle size, the L cross section (cross section parallel to the rolling direction and the plate thickness direction of the thick steel plate) at the plate thickness 1/4 position of the thick steel plate is mirror-polished, and then the electron backscatter diffraction method is performed. (EBSD) is used to measure the crystal orientation of an arbitrary 1.0 mm ⁇ 0.4 mm region at one location, and the region where the orientation difference between adjacent grains is 15 ° or more is defined as one crystal grain, and each crystal grain is defined as one crystal grain. The particle size of is calculated as the diameter equivalent to a circle. The area average calculated based on all the calculated crystal grains is determined as the "average particle size of the crystal grains".
  • EBSD electron backscatter diffraction method
  • area average (d) is calculated by the following equation 3 from the area of each crystal grain occupies (a i) and the circle equivalent diameter of each grain (d i).
  • d ⁇ (a i ⁇ d i) / ⁇ a i ⁇ Formula 3
  • the aspect ratio of the crystal grains is not particularly limited, but may be, for example, 1.8 or less, 1.6 or less, or 1.5 or less.
  • the aspect ratio of crystal grains is determined as follows. First, the L cross section (cross section parallel to the rolling direction and the plate thickness direction of the thick steel plate) at the plate thickness 1/4 position of the thick steel plate is mirror-polished, and then an arbitrary 1.0 mm by electron backscatter diffraction method (EBSD).
  • EBSD electron backscatter diffraction method
  • the crystal orientation of a region of ⁇ 0.4 mm is measured at one location, and the region where the orientation difference of adjacent grains is 15 ° or more is defined as one crystal grain, and the rolling direction length and plate thickness direction of each crystal grain are defined.
  • the length is measured and the aspect ratio of each crystal grain is calculated.
  • the arithmetic mean of the calculated aspect ratios of all the crystal grains is determined as the "aspect ratio of the crystal grains".
  • the structure of the thick steel sheet according to the present embodiment is mainly composed of ferrite.
  • the method for producing a thick steel sheet which will be described later, describes a tempering process, even if such a tempering process is performed, the structure of the thick steel sheet is mainly composed of ferrite, for example, tempered martensite in the structure. And the content of tempered lower bainite is 30% or less in total.
  • the thick steel plate according to this embodiment has a plate thickness of 70 mm or more.
  • the thickness of the thick steel plate is not particularly limited, but may be 80 mm or more, 90 mm or more, or 100 mm or more.
  • the upper limit is not particularly limited, but in general, the thickness of the thick steel plate is 150 mm or less.
  • tensile strength (TS) 580 to 730 MPa
  • the tensile strength is preferably 600 MPa or more, more preferably 650 MPa or more.
  • the tensile strength may be 700 MPa or less or 680 MPa or less.
  • high strength can be maintained even after PWHT, and for example, even when heated at 650 ° C. for 15 hours (corresponding to PWHT), a tensile strength (TS) of 580 to 730 MPa can be obtained. Can be achieved.
  • the tensile strength after heating at 650 ° C. for 15 hours is preferably 600 MPa or more, more preferably 650 MPa or more, and may be 700 MPa or less or 680 MPa or less.
  • the thick steel plate of the present embodiment it is possible to similarly achieve an excellent yield strength (YS) regardless of the presence or absence of PWHT.
  • the thick steel sheet of the present embodiment is 400 MPa or more, preferably 450 MPa or more, more preferably both when heated at 650 ° C. for 15 hours (corresponding to PWHT) and when such heat treatment is not performed. Can achieve a yield strength of 500 MPa or more.
  • the thick steel plate of the present embodiment it is possible to similarly achieve excellent low temperature toughness with or without PWHT. More specifically, the thick steel sheet of the present embodiment is subjected to JIS No. 4 Charpy impact at ⁇ 35 ° C. both when heated at 650 ° C. for 15 hours (corresponding to PWHT) and when such heat treatment is not performed. It is possible to achieve low temperature toughness having an average value of absorbed energy (vE -35 ) of 70 J or more, preferably 100 J or more, and more preferably 150 J or more.
  • vE -35 average value of absorbed energy
  • the thick steel plate according to the present embodiment exhibits excellent strength and low temperature toughness not only before PWHT but also after PWHT as described above, and is therefore used in structures such as bridges, buildings, shipbuilding and pressure vessels. Very suitable for.
  • the steel plate for a pressure vessel is extremely useful in applications such as a pressure vessel in which various gases are reacted in a low temperature region of ⁇ 10 ° C. or lower.
  • Tensile strength (TS) and yield strength (YS) are determined by performing a tensile test in accordance with JIS Z2241: 2011 based on JIS No. 5 test pieces collected from the direction parallel to the plate width direction (C direction) of the thick steel plate. Be measured.
  • the average value of Charpy impact absorption energy (vE -35 ) is based on the JIS No. 4 test piece taken from the C direction of the thick steel plate, and in accordance with the regulations of JIS Z2242: 2005, using an impact blade with a radius of 2 mm. It is calculated by measuring three Charpy impact absorption energies at ⁇ 35 ° C. and averaging them.
  • the method for producing a thick steel sheet according to the first embodiment includes a homogenizing heat treatment step, a hot rolling step, a quenching step, an intermediate heat treatment step, and a tempering step.
  • a homogenizing heat treatment step for melting steel
  • a hot rolling step for rolling steel
  • a quenching step for intermediate heat treatment
  • a tempering step for tempering steel
  • each step will be described in more detail.
  • the steel piece to be used in the present production method is not particularly limited as long as it is within the range of the chemical composition of the present embodiment, and a steel piece produced under any suitable casting conditions known to those skilled in the art is used. be able to.
  • the piece of steel may be an ingot-slab slab or a continuously cast slab. From the viewpoint of production efficiency, yield and energy saving, it is preferable to use a continuously cast slab as the steel piece.
  • the steel pieces having the chemical composition specified in the first embodiment are heated for homogenization in the homogenization heat treatment step before the hot rolling step.
  • the use and control of pinned particles is important to suppress the formation of coarse tissue.
  • NbCN is used as the pinning particles.
  • Nb segregation is diffused by performing homogenization heat treatment before hot rolling under conditions according to the Nb content, and the formed NbCN effectively functions as pinning particles. It is possible to suppress the formation of coarse particles, particularly the formation of coarse particles in the negative segregation portion.
  • the product S of the time spent at the temperature T ° C. or higher determined by the following formula 1 and the average temperature (° C.) at the residence time (h) during the homogenizing heat treatment is 20000 to 100,000 (° C.).
  • T 4500 / (2-log [Nb])-200 ... Equation 1
  • [Nb] is the Nb content (mass%).
  • the reheating temperature is preferably 1000 ° C. or higher from the viewpoint of reducing the load on the rolling roll, and preferably 1250 ° C. or lower from the viewpoint of suppressing coarsening of the structure.
  • the hot-rolled steel sheet is cooled from 800 ° C. to 500 ° C. at an average cooling rate of 0.050 ° C./s or less.
  • the ferrite fraction can be increased in the structure after hot rolling, the final structure can be made finer, and the toughness can be improved.
  • the average cooling rate from 800 ° C. to 500 ° C. exceeds 0.05 ° C./s, the bainite fraction increases in the structure after hot rolling, and the final structure may become coarse and the toughness may decrease. be.
  • the steel sheet After the hot rolling process, the steel sheet is once cooled to 150 ° C or lower, then reheated to a temperature of 800 ° C or higher (quenching temperature), and then to 200 ° C or lower at an average cooling rate of 1.0 ° C / s or higher. It is cooled.
  • quenching temperature a temperature of 800 ° C or higher
  • 200 ° C or lower at an average cooling rate of 1.0 ° C / s or higher. It is cooled.
  • desired strength tensile strength and yield strength
  • the structure can be made finer to secure desired strength (tensile strength and yield strength) even after PWHT, and to improve low temperature toughness.
  • desired strength tensile strength and yield strength
  • the steel sheet is heated to 650 to 850 ° C. in the intermediate heat treatment step, and then cooled to 200 ° C. or lower at an average cooling rate of 1.0 ° C./s or more.
  • the intermediate heat treatment step may be omitted if sufficient tempering can be performed in the tempering step described later.
  • the steel sheet is tempered in the tempering step, specifically heated at a tempering temperature of 550 to 700 ° C. for 30 minutes to 1 hour.
  • a tempering temperature 550 to 700 ° C. for 30 minutes to 1 hour.
  • the cooling rate after tempering is not particularly limited, and cooling may be performed by, for example, air cooling.
  • the thick steel plate according to the second embodiment of the present invention is based on mass%.
  • C 0.050 to 0.130%, Si: 0.100 to 0.600%, Mn: 1.100 to 1.800%, P: 0.0200% or less, S: 0.0100% or less, Mo: 0.050 to 0.500%, V: 0.005 to 0.100%, Nb: 0.005 to 0.100%, Al: 0.001 to 0.080%, B: 0.0003 to 0.0030%, N: 0.0100% or less, O: 0.0100% or less, Cu: 0 to 0.500%, Ni: 0 to 0.800%, Cr: 0 to 0.50%, W: 0 to 0.50%, Ti: 0.005 to 0.100%, Sn: 0 to 0.050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0100%, and balance: containing a chemical composition consisting of Fe and impurities.
  • It contains a structure in which the coarse particle size of the crystal grains surrounded by the boundary having an orientation difference of 15 ° or more is 45 ⁇ m or less and the average particle size of the crystal grains is 25 ⁇ m or less. It has a tensile strength of 580 to 730 MPa and has a tensile strength of 580 to 730 MPa. It is characterized by having a plate thickness of 70 mm or more.
  • the Nb content is set in the range of 0.005 to 0.100%, and under the conditions corresponding to the Nb content, before the hot rolling.
  • NbSegregation is diffused by performing homogenization heat treatment, and NbCN (NbCN is NbC, NbN, NbC in which part of C is replaced with N, and / or part of N in NbN is C.
  • the present inventors have used boron (B) in combination with titanium (Ti), which improves hardenability but may form boron nitride (BN) and reduce toughness. It has been found that, as in the case of the first embodiment, the structure of the thick steel plate can be refined to achieve significantly improved low temperature toughness even after PWHT.
  • Carbon (C) is an element necessary for ensuring the strength of the base material, and is also an element constituting NbCN, which is a pinning particle.
  • the C content is set to 0.050% or more.
  • the C content may be 0.060% or more, 0.070% or more, or 0.080% or more.
  • the toughness of the weld heat-affected zone (HAZ), especially the HAZ near the melt line (FL) may be significantly deteriorated in addition to the base metal, and the strength tends to be excessive. There is also. Therefore, the C content is set to 0.130% or less.
  • the C content may be 0.120% or less, 0.110% or less, or 0.100% or less.
  • Si is a deoxidizing element and is an element that also contributes to the improvement of strength. In order to obtain these effects sufficiently, the Si content is set to 0.100% or more. The Si content may be 0.150% or more, 0.200% or more, or 0.250% or more. On the other hand, if Si is excessively contained, island-shaped martensite may be formed and the toughness may be lowered. Therefore, the Si content is set to 0.600% or less. The Si content may be 0.500% or less, 0.400% or less, or 0.350% or less.
  • Mn Manganese (Mn) is a deoxidizing element and is also an element that improves hardenability.
  • the Mn content shall be 1.100% or more.
  • the Mn content may be 1.200% or more, 1.250% or more, or 1.350% or more.
  • the Mn content is set to 1.800% or less.
  • the Mn content may be 1.700% or less, 1.650% or less, or 1.600% or less.
  • Phosphorus (P) is an impurity that segregates at grain boundaries and reduces toughness. Therefore, the P content is set to 0.0200% or less.
  • the P content is preferably 0.0150% or less, more preferably 0.0100% or less, and most preferably 0.0080% or less. Since the smaller the P content is, the more preferable it is, the lower limit is not particularly specified. However, from the viewpoint of manufacturing cost, the P content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • S 0.0100% or less
  • Sulfur (S) is an impurity that promotes central segregation and may cause the formation of stretched MnS, which is the starting point of brittle fracture. Therefore, the S content is set to 0.0100% or less.
  • the S content is preferably 0.0080%, more preferably 0.0060%, and most preferably 0.0050% or less. Since the smaller the S content is, the more preferable it is, the lower limit is not particularly specified. However, from the viewpoint of manufacturing cost, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • Molybdenum is an element that improves the strength and toughness of the base metal. In order to obtain such an effect sufficiently, the Mo content is set to 0.050% or more. The Mo content may be 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if Mo is excessively contained, the strength of the base metal may be excessively increased and the toughness may be impaired. Therefore, the Mo content is set to 0.500% or less. The Mo content may be 0.450% or less, 0.400% or less, or 0.350% or less.
  • Vanadium (V) is an element that precipitates carbonitride in the tempering step and the post-welding heat treatment step, and contributes to the improvement of the strength of the base metal.
  • the V content is set to 0.005% or more.
  • the V content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • the V content is set to 0.100% or less.
  • the V content may be 0.080% or less, 0.060% or less, or 0.040% or less.
  • Niobium is an element that refines the structure by the pinning effect in the form of NbCN and improves low temperature toughness. In order to obtain such an effect, the Nb content needs to be 0.005% or more.
  • the Nb content is preferably 0.010% or more, more preferably 0.015% or more, and most preferably 0.020% or more.
  • the Nb content is set to 0.100% or less.
  • the Nb content may be 0.080% or less, 0.070% or less, or 0.050% or less.
  • Aluminum (Al) is a deoxidizing element and is also an element that suppresses the formation of cementite. Further, Al contributes to fine particle formation as pinning particles AlN. In order to obtain at least one of these effects, the Al content is set to 0.001% or more. The Al content may be 0.015% or more, 0.020% or more, or 0.025% or more. On the other hand, if Al is excessively contained, the amount of inclusions increases, which may lead to a decrease in toughness. Therefore, the Al content is set to 0.080% or less. The Al content may be 0.070% or less, 0.060% or less, or 0.050% or less.
  • B Boron
  • B is an element that segregates at grain boundaries to improve hardenability, and also has the effect of making the final structure finer. In order to obtain these effects sufficiently, the B content is set to 0.0003% or more. The B content may be 0.0005% or more, 0.0007% or more, 0.0010% or more. On the other hand, if B is excessively contained, boron nitride (BN) may be formed, resulting in a decrease in toughness. Therefore, the B content is set to 0.0030% or less. The B content may be 0.0028% or less, 0.0025% or less, or 0.0020% or less.
  • N 0.0100% or less
  • Nitrogen (N) is an element that forms a nitride, and particularly when BN is formed, it may hinder the hardenability improving effect of B. Therefore, the N content is set to 0.0100% or less.
  • the N content is preferably 0.0080% or less, more preferably 0.0060% or less, and most preferably 0.0050% or less.
  • the N content is preferably 0.0003% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • Oxygen (O) is an impurity, so it should be 0.0100% or less.
  • the O content is preferably 0.0060% or less, more preferably 0.0040% or less, and most preferably 0.0030% or less. It is preferable to reduce O as much as possible, but from the viewpoint of deoxidation cost, the O content may be 0.0001% or more, 0.0002% or more, or 0.0003% or more.
  • Titanium (Ti) is an element effective in inhibiting B from binding to solid solution nitrogen to form BN by forming titanium nitride (TiN) and consuming the solid solution nitrogen in the steel. .. In order to obtain such an effect sufficiently, the Ti content is set to 0.005% or more. The Ti content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • Ti oxide and Ti-Al oxide may be formed to reduce the dispersion density, and the effect of refining the structure of the weld heat affected zone of small heat input may be reduced. .. Therefore, the Ti content is set to 0.100% or less.
  • the Ti content may be 0.080% or less, 0.060% or less, or 0.050% or less.
  • the thick steel plate may contain one or more of the following optional elements, if necessary. Hereinafter, these optional elements will be described in detail.
  • Copper (Cu) is an element that contributes to the increase in strength.
  • the Cu content may be 0%, but in order to obtain such an effect, the Cu content is preferably 0.050% or more.
  • the Cu content may be 0.150% or more, 0.200% or more, or 0.250% or more.
  • the toughness of the base metal may decrease. Therefore, the Cu content is set to 0.500% or less.
  • the Cu content may be 0.450% or less, 0.400% or less, or 0.350% or less.
  • Nickel (Ni) is an effective element for ensuring toughness.
  • the Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.100% or more.
  • the Ni content may be 0.200% or more, 0.250% or more, or 0.300% or more.
  • the Ni content is set to 0.800% or less.
  • the Ni content may be 0.700% or less, 0.650% or less, or 0.600% or less.
  • Chromium (Cr) is an element that contributes to the improvement of carbon dioxide corrosion resistance and hardenability and affects the strength.
  • the Cr content may be 0%, but in order to obtain these effects, the Cr content is preferably 0.05% or more.
  • the Cr content may be 0.10% or more, 0.15% or more, or 0.20% or more.
  • the toughness of HAZ may decrease. Therefore, the Cr content is set to 0.50% or less.
  • the Cr content may be 0.45% or less, 0.40% or less, or 0.35% or less.
  • Tungsten is an element that contributes to the improvement of corrosion resistance and affects the strength.
  • the W content may be 0%, but in order to obtain these effects, the W content is preferably 0.05% or more.
  • the W content may be 0.10% or more, 0.15% or more, or 0.20% or more.
  • the toughness of HAZ may decrease. Therefore, the W content is set to 0.50% or less.
  • the W content may be 0.45% or less, 0.40% or less, or 0.35% or less.
  • Tin (Sn) is an element that affects strength.
  • the Sn content may be 0%, but in order to obtain this effect, the Sn content is preferably 0.005% or more.
  • the Sn content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • the toughness may decrease. Therefore, the Sn content is set to 0.050% or less.
  • the Sn content may be 0.045% or less, 0.040% or less, or 0.035% or less.
  • Ca is an element that controls the morphology of oxides and sulfides.
  • the Ca content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive Ca content may saturate the above effects and impair toughness due to the formation of inclusions. Therefore, the Ca content is set to 0.0050% or less.
  • the Ca content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.
  • Mg Magnesium (Mg) is an element that controls the morphology of oxides and sulfides.
  • the Mg content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the Mg content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive content of Mg saturates the above effects and may impair toughness due to the formation of inclusions. Therefore, the Mg content is set to 0.0050% or less.
  • the Mg content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.
  • Rare earth metals are elements that control the morphology of oxides and sulfides.
  • the REM content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the REM content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive content of REM saturates the above effects and may impair toughness due to the formation of inclusions. Therefore, the REM content is set to 0.0100% or less.
  • the REM content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
  • the REMs in the present specification are lutetium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to 71, which are lanthanoids. It is one or more elements selected from the group consisting of lutetium (Lu), and the REM content is the total content of these elements.
  • the balance other than the above elements is Fe and impurities.
  • Impurities are components that are mixed in by various factors in the manufacturing process, including raw materials such as ore and scrap, when thick steel sheets are industrially manufactured.
  • the carbon equivalent (Ceq) of the thick steel sheet, the coarse grain size of the crystal grains, the average grain size of the crystal grains, the aspect ratio of the crystal grains, the structure of the thick steel sheet, the plate thickness, and the mechanical properties according to the second embodiment are described in the embodiment. As explained above in relation to 1.
  • the method for producing a thick steel sheet according to the second embodiment includes a homogenizing heat treatment step, a hot rolling step, a quenching step, an intermediate heat treatment step, and a tempering step.
  • a homogenizing heat treatment step for melting steel
  • a hot rolling step for rolling steel
  • a quenching step for intermediate heat treatment
  • a tempering step for tempering steel
  • each step will be described in more detail.
  • the steel piece to be used in the present production method is not particularly limited as long as it is within the range of the chemical composition of the present embodiment, and a steel piece produced under any suitable casting conditions known to those skilled in the art is used. be able to.
  • the piece of steel may be an ingot-slab slab or a continuously cast slab. From the viewpoint of production efficiency, yield and energy saving, it is preferable to use a continuously cast slab as the steel piece.
  • the steel pieces having the chemical composition specified in the second embodiment are heated for homogenization in the homogenization heat treatment step before the hot rolling step.
  • the use and control of pinned particles is important to suppress the formation of coarse tissue.
  • NbCN is used as the pinning particles.
  • Nb segregation is diffused by performing homogenization heat treatment before hot rolling under conditions according to the Nb content, and the formed NbCN effectively functions as pinning particles. It is possible to suppress the formation of coarse particles, particularly the formation of coarse particles in the negative segregation portion.
  • the product S of the time spent at the temperature T ° C. or higher determined by the following formula 1 and the average temperature (° C.) at the residence time (h) during the homogenizing heat treatment is 20000 to 100,000 (° C.).
  • T 4500 / (2-log [Nb])-200 ... Equation 1
  • [Nb] is the Nb content (mass%).
  • the reheating temperature is preferably 1000 ° C. or higher from the viewpoint of reducing the load on the rolling roll, and preferably 1250 ° C. or lower from the viewpoint of suppressing coarsening of the structure.
  • the hot-rolled steel sheet is cooled from 800 ° C. to 500 ° C. at an average cooling rate of 2.0 ° C./s or higher.
  • the ferrite fraction can be increased in the structure after hot rolling, the final structure can be made finer, and the toughness can be improved.
  • the thick steel sheet according to the second embodiment containing 0.0003% or more of B when cooling after hot rolling is performed at a relatively slow average cooling rate of less than 2.0 ° C./s, the structure after hot rolling is obtained. In some cases, the bainite fraction increases, the final structure becomes coarse, and the toughness decreases.
  • the steel sheet After the hot rolling process, the steel sheet is once cooled to 150 ° C or lower, then reheated to a temperature of 800 ° C or higher (quenching temperature), and then to 200 ° C or lower at an average cooling rate of 1.0 ° C / s or higher. It is cooled.
  • quenching temperature a temperature of 800 ° C or higher
  • 200 ° C or lower at an average cooling rate of 1.0 ° C / s or higher. It is cooled.
  • desired strength tensile strength and yield strength
  • the structure can be made finer to secure desired strength (tensile strength and yield strength) even after PWHT, and to improve low temperature toughness.
  • desired strength tensile strength and yield strength
  • the steel sheet is heated to 650 to 850 ° C. in the intermediate heat treatment step, and then cooled to 200 ° C. or lower at an average cooling rate of 1.0 ° C./s or more.
  • the intermediate heat treatment step may be omitted if sufficient tempering can be performed in the tempering step described later.
  • the steel sheet is tempered in the tempering step, specifically heated at a tempering temperature of 550 to 700 ° C. for 30 minutes to 1 hour.
  • a tempering temperature 550 to 700 ° C. for 30 minutes to 1 hour.
  • the cooling rate after tempering is not particularly limited, and cooling may be performed by, for example, air cooling.
  • the thick steel plate according to the third embodiment of the present invention is based on mass%.
  • C 0.050 to 0.130%, Si: 0.100 to 0.600%, Mn: 1.100 to 1.800%, P: 0.0200% or less, S: 0.0100% or less, Mo: 0.050 to 0.500%, V: 0.005 to 0.100%, Nb: 0 to 0.100%, Al: 0.081 to 0.120%, B: Less than 0.0003%, N: 0.0100% or less, O: 0.0100% or less, Cu: 0 to 0.500%, Ni: 0 to 0.800%, Cr: 0 to 0.50%, W: 0 to 0.50%, Ti: 0 to 0.100%, Sn: 0 to 0.050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0100%, and balance: containing a chemical composition consisting of Fe and impurities.
  • the presence of coarse AlN not only does not contribute to pinning, but also has a problem that the crystal grains are partially coarsened. Therefore, the present inventors set the Al and N contents within an appropriate range, and once solid-dissolve AlN by heating under temperature conditions corresponding to these contents before the hot rolling step. It was found that by preventing the coarse AlN from remaining after the hot rolling step, the AlN can effectively function as pinning particles and the formation of the coarse grains can be suppressed.
  • the present inventors heat the steel piece at an AlN solid solution temperature of Ts ° C. or higher and 1300 ° C. or lower, which is obtained by the following formula 4 during heating before the hot rolling process.
  • the AlN is effectively functioned as pinning particles without leaving coarse AlN, and the coarse particle size of the crystal grains is 45 ⁇ m or less and the average particle size of the crystal grains is 45 ⁇ m or less in the thick steel sheet. It has been found that a fine structure having a size of 25 ⁇ m or less can be formed.
  • Carbon (C) is an element necessary for ensuring the strength of the base metal.
  • the C content is set to 0.050% or more.
  • the C content may be 0.060% or more, 0.070% or more, or 0.080% or more.
  • the toughness of the weld heat-affected zone (HAZ), especially the HAZ near the melt line (FL) may be significantly deteriorated in addition to the base metal, and the strength tends to be excessive. There is also. Therefore, the C content is set to 0.130% or less.
  • the C content may be 0.120% or less, 0.110% or less, or 0.100% or less.
  • Si is a deoxidizing element and is an element that also contributes to the improvement of strength. In order to obtain these effects sufficiently, the Si content is set to 0.100% or more. The Si content may be 0.150% or more, 0.200% or more, or 0.250% or more. On the other hand, if Si is excessively contained, island-shaped martensite may be formed and the toughness may be lowered. Therefore, the Si content is set to 0.600% or less. The Si content may be 0.500% or less, 0.400% or less, or 0.350% or less.
  • Mn Manganese (Mn) is a deoxidizing element and is also an element that improves hardenability.
  • the Mn content shall be 1.100% or more.
  • the Mn content may be 1.200% or more, 1.250% or more, or 1.350% or more.
  • the Mn content is set to 1.800% or less.
  • the Mn content may be 1.700% or less, 1.650% or less, or 1.600% or less.
  • Phosphorus (P) is an impurity that segregates at grain boundaries and reduces toughness. Therefore, the P content is set to 0.0200% or less.
  • the P content is preferably 0.0150% or less, more preferably 0.0100% or less, and most preferably 0.0080% or less. Since the smaller the P content is, the more preferable it is, the lower limit is not particularly specified. However, from the viewpoint of manufacturing cost, the P content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • S 0.0100% or less
  • Sulfur (S) is an impurity that promotes central segregation and may cause the formation of stretched MnS, which is the starting point of brittle fracture. Therefore, the S content is set to 0.0100% or less.
  • the S content is preferably 0.0080%, more preferably 0.0060%, and most preferably 0.0050% or less. Since the smaller the S content is, the more preferable it is, the lower limit is not particularly specified. However, from the viewpoint of manufacturing cost, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • Molybdenum is an element that improves the strength and toughness of the base metal. In order to obtain such an effect sufficiently, the Mo content is set to 0.050% or more. The Mo content may be 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if Mo is excessively contained, the strength of the base metal may be excessively increased and the toughness may be impaired. Therefore, the Mo content is set to 0.500% or less. The Mo content may be 0.450% or less, 0.400% or less, or 0.350% or less.
  • Vanadium (V) is an element that precipitates carbonitride in the tempering step and the post-welding heat treatment step, and contributes to the improvement of the strength of the base metal.
  • the V content is set to 0.005% or more.
  • the V content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • the V content is set to 0.100% or less.
  • the V content may be 0.080% or less, 0.060% or less, or 0.040% or less.
  • Aluminum (Al) is a deoxidizing element and is also an element that suppresses the formation of cementite. Further, Al contributes to fine particle formation as pinning particles AlN. In order to obtain these effects sufficiently, the Al content is 0.081% or more. The Al content may be 0.085% or more, 0.090% or more, or 0.095% or more. On the other hand, if Al is excessively contained, it becomes difficult to dissolve AlN in solid solution, and the coarse AlN itself becomes the starting point of brittle fracture, which may lead to a decrease in toughness. Therefore, the Al content is set to 0.120% or less. The Al content may be 0.115% or less, 0.110% or less, or 0.105% or less.
  • B is an element that segregates at grain boundaries to improve hardenability. However, if B is contained excessively, the strength may become too high, or the formation of upper bainite may be promoted, resulting in a decrease in toughness. Therefore, the B content is set to less than 0.0003%.
  • the B content may be 0.0002% or less, 0.0001% or less, or 0%.
  • N Nitrogen (N) is an element that forms a nitride, and if it is contained in an excessive amount, a coarse nitride is formed, which causes a decrease in toughness. Therefore, the N content is set to 0.0100% or less.
  • the N content is preferably 0.0090% or less, more preferably 0.0080% or less, and most preferably 0.0070% or less.
  • the N content is preferably 0.0027% or more, and may be 0.0030% or more, 0.0035% or more, or 0.0040% or more.
  • Oxygen (O) is an impurity, so it should be 0.0100% or less.
  • the O content is preferably 0.0060% or less, more preferably 0.0040% or less, and most preferably 0.0030% or less. It is preferable to reduce O as much as possible, but from the viewpoint of deoxidation cost, the O content may be 0.0001% or more, 0.0002% or more, or 0.0003% or more.
  • AlN [[Al] ⁇ [N] ⁇ 3.2 ⁇ 10 -4 ]
  • the contents of Al and N are specified separately, it may not be possible to generate AlN as pinning particles in an appropriate amount and size.
  • the Al and N contents are kept within an appropriate range in terms of the product of those contents, and as will be described in detail later, the temperature conditions according to the product of the contents are before the hot rolling step. By heating underneath, AlN can be effectively functioned as pinning particles and the formation of coarse particles can be suppressed.
  • the contents of Al and N are [Al] ⁇ [N] ⁇ 3.2 ⁇ 10 -4 , preferably [Al] ⁇ [N] ⁇ 4.0 ⁇ 10. It is necessary to satisfy -4.
  • the upper limit of [Al] ⁇ [N] is not particularly limited, but if the value of [Al] ⁇ [N] becomes too high, the heating temperature before the hot rolling step for solid-solving AlN becomes high. Therefore, in general, it is preferable that the contents of Al and N satisfy [Al] ⁇ [N] ⁇ 9.5 ⁇ 10 -4.
  • the thick steel plate may contain one or more of the following optional elements, if necessary. Hereinafter, these optional elements will be described in detail.
  • Niobium is an element that refines the structure by the pinning effect in the form of NbCN and improves low temperature toughness.
  • the Nb content may be 0%, but in order to obtain such an effect, the Nb content is preferably 0.005% or more.
  • the Nb content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • the Nb content is set to 0.100% or less.
  • the Nb content may be 0.080% or less, 0.070% or less, or 0.050% or less.
  • Copper (Cu) is an element that contributes to the increase in strength.
  • the Cu content may be 0%, but in order to obtain such an effect, the Cu content is preferably 0.050% or more.
  • the Cu content may be 0.150% or more, 0.200% or more, or 0.250% or more.
  • the toughness of the base metal may decrease. Therefore, the Cu content is set to 0.500% or less.
  • the Cu content may be 0.450% or less, 0.400% or less, or 0.350% or less.
  • Nickel (Ni) is an effective element for ensuring toughness.
  • the Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.100% or more.
  • the Ni content may be 0.200% or more, 0.250% or more, or 0.300% or more.
  • the Ni content is set to 0.800% or less.
  • the Ni content may be 0.700% or less, 0.650% or less, or 0.600% or less.
  • Chromium (Cr) is an element that contributes to the improvement of carbon dioxide corrosion resistance and hardenability and affects the strength.
  • the Cr content may be 0%, but in order to obtain these effects, the Cr content is preferably 0.05% or more.
  • the Cr content may be 0.10% or more, 0.15% or more, or 0.20% or more.
  • the toughness of HAZ may decrease. Therefore, the Cr content is set to 0.50% or less.
  • the Cr content may be 0.45% or less, 0.40% or less, or 0.35% or less.
  • Tungsten is an element that contributes to the improvement of corrosion resistance and affects the strength.
  • the W content may be 0%, but in order to obtain these effects, the W content is preferably 0.05% or more.
  • the W content may be 0.10% or more, 0.15% or more, or 0.20% or more.
  • the toughness of HAZ may decrease. Therefore, the W content is set to 0.50% or less.
  • the W content may be 0.45% or less, 0.40% or less, or 0.35% or less.
  • Ti 0 to 0.100%
  • Ti titanium
  • the Ti content may be 0%, but in order to obtain the above effects, the Ti content is preferably 0.005% or more.
  • the Ti content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • Ti oxide and Ti-Al oxide may be formed to reduce the dispersion density, and the effect of refining the structure of the weld heat affected zone of small heat input may be reduced. .. Therefore, the Ti content is set to 0.100% or less.
  • the Ti content may be 0.080% or less, 0.060% or less, or 0.050% or less.
  • Tin (Sn) is an element that affects strength.
  • the Sn content may be 0%, but in order to obtain this effect, the Sn content is preferably 0.005% or more.
  • the Sn content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • the toughness may decrease. Therefore, the Sn content is set to 0.050% or less.
  • the Sn content may be 0.045% or less, 0.040% or less, or 0.035% or less.
  • Ca is an element that controls the morphology of oxides and sulfides.
  • the Ca content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive Ca content may saturate the above effects and impair toughness due to the formation of inclusions. Therefore, the Ca content is set to 0.0050% or less.
  • the Ca content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.
  • Mg Magnesium (Mg) is an element that controls the morphology of oxides and sulfides.
  • the Mg content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the Mg content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive content of Mg saturates the above effects and may impair toughness due to the formation of inclusions. Therefore, the Mg content is set to 0.0050% or less.
  • the Mg content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.
  • Rare earth metals are elements that control the morphology of oxides and sulfides.
  • the REM content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the REM content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive content of REM saturates the above effects and may impair toughness due to the formation of inclusions. Therefore, the REM content is set to 0.0100% or less.
  • the REM content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
  • the REMs in the present specification are lutetium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to 71, which are lanthanoids. It is one or more elements selected from the group consisting of lutetium (Lu), and the REM content is the total content of these elements.
  • the balance other than the above elements is Fe and impurities.
  • Impurities are components that are mixed in by various factors in the manufacturing process, including raw materials such as ore and scrap, when thick steel sheets are industrially manufactured.
  • the carbon equivalent (Ceq) of the thick steel sheet, the coarse grain size of the crystal grains, the average grain size of the crystal grains, the aspect ratio of the crystal grains, the structure of the thick steel sheet, the plate thickness, and the mechanical properties according to the third embodiment are described in the embodiment. As explained above in relation to 1.
  • the method for manufacturing a thick steel sheet according to the third embodiment includes a heating step, a hot rolling step, a quenching step, and a tempering step.
  • a heating step a hot rolling step
  • a quenching step a quenching step
  • a tempering step a tempering step.
  • each step will be described in more detail.
  • the steel piece to be used in the present production method is not particularly limited as long as it is within the range of the chemical composition of the present embodiment, and a steel piece produced under any suitable casting conditions known to those skilled in the art is used. be able to.
  • the piece of steel may be an ingot-slab slab or a continuously cast slab. From the viewpoint of production efficiency, yield and energy saving, it is preferable to use a continuously cast slab as the steel piece.
  • the steel pieces having the chemical composition specified in the third embodiment are heated before the hot rolling step.
  • the use and control of pinned particles is important to suppress the formation of coarse tissue. This is because the coarse austenite ( ⁇ ) particles are generated when the pin is released.
  • ⁇ coarse austenite
  • AlN is used as the pinning particles, but the presence of coarse AlN not only does not contribute to pinning, but also has a problem that the crystal grains are partially coarsened.
  • AlN is once solid-dissolved by heating under a temperature condition corresponding to the product of Al and N contents before the hot rolling step, and coarse AlN does not remain after the hot rolling step.
  • AlN can be effectively functioned as pinning particles and the formation of coarse particles can be suppressed.
  • the heating temperature is set to 1300 ° C. or lower.
  • Ts 7400 / (1.95-log ([Al] ⁇ [N]))-273 ⁇ ⁇ ⁇ Equation 4
  • [Al] and [N] are the contents (mass%) of Al and N, respectively.
  • the steel pieces are generally hot rolled in the hot rolling step at a rolling reduction of 50% or more, and then cooled from 800 ° C. to 500 ° C. at an average cooling rate of 0.10 ° C./s or more.
  • an average cooling rate 0.10 ° C./s or more.
  • the steel sheet After the hot rolling process, the steel sheet is once cooled to 150 ° C or lower, then reheated to a temperature of 850 ° C or higher (quenching temperature), and then to 200 ° C or lower at an average cooling rate of 1.0 ° C / s or higher. It is cooled.
  • quenching temperature a temperature of 850 ° C or higher
  • 200 ° C or lower at an average cooling rate of 1.0 ° C / s or higher. It is cooled.
  • desired strength tensile strength and yield strength
  • the steel sheet is tempered in the tempering step, specifically heated at a tempering temperature of 550 to 700 ° C. for 30 minutes to 1 hour.
  • a tempering temperature 550 to 700 ° C. for 30 minutes to 1 hour.
  • the cooling rate after tempering is not particularly limited, and cooling may be performed by, for example, air cooling.
  • the thick steel plate according to the fourth embodiment of the present invention is based on mass%.
  • It contains a structure in which the coarse particle size of the crystal grains surrounded by the boundary having an orientation difference of 15 ° or more is 45 ⁇ m or less and the average particle size of the crystal grains is 25 ⁇ m or less. It has a tensile strength of 580 to 730 MPa and has a tensile strength of 580 to 730 MPa. It is characterized by having a plate thickness of 70 mm or more.
  • the use and control of pinned particles may be effective in suppressing the formation of coarse tissue.
  • the present inventors have focused on and studied the use of TiN as pinning particles in relation to the miniaturization of crystal grains. Therefore, the present inventors set the Ti and N contents within an appropriate range, and suppress the formation of coarse grains by rolling under conditions corresponding to these contents in the hot rolling step. I found that I could do it.
  • the present inventors hot-rolled so that the parameter Z obtained by the following formula 5 is 7 or more during the hot-rolling step, so that the ferrite content in the structure after the hot-rolling is
  • the coarse grain size of the crystal grains is 45 ⁇ m or less and the average grain size of the crystal grains is 25 ⁇ m or less in the thick steel sheet.
  • according to the fourth embodiment of the present invention even after PWHT in which the low temperature toughness generally decreases, as compared with the case of a structure in which the coarse particle size and the average particle size of the crystal grains are not within the above ranges.
  • Z 0.08 ⁇ ⁇ + 300 ⁇ f + 10 ⁇ ⁇ ⁇ f ⁇ ⁇ Equation 5
  • ⁇ is the cumulative reduction rate (%) at 800 ° C. or lower
  • f is the smaller value of the Ti content (mass%) and the 3.4 ⁇ N content (mass%).
  • Carbon (C) is an element necessary for ensuring the strength of the base metal.
  • the C content is set to 0.050% or more.
  • the C content may be 0.060% or more, 0.070% or more, or 0.080% or more.
  • the toughness of the weld heat-affected zone (HAZ), especially the HAZ near the melt line (FL) may be significantly deteriorated in addition to the base metal, and the strength tends to be excessive. There is also. Therefore, the C content is set to 0.130% or less.
  • the C content may be 0.120% or less, 0.110% or less, or 0.100% or less.
  • Si is a deoxidizing element and is an element that also contributes to the improvement of strength. In order to obtain these effects sufficiently, the Si content is set to 0.100% or more. The Si content may be 0.150% or more, 0.200% or more, or 0.250% or more. On the other hand, if Si is excessively contained, island-shaped martensite may be formed and the toughness may be lowered. Therefore, the Si content is set to 0.600% or less. The Si content may be 0.500% or less, 0.400% or less, or 0.350% or less.
  • Mn Manganese (Mn) is a deoxidizing element and is also an element that improves hardenability.
  • the Mn content shall be 1.100% or more.
  • the Mn content may be 1.200% or more, 1.250% or more, or 1.350% or more.
  • the Mn content is set to 1.800% or less.
  • the Mn content may be 1.700% or less, 1.650% or less, or 1.600% or less.
  • Phosphorus (P) is an impurity that segregates at grain boundaries and reduces toughness. Therefore, the P content is set to 0.0200% or less.
  • the P content is preferably 0.0150% or less, more preferably 0.0100% or less, and most preferably 0.0080% or less. Since the smaller the P content is, the more preferable it is, the lower limit is not particularly specified. However, from the viewpoint of manufacturing cost, the P content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • S 0.0100% or less
  • Sulfur (S) is an impurity that promotes central segregation and may cause the formation of stretched MnS, which is the starting point of brittle fracture. Therefore, the S content is set to 0.0100% or less.
  • the S content is preferably 0.0080%, more preferably 0.0060%, and most preferably 0.0050% or less. Since the smaller the S content is, the more preferable it is, the lower limit is not particularly specified. However, from the viewpoint of manufacturing cost, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • Molybdenum is an element that improves the strength and toughness of the base metal. In order to obtain such an effect sufficiently, the Mo content is set to 0.050% or more. The Mo content may be 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if Mo is excessively contained, the strength of the base metal may be excessively increased and the toughness may be impaired. Therefore, the Mo content is set to 0.500% or less. The Mo content may be 0.450% or less, 0.400% or less, or 0.350% or less.
  • Vanadium (V) is an element that precipitates carbonitride in the tempering step and the post-welding heat treatment step, and contributes to the improvement of the strength of the base metal.
  • the V content is set to 0.005% or more.
  • the V content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • the V content is set to 0.100% or less.
  • the V content may be 0.080% or less, 0.060% or less, or 0.040% or less.
  • Aluminum (Al) is a deoxidizing element and is also an element that suppresses the formation of cementite. Further, Al contributes to fine particle formation as pinning particles AlN. In order to obtain at least one of these effects, the Al content is set to 0.001% or more. The Al content may be 0.015% or more, 0.020% or more, or 0.025% or more. On the other hand, if Al is excessively contained, the amount of inclusions increases, which may lead to a decrease in toughness. Therefore, the Al content is set to 0.080% or less. The Al content may be 0.070% or less, 0.060% or less, or 0.050% or less.
  • N Nitrogen (N) is an element that forms a nitride, and if it is contained in an excessive amount, a coarse nitride is formed, which causes a decrease in toughness. Therefore, the N content is set to 0.0100% or less.
  • the N content is preferably 0.0080% or less, more preferably 0.0060% or less, and most preferably 0.0050% or less.
  • the N content is preferably 0.0003% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • Oxygen (O) is an impurity, so it should be 0.0100% or less.
  • the O content is preferably 0.0060% or less, more preferably 0.0040% or less, and most preferably 0.0030% or less. It is preferable to reduce O as much as possible, but from the viewpoint of deoxidation cost, the O content may be 0.0001% or more, 0.0002% or more, or 0.0003% or more.
  • Titanium (Ti) contributes to fine graining as pinning particles TiN and improves toughness.
  • the Ti content is set to 0.005% or more.
  • the Ti content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • the Ti content is set to 0.100% or less.
  • the Ti content may be 0.080% or less, 0.060% or less, or 0.050% or less.
  • Ti is added to make B function effectively, whereas in the fourth embodiment, Ti is added to form pinning particles TiN. Therefore, in the fourth embodiment, the Ti content can be appropriately determined from the range of 0.005 to 0.100% regardless of the B content.
  • the thick steel plate may contain one or more of the following optional elements, if necessary. Hereinafter, these optional elements will be described in detail.
  • Copper (Cu) is an element that contributes to the increase in strength.
  • the Cu content may be 0%, but in order to obtain such an effect, the Cu content is preferably 0.050% or more.
  • the Cu content may be 0.150% or more, 0.200% or more, or 0.250% or more.
  • the toughness of the base metal may decrease. Therefore, the Cu content is set to 0.500% or less.
  • the Cu content may be 0.450% or less, 0.400% or less, or 0.350% or less.
  • Nickel (Ni) is an effective element for ensuring toughness.
  • the Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.100% or more.
  • the Ni content may be 0.200% or more, 0.250% or more, or 0.300% or more.
  • the Ni content is set to 0.800% or less.
  • the Ni content may be 0.700% or less, 0.650% or less, or 0.600% or less.
  • Chromium (Cr) is an element that contributes to the improvement of carbon dioxide corrosion resistance and hardenability and affects the strength.
  • the Cr content may be 0%, but in order to obtain these effects, the Cr content is preferably 0.05% or more.
  • the Cr content may be 0.10% or more, 0.15% or more, or 0.20% or more.
  • the toughness of HAZ may decrease. Therefore, the Cr content is set to 0.50% or less.
  • the Cr content may be 0.45% or less, 0.40% or less, or 0.35% or less.
  • Tungsten is an element that contributes to the improvement of corrosion resistance and affects the strength.
  • the W content may be 0%, but in order to obtain these effects, the W content is preferably 0.05% or more.
  • the W content may be 0.10% or more, 0.15% or more, or 0.20% or more.
  • the toughness of HAZ may decrease. Therefore, the W content is set to 0.50% or less.
  • the W content may be 0.45% or less, 0.40% or less, or 0.35% or less.
  • Tin (Sn) is an element that affects strength.
  • the Sn content may be 0%, but in order to obtain this effect, the Sn content is preferably 0.005% or more.
  • the Sn content may be 0.010% or more, 0.015% or more, or 0.020% or more.
  • the toughness may decrease. Therefore, the Sn content is set to 0.050% or less.
  • the Sn content may be 0.045% or less, 0.040% or less, or 0.035% or less.
  • Ca is an element that controls the morphology of oxides and sulfides.
  • the Ca content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive Ca content may saturate the above effects and impair toughness due to the formation of inclusions. Therefore, the Ca content is set to 0.0050% or less.
  • the Ca content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.
  • Mg Magnesium (Mg) is an element that controls the morphology of oxides and sulfides.
  • the Mg content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the Mg content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive content of Mg saturates the above effects and may impair toughness due to the formation of inclusions. Therefore, the Mg content is set to 0.0050% or less.
  • the Mg content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.
  • Rare earth metals are elements that control the morphology of oxides and sulfides.
  • the REM content may be 0%, but in order to obtain such an effect, it is preferably 0.0001% or more.
  • the REM content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. Excessive content of REM saturates the above effects and may impair toughness due to the formation of inclusions. Therefore, the REM content is set to 0.0100% or less.
  • the REM content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
  • the REMs in the present specification are lutetium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to 71, which are lanthanoids. It is one or more elements selected from the group consisting of lutetium (Lu), and the REM content is the total content of these elements.
  • the balance other than the above elements is Fe and impurities.
  • Impurities are components that are mixed in by various factors in the manufacturing process, including raw materials such as ore and scrap, when thick steel sheets are industrially manufactured.
  • the carbon equivalent (Ceq) of the thick steel sheet, the coarse grain size of the crystal grains, the average grain size of the crystal grains, the aspect ratio of the crystal grains, the structure of the thick steel sheet, the plate thickness, and the mechanical properties according to the fourth embodiment are described in the embodiment. As explained above in relation to 1.
  • the method for producing a thick steel sheet according to the fourth embodiment includes a hot rolling step, a quenching step, an intermediate heat treatment step, and a tempering step after the homogenizing heat treatment.
  • the above homogenization heat treatment is optional and may or may not be performed.
  • each of the other steps will be described in more detail.
  • the steel piece to be used in the present production method is not particularly limited as long as it is within the range of the chemical composition of the present embodiment, and a steel piece produced under any suitable casting conditions known to those skilled in the art is used. be able to.
  • the piece of steel may be an ingot-slab slab or a continuously cast slab. From the viewpoint of production efficiency, yield and energy saving, it is preferable to use a continuously cast slab as the steel piece.
  • the steel pieces having the chemical composition specified in the fourth embodiment are heated in the hot rolling step, and then hot rolled so that the parameter Z obtained by the following formula 5 is 7 or more.
  • the heating temperature is preferably 1000 ° C. or higher from the viewpoint of reducing the load on the rolling roll, and preferably 1250 ° C. or lower from the viewpoint of suppressing coarsening of the structure.
  • Z 0.08 ⁇ ⁇ + 300 ⁇ f + 10 ⁇ ⁇ ⁇ f ⁇ ⁇ Equation 5
  • ⁇ is the cumulative reduction rate (%) at 800 ° C. or lower
  • f is the smaller value of the Ti content (mass%) and the 3.4 ⁇ N content (mass%).
  • the ferrite fraction is increased in the structure after hot rolling by hot rolling so that the parameter Z obtained by the above formula 5 is 7 or more in the hot rolling step.
  • the coarse grain size of the crystal grains is 45 ⁇ m or less and the average grain size of the crystal grains is 25 ⁇ m or less in the thick steel sheet. It is possible to form a fine structure.
  • the bainite fraction may increase in the structure after hot rolling, and the final structure may become coarse and the toughness may decrease.
  • the hot-rolled steel sheet is cooled from 800 ° C. to 500 ° C. at an average cooling rate of 0.20 ° C./s or less.
  • the ferrite fraction can be increased in the structure after hot rolling, the final structure can be made finer, and the toughness can be improved.
  • the average cooling rate exceeds 0.20 ° C./s, the bainite fraction increases in the structure after hot rolling, and the final structure may become coarse and the toughness may decrease.
  • the steel sheet After the hot rolling process, the steel sheet is once cooled to 150 ° C or lower, then reheated to a temperature of 800 ° C or higher (quenching temperature), and then to 200 ° C or lower at an average cooling rate of 1.0 ° C / s or higher. It is cooled.
  • quenching temperature a temperature of 800 ° C or higher
  • 200 ° C or lower at an average cooling rate of 1.0 ° C / s or higher. It is cooled.
  • desired strength tensile strength and yield strength
  • the structure can be made finer to secure desired strength (tensile strength and yield strength) even after PWHT, and to improve low temperature toughness.
  • desired strength tensile strength and yield strength
  • the steel sheet is heated to 650 to 850 ° C. in the intermediate heat treatment step, and then cooled to 200 ° C. or lower at an average cooling rate of 1.0 ° C./s or more.
  • the intermediate heat treatment step may be omitted if sufficient tempering can be performed in the tempering step described later.
  • the steel sheet is tempered in the tempering step, specifically heated at a tempering temperature of 550 to 700 ° C. for 30 minutes to 1 hour.
  • a tempering temperature 550 to 700 ° C. for 30 minutes to 1 hour.
  • the cooling rate after tempering is not particularly limited, and cooling may be performed by, for example, air cooling.
  • the thick steel plates according to the first to fourth embodiments were manufactured in Examples A to D, respectively, and the mechanical properties of the obtained thick steel plates were investigated.
  • the coarse particle size, average particle size, aspect ratio, and mechanical properties of the crystal grains in the thick steel sheets obtained in Examples A to D were determined by the following methods.
  • the coarse particle size of the crystal grains was determined as follows. First, the L cross section (cross section parallel to the rolling direction and the plate thickness direction of the thick steel plate) at the plate thickness 1/4 position of the thick steel plate is mirror-polished, and then an arbitrary 1.0 mm by electron backscatter diffraction method (EBSD). The crystal orientation of a region of ⁇ 0.4 mm is measured at one location, the region where the orientation difference of adjacent grains is 15 ° or more is defined as one crystal grain, and the particle size of each crystal grain is calculated as the equivalent diameter of a circle. did. Next, 10 of these crystal grains having a large circle-equivalent diameter were selected, and the average value of those circle-equivalent diameters was determined as the "coarse grain size of the crystal grains".
  • EBSD electron backscatter diffraction method
  • the average particle size of the crystal grains was determined as follows. First, as in the case of coarse particle size, the L cross section (cross section parallel to the rolling direction and the plate thickness direction of the thick steel plate) at the plate thickness 1/4 position of the thick steel plate is mirror-polished, and then the electron backscatter diffraction method is performed. (EBSD) is used to measure the crystal orientation of an arbitrary 1.0 mm ⁇ 0.4 mm region at one location, and the region where the orientation difference between adjacent grains is 15 ° or more is defined as one crystal grain, and each crystal grain is defined as one crystal grain. Was calculated as a circle-equivalent diameter.
  • EBSD electron backscatter diffraction method
  • the aspect ratio of the crystal grains was determined as follows. First, the L cross section (cross section parallel to the rolling direction and the plate thickness direction of the thick steel plate) at the plate thickness 1/4 position of the thick steel plate is mirror-polished, and then an arbitrary 1.0 mm by electron backscatter diffraction method (EBSD). The crystal orientation of a region of ⁇ 0.4 mm is measured at one location, and the region where the orientation difference of adjacent grains is 15 ° or more is defined as one crystal grain, and the rolling direction length and plate thickness direction of each crystal grain are defined. The length was measured and the aspect ratio of each crystal grain was calculated. Next, the arithmetic mean of the calculated aspect ratios of all the crystal grains was determined as the "aspect ratio of the crystal grains”.
  • EBSD electron backscatter diffraction method
  • TS Tensile strength
  • YS yield strength
  • Charpy when the obtained thick steel sheet is heat-treated at 650 ° C. for 15 hours, which corresponds to PWHT, in order to evaluate the mechanical properties of the thick steel sheet after PWHT.
  • the average value of shock absorption energy (vE -35) was measured.
  • TS and YS were measured by performing a tensile test in accordance with JIS Z2241: 2011 based on a JIS No. 5 test piece collected from a direction (C direction) parallel to the plate width direction of the heat-treated thick steel sheet.
  • the average value of vE- 35 is based on the JIS No.
  • test piece taken from the C direction of the thick steel sheet that has been similarly heat-treated, and in accordance with the provisions of JIS Z2242: 2005, using an impact blade with a radius of 2 mm- It was calculated by measuring three Charpy impact absorption energies at 35 ° C. and averaging them.
  • Example A (corresponding to Embodiment 1)
  • a slab having the chemical composition shown in Table 1 was cast by a continuous casting method.
  • a thick steel plate having a plate thickness of 70 mm or more was produced from these slabs under the production conditions shown in Table 2.
  • the rest other than the components shown in Table 1 are Fe and impurities.
  • Hot rolling was carried out at a rolling reduction of 50% or more, and after the hot rolling step, the steel sheet was once cooled to 150 ° C. or lower, and then reheated to the quenching temperature shown in Table 2.
  • Table 3 Although not shown in Table 3, the aspect ratio of the crystal grains was 1.5 or less in all the examples in Table 3.
  • Comparative Examples 133 to 137 sufficient TS could not be obtained because the content of C, Si, Mn, Mo or V was low.
  • Comparative Example 138 since the Nb content was low, the pinning effect of NbCN was not sufficiently obtained, the crystal grains were coarsened, and the low temperature toughness was lowered.
  • Comparative Example 139 since Al was not contained, the pinning effect by AlN could not be obtained, the crystal grains were coarsened, and the low temperature toughness was lowered.
  • Comparative Examples 140, 142 and 146 to 148 the C, Mn, Ni, Mo or V content was high, so that the strength was excessive and the low temperature toughness was lowered.
  • Comparative Example 141 since the Si content was high, island-shaped martensite was formed and the low temperature toughness was lowered.
  • Comparative Examples 143 to 145 the low temperature toughness was lowered because the P, S or Cu content was high.
  • Comparative Examples 149 and 150 since the Nb or Al content was high, coarse precipitates were precipitated and the amount of inclusions was large, resulting in a decrease in low temperature toughness.
  • Comparative Example 151 since the B content was high, the strength became too high and the low temperature toughness decreased.
  • Comparative Examples 152 and 153 since the N or O content was high, a large amount of inclusions and the like were generated, and the low temperature toughness decreased.
  • Comparative Examples 154 to 156 since the control in the homogenization heat treatment step was not appropriate, the pinning effect by NbCN was not sufficiently obtained, the crystal grains were coarsened, and the low temperature toughness was lowered. In Comparative Examples 157 and 158, the bainite fraction increased due to the high average cooling rate after hot rolling, the final structure became coarse, and the low temperature toughness decreased.
  • the thick steel sheets according to all the examples are mainly composed of ferrite, and the total content of tempered martensite and tempered lower bainite in the structure is 30% or less. It turned out that there was.
  • Example B (corresponding to Embodiment 2)
  • a slab having the chemical composition shown in Table 4 was cast by a continuous casting method.
  • a thick steel plate having a plate thickness of 70 mm or more was produced from these slabs under the production conditions shown in Table 5.
  • the rest other than the components shown in Table 4 are Fe and impurities.
  • Hot rolling was carried out at a rolling reduction of 50% or more, and after the hot rolling step, the steel sheet was once cooled to 150 ° C. or lower, and then reheated to the quenching temperature shown in Table 2. The results are shown in Table 6. Although not shown in Table 6, the aspect ratio of the crystal grains was 1.5 or less in all the examples in Table 6.
  • Comparative Examples 228 to 231 and 233 With reference to Tables 4 to 6, in Comparative Examples 228 to 231 and 233, sufficient TS could not be obtained because the C, Si, Mn, Mo or V contents were low.
  • Comparative Example 232 since the Nb content was low, the pinning effect of NbCN was not sufficiently obtained, the crystal grains were coarsened, and the low temperature toughness was lowered.
  • Comparative Example 234 it is considered that since the Ti content was low, the consumption of solid solution nitrogen in the steel due to the formation of TiN was small, and as a result, a large amount of BN was generated and the low temperature toughness was lowered.
  • Comparative Example 235 since the B content was low, the effect of fine graining by B was not sufficiently obtained, the crystal grains were coarsened, and the low temperature toughness was lowered.
  • Comparative Example 236 since Al was not contained, the pinning effect by AlN could not be obtained, the crystal grains were coarsened, and the low temperature toughness was lowered.
  • Comparative Examples 240 and 241 the low temperature toughness decreased due to the high P or S content.
  • Comparative Examples 245 and 249 since the Nb or Al content was high, coarse precipitates were precipitated and the amount of inclusions was large, resulting in a decrease in low temperature toughness.
  • Comparative Example 247 since the Ti content was high, Ti oxide and the like were formed and the low temperature toughness was lowered.
  • Comparative Examples 250 and 251 since the N or O content was high, a large amount of inclusions and the like were generated, and the low temperature toughness was lowered.
  • Comparative Examples 252 and 253 since the control in the homogenization heat treatment step was not appropriate, the pinning effect by NbCN was not sufficiently obtained, the crystal grains were coarsened, and the low temperature toughness was lowered. In Comparative Examples 254 and 255, the bainite fraction increased due to the slow average cooling rate after hot rolling, the final structure became coarse, and the low temperature toughness decreased.
  • the thick steel sheets according to all the examples are mainly composed of ferrite, and the total content of tempered martensite and tempered lower bainite in the structure is 30% or less. It turned out that there was.
  • Example C (corresponding to Embodiment 3)
  • a slab having the chemical composition shown in Table 7 was cast by a continuous casting method.
  • a thick steel plate having a plate thickness of 70 mm or more was produced from these slabs under the production conditions shown in Table 8.
  • the rest other than the components shown in Table 7 are Fe and impurities.
  • Hot rolling is carried out at a rolling reduction of 50% or more, and after the hot rolling process, the steel sheet is once cooled to 150 ° C. or lower, then reheated to the quenching temperature shown in Table 8, and then 1.0 ° C./s. It was cooled to 200 ° C. or lower at the above average cooling rate.
  • the results are shown in Table 8.
  • the aspect ratio of the crystal grains was 1.5 or less in all the examples in Table 8.
  • Comparative Examples 311, 313, 315, 320 and 322 in Comparative Examples 311, 313, 315, 320 and 322, sufficient TS could not be obtained because the C, Si, Mn, Mo or V content was low.
  • Comparative Examples 317 and 318 the low temperature toughness decreased due to the high P or S content.
  • Comparative Example 319 since the Ni content was high, the hardenability became excessive and the low temperature toughness decreased.
  • Comparative Example 324 since the Al content was low, the pinning effect of AlN was not sufficiently obtained, the crystal grains were coarsened, and the low temperature toughness was lowered. On the other hand, in Comparative Example 325, since the Al content was high, coarse AlN was formed and the low temperature toughness was lowered.
  • Comparative Example 326 since the B content was high, the strength became excessive and the low temperature toughness decreased.
  • Comparative Example 327 since [Al] ⁇ [N] was low, the pinning effect by AlN could not be sufficiently obtained, the crystal grains were coarsened, and the low temperature toughness was lowered.
  • Comparative Examples 328 to 330 since the control in the heating step was not appropriate, coarse AlN remained after the hot rolling step, the pinning effect by the AlN was not sufficiently obtained, and the crystal grains became coarse and the temperature was low. The toughness has decreased.
  • Comparative Examples 331 and 332 since the average cooling rate after hot rolling was slow, coarsening of AlN occurred and low temperature toughness decreased.
  • the thick steel sheets according to all the examples are mainly composed of ferrite, and the total content of tempered martensite and tempered lower bainite in the structure is 30% or less. It turned out that there was.
  • Example D (corresponding to Embodiment 4)
  • a slab having the chemical composition shown in Table 9 was cast by a continuous casting method.
  • a thick steel plate having a plate thickness of 70 mm or more was produced from these slabs under the production conditions shown in Table 10.
  • the rest other than the components shown in Table 9 are Fe and impurities.
  • the steel sheet was once cooled to 150 ° C. or lower, and then reheated to the quenching temperature shown in Table 10. The results are shown in Table 11.
  • the aspect ratio of the crystal grains was 1.5 or less in all the examples in Table 11.
  • Comparative Examples 422 to 426 With reference to Tables 9 to 11, in Comparative Examples 422 to 426, sufficient TS could not be obtained because the C, Si, Mn, Mo or V contents were low.
  • Comparative Example 427 since the Ti content was low, the pinning effect of TiN was not sufficiently obtained, the crystal grains were coarsened, and the low temperature toughness was lowered.
  • Comparative Example 428 since the Al content was low, the pinning effect of AlN was not sufficiently obtained, and the low temperature toughness was lowered.
  • Comparative Examples 429 and 431 since the C or Mn content was high, the strength became excessive and the low temperature toughness decreased.
  • Comparative Examples 430 and 432 to 434 the low temperature toughness decreased due to the high content of Si, P, S or Cu.
  • Comparative Examples 435 to 437 the Ni, Mo or V content was high, so that the strength was excessive and the low temperature toughness was lowered.
  • Comparative Example 438 since the Ti content was high, a large amount of TiN was generated and the low temperature toughness decreased. In Comparative Example 439, since the Al content was high, the amount of inclusions was large and the low temperature toughness was lowered. In Comparative Example 440, since the N content was high, a large amount of inclusions and the like were generated, and the low temperature toughness decreased. In Comparative Example 441, since the parameter Z in the hot rolling step was not appropriate, the bainite fraction increased in the structure after hot rolling, the final structure became coarse, and the low temperature toughness decreased. In Comparative Example 442, since the average cooling rate after hot rolling was high, the bainite fraction also increased in the structure after hot rolling, the final structure became coarse, and the low temperature toughness decreased.
  • the thick steel sheets according to all the examples are mainly composed of ferrite, and the total content of tempered martensite and tempered lower bainite in the structure is 30% or less. It turned out that there was.

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JPWO2024161737A1 (https=) * 2023-01-30 2024-08-08

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KR20250092395A (ko) 2023-12-14 2025-06-24 주식회사 포스코 구조용 강판 및 이의 제조방법

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