WO2013150687A1 - アレスト性に優れた高強度厚鋼板 - Google Patents

アレスト性に優れた高強度厚鋼板 Download PDF

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WO2013150687A1
WO2013150687A1 PCT/JP2012/082669 JP2012082669W WO2013150687A1 WO 2013150687 A1 WO2013150687 A1 WO 2013150687A1 JP 2012082669 W JP2012082669 W JP 2012082669W WO 2013150687 A1 WO2013150687 A1 WO 2013150687A1
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plate thickness
steel plate
area ratio
grain boundary
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PCT/JP2012/082669
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English (en)
French (fr)
Japanese (ja)
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中島　清孝
鉄平 大川
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新日鐵住金株式会社
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Priority to CN201280057711.1A priority Critical patent/CN103958715B/zh
Priority to KR1020147013890A priority patent/KR101444646B1/ko
Priority to JP2013513475A priority patent/JP5445720B1/ja
Publication of WO2013150687A1 publication Critical patent/WO2013150687A1/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
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • 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
    • C21D2201/00—Treatment for obtaining particular effects
    • C21D2201/05—Grain orientation
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a high-strength thick steel plate excellent in arrestability.
  • arrestability the ability to suppress the propagation of brittle fracture in order to suppress brittle fracture of structures
  • a method for controlling the crystal grain size for example, a method for controlling the embrittled second phase, and a method for controlling the texture are known.
  • Patent Documents 1 to 3, 21 As a method for controlling the crystal grain size, there are techniques described in Patent Documents 1 to 3, 21.
  • the arrestability is improved by using ferrite as a parent phase and making the ferrite finer.
  • cooling is performed so that 1/8 or more of the slab thickness becomes Ar3 or less from the front and back layer portions toward the center of the slab thickness, and rolling is performed in a cryogenic region, and then Ac3 is It is necessary to reheat to a temperature exceeding that and recrystallize the ferrite.
  • fine ferrite recrystallized grains are formed by rolling in the process where the surface layer part is once cooled to Ar1 or less and then the surface layer part is reheated, using ferrite as a parent phase. obtain.
  • the average crystal grain size in the major axis direction of ferrite is 5 ⁇ m or more and the aspect ratio is 2 or more, or the average grain size in the major axis direction of prior austenite grains is 10 ⁇ m or more and the aspect ratio is By setting it to 2 or more, the brittle crack propagation stopping property is enhanced.
  • Patent Document 4 As a method for controlling the embrittled second phase, there is a technique described in Patent Document 4.
  • a fine embrittled second phase (for example, martensite) is dispersed in a ferrite as a parent phase, whereby a microcrack is formed in the embrittled second phase at the brittle crack tip. To relieve the stress state at the crack tip.
  • Patent Documents 5 to 17 As a method for controlling the texture, there are techniques described in Patent Documents 5 to 17. In the techniques described in Patent Documents 5 to 17, by controlling the X-ray plane intensity ratio as a texture at each plate thickness position, for example, the surface layer portion, 1/4 portion of the plate thickness, and 1/2 portion of the plate thickness. , Change the crack propagation direction, improve arrestability.
  • Patent Documents 18 to 20 there are techniques described in Patent Documents 18 to 20.
  • the arrestability is improved by controlling the crystal grain size and the X-ray plane intensity ratio by setting the ferrite fraction at 1 ⁇ 2 part of the plate thickness to 80% or more.
  • the arrestability is improved by controlling the texture size ratio measured by X-rays and the crystal grain size of 1 ⁇ 2 part of the surface layer and the plate thickness.
  • the arrestability is improved by controlling the crystal grain size of the surface layer, 1 ⁇ 2 part of the plate thickness, and the area ratio of the ⁇ 100 ⁇ plane perpendicular to the external stress.
  • Patent Documents 5 to 17, 19 and 21 do not control the crystal grain size, which is the most effective factor for improving the arrestability of a steel plate having high strength and a large thickness. That is, only by controlling the texture, the arrestability cannot be dramatically improved with a steel plate having a high strength and a large thickness. Moreover, since the X-ray plane intensity ratio represents a local texture, the variation is large. These techniques are not techniques for improving arrestability and obtaining high productivity during hot rolling. In the first place, the techniques of Patent Documents 5 to 8, 11 and 21 are techniques for improving the brittle crack propagation stopping characteristics in the thickness direction, and are parallel to the steel sheet surface as in the present application, for example, perpendicular or parallel to the rolling direction.
  • the cumulative reduction rate is 30% or more and the average pass pressure is reduced in the temperature range where the temperature at the center of the plate thickness is Ar3 point -10 ° C or lower and Ar3 point -50 ° C or higher.
  • Rolling with a rate of 8% or more is necessary. That is, rolling at a very low temperature is indispensable, productivity at the time of rolling is very low, and mass production is difficult.
  • the arrestability in the plate thickness direction of the steel sheet disclosed in Patent Documents 14, 15 and 17 is the large-sized hybrid ESSO test (running plate length: 1600 mm, test plate length: 800 mm, specimen width: 2400 mm, load stress: 235 kg. -Mm -0.5 ).
  • the Kca at ⁇ 20 ° C. is considered to be 6000 N ⁇ mm ⁇ 0.5 or less.
  • rolling at a low temperature is indispensable, and mass production is difficult.
  • the present invention has been made in consideration of the above-mentioned circumstances, and the purpose thereof is an arrest that has low manufacturing cost, high productivity, high strength, thick plate thickness, and no degradation of HAZ toughness.
  • An object of the present invention is to provide a high-strength thick steel plate having excellent properties.
  • the gist of the present invention is as follows.
  • the high-strength thick steel plate according to one embodiment of the present invention is, in mass%, C: 0.04 to 0.16%, Si: 0.01 to 0.5%, Mn: 0.75 to 2. 5%, Al: 0.001 to 0.1%, Nb: 0.003 to 0.05%, Ti: 0.003 to 0.05%, N: 0.001 to 0.008%, P is 0.03% or less, S is 0.02% or less, Cu is 1% or less, Ni is 2% or less, Cr is 1% or less, Mo is 0.5% or less, V is 0.15% or less, B is 0.005% or less, Ca is 0.01% or less, Mg is 0.01% or less, REM is limited to 0.01% or less, and the balance contains iron and unavoidable impurities.
  • Carbon equivalent Ceq. Has a component composition of 0.30 to 0.50%, has a microstructure containing 70% or less of ferrite in area ratio, and bainite of 30% or more in area ratio, and is 1/4 of the plate thickness.
  • the crystal grain boundary density which is the total length per unit area of the crystal grain boundaries having a crystal orientation difference of 15 ° or more, is 400 to 1000 mm / mm 2 and is 15 ° to the plane perpendicular to the main rolling direction.
  • the area ratio of the ⁇ 110 ⁇ plane forming an angle of 15 ° or less with respect to the plane perpendicular to the plane is 40 to 70%.
  • Ceq. C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ...
  • the high-strength thick steel plate described in the above (1) may have a thickness of 60 to 95 mm.
  • the high strength thick steel plate described in (1) or (2) above may have a yield stress of 390 to 690 MPa.
  • the microstructure may contain pearlite having an area ratio of 10% or less.
  • the microstructure has a ferrite area ratio of less than 50%, a pearlite area ratio of 5% or less, and a bainite area. The rate may be 50% or more.
  • the crystal grain boundary density at the 1 ⁇ 4 part of the plate thickness is 500 to 900 mm / mm 2
  • the plate thickness 1 / 2 parts of the grain boundary density may be 400 to 800 mm / mm 2 .
  • the Cu is 0.5% or less
  • the Ni is 1% or less
  • the Cr is 0.5% or less
  • the Mo may be further limited to 0.2% or less
  • the V may be further limited to 0.07% or less.
  • the B may be further limited to 0.002% or less.
  • the Ca is 0.003% or less
  • the Mg is 0.003% or less
  • the REM is 0.003%. You may further restrict
  • a steel sheet that is extremely excellent in arrestability in a direction parallel to the surface of the steel sheet for example, a direction perpendicular to or parallel to the rolling direction, has high strength even when the plate thickness is thick, and has no deterioration in HAZ toughness. Therefore, it is possible to reduce the cost and improve the safety of the welded steel structure.
  • the present inventors have intensively studied to solve the above problems, and as a result, by controlling the component composition, microstructure, grain boundary density in the plate thickness direction, and texture in the plate thickness direction of the high-strength steel plate.
  • the present inventors have found that a high-strength steel sheet having high productivity during hot rolling and improved arrestability in a direction parallel to the steel sheet surface, for example, a direction perpendicular to or parallel to the rolling direction can be obtained.
  • the steel sheet according to this embodiment controls the component composition, the microstructure, the grain boundary density in the plate thickness direction, and the texture in the plate thickness direction, thereby controlling the direction parallel to the steel plate surface, for example, the rolling direction. Improve the arrestability in the direction perpendicular to or parallel to.
  • the steel sheet according to the present embodiment is a mixed structure of ferrite and bainite, or a mixed structure of ferrite, pearlite, and bainite, and has a microstructure with a ferrite area ratio of 70% or less and a bainite area ratio of 30% or more. If the ferrite area ratio is more than 70%, it is difficult to obtain a steel plate having a large plate thickness and high strength. If a steel plate having a desired thickness and strength can be obtained, bainite, pearlite and bainite can be used as the second phase.
  • the present invention is intended for thick high-strength steel, and the upper limit of the ferrite area ratio may be limited to less than 50%, less than 30%, less than 20%, or less than 10%.
  • the upper limit of the bainite area ratio may be 95% in order to ensure the ferrite area ratio and increase the grain boundaries that hinder brittle crack propagation.
  • the present invention is intended for thick high-strength steel, and the lower limit of the bainite area ratio may be limited to 50% or more, 60% or more, 70% or more, or 80% or more.
  • Perlite may be contained as long as a steel plate having a desired thickness and strength can be obtained. Therefore, the pearlite area ratio may be limited to 10% or less, 5% or less, or 3% or less.
  • the lower limit of the pearlite area ratio is 0%.
  • MA fine island martensite
  • the MA area ratio may be limited to 3% or less, 2% or less, or 1% or less, and 0% is most desirable.
  • the grain boundary density is 400 to 1000 mm / mm 2 at 1 ⁇ 4 part of the plate thickness
  • the grain boundary density is set to 300 to 900 mm / mm 2 at 1 ⁇ 2 part of the plate thickness.
  • crystal grain boundary density means “total length per unit area of crystal grain boundaries having a crystal orientation difference of 15 ° or more”. The reason why the crystal orientation difference is set to 15 ° or more is that if it is less than 15 °, the crystal grain boundary is unlikely to be an obstacle to brittle crack propagation, and the effect of improving arrestability is reduced.
  • the arrest toughness value (Kca) at ⁇ 20 ° C. is 6000 N ⁇ s when the grain boundary density satisfies the requirements of 400 and 300 mm / mm 2 or more at 1 ⁇ 4 part and 1 ⁇ 2 part of the plate thickness, respectively.
  • the grain boundary density is preferably set to 500 or 400 mm / mm 2 or more at 1/4 part or 1/2 part of the plate thickness, or 600 or 400 mm, respectively. / Mm 2 or more is more preferable.
  • the arrestability improves as the grain boundary density increases, but excessively increasing the rolling load increases the productivity, so the upper limit of the grain boundary density is 1/4 of the plate thickness.
  • Part and 1/2 part are 1000 and 900 mm / mm 2 , respectively. Each limit may be respectively limited 900,800mm / mm 2 or, respectively 800,700mm / mm 2.
  • the grain boundary density is defined by 1/4 part and 1/2 part of the plate thickness. This is because by controlling 1 ⁇ 4 part and 1 ⁇ 2 part of the thickness, it is possible to obtain a representative value of the average grain thickness of the grain boundary.
  • the other plate thickness positions inevitably have a low temperature and a high cooling rate. Since the field density tends to increase, it is not necessary to limit the special value. However, depending on the heating method, a large temperature gradient may occur in the thickness direction, and the grain boundary density at 1/4 and 1/2 parts of the thickness may be reversed. ing.
  • the crystal grain boundary For the measurement of the crystal grain boundary, it is preferable to use an EBSD (Electron Back Scatter Diffraction Pattern) method capable of accurately measuring crystal orientation information with a wide field of view.
  • EBSD Electro Back Scatter Diffraction Pattern
  • the crystal grain boundary density was measured by measuring the area of 500 ⁇ m ⁇ 500 ⁇ m at 1 ⁇ 4 part and 1 ⁇ 2 part of the plate thickness at 1 ⁇ m pitch by EBSD method, and the crystal orientation difference between adjacent grains was 15 It can be determined by defining a boundary of more than 0 ° as a grain boundary and dividing the total length of the grain boundary at that time by the measurement area.
  • the highest stress externally applied to the steel structure is defined as external stress.
  • the external stress is applied substantially parallel to the main rolling direction of the steel sheet.
  • a surface perpendicular to the external stress can be handled as a surface perpendicular to the main rolling direction of the steel sheet.
  • the main rolling direction of the steel sheet can be specified by, for example, corroding the steel sheet surface with picric acid and measuring the aspect ratio of prior austenite. That is, the direction in which the aspect ratio of the prior austenite is large can be specified as the main rolling direction of the steel sheet.
  • the texture of the ⁇ 110 ⁇ plane forming an angle of 15 ° or less with respect to the plane perpendicular to the main rolling direction of the steel sheet is 40 to 70% in terms of the area ratio at 1/2 part of the sheet thickness. It has been found that the driving force of crack propagation can be reduced by the fact that the brittle crack in the vicinity of 1 ⁇ 2 part does not propagate straight but propagates in an inclined manner. However, if a similar texture is developed in plate thickness parts other than 1/2 part of the plate thickness, the crack propagates in an inclined state, and a sufficient arrestability improving effect cannot be exhibited.
  • the area ratio of the ⁇ 100 ⁇ plane that forms an angle of 15 ° or less with respect to the plane perpendicular to the main rolling direction is 10 to 40%
  • the area ratio of the ⁇ 110 ⁇ plane that forms an angle of 15 ° or less with respect to the plane perpendicular to the main rolling direction is set to 40 to 70% at 1/2 part of the plate thickness.
  • FIG. 1A is a photograph showing a mode of crack propagation generated by applying an impact from a V-notch in the left direction of the steel plate according to one embodiment of the present invention
  • FIG. 1B is a fracture view of the crack. It is a photograph which shows a cross section.
  • FIG. 2A is a photograph for a steel sheet with an area ratio of the ⁇ 100 ⁇ plane exceeding 40% at an angle of 15 ° or less with respect to a plane perpendicular to the main rolling direction at 1 ⁇ 4 part of the plate thickness.
  • FIG. 2B is a photograph showing an aspect of crack propagation generated by applying an impact from the left V-notch
  • FIG. 2B is a photograph showing a fracture surface of the crack.
  • the area ratio of the ⁇ 100 ⁇ plane that forms an angle of 15 ° or less with respect to the plane perpendicular to the main rolling direction at 1/4 part of the plate thickness is preferably 13 to 37%, more preferably 15 to 35%.
  • the reason why the area ratio of the ⁇ 110 ⁇ plane that forms an angle within 15 ° with respect to the plane perpendicular to the main rolling direction at 1 ⁇ 2 part of the plate thickness is 70% or less is shown in FIGS. 3A and 3B. As shown in the figure, when it exceeds 70%, the arrestability is deteriorated by propagating while being inclined without receiving a resistance of 1 ⁇ 4 part. Note that FIG.
  • FIG. 3A is a photograph for a steel sheet in which the area ratio of the ⁇ 110 ⁇ plane, which forms an angle of 15 ° or less with respect to the plane perpendicular to the main rolling direction, is more than 70% at 1/2 part of the plate thickness.
  • FIG. 3B is a photograph showing an aspect of crack propagation generated by applying an impact from the left V-notch
  • FIG. 3B is a photograph showing a fracture surface of the crack.
  • the area ratio of the ⁇ 110 ⁇ plane that forms an angle of 15 ° or less with respect to the plane perpendicular to the main rolling direction is preferably 45 to 65%, more preferably 40%. ⁇ 60%.
  • the texture is preferably measured by the EBSD method.
  • the measures for improving the arrestability as described above can be applied to a steel plate having a yield stress of 390 to 690 MPa, a tensile strength of 500 to 780 MPa, and a steel plate having a thickness of 60 to 95 mm. This is because in a region where the yield stress is less than 390 MPa or the plate thickness is less than 60 mm, it is relatively easy to improve the arrestability without relying on the means of the present invention, the yield stress exceeds 690 MPa, the plate thickness is In the region exceeding 95 mm, even if the grain boundary density and texture defined in the present invention are formed, the mechanical condition becomes severe, so the arrest toughness value (Kca) at ⁇ 20 ° C. is 6000 N ⁇ mm ⁇ 0.5.
  • the lower limit of the yield stress may be limited to 440 MPa or 470 MPa, and the upper limit may be limited to 640 MPa or 590 MPa.
  • the lower limit of the tensile strength may be limited to 520 MPa, 540 MPa, or 560 MPa, and the upper limit may be limited to 730 MPa, 680 MPa, or 630 MPa.
  • Component composition Hereinafter, the component composition of the steel plate according to the present embodiment will be described. “%” For a component means mass%.
  • C 0.04 to 0.16% C is contained in an amount of 0.04% or more in order to ensure the strength and toughness of the thick base material. If the C content exceeds 0.16%, it becomes difficult to ensure good HAZ toughness, so the C content is set to 0.16% or less. Therefore, the lower limit value of C is 0.04%, preferably 0.05%, more preferably 0.06%, and the upper limit value of C is 0.16%, preferably 0.14%, more preferably 0. .12%.
  • Si 0.01 to 0.5% Since Si is effective as a deoxidizing element and a strengthening element, it is contained in an amount of 0.01% or more. If the Si content exceeds 0.5%, the HAZ toughness is greatly deteriorated, so the Si content is 0.5% or less. Accordingly, the lower limit of Si is 0.01%, preferably 0.03%, more preferably 0.05%, and the upper limit of Si is 0.5%, preferably 0.4%, more preferably 0. .35% or 0.3%.
  • Mn 0.75 to 2.5% Mn is contained in an amount of 0.75% or more in order to economically secure the strength and toughness of the thick base material. If the Mn content exceeds 2.5%, the center segregation becomes prominent, and the toughness of the base material and the HAZ where the center segregation has occurred deteriorates, so the Mn content is 2.5% or less. . Therefore, the lower limit value of Mn is 0.75%, preferably 0.9%, more preferably 1.2%, and the upper limit value of Mn is 2.5%, preferably 2.0%, more preferably 1%. .8% or 1.6%.
  • P Limited to 0.03% or less P is one of the impurity elements.
  • the P content may be limited to 0.03% or less. Preferably, it is 0.02% or less, more preferably 0.015% or less.
  • the lower limit value is 0%, but considering the cost for reducing the P content, 0.0001% may be set as the lower limit value.
  • S Limited to 0.02% or less S is one of the impurity elements.
  • the S content may be limited to 0.02% or less. Preferably, it is 0.01% or less, More preferably, it is 0.008% or less.
  • the lower limit is 0%, but 0.0001% may be set as the lower limit in consideration of the cost for reducing the S content.
  • Al 0.001 to 0.1% Al is responsible for deoxidation and reduces O, which is one of the impurity elements.
  • Mn and Si also contribute to deoxidation.
  • the Al content is less than 0.001%, O cannot be stably reduced.
  • the Al content exceeds 0.1%, alumina-based coarse oxides and clusters thereof are generated, and the base material and the HAZ toughness are impaired. Therefore, the Al content is 0.1% or less.
  • the lower limit of Al is 0.001%, preferably 0.01%, more preferably 0.015%
  • the upper limit of Al is 0.1%, preferably 0.08%, more preferably 0. .05%.
  • Nb 0.003 to 0.05%
  • Nb is an important element in the present invention.
  • Nb is an effective element for expanding the non-recrystallization temperature range, and raises the rolling temperature and contributes to productivity improvement. In order to acquire this effect, it is necessary to make it contain 0.003% or more.
  • the Nb content exceeds 0.05%, the HAZ toughness and weldability deteriorate, so the Nb content is set to 0.05% or less. Therefore, the lower limit value of Nb is 0.003%, preferably 0.005%, more preferably 0.008%, and the upper limit value of Nb is 0.05%, preferably 0.025%, more preferably 0. .018%.
  • Ti 0.003 to 0.05%
  • TiN is formed by containing Ti, and it suppresses that an austenite particle size becomes large at the time of steel bill heating.
  • the austenite grain size is increased, the crystal grain size of the transformed structure is also increased, so that it becomes difficult to obtain a predetermined grain boundary density, and toughness and arrestability are deteriorated.
  • the Ti content exceeds 0.05%, TiC is formed and the HAZ toughness decreases, so the Ti content is set to 0.05% or less. Therefore, the lower limit value of Ti is 0.003%, preferably 0.006%, more preferably 0.008%, and the upper limit value of Ti is 0.05%, preferably 0.02%, more preferably 0. .015%.
  • N 0.001 to 0.008% N is an important element in the present invention.
  • N content if the N content exceeds 0.008%, the steel material becomes brittle, so the N content is set to 0.008% or less. Therefore, the lower limit value of N is 0.001%, preferably 0.0015%, more preferably 0.002%, and the upper limit value of N is 0.008%, preferably 0.0065%, more preferably 0. 0.006%.
  • the balance of the elements described above may be Fe and inevitable impurities.
  • the component composition of the steel plate according to the present embodiment may contain at least one of Cu, Ni, Cr, Mo, V, B, Ca, Mg, and REM as necessary.
  • the lower limit value of the content of these elements is 0%, but a lower limit value may be set in order to stably obtain the effect of addition.
  • the addition effect and content of each element will be described. Even if these elements are intentionally added, even if they are mixed as unavoidable impurities, the steel sheet whose content is within the scope of claims is considered within the scope of the present invention.
  • the strength and toughness of the base material can be improved.
  • the lower limit value of Cu is 0%, but the lower limit value may be 0.1% in order to stably obtain the effect of addition. Therefore, the lower limit of Cu is 0%.
  • the lower limit may be 0.1% or 0.2%.
  • the upper limit value of Cu may be limited to 1%, 0.8%, 0.5%, or 0.3% as necessary.
  • Ni 0-2% By adding Ni, the strength and toughness of the base material can be improved. However, if the Ni content is too large, the HAZ toughness and weldability deteriorate, so 2% is made the upper limit.
  • the lower limit value of Ni is 0%, but the lower limit value may be 0.1% in order to stably obtain the effect of addition. Therefore, the lower limit of Ni is 0%. In order to improve the strength and toughness of the base material, the lower limit may be 0.1% or 0.2%.
  • the upper limit value of Ni may be limited to 2%, 1%, 0.5%, or 0.3% as necessary.
  • the strength and toughness of the base material can be improved.
  • the Cr content is too high, the HAZ toughness and weldability deteriorate, so 1% is made the upper limit.
  • the lower limit of Cr is 0%, the lower limit may be set to 0.1% or 0.2% in order to stably obtain the effect of addition.
  • the upper limit value of Cr may be limited to 1%, 0.8%, 0.5%, or 0.3% as necessary.
  • Mo 0 to 0.5%
  • the strength and toughness of the base material can be improved.
  • the Mo content is too large, the HAZ toughness and weldability deteriorate, so 0.5% is made the upper limit.
  • the lower limit of Mo is 0%, the lower limit may be set to 0.01% or 0.02% in order to stably obtain the effect of addition.
  • the upper limit value of Mo may be limited to 0.5%, 0.3%, 0.2%, or 0.1% as necessary.
  • V 0 to 0.15%
  • the lower limit value of V is 0%, but the lower limit value may be 0.01% or 0.02% in order to stably obtain the effect of addition.
  • the upper limit value of V may be limited to 0.15%, 0.1%, 0.07%, or 0.05% as necessary.
  • B 0 to 0.005%
  • the lower limit value of B is 0%, but the lower limit value may be 0.0002% or 0.0003% in order to stably obtain the effect of addition.
  • the upper limit value of B may be limited to 0.005%, 0.003%, 0.002%, or 0.001% as necessary.
  • Ca 0 to 0.01%
  • the lower limit value of Ca is 0%, but the lower limit value may be 0.0002% or 0.0003% in order to stably obtain the addition effect.
  • the upper limit value of Ca may be limited to 0.01%, 0.005%, 0.003%, or 0.001% as necessary.
  • Mg 0 to 0.01%
  • the lower limit value of Mg is 0%, but the lower limit value may be 0.0002% or 0.0003% in order to stably obtain the effect of addition.
  • the upper limit value of Mg may be limited to 0.01%, 0.005%, 0.003%, or 0.001% as necessary.
  • REM 0 to 0.01%
  • HAZ toughness is improved.
  • the lower limit value of REM is 0%, but the lower limit value may be 0.0003% or 0.0005% in order to stably obtain the addition effect.
  • the upper limit of REM may be limited to 0.01%, 0.005%, 0.003%, or 0.001% as necessary.
  • the above-mentioned selective elements can be intentionally added to improve the strength and toughness of the base material. However, it is not necessary to add any of these selective elements in order to reduce alloy costs. Even if these elements are not intentionally added, Cu: 0.1% or less, Ni: 0.1% or less, Cr: 0.1% or less, Mo: 0.01 % Or less, V: 0.01% or less, B: 0.0002% or less, Ca: 0.0003% or less, Mg: 0.0003% or less, REM: 0.0003% or less can be contained in the steel. .
  • the steel plate according to the present embodiment has a carbon equivalent Ceq. Is 0.30 to 0.50%.
  • Ceq. C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ...
  • each component is the mass% of each component contained in the steel plate.
  • the lower limit value of the carbon equivalent is 0.30%, preferably 0.32%, more preferably 0.34%, still more preferably 0.36%
  • the upper limit value of the carbon equivalent is 0.50%, preferably Is 0.44%, more preferably 0.42%, still more preferably 0.40%.
  • molten steel adjusted to a desired component composition is melted by a known melting method using a converter or the like, and a steel piece is obtained by a known casting method such as continuous casting.
  • the steel slab is cooled to a plate thickness center temperature of 600 ° C. or lower, and then charged in a heating furnace having an ambient temperature of 1000 to 1250 ° C. for 30 to 600 minutes, and extracted at a plate thickness center temperature of 950 to 1150 ° C.
  • the charging to the heating furnace with the temperature of the cooled slab exceeding 600 ° C. has not yet completed the transformation from austenite to ferrite during cooling, so there is a refining effect due to reverse transformation to austenite during heating. This is because it is difficult to obtain and it is difficult to increase the grain boundary density after rolling with coarse austenite grains.
  • it is 500 degrees C or less. If the atmospheric temperature of heating is less than 1000 ° C., sufficient heating cannot be performed and solution formation becomes insufficient.
  • a preferable range of the atmospheric temperature is 1050 to 1200 ° C. This is because, when the charging time into the heating furnace is less than 30 minutes, solution formation is insufficient, and when more than 600 minutes, austenite grains become coarse.
  • a preferred charging time range is 40 to 500 minutes.
  • a preferable range of the heat extraction temperature is 1000 to 1100 ° C.
  • the 1-pass reduction rate is 3 to 30% for 4 to 15 passes, less than 3% is within 3 passes (including 0), and the cumulative reduction rate is 15 to 70%. Rough rolling is performed.
  • the plate thickness center temperature exceeds 1150 ° C.
  • the recrystallized austenite grains cannot be made fine even in the subsequent finish rolling.
  • the productivity is lowered.
  • a preferred thickness center temperature is 900 to 1000 ° C. If the one-pass rolling reduction is less than 3%, austenite grains grow abnormally, so it is necessary to avoid them as much as possible. However, if rolling with a 1-pass reduction rate of less than 3% is limited to 3 passes and rolling with a 1-pass reduction rate of 3 to 30% is performed for 4 passes or more, refinement by sufficient recrystallization can be achieved. However, if it exceeds 30%, the load on the rolling mill is large, and if it exceeds 15 passes, the productivity is lowered.
  • the 1-pass rolling reduction is limited to 30% and the number of passes is 4 to 15 passes. Rolling with a 1-pass reduction of 5 to 25% is preferably 6 to 13 passes.
  • the reason why the cumulative rolling reduction of rough rolling is 15 to 70% is that when the cumulative rolling reduction is less than 15%, it is difficult to refine by austenite recrystallization, porosity remains, internal cracks and ductility, and This is because deterioration of toughness may occur, and if it exceeds 70%, the number of passes increases and productivity decreases.
  • a preferred cumulative rolling reduction is 30 to 60%.
  • the plate thickness center temperature exceeds 850 ° C., it does not sufficiently enter the non-recrystallized region, the increase in dislocation is suppressed, and the crystal grain boundary density cannot be increased.
  • the sheet thickness center temperature is less than 750 ° C., the productivity is lowered and part of the processed ferrite is included, so that it is within 15 ° with respect to the plane perpendicular to the main rolling direction of the steel sheet having a half thickness. It is difficult to make the area ratio of the ⁇ 110 ⁇ plane forming the angle of 40% or more.
  • a preferred thickness center temperature is 760 to 840 ° C. In rolling with less than 4 passes, it is difficult to ensure a shape ratio of 1 or less, and when it exceeds 15 passes, productivity decreases.
  • the shape ratio of the equation (2) is an index representing what kind of strain component is imparted to the steel sheet by rolling.
  • the shape ratio is small, a shear strain component is applied, and when the shape ratio is large, a large amount of compressive strain component is applied. Since the change of the strain component due to the change in the shape ratio has a great influence on the formation of the texture having a quarter of the plate thickness, the range is set as described above.
  • the reason for setting the average value of the shape ratio to 0.5 to 1 is that when the thickness is less than 0.5 at 1/4 part of the plate thickness, the shear strain of rolling becomes dominant, and the ⁇ 100 ⁇ texture is developed.
  • a preferable range of the average shape ratio is 0.6 to 0.9. If the cumulative rolling reduction is less than 40%, it is difficult to increase the grain boundary density due to the accumulation of dislocations and develop the specified texture. If the cumulative rolling reduction exceeds 80%, the effect of increasing the grain boundary density due to the accumulation of dislocations is saturated. In addition, the productivity decreases, so 40 to 80%. A preferable range of the cumulative rolling reduction is 45 to 75%.
  • accelerated cooling is performed from a plate thickness center temperature of 700 ° C. or more to a temperature of 550 ° C. or less at a plate thickness center cooling rate of 2 to 10 ° C./s.
  • the plate thickness center temperature at the start of cooling is less than 700 ° C.
  • ferrite transformation proceeds and coarsens, so it is difficult to increase the grain boundary density.
  • the sheet thickness center cooling rate is less than 2 ° C./s, it is difficult to increase the grain boundary density.
  • the plate thickness center cooling rate exceeding 10 ° C./s is difficult to achieve with a steel plate having a plate thickness of 60 mm or more, so this is the upper limit.
  • the cooling stop temperature exceeds 550 ° C., it is difficult to increase the grain boundary density.
  • the preferred accelerated cooling conditions are a plate thickness center temperature at the start of cooling of 720 ° C. or higher, a cooling rate of 3 to 8 ° C./s, and a cooling stop temperature of 500 ° C. or lower.
  • the steel plate which concerns on this embodiment can be manufactured by controlling manufacture using the plate
  • the plate thickness center temperature it is possible to properly control the manufacturing conditions even when the plate thickness changes compared to the case where the surface temperature of the steel plate is used. Can be manufactured efficiently.
  • the temperature distribution inside the steel sheet is calculated while measuring the surface temperature of the steel sheet from heating to rolling, and the rolling control is performed while predicting the rolling reaction force from the calculation result of the temperature distribution. It is carried out.
  • the steel plate center temperature can be easily obtained during rolling.
  • the accelerated cooling is controlled while predicting the temperature distribution inside the plate thickness.
  • tempering at 300 to 650 ° C. may be performed as necessary.
  • Tempering at less than 300 ° C makes it difficult to obtain the effect of tempering.
  • the tempering temperature exceeds 650 ° C., the amount of softening increases and it becomes difficult to ensure the strength.
  • a preferable tempering temperature is 400 to 600 ° C.
  • the steel plate according to this embodiment can be applied as a steel plate having a thickness of 60 to 95 mm and a yield stress of 390 to 690 MPa.
  • the present invention is applicable to the production of steel sheets having a yield stress of 390 MPa class, 460 MPa class or higher for hulls and offshore structures.
  • the arrestability can be improved to Kca at ⁇ 20 ° C. exhibiting arrestability of 6000 N ⁇ mm ⁇ 0.5 or more.
  • it can be set as the high strength thick steel plate which is low in manufacturing cost, has high productivity, has no HAZ toughness deterioration, and has excellent arrestability.
  • steel slabs A to Z were produced by continuous casting.
  • Steel slabs A to O are invention steels
  • steel slabs P to Z are comparative steels.
  • Example 1 to 20 and Comparative Examples 21 to 55 the steel slabs A to Z are reheated and further subjected to thick plate rolling to obtain a thick steel plate having a thickness of 60 to 95 mm. Subsequently, the thick steel plate is cooled with water. did. However, in Comparative Example 53, air cooling was performed instead of water cooling. Thereafter, heat treatment was performed as necessary.
  • Tables 1 and 2 show the composition of steel slabs A to Z.
  • the underline in Table 1 and Table 2 indicates that the numerical value is outside the range of the present invention, and the italic type indicates the analytical value of the amount contained as an unavoidable impurity.
  • Tables 3 to 6 show the production methods. For rolling, a rolling mill having a roll radius of 600 mm was used. Productivity was evaluated by the time required from the time of extraction from the heating furnace to the start of cooling after completion of rolling, and the production time of less than 1000 s was defined as good. Underlines in Tables 3 to 6 indicate that the conditions are not preferable, or that the productivity is outside the value defined as good.
  • the temperature and cooling rate in the manufacturing method are values at the center position of the plate thickness, and were obtained from the measured surface temperature by heat conduction analysis using a known differential method.
  • microstructure fraction For each thick steel plate produced, the microstructure fraction, texture, grain boundary density, and mechanical properties were measured.
  • the microstructure phase fraction was obtained by photographing the microstructure at a magnification of 500 times at 1/2 part of the plate thickness with an optical microscope, obtaining the total area of each phase by image analysis, and dividing by the measured area.
  • the grain boundary density was measured by measuring the area of 1 ⁇ 4 part and 1 ⁇ 2 part of the thickness of 500 ⁇ m ⁇ 500 ⁇ m at 1 ⁇ m pitch by EBSD method, and the boundary where the crystal orientation difference between adjacent grains was 15 ° or more. It was defined as a crystal grain boundary, and was obtained by dividing the total length of the crystal grain boundary at that time by the measurement area.
  • the texture is ⁇ 100 ⁇ plane that forms an angle of 15 ° or less with respect to the plane perpendicular to the main rolling direction of the steel sheet at 1 ⁇ 4 part of the plate thickness, and ⁇ 110 ⁇ plane at 1 ⁇ 2 part of the plate thickness.
  • Each map was created, and the total area was divided by the measured area to obtain the area ratio.
  • the yield stress and Charpy absorbed energy of the base material were tested using test pieces taken from the center of the plate thickness, and the results were used as representative values for each steel plate.
  • the tensile test was performed in accordance with “Metal Material Tensile Test Method” of JIS Z 2241 (1998), and two of them were tested and the average value was obtained.
  • the tensile test piece was a JIS Z 2201 (1998) No. 4 test piece.
  • Charpy absorbed energy was measured in accordance with JIS Z 2242 (2005) “Charpy impact test method for metal materials” using a 2 mm V notch Charpy impact test piece. The average value was obtained.
  • the arrest toughness value Kca at ⁇ 20 ° C. was obtained by a temperature gradient type standard ESSO test (original thickness and plate width of 500 mm).
  • a butt welded joint was produced by a submerged arc welding method with a welding heat input of 10 kJ / mm, and a notch of a 2 mm V-notch Charpy impact test piece was inserted along the fusion line (FL) at 1/4 part of the plate thickness. The average value of the absorbed energy of each three at -20 ° C. was determined.
  • the Charpy impact test was based on “Charpy impact test method for metal materials” of JIS Z 2242 (2005).
  • Examples 1 to 20 satisfy all the conditions of the present invention, the strength, toughness, arrestability, joint toughness, and productivity are good.
  • Comparative Examples 21 to 31 had a problem in at least one of strength, toughness, arrestability, and joint toughness because the component range was outside the scope of the present invention.
  • Comparative Example 36 had low productivity, low grain boundary density, low toughness, and arrestability because the heat extraction temperature was too high.
  • Comparative Example 37 had a high ferrite fraction and a low strength because the heat extraction temperature was too low.
  • Comparative Example 41 had a high heat extraction temperature and a correspondingly high rolling temperature, so that the grain boundary density was small, and the toughness, arrestability, and productivity were low.
  • Comparative Example 46 had a small number of finish rolling passes, and the shape ratio was too large. Accordingly, the ⁇ 100 ⁇ area ratio of 1 ⁇ 4 part of the plate thickness was small, and the arrestability was low.
  • Comparative Example 47 was low in productivity because there were too many finishing rolling passes.
  • Comparative Example 51 was low in productivity because the cumulative rolling reduction of finish rolling was too large.
  • Comparative Example 53 is cooling by air cooling, the ⁇ 100 ⁇ area ratio of 1 ⁇ 4 part of the plate thickness, the ⁇ 110 ⁇ area ratio of 1 ⁇ 2 part of the plate thickness, and the grain boundary density are small, and the strength, toughness, And arrestability was low.
  • Comparative Example 55 was low in strength because the tempering temperature was too high.
  • the present invention it is possible to provide a high-strength thick steel plate that is low in manufacturing cost, high in productivity, high in strength, thick in plate thickness, and free from deterioration of HAZ toughness and excellent in arrestability.

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