WO2019069771A1 - Steel sheet and method for producing steel sheet - Google Patents

Steel sheet and method for producing steel sheet Download PDF

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Publication number
WO2019069771A1
WO2019069771A1 PCT/JP2018/035778 JP2018035778W WO2019069771A1 WO 2019069771 A1 WO2019069771 A1 WO 2019069771A1 JP 2018035778 W JP2018035778 W JP 2018035778W WO 2019069771 A1 WO2019069771 A1 WO 2019069771A1
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WO
WIPO (PCT)
Prior art keywords
steel plate
steel
less
phase
amount
Prior art date
Application number
PCT/JP2018/035778
Other languages
French (fr)
Japanese (ja)
Inventor
和寿 柳田
中島　清孝
浩司 石田
大貴 今城
Original Assignee
新日鐵住金株式会社
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Publication date
Application filed by 新日鐵住金株式会社 filed Critical 新日鐵住金株式会社
Priority to JP2019500684A priority Critical patent/JP6536761B1/en
Priority to KR1020197025144A priority patent/KR102027871B1/en
Publication of WO2019069771A1 publication Critical patent/WO2019069771A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/10—Supplying or treating molten metal
    • B22D11/108—Feeding additives, powders, or the like
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/12—Accessories for subsequent treating or working cast stock in situ
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/12—Accessories for subsequent treating or working cast stock in situ
    • B22D11/128—Accessories for subsequent treating or working cast stock in situ for removing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/16—Controlling or regulating processes or operations
    • B22D11/20—Controlling or regulating processes or operations for removing cast stock
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/06—Deoxidising, e.g. killing
    • 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
    • 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—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
    • C22C38/50—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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present disclosure relates to a steel plate and a method of manufacturing a steel plate.
  • Examples of applications of the steel plate include ships, buildings, bridges, marine structures, LNG storage tanks, other large tanks, line pipes and the like (see, for example, Patent Documents 1 to 3).
  • the steel plate applied to these applications is a brittle crack propagation arresting property (BCA: stop a brittle crack at the base material even if a brittle crack occurs in a welded joint).
  • Brittle Crack Arrest hereinafter, may be referred to as "arrest”.
  • a steel plate applied to ships for example, it may be applied to an important member (for example, Hatch Side Coating, Upper Deck) of a container ship.
  • container ships have been increased in size for the purpose of reducing environmental load and increasing the efficiency of marine transportation.
  • thick steel plates are used as important members of container vessels. It has been found that when the plate thickness is increased, the crack may penetrate the hull if a brittle crack occurs in the welded joint.
  • CTOD Crack Tip Opening Displacement
  • Patent Document 1 discloses a high strength thick steel plate excellent in arrestability by controlling the composition of the high strength steel plate, microstructure, grain boundary density in the thickness direction, and texture in the thickness direction. ing.
  • the steel plate disclosed in Patent Document 1 has a specific chemical composition, and has a microstructure of 70% or less of ferrite and 30% or more of bainite in area ratio.
  • the grain boundary density at a quarter of the plate thickness is 400 mm / mm 2 to 1000 mm / mm 2
  • the area ratio of the ⁇ 100 ⁇ plane at a quarter of the plate thickness is 10% to 40%. .
  • the grain boundary density in half of the plate thickness is 300 mm / mm 2 to 900 mm / mm 2
  • the area ratio of ⁇ 110 ⁇ plane in half of the plate thickness is 40% to 70%.
  • the yield stress is 390 MPa to 690 MPa.
  • Patent Document 1 focuses on arrestability, it does not focus on joint CTOD characteristics in particular.
  • the melting method and the casting method have not been particularly studied.
  • the steel plate of the technology disclosed in Patent Document 1 satisfies both arrestability and joint CTOD characteristics at a high level. It was found that there is room for further improvement.
  • Patent Document 2 grain boundary ferrite and ferrite side plates formed along the ⁇ grain boundary of the weld heat affected zone near the melting line are refined, and intragranular transformation within the ⁇ grain of the weld heat affected zone near the melting line.
  • a steel plate having a yield strength of 460 MPa or more excellent in the CTOD characteristics of the weld heat affected zone is disclosed.
  • the steel plate disclosed in Patent Document 2 has a specific chemical composition, and further includes a specific TiN containing an Mg-Al oxide and a complex of sulfide and an oxide. There is a specific amount of specific particles contained.
  • Patent Document 2 focuses on the joint CTOD characteristics, it does not focus on the arrestability in particular. Further, the melting method and the casting method have not been particularly studied, and furthermore, the steel making method has not been specifically studied. Then, as a result of examining the technology disclosed in Patent Document 2 by the present inventors, the steel plate of the technology disclosed in Patent Document 2 also satisfies both the arrestability and joint CTOD characteristics at a high level. It was found that there is room for further improvement.
  • Patent Document 3 suppresses the hardening of the center segregated portion of the steel plate, uniformly disperses TiN in the steel uniformly, and crystallizes Ca sulfide, so that the yield stress is 420 MPa or more, and the multilayer welded portion
  • a steel plate which is excellent in the low temperature toughness of the weld heat affected zone.
  • the steel plate disclosed in Patent Document 3 has a specific chemical composition, and satisfies the relationship of H Vmax / H Vave ⁇ 1.35 + 0.006 / [C] ⁇ t / 500.
  • Patent Document 3 focuses on the joint CTOD characteristics, it does not focus on the arrestability in particular.
  • the melting method and the casting method have not been particularly studied.
  • the steel plate of the technology disclosed in Patent Document 3 also satisfies both the arrestability and the joint CTOD characteristics at a high level. It turned out that there is room for improvement.
  • the inventors of the present invention examined the steel plates disclosed in the above Patent Documents 1 to 3 and as a result, they have both high arrestability and joint CTOD characteristics at a high level that satisfy the more stringent standards than before, It was found that there is room for further improvement in order to obtain a steel plate with higher strength. Thus, a steel plate which is superior in both arrestability and joint CTOD characteristics and has higher strength than conventional steel plates which satisfy the more stringent standards has not been established yet. .
  • the present disclosure has been made in view of such circumstances, and has high strength and steel sheet that has both excellent arrestability and excellent joint CTOD characteristics that can satisfy even more stringent standards. It is provided.
  • the present disclosure includes the following aspects.
  • the surface temperature of the steel plate is Ar 3 point ⁇ 30 ° C. to recrystallization on the steel plate after rough rolling
  • finish rolling with a cumulative rolling reduction of 50% to 75% in a temperature range of a temperature T rex
  • the steel sheet after finish rolling has a cross section along the thickness direction of the steel sheet until the temperature at a quarter position from the surface of the steel sheet in the thickness direction reaches a temperature range of 0 ° C. to 600 ° C. Cooling at an average cooling rate of 1 ° C./s to 20 ° C./s at The manufacturing method of the steel plate which has.
  • a steel plate that is superior in both arrestability and joint CTOD characteristics and satisfies more stringent standards than conventional steel plates and that has high strength is required.
  • the present inventors further tightened by controlling the microstructure (tissue morphology, tissue particle size, amount of inclusions, and center segregation). It was found that a steel plate having both excellent arrestability and excellent joint CTOD characteristics that can satisfy even the above standards and having high strength can be obtained, and the steel plate according to the present disclosure is completed.
  • the present inventors also examined in detail the method of manufacturing a steel plate in order to control the microstructure (the structure morphology, the particle size of the structure, the amount of inclusions, and the center segregation).
  • a numerical range represented using “to” means a range including numerical values described before and after “to” as the lower limit value and the upper limit value.
  • “%” indicating the content of a component (element) means “mass%”.
  • the content of C (carbon) may be denoted as “C amount”.
  • the term “step” is not limited to an independent step, and if the intended purpose of the step is achieved even if it can not be clearly distinguished from other steps, this term included.
  • the steel plate according to the present disclosure is in mass%, C: 0.03% to 0.14%, Si: 0.01% to 0.50%, Mn: 1.20% to 2.50%, P: 0.030% or less, S: 0.020% Nb: 0.003% to 0.050%, Ti: 0.003% to 0.050%, Al: 0.001% to 0.100%, N: 0.0010% to 0.0080%, O (oxygen): 0.0050% or less, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, REM: 0% to 0.0100%, Zr: 0% to 0.0100% , Te: 0% to 0.0100%, V: 0% to 0.150%, Cu: 0% to 1.00%, Ni: 0% to 2.00%, Cr: 0% to 1.00% Mo: 0% to 0.50%, B: 0% to 0.0050%, and the balance: Fe and impurities have a chemical composition
  • the steel plate according to the present disclosure may or may not include at least one selected from the group consisting of Ca, Mg, REM, Zr, Te, V, Cu, Ni, Cr, Mo, and B. May be Therefore, the lower limit value of these elements is 0%. When these elements are contained, the lower limit may be more than 0%.
  • C 0.03% to 0.14%
  • base material the required strength of the steel plate
  • the amount of C is set to 0.03% to 0.14%.
  • the preferable lower limit of the amount of C is 0.04% or more, more preferably 0.05% or more.
  • the preferable upper limit of the amount of C is 0.12% or less, more preferably 0.10% or less.
  • Si 0.01% to 0.50%
  • Si is a deoxidizing element and is an element effective for solid solution strengthening. If the amount of Si is less than 0.01%, the effect of containing Si can not be obtained, so the lower limit of the amount of Si is made 0.01% or more. On the other hand, if the Si content exceeds 0.50%, the weldability and the required joint CTOD characteristics become inferior. Therefore, the amount of Si is set to 0.01% to 0.50%.
  • the preferable lower limit of the amount of Si is 0.05% or more, more preferably 0.10% or more.
  • the upper limit of the amount of Si is preferably 0.40% or less, more preferably 0.30% or less.
  • Mn is an effective element for improving the strength and toughness of the base material, and the content of Mn is 1.20% or more.
  • the content of Mn is 1.20% or more.
  • the upper limit is 2.50%. Therefore, the amount of Mn is set to 1.20% to 2.50%.
  • the preferable lower limit of the amount of Mn is 1.40% or more, more preferably 1.60% or more.
  • the preferable upper limit of the amount of Mn is 2.20% or less, more preferably 2.00% or less.
  • (P: 0.030% or less) P is present in the steel sheet as an impurity.
  • the upper limit of the amount of P is set to 0.030% or less.
  • the upper limit of the amount of P is preferably 0.020% or less, more preferably 0.010% or less.
  • the lower the amount of P the lower the amount. However, from the viewpoint of the manufacturing cost, it may be 0.001% or more.
  • (S: 0.020% or less) S is present in the steel sheet as an impurity.
  • the upper limit of the amount of S is made 0.020% or less.
  • the upper limit of the amount of S is preferably 0.015% or less, more preferably 0.010% or less, still more preferably 0.005% or less, and still more preferably 0.002% or less.
  • the lower the amount of S the lower the amount. However, from the viewpoint of the manufacturing cost, it may be 0.001% or more.
  • Nb suppresses the recrystallization temperature, contributes to the refinement of the structure by the addition of a small amount, and is an element effective for securing the strength of the base material.
  • the Nb content is set to 0.003% to 0.050%.
  • the preferable lower limit of the amount of Nb is 0.006% or more, more preferably 0.010% or more.
  • the upper limit of the Nb content is preferably 0.040% or less, more preferably 0.030% or less.
  • Ti is an element that contributes to the improvement of toughness through the refinement of the structure of the base material and the weld by the addition of a small amount. In addition, Ti also functions as a deoxidizing element. On the other hand, excessive addition of Ti hardens the weld and significantly deteriorates toughness, resulting in inferior joint CTOD characteristics. Therefore, the amount of Ti is set to 0.003% to 0.050%.
  • the preferable lower limit of the amount of Ti is 0.006% or more, more preferably 0.010% or more.
  • the preferable upper limit of the amount of Ti is 0.035% or less, more preferably 0.020% or less.
  • Al 0.001% to 0.100% Since Al is a deoxidizing element, the amount of Al is made 0.001% or more. On the other hand, the excessive addition of Al impairs the surface quality of the steel slab and forms an inclusion harmful to toughness, so the upper limit of the amount of Al is 0.100%. Therefore, the Al content is set to 0.001% to 0.100%.
  • the preferable lower limit of the amount of Al is 0.010% or more, more preferably 0.020% or more.
  • the upper limit of the amount of Al is preferably 0.060% or less, more preferably 0.040% or less.
  • N (N: 0.0010% to 0.0080%) N forms a nitride together with Al to improve the joint toughness, so the lower limit of the N amount is made 0.0010% or more. However, if the content of N is excessive, embrittlement and elongation decrease due to solid solution N occur, so the upper limit of the amount of N is made 0.0080% or less. Therefore, the N content is set to 0.0010% to 0.0080%.
  • the preferable lower limit of the N amount is 0.0015% or more, more preferably 0.0020% or more.
  • the preferable upper limit of the amount of N is 0.0060% or less, more preferably 0.0040% or less.
  • O oxygen
  • the upper limit of the amount of O is made 0.0050% or less.
  • the lower limit of O is not particularly defined because the amount of O is preferably as small as possible.
  • the lower limit of the amount of O may be 0.0005% or more.
  • the preferable lower limit of the amount of O may be 0.0007% or more, and may be 0.0010% or more.
  • the upper limit of the amount of O is preferably 0.0040% or less, more preferably 0.0030% or less.
  • Ca 0% to 0.0100%
  • Ca is a deoxidizing element, and is an element that suppresses the formation of coarse inclusions by forming sulfides, forms a fine oxide, and suppresses the formation of harmful inclusions. Therefore, Ca may be contained.
  • the amount of Ca exceeds 0.0100%, coarse oxides, sulfides and acid sulfides are formed, and the joint CTOD characteristics become inferior. Therefore, the amount of Ca is set to 0% to 0.0100%.
  • the upper limit of the amount of Ca is preferably 0.0070% or less, more preferably 0.0040% or less.
  • the lower limit of the amount of Ca is preferably made 0.0001% or more. More preferably, it is 0.0010% or more, particularly preferably 0.0020% or more.
  • Mg is a deoxidizing element, and is an element that suppresses the formation of coarse inclusions by forming sulfides, forms a fine oxide, and suppresses the formation of harmful inclusions. Therefore, Mg may be contained. However, when the Mg content exceeds 0.0100%, coarse oxides, sulfides and acid sulfides are formed, and the joint CTOD characteristics become inferior. Therefore, the amount of Mg is set to 0% to 0.0100%.
  • the preferable upper limit of the amount of Mg is 0.0070% or less, more preferably 0.0040% or less. In order to obtain the effect by the above-mentioned effect more certainly, it is preferable to make the lower limit of the amount of Mg into 0.0001% or more. More preferably, it is 0.0010% or more, particularly preferably 0.0020% or more.
  • REM 0% to 0.0100%
  • REM is a deoxidizing element, is an element that suppresses the formation of coarse inclusions by forming sulfides, forms a fine oxide, and suppresses the formation of harmful inclusions. Therefore, REM may be contained.
  • the REM amount is 0% to 0.0100%.
  • the upper limit of the REM amount is preferably 0.0070% or less, more preferably 0.0040% or less.
  • the lower limit of the amount of REM is preferably made 0.0001% or more.
  • REM is a general term for Sc, Y, and a total of 17 elements of lanthanoids. As REM, one or more elements out of 17 elements in total may be included. The content of REM refers to the total content of these elements.
  • Zr 0% to 0.0100%
  • Zr is an element that contributes to the improvement of toughness through the refinement of the structure of the base material and the weld by the addition of a small amount.
  • Zr also functions as a deoxidizing element. Therefore, Zr may be contained.
  • the amount of Zr is set to 0% to 0.0100%.
  • the preferable upper limit of the amount of Zr is 0.0070% or less, more preferably 0.0040% or less.
  • the lower limit of the amount of Zr is preferably made 0.0001% or more. More preferably, it is 0.0010% or more, particularly preferably 0.0020% or more.
  • Te 0% to 0.0100%
  • Te is an element that contributes to the improvement of toughness by grain refinement. Therefore, Te may be contained. However, the effect is saturated even if the amount of Te exceeds 0.0100%. Therefore, the Te content is set to 0% to 0.0100%.
  • the upper limit of the amount of Te is preferably 0.0070% or less, more preferably 0.0040% or less.
  • the lower limit of the amount of Te is preferably set to 0.0001% or more in order to obtain the effect by the above action more reliably. More preferably, it is 0.0010% or more, particularly preferably 0.0020% or more.
  • V is an element that contributes to the increase in strength of the base material by precipitation strengthening. Therefore, V may be contained. However, if the V content exceeds 0.150%, the joint toughness is impaired. Therefore, the V amount is set to 0% to 0.150%.
  • the preferable upper limit of the amount of V is 0.080% or less, more preferably 0.060% or less.
  • the lower limit of the amount of V is preferably set to 0.001% or more in order to obtain the effect by the above action more reliably. More preferably, it is 0.010% or more, and particularly preferably 0.020% or more.
  • Cu 0% to 1.00%
  • Cu is an element which improves the hardenability and is effective for strengthening the base material. Therefore, Cu may be contained. However, if the amount of Cu exceeds 1.00%, the decrease in toughness accompanying the increase in hardness of the joint becomes remarkable. Therefore, the amount of Cu is set to 0% to 1.00%.
  • the upper limit of the amount of Cu is preferably 0.80% or less, more preferably 0.60% or less.
  • the lower limit of the amount of Cu is preferably 0.01% or more in order to obtain the effect by the above action more reliably. More preferably, it is 0.05% or more, particularly preferably 0.10% or more.
  • Ni is an element effective for securing the strength and improving the toughness of the base material. Therefore, Ni may be contained. However, even if the amount of Ni exceeds 2.00%, the effect of containing Ni saturates and the cost increases. Therefore, the amount of Ni is set to 0% to 2.00%.
  • the upper limit of the amount of Ni is preferably 1.50% or less, more preferably 1.00% or less.
  • the lower limit of the amount of Ni is preferably 0.01% or more. More preferably, it is 0.10% or more, particularly preferably 0.20% or more.
  • Cr 0% to 1.00% Cr is an element which improves the hardenability and is effective in increasing the strength of the base material. Therefore, Cr may be contained. However, if the amount of Cr exceeds 1.00%, the toughness decreases with the increase in the hardness of the joint. Therefore, the amount of Cr is set to 0% to 1.00%.
  • the upper limit of the amount of Cr is preferably 0.80% or less, more preferably 0.60% or less.
  • the lower limit of the amount of Cr is preferably 0.01% or more. More preferably, it is 0.05% or more, particularly preferably 0.10% or more.
  • Mo 0% to 0.50%
  • Mo is an element that improves the hardenability and is effective in increasing the strength of the base material. Therefore, Mo may be contained. However, when the Mo content exceeds 0.50%, the toughness decreases with the increase in hardness of the joint. Therefore, the Mo content is 0% to 0.50%.
  • the preferable upper limit of the Mo amount is 0.40% or less, more preferably 0.30% or less. In order to obtain the effect by the above-mentioned effect more certainly, it is preferable to make the lower limit of Mo amount into 0.01% or more. More preferably, it is 0.05% or more, particularly preferably 0.10% or more.
  • B is an element contributing to the improvement of the base material strength by enhancing the hardenability by the addition of a small amount. Therefore, B may be contained. However, when the B content exceeds 0.0050%, the joint CTOD characteristics become inferior. Therefore, the B content is 0% to 0.0050%.
  • the upper limit of the amount of B is preferably 0.0040% or less, more preferably 0.0030% or less. In order to obtain the effect by the above action more reliably, the lower limit of the amount of B is preferably made 0.0001% or more. More preferably, it is 0.0005% or more, particularly preferably 0.0010% or more.
  • the balance is Fe and impurities.
  • the impurity refers to a component contained in the raw material or a component mixed in the process of production, which is not intentionally contained in the steel sheet.
  • the steel plate according to the present disclosure has a carbon equivalent Ceq of 0.30% to 0.55% determined by the following equation (1).
  • C, Mn, Cr, Mo, V, Cu, and Ni in Formula (1) represent the content (mass%) of each element contained in a steel plate.
  • 0 mass% is substituted and calculated as content of the applicable element in Formula (1).
  • the carbon equivalent When the carbon equivalent is less than 0.30%, it becomes difficult to satisfy the strength characteristics (tensile strength, yield stress) required for the steel plate as the base material. When the carbon equivalent exceeds 0.55%, it becomes difficult to satisfy the required arrestability and joint CTOD characteristics.
  • the lower limit value of the carbon equivalent is preferably 0.35% or more, more preferably 0.40% or more.
  • the upper limit of the carbon equivalent is preferably 0.52% or less, more preferably 0.49% or less.
  • the steel plate according to the present disclosure has a metallographic structure (micro-structure at a quarter position in the plate thickness direction from the steel plate surface of the cross section along the plate thickness direction (hereinafter may be referred to as "quarter plate thickness”). Structure, area fraction, 10.0% to 75.0% of ferrite phase, 10.0% to 90.0% of bainite phase, 0% to 15.0% of pearlite phase, and martensite / austenite mixed phase (Hereinafter, it may be referred to as "MA phase”.) It is composed of 0% to 1.0% tissue morphology.
  • the average particle diameter (diameter) measured by the electron beam backscattering diffraction method of all the phases (ferrite phase, bainite phase, pearlite phase, and MA phase) is 20 micrometers or less.
  • the sum of the area fractions of the ferrite phase, the bainite phase, the pearlite phase, and the MA phase is 100%.
  • the reasons for limitation of the microstructure (the morphology of the steel sheet, the average particle size of the structure, the amount of inclusions, and the center segregation) of the steel plate according to the present disclosure will be described.
  • the 1 ⁇ 4 position in the thickness direction from the surface of the steel plate of the cross section along the thickness direction represents a 2 mm square range around the 1 ⁇ 4 position.
  • the ferrite phase contributes to the strength and arrestability of the base material.
  • the area fraction exceeds 75.0%, the strength of the base material becomes inferior.
  • the area fraction of the ferrite phase is set to 10.0% to 75.0%.
  • the preferred range of the area fraction of the ferrite phase is 20.0% to 50.0%.
  • the bainite phase mainly contributes to the strength of the base material. If the area fraction of the bainite phase is less than 10.0%, the strength of the base material becomes inferior. On the other hand, when the area fraction of the bainite phase exceeds 90.0%, arrestability becomes inferior. Therefore, the area fraction of the bainite phase is 10.0% to 90.0%.
  • the preferred range of the area fraction of the bainite phase is 50.0% to 80.0%.
  • the perlite phase may be contained in the microstructure.
  • the area fraction of the pearlite phase becomes excessive, the strength of the base material becomes inferior. Therefore, when the pearlite phase is included, the area fraction of the pearlite phase is 15.0% or less.
  • the preferable upper limit of the area fraction of the pearlite phase is 10.0% or less.
  • the pearlite phase may not be contained. That is, the lower limit value of the pearlite phase is 0%.
  • MA phase 0% to 1.0% Since the MA phase lowers the joint CTOD characteristics, when the area fraction of the MA phase becomes excessive, the joint CTOD characteristics become inferior. Therefore, when the MA phase is included, the area fraction of the MA phase is 1.0% or less.
  • the lower limit is not particularly defined as the amount of MA phase is as small as possible. The MA phase may not be included.
  • the area fractions of the ferrite phase, the bainite phase, the pearlite phase, and the MA phase are measured as follows.
  • the metallographic structure of a cross section in the direction perpendicular to the rolling direction of the steel plate (so-called C direction cross section) and a cross section perpendicular to the width direction of the steel plate (so-called L direction cross section) It is determined by taking a picture and analyzing the image.
  • a cross section in the direction perpendicular to the rolling direction of the steel plate C direction cross section
  • a cross section in the direction perpendicular to the width direction of the steel plate L direction cross section
  • the L-direction cross-section observation sample and the C-direction cross-section observation sample are collected from the 1 ⁇ 4 position from the width direction end face of the steel plate at four positions.
  • the collected sample is subjected to nital etching, and after etching, eight fields of view are taken at a magnification of 500 with a total of L direction cross section (four fields of view) and C direction cross section (four fields of view) using an optical microscope.
  • binarization processing is performed on the obtained tissue photograph using image analysis software, and image analysis is performed.
  • the area ratio is determined using the phase that appears white as a ferrite phase, the phase that appears black as a pearlite phase, and the phase that appears gray as a bainite phase or an MA phase (martensitic / austenitic mixed phase).
  • the nital-etched portion is repeller-etched, and image analysis is performed on a portion that appears gray by nital etching to determine what is white as the MA phase (martensitic-austenitic mixed phase) and the area ratio.
  • the area ratio of the above-mentioned MA phase (the martensite / austenite mixed phase) is subtracted from the area ratio which appeared gray by nital etching, and this is defined as the area ratio of the bainite phase.
  • the average particle diameter of all the phases is set to 20 ⁇ m or less (preferably 15 ⁇ m or less, more preferably 10 ⁇ m or less). The smaller the average particle size of all the phases, the better the improvement in arrestability.
  • the lower limit of the average particle diameter of all phases is not particularly limited, and, for example, 1 ⁇ m or more can be mentioned, and 5 ⁇ m or more can be mentioned.
  • the average grain size of all phases is measured by the electron back scattering pattern (EBSP) which can measure the crystal orientation information with a wide field of view with high accuracy.
  • the EBSP method can also measure the grain size of complex structures such as bainite. Specifically, it is measured by the following method. The EBSP method is repeated while moving the measurement position by 1 ⁇ m for a 500 ⁇ m ⁇ 500 ⁇ m area of 1/4 part of plate thickness.
  • a boundary where the crystal orientation difference between adjacent grains is 15 ° or more is defined as a grain boundary, and the area fraction weighted average value of the equivalent circle diameter (diameter) surrounded by the grain boundaries is determined, Average particle size.
  • the steel plate according to the present disclosure is an oxide, a sulfide, and an oxysulfide having a circle equivalent diameter of 2 ⁇ m or more at a quarter position in the plate thickness direction from the surface of the steel plate in a cross section along the plate thickness direction.
  • the sum total of content of an oxide, a sulfide, and an acid sulfide whose circle equivalent diameter is 2 micrometers or more is 50 pieces / mm ⁇ 2 > or less as a number density.
  • an acid sulfide represents the complex of an oxide and a sulfide.
  • oxides, sulfides and acid sulfides contained in 1 ⁇ 4 part of plate thickness mean inclusions (hereinafter, oxides, sulfides and acid sulfides are collectively referred to simply as “inclusions”. There is a case. Hereinafter, the reasons for limitation of inclusions will be described.
  • the lower limit value of the content of the above-mentioned inclusions contained in 1/4 part of board thickness is not particularly limited, and, for example, 10 / mm 2 or more can be mentioned.
  • the equivalent circle diameter and the content (number density) of oxides, sulfides and oxysulfides contained in 1/4 part of plate thickness of steel sheet are measured using a scanning electron microscope (SEM: Scanning Electron Microscope) It is preferable to measure with a wide visual field. Specifically, it is measured by the following method. Specifically, from the surface of the steel plate to the thickness direction 1 ⁇ 4 of the cross section in the direction perpendicular to the rolling direction of the steel plate (C direction cross section) and the cross section in the direction perpendicular to the width direction of the steel plate (L direction cross section) Take a sample. Then, a cross section perpendicular to the rolling direction of the steel plate (so-called C direction cross section) is polished and observed.
  • SEM Scanning Electron Microscope
  • a cross section perpendicular to the width direction of the steel plate is polished and observed.
  • the acceleration voltage is 15 kV
  • the current is 89 ⁇ A to 91 ⁇ A
  • the L-direction cross section and the C direction cross-section make the observation field area 180 mm 2 to 200 mm 2 .
  • Each observation visual field area of the cross section in the C direction is 90 mm 2 to 100 mm 2 .
  • the SEM / WDX analysis first identifies particles having a circle equivalent diameter (diameter) of 2 ⁇ m or more which is observed in the observation field of view.
  • the composition of each particle is then analyzed by WDX to identify oxides, sulfides and acid sulfides according to the following definition.
  • the particle density of the inclusions is determined by taking a SEM particle image of the particles and counting the number of each particle by image analysis.
  • the analysis of inclusions may be automatically and continuously measured using software attached to the SEM.
  • inclusions corresponding to the following conditions are determined as an oxide, a sulfide, and an acid sulfide. That is, as inclusions, all elements other than Fe contained in the steel plate according to the present disclosure are elements to be analyzed, and the mass% for each object to be analyzed is calculated. The total of all elements to be analyzed is 100% by mass.
  • inclusions with an O (oxygen) content of 10% by mass or more as oxides, inclusions with an S content of 3% by mass or more as sulfides, and O (oxygen) content as 10% Inclusions having a% or more content and a S content of 3% by mass or more are referred to as an acid sulfide.
  • the steel plate according to the present disclosure it is important that central segregation be controlled to a suitable range.
  • the steel plate according to the present disclosure adopts the following index as an index representing central segregation.
  • the steel plate according to the present disclosure has a cross section along the thickness direction, with respect to a Mn concentration (Mn (1/4) ) at a 1/4 position from the surface of the steel plate to the thickness direction,
  • the ratio (Mn (1/2) / (Mn (1/2) ) ratio of Mn concentration (Mn (1/2) ) at half position in the plate thickness direction from the surface of the steel sheet (hereinafter sometimes referred to as "plate thickness 1/2 part" ) Mn (1/4) is 0.90 to 1.80.
  • the 1 ⁇ 2 position in the thickness direction from the surface of the steel plate of the cross section along the thickness direction represents a 2 mm square range around the 1 ⁇ 2 position.
  • the 1 ⁇ 4 position in the thickness direction from the surface of the steel plate of the cross section along the thickness direction represents a 2 mm square range around the 1 ⁇ 4 position.
  • Mn is centrally segregated during continuous casting to form an embrittled zone at the center of the plate thickness, which affects the joint CTOD characteristics. Therefore, the ratio (Mn (1/2) / Mn) of the concentration (Mn (1/2) ) of Mn in the plate thickness 1/2 part to the concentration (Mn (1/4) ) of Mn in the plate thickness 1/4 part Let (1/4) ) be 0.90 to 1.80. If (Mn (1/2) / Mn (1/4) ) exceeds 1.80, the Mn concentration at the center of the plate thickness becomes too high, so the joint CTOD characteristics deteriorate.
  • Mn (1/2) / Mn (1/4) it is practically difficult to stably obtain a steel sheet with Mn (1/2) / Mn (1/4) less than 0.90.
  • the preferred range of Mn (1/2) / Mn (1/4) is 0.95 to 1.70, and the more preferred range is 1.00 to 1.60.
  • Mn (1/2) / Mn (1/4) is determined as follows. First, a sample for measurement of Mn concentration collected from 1/4 part of plate thickness and a sample for measurement of Mn concentration collected from 1/2 part of plate thickness are prepared. Next, for each sample, an electron probe microanalyzer (EPMA: Electron Probe MicroAnalyzer, measurement conditions; acceleration voltage: 15 kV, beam diameter: 20 ⁇ m, irradiation time: 20 ms, taking a 2 mm square range of the sample as a measurement range) The maximum value of the concentration of Mn when the measurement range of 2 mm square is measured is measured by the measurement pitch: 20 ⁇ m.
  • EPMA Electron Probe MicroAnalyzer
  • Mn (1/2) / Mn (1/4) is calculated with the maximum concentrations obtained from the respective samples as Mn (1/2) and Mn (1/4) , respectively.
  • Each sample is taken from the C direction cross section and the L direction cross section, and the measurement is performed on the C direction cross section and the L direction cross section.
  • the thickness of the steel plate according to the present disclosure is not particularly limited, and may be, for example, 50 mm or more, and may further be 50 mm to 100 mm.
  • the tensile strength (TS) of the steel plate according to the present disclosure is not particularly limited, and is preferably 510 MPa or more (preferably 510 MPa to 720 MPa, more preferably 570 MPa to 720 MPa) from the viewpoint of achieving high strength.
  • the yield stress (YP) is preferably 390 MPa or more (preferably 390 MPa to 650 MPa, more preferably 460 MPa to 650 MPa).
  • the tensile strength (TS) of the steel plate according to the present disclosure is measured using a No. 1B tensile test piece of JIS Z 2241 (2011).
  • the yield stress (YP) means the proof stress of the permanent elongation method at the time of permanent elongation 0.2% of JIS Z2241 (2011).
  • brittle crack propagation stopping toughness value at test temperature minus 10 ° C. in a temperature gradient type ESSO test K ca (hereinafter also referred to as "arrest toughness value K ca-10 ° C.”.)
  • Arrest toughness value K ca-10 ° C There 6000 N / mm 1.5 or more (preferably, 8000 N / mm 1.5 or more) may be a.
  • Arrest toughness value K ca -10 ° C is defined by the NK Classification Society Steel Ship Regulations Inspection Guideline K, Appendix K3.12.2-1. Measure according to (2016) “Testing Procedure for Thermal Gradient ESSO Test and Thermal Gradient Double Tensile Test”.
  • the steel plate according to the present disclosure has an opening displacement ⁇ c (hereinafter referred to as “opening” at a test temperature minus 10 ° C. in a crack tip opening displacement test of the welded part (hereinafter may be referred to as “CTOD test of welded part”).
  • the displacement ⁇ c ⁇ 10 ° C. may be referred to as“ 0.10 mm or more (preferably 0.20 mm or more) ”.
  • the opening displacement ⁇ c ⁇ 10 ° C. is measured in accordance with BS 7448 standard (British Standard) Part 1 (1991) and BS 7448 standard (British Standard) Part 2 (1997).
  • submerged arc welding SAW welding
  • SAW welding submerged arc welding
  • the form of the metal structure, the average particle diameter of each phase of the metal structure, the amount of inclusions, and the center segregation are controlled to satisfy the above-mentioned conditions.
  • Tensile strength (TS), yield stress (YP), arrest toughness value K ca -10 ° C , and opening displacement ⁇ c -10 ° C even when the plate thickness is 50 mm or more (for example, 50 mm to 100 mm)
  • the value of the above range can be satisfied.
  • the steel plate according to the present disclosure can be obtained, for example, by a manufacturing method having the following steps.
  • a step of performing deoxidation treatment by adding a deoxidizing element so that the final dissolved oxygen amount becomes 20 ppm or less in a molten steel whose dissolved oxygen amount has been adjusted to 50 ppm or less (deoxidation treatment step); Continuous casting of the deoxidized molten steel at a throughput of 0.80 to 5.00 tons (0.80 ton / 5.00 to 5.00 ton / min) per minute, and center solidification of the slab during continuous casting
  • the phase ratio is in the range of 0.5 to 1.0
  • the amount of reduction is 1 to 1.5 mm (1 mm / m to 1.5 mm / m) with a reduction gradient of 1 mm to 1 mm in the casting advancing direction with respect to the slab.
  • a step of obtaining a billet by applying a light pressure of 1 mm to 15 mm (casting step), The billet is heated in a temperature range of 950 ° C. to 1150 ° C. and rough rolling is performed on the heated billet, and then the surface temperature of the steel plate is Ar 3 point ⁇ 30 ° C. to recrystallization temperature on the steel plate after rough rolling
  • a step (finishing step) of performing finish rolling with a cumulative rolling reduction of 50% to 75% in a temperature range of T rex The steel sheet after finish rolling has a cross section along the thickness direction of the steel sheet until the temperature at a quarter position from the surface of the steel sheet in the thickness direction reaches a temperature range of 0 ° C. to 600 ° C. Cooling at an average cooling rate of 1 ° C./s to 20 ° C./s (cooling step). Each step will be described below.
  • adjustment of the amount of dissolved oxygen in molten steel before adding deoxidizing element is performed.
  • the amount of dissolved oxygen in the molten steel exceeds 50 ppm, it is difficult to adjust the final amount of dissolved oxygen in the molten steel to 20 ppm or less by adding a deoxidizing element described later. Therefore, adjustment of the amount of dissolved oxygen in molten steel before adding a deoxidizing element is 50 ppm or less (preferably 40 ppm or less, more preferably 30 ppm or less).
  • the lower limit of the amount of dissolved oxygen in the molten steel before adding the deoxidizing element is not particularly limited, and may be adjusted to, for example, more than 20 ppm. Note that ppm is on a mass basis.
  • the method for adjusting the dissolved oxygen content of the molten steel to 50 ppm or less is not particularly limited, and examples thereof include performing using a RH (Ruhrstahl-Heraeus) vacuum degassing apparatus.
  • RH Rasterstahl-Heraeus
  • a deoxidizing element is added to the molten steel after the dissolved oxygen content of the molten steel becomes 50 ppm or less, and the final dissolved oxygen content in the molten steel after addition of the deoxidizing element is adjusted to 20 ppm or less.
  • the final amount of dissolved oxygen in the molten steel after addition of the deoxidizing element exceeds 20 ppm, it becomes difficult to control the number density of inclusions of oxides, sulfides and acid sulfides to a predetermined amount or less.
  • the final amount of dissolved oxygen in the molten steel after addition of the deoxidizing element should be as low as possible.
  • the amount of final dissolved oxygen in the molten steel after deoxidation element addition 15 ppm or less is preferable, for example, and 10 ppm or less is more preferable.
  • the lower limit of the final amount of dissolved oxygen in the molten steel after addition of the deoxidizing element is not particularly limited, and may be, for example, 5 ppm or more.
  • the deoxidizing element at least one selected from the group consisting of Al, Ca, Si, Ti, Mg, Zr, and REM is used.
  • the addition order of the deoxidizing elements is not particularly limited.
  • Al and a deoxidizing element other than Al may be simultaneously added, or after adding Al, a deoxidizing element other than Al may be added.
  • RH vacuum degassing apparatus For adjustment of the final dissolved oxygen content of the molten steel after adding the deoxidizing element by the RH vacuum degassing apparatus, for example, it is preferable to reflux the molten steel for 10 to 60 minutes at a degree of vacuum of 1 to 5 torr ( ⁇ 133 Pa to 667 Pa). .
  • a chemical composition is adjusted in consideration of a yield so that each element may become desired content.
  • the method of adding each element is not particularly limited as long as it can be contained in the steel sheet so that the chemical composition satisfies the above conditions.
  • the measurement of the amount of dissolved oxygen before adding the deoxidizing element and the final amount of dissolved oxygen after adding the deoxidizing element are free from existing oxygen as a simple substance which is not oxidized as a compound with other compounds. Measure the state oxygen. Specifically, the amount of dissolved oxygen before adding the deoxidizing element and the final amount of dissolved oxygen after adding the deoxidizing element are determined by a known method of electromotive force measurement (for example, “in converter furnace by electromotive force measurement "Rapid analysis of molten steel oxygen” (see Ikuta et al., Iron and Steel, Japan Iron and Steel Institute, 1972, No. 10, pp. 125-132).
  • molten steel having a predetermined chemical composition which has undergone deoxidation treatment, is continuously cast at a throughput of 0.80 ton / min to 5.00 ton / min.
  • the throughput of the molten steel is preferably 3.00 ton / min to 4.50 ton / min in that the number density of inclusions is further controlled.
  • throughput refers to the production rate (ton / min) of slabs produced per minute.
  • the throughput can be obtained by the following equation.
  • Throughput (ton / min) casting width (mm) ⁇ casting thickness (mm) ⁇ casting speed (mm / min) ⁇ molten steel density (ton / mm 3 )
  • the molten steel density is determined by the metal type of the molten steel, but in the present disclosure, the molten steel density is 7.85 ⁇ 10 ⁇ 9 (ton / mm 3 ).
  • the light reduction is 1.0 mm / m to 1.5 mm / m (preferably) to the slab when the central solid phase ratio of the slab at the end of solidification of the slab is in the range of 0.5 to 1.0.
  • the reduction is always 1 mm to 15 mm with a reduction gradient of 1.0 mm / m to 1.5 mm / m. There is no need to carry out a light reduction.
  • the light reduction is 1 mm to 15 mm at a reduction gradient of 1.0 mm / m to 1.5 mm / m at any timing during which the central solid phase ratio is in the range of 0.5 to 1.0.
  • the light reduction to be For example, while the central solid phase ratio is in the range of 0.5 to 0.8, or in the range of 0.6 to 0.8, or in the range of 0.6 to 0.7, At the timing, the pressure reduction gradient may be in the range of 1.0 mm / m to 1.5 mm / m, and the light reduction may be performed so that the amount of pressure reduction is 1 mm to 15 mm.
  • the central solid phase ratio can be defined as the solid phase ratio of the molten portion in the central portion of the cast slab thickness direction and in the cast slab width direction.
  • the central solid phase rate can be determined by heat transfer and solidification calculation. As heat transfer and solidification calculation, enthalpy method, equivalent specific heat method, etc. are widely known, and any method may be used.
  • Rolling process Next, the steel slab obtained through the casting process is heated at a predetermined temperature, and rough rolling is performed on the heated steel slab. Then, finish rolling is performed on the steel plate after rough rolling so that the surface temperature of the steel plate becomes a predetermined cumulative rolling reduction at a predetermined temperature.
  • This process is a process of refining austenite grains by heating a steel piece, efficiently accumulating strain in austenite by predetermined finish rolling, and contributing to the grain refinement of a microstructure. And, this process affects the arrestability.
  • the billet is heated in a temperature range of 950 ° C. to 1150 ° C. (preferably 1000 ° C. to 1100 ° C.).
  • a temperature range of 950 ° C. to 1150 ° C. (preferably 1000 ° C. to 1100 ° C.).
  • austenitization becomes sufficient and austenite grains are refined.
  • the heating temperature of the steel piece is set to 1150 ° C. or less, coarsening of austenite is suppressed and austenite grains are refined. And, by setting this temperature range, excellent arrestability can be obtained.
  • heating temperature represents the average temperature of the total thickness of a billet.
  • the steel slab after heating is roughly rolled, and the steel plate after rough rolling is subjected to finish rolling.
  • the finish rolling is performed such that the cumulative rolling reduction is 50% to 75% in the temperature range where the surface temperature of the steel sheet is Ar 3 point ⁇ 30 ° C. to the recrystallization temperature T rex .
  • the temperature range of the finish rolling represents the surface temperature of the steel plate, and the rolling is started after the surface temperature of the steel plate reaches Ar 3 point ⁇ 30 ° C. to the recrystallization temperature T rex .
  • the surface temperature of the steel sheet is in a temperature range of Ar 3 point ⁇ 30 ° C. to the recrystallization temperature T rex , the coarsening of each phase of the microstructure is suppressed.
  • ferrite is finely divided to obtain good arrestability. If the start temperature of finish rolling is Ar 3 -30 ° C. or higher, two-phase zone rolling (rolling in a temperature range where ferrite + austenite two phase exists) will not be performed, so coarse worked ferrite (ferrite formed during rolling) Generation is suppressed. On the other hand, if the recrystallization temperature is equal to or lower than the Rex temperature, rolling in the non-recrystallization region is performed, and coarsening of ferrite is suppressed.
  • the cumulative rolling reduction is set to 50% to 75% (preferably 55% to 65%).
  • Ar 3 is represented by the following formula (2)
  • the recrystallization temperature T rex is represented by the following formula (3).
  • T rex -91900 [Nb *] 2 + 9400 [Nb *] + 770 (However, in Formula (3), [Nb *] is represented by following formula (4).
  • T represents the heating temperature of a billet, and a unit represents celsius temperature (degreeC).
  • Formula (4) Sol 910-310 [ C] +65 [Si] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo]
  • T rex -91900 [Nb *] 2 + 9400 [Nb *] + 770
  • Nb (10 ( -6770 / (T + 273) +2.26 )) / (C + 12/14 ⁇ N)
  • [Nb] ⁇ [Sol. In the case of Nb], the relationship of [Nb *] [Nb] is satisfied.
  • [Nb] represents the Nb content (% by mass), and [Sol. Nb] is Sol. Nb (solid solution Nb) (mass%) is represented. )
  • the plate thickness after rough rolling is the same as the plate thickness at the time of starting finish rolling.
  • the steel plate after finish rolling has a cooling rate of 1 ° C / s to 20 ° C / s at a plate thickness of 1/4 of the steel plate, and a temperature of 0 ° C to 600 ° C at a plate thickness of 1/4 of a steel plate Cool down to area. That is, the cooling stop temperature is a temperature range of 0 ° C. to 600 ° C. as a temperature at a thickness of 1/4 of the steel plate.
  • the cooling method is not particularly limited, and examples thereof include methods such as water cooling.
  • a cooling rate By cooling at a cooling rate of 1 ° C / sec to 20 ° C / sec at a plate thickness of 1 ⁇ 4 part and cooling to a temperature range of 0 ° C to 600 ° C at a plate thickness of 1 ⁇ 4 part A predetermined amount of ferrite phase and a predetermined amount of bainite phase are obtained.
  • the cooling rate and the cooling stop temperature are the calculated cooling rate and the calculated cooling stop temperature at a quarter of the plate thickness.
  • a cooling rate is an average cooling rate from a cooling start to a cooling stop.
  • the preferable manufacturing method for obtaining the steel plate according to the present disclosure further includes a step (tempering step) of tempering the steel plate after the cooling step in a temperature range of 350 ° C. to 650 ° C., if necessary. It is also good.
  • tempering process After cooling the steel plate, if necessary, it may be tempered by heat treatment in a temperature range of 350 ° C. to 650 ° C. (preferably 450 ° C. to 550 ° C.) to adjust the strength and toughness of the steel plate.
  • the tempering temperature is 350 ° C. or more, the effect of improving the toughness by strain removal is enhanced.
  • the tempering temperature is set to 650 ° C. or less, strength reduction can be suppressed.
  • the manufacturing method of the steel plate which concerns on this indication is not limited to the above-mentioned manufacturing method. Even if the manufacturing method of a steel plate is a manufacturing method other than the above-mentioned, if the steel plate is within the specified range, the steel plate is considered to be included in the range of the steel plate according to the present disclosure.
  • an example demonstrates in more detail an example of a desirable embodiment in a manufacturing method of a steel plate concerning this indication, and a steel plate.
  • the following example is not a thing of the property which limits the manufacturing method of the steel plate which concerns on this indication, and a steel plate.
  • the steel sheet and the method for producing the steel sheet according to the present disclosure can be modified and implemented as long as they can be applied to the purpose described above or later, and all of them are included in the technical scope of the present disclosure. .
  • Example 1 shows the chemical composition of the steel plate.
  • the dissolved oxygen amount before the addition of the deoxidizing element and the final dissolved oxygen amount of the molten steel after the addition of the deoxidizing element are adjusted so as to obtain the values shown in Table 2;
  • the chemical composition of the molten steel was adjusted so that Next, the prepared molten steel was continuously cast to obtain a billet so as to have the throughput, the reduction gradient, and the reduction amount shown in Table 2.
  • the steel slab was heated under the heating conditions shown in Table 2, and subjected to finish rolling and cooling under the finish rolling conditions and cooling conditions shown in Table 2 to obtain steel plates. And about the temperature shown in Table 2, it tempered about a part of obtained steel plate.
  • the plate thicknesses of the obtained steel plates are shown in Table 3.
  • the number density of the above inclusions (oxide, sulfide, and acid sulfide) was measured according to the method described above.
  • the Mn concentration (Mn (1/4) ) at a 1/4 position from the surface of the steel plate to the cross section along the thickness direction The ratio (Mn (1/2) / Mn (1/4) 2) of the Mn concentration (Mn (1/2) ) at the 1/2 position in the thickness direction was measured according to the method described above.
  • the opening displacement ⁇ c (opening displacement ⁇ c ⁇ 10 ° C. ) at a test temperature of ⁇ 10 ° C. in the crack tip opening displacement test of the weld was measured according to the method described above. The results of measurement of these characteristics are shown in Table 3.
  • ⁇ fraction indicates the area fraction of ferrite phase
  • B fraction indicates the area fraction of bainite phase
  • P fraction indicates the area fraction of pearlite phase
  • MA fraction Represents the area fraction of the martensite / austenite mixed phase, respectively.
  • Average particle size represents the average particle size (diameter) of all phases of the ferrite phase, the bainite phase, the pearlite phase, and the martensite-austenite mixed phase.
  • Numberer of inclusions of 2 ⁇ m or more represents the total number density of oxides, sulfides, and oxysulfides having a circle equivalent diameter (diameter) of 2 ⁇ m or more.
  • T / 2 Mn ⁇ t / 4 Mn is the Mn concentration (Mn (1/2) ) at the plate thickness 1/2 part to the Mn concentration (Mn ( 1/4) ) at the plate thickness 1/4 . It represents the ratio (Mn (1/2) / Mn (1/4) ).
  • the plate thickness is 50 mm-100 mm
  • arresting toughness value K ca-10 ° C. is, it is 6000 N / mm 1.5 or more
  • the opening displacement .delta.c - It is understood that 10 ° C. is 0.10 mm or more.
  • Nos. 1 to 21 have a tensile strength of 510 MPa to 720 MPa and a yield stress of 390 MPa to 650 MPa. That is, Nos. 1 to 21 within the range defined by the steel plate according to the present disclosure have both excellent arrestability and excellent joint CTOD characteristics even with a plate thickness of 50 mm to 100 mm, and have high strength. It turns out that it is a certain steel plate.
  • the joint CTOD characteristics were particularly inferior in all steel plates.
  • No. 22, No. 24 to No. 27, and No. 29 to No. 32 were within the component range defined by the steel plate according to the present disclosure, the microstructure was a structure outside the defined range, and therefore arrested And at least one of the joint CTOD characteristics was inferior.
  • No. 23 and No. 28 were within the component range defined in the steel plate according to the present disclosure, No. 28 is outside the prescribed range of the ferrite phase, and No. 23 is 0% of the area ratio of the bainite phase It is. Therefore, the steel sheet has inferior strength (at least one of tensile strength and yield stress) although it has excellent arrestability and excellent joint CTOD characteristics.
  • the steel plate according to the present disclosure is a steel plate having high strength that has both excellent arrestability and excellent joint CTOD characteristics that can satisfy even more stringent standards. Therefore, according to the steel plate according to the present disclosure, in particular, it can be suitably applied to important members of a container ship (for example, Hatch Side Coating, Upper Deck).

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  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
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  • Treatment Of Steel In Its Molten State (AREA)

Abstract

This steel sheet has a predetermined chemical composition, and the carbon equivalent represented by a specific formula falls within a predetermined numerical range. The metal structure at a position of ¼ of the thickness from the steel sheet surface (1/4 of the sheet thickness) has, in area fraction, 10-75% ferrite phase, 10-90% bainite phase, 0-15% pearlite phase and 0-1% martensite/austenite mixed phase, with the average particle size (diameter) measured by electron backscatter analysis of all phases not exceeding 20 µm. The circle-equivalent diameter (diameter) contained in ¼ of the thickness of the steel sheet is at least 2 µm, the total of oxides, sulfides and oxysulfides does not exceed 50 units/mm2, the ratio (Mn(1/2)/Mn(1/4)) of the Mn concentration (Mn(1/2)) at a position at half of the sheet thickness (half sheet thickness) from the steel sheet surface with respect to the Mn concentration (Mn(1/4)) at a position at a quarter of the sheet thickness (quarter sheet thickness) is 0.90-1.80.

Description

鋼板および鋼板の製造方法Steel sheet and method of manufacturing steel sheet
　本開示は、鋼板および鋼板の製造方法に関する。 The present disclosure relates to a steel plate and a method of manufacturing a steel plate.
　鋼板の用途として、例えば、船舶、建築物、橋梁、海洋構造物、ＬＮＧ貯蔵タンク、その他の大型タンク、ラインパイプ等が挙げられる（例えば、特許文献１～３参照）。
　これらの用途に適用される鋼板は、脆性破壊を抑制するために、万が一、脆性き裂が溶接継手箇所に発生した場合でも、脆性き裂を母材で停止させる脆性亀裂伝播停止特性（ＢＣＡ：Ｂｒｉｔｔｌｅ　Ｃｒａｃｋ　Ａｒｒｅｓｔ；以下、「アレスト性」と称する場合がある。）が求められる。 Examples of applications of the steel plate include ships, buildings, bridges, marine structures, LNG storage tanks, other large tanks, line pipes and the like (see, for example, Patent Documents 1 to 3).
In order to suppress brittle fracture, the steel plate applied to these applications is a brittle crack propagation arresting property (BCA: stop a brittle crack at the base material even if a brittle crack occurs in a welded joint). Brittle Crack Arrest; hereinafter, may be referred to as "arrest".
　船舶に適用される鋼板としては、例えば、コンテナ船の重要部材（例えば、Ｈａｔｃｈ　Ｓｉｄｅ　Ｃｏａｍｉｎｇ、Ｕｐｐｅｒ　Ｄｅｃｋ）に適用される場合がある。
　近年、コンテナ船は、環境負荷低減、海上輸送の高効率化等を目的として、大型化が進んでいる。コンテナ船の大型化に伴い、コンテナ船の重要部材には、厚板鋼板が使用される。板厚が厚くなると、溶接継手に脆性亀裂が発生した場合、亀裂が船体を貫通する場合があることが判明している。 As a steel plate applied to ships, for example, it may be applied to an important member (for example, Hatch Side Coating, Upper Deck) of a container ship.
In recent years, container ships have been increased in size for the purpose of reducing environmental load and increasing the efficiency of marine transportation. With the increase in size of container vessels, thick steel plates are used as important members of container vessels. It has been found that when the plate thickness is increased, the crack may penetrate the hull if a brittle crack occurs in the welded joint.
日本国特許第５４４５７２０号公報Japanese Patent No. 5445720 日本国特許第３６９９６５７号公報Japanese Patent No. 3699657 日本国特許第５８４６３１１号公報Japanese Patent No. 5846311
　これまで、コンテナ船の建造において、使用する鋼材および溶接部の検査に特別な規定はなかった。しかしながら、国際船級協会連合会（ＩＡＣＳ：Ｉｎｔｅｒｎａｔｉｏｎａｌ　Ａｓｓｏｃｉａｔｉｏｎ　ｏｆ　Ｃｌａｓｓｉｆｉｃａｔｉｏｎ　Ｓｏｃｉｅｔｉｅｓ）において、コンテナ船の重要部材への脆性亀裂伝播停止特性を高めた高アレスト鋼の適用と、船舶建造時の溶接部超音波探傷検査方法に、従来実施されてきた検査方法よりも、さらに厳しい検査法の適用が規定化された。 So far, there have been no special provisions in the inspection of steel materials and welds used in the construction of container ships. However, in the International Association of Classification Societies (IACS), application of high arrest steel with enhanced brittle crack arresting properties to important members of container ships, and ultrasonic inspection of welds at ship construction In the method, the application of the inspection method more rigorous than that of the conventionally implemented inspection method is specified.
　これに伴い、安全性の向上及び厳格化された超音波探傷検査方法にも対応し得る鋼材が要求されている。このような鋼材として、アレスト性に優れるとともに、溶接継手からの脆性き裂の発生を抑制する脆性亀裂発生特性（ＣＴＯＤ：Ｃｒａｃｋ　Ｔｉｐ　Ｏｐｅｎｉｎｇ　Ｄｉｓｐｌａｃｅｍｅｎｔ；以下、「継手ＣＴＯＤ特性」と称する場合がある。）に優れた高強度である鋼材の開発が求められている。 Along with this, there is a demand for a steel material that can cope with the improved safety and the stringent ultrasonic flaw inspection method. As such a steel material, a brittle crack initiation characteristic (CTOD: Crack Tip Opening Displacement; hereinafter sometimes referred to as “joint CTOD characteristic”) which is excellent in arrestability and which suppresses the occurrence of a brittle crack from a welded joint. There is a need for the development of steel materials that have excellent high strength.
　特許文献１には、高強度鋼板の成分組成、ミクロ組織、板厚方向の結晶粒界密度、および板厚方向の集合組織を制御することにより、アレスト性に優れた高強度厚鋼板が開示されている。特許文献１に開示された鋼板は、具体的には、特定の化学組成を有し、面積率で、フェライト７０％以下およびベイナイト３０％以上のミクロ組織を有する。さらに、板厚の１／４部での結晶粒界密度が４００ｍｍ／ｍｍ２～１０００ｍｍ／ｍｍ２、及び板厚の１／４部における｛１００｝面の面積率が１０％～４０％である。また、板厚の１／２部での結晶粒界密度が３００ｍｍ／ｍｍ２～９００ｍｍ／ｍｍ２、及び板厚の１／２部における｛１１０｝面の面積率が４０％～７０％である。そして、降伏応力３９０ＭＰａ～６９０ＭＰａである。 Patent Document 1 discloses a high strength thick steel plate excellent in arrestability by controlling the composition of the high strength steel plate, microstructure, grain boundary density in the thickness direction, and texture in the thickness direction. ing. Specifically, the steel plate disclosed in Patent Document 1 has a specific chemical composition, and has a microstructure of 70% or less of ferrite and 30% or more of bainite in area ratio. Furthermore, the grain boundary density at a quarter of the plate thickness is 400 mm / mm 2 to 1000 mm / mm 2 , and the area ratio of the {100} plane at a quarter of the plate thickness is 10% to 40%. . In addition, the grain boundary density in half of the plate thickness is 300 mm / mm 2 to 900 mm / mm 2 , and the area ratio of {110} plane in half of the plate thickness is 40% to 70%. . And, the yield stress is 390 MPa to 690 MPa.
　しかしながら、特許文献１に記載の技術は、アレスト性に着目されているが、継手ＣＴＯＤ特性については、特に着目されていなかった。また、溶製方法および鋳造方法については特に検討されていなかった。そして、本発明者らが特許文献１に開示される技術を検討した結果、特許文献１に開示される技術の鋼板には、アレスト性と継手ＣＴＯＤ特性との両特性を高レベルで満足させるために、さらなる改善の余地があることが分かった。 However, although the technique described in Patent Document 1 focuses on arrestability, it does not focus on joint CTOD characteristics in particular. In addition, the melting method and the casting method have not been particularly studied. Then, as a result of examining the technology disclosed in Patent Document 1 by the present inventors, the steel plate of the technology disclosed in Patent Document 1 satisfies both arrestability and joint CTOD characteristics at a high level. It was found that there is room for further improvement.
　特許文献２には、溶融線近傍の溶接熱影響部のγ粒界に沿って生成する粒界フェライトおよびフェライトサイドプレートの微細化、溶融線近傍の溶接熱影響部のγ粒内を粒内変態フェライトの生成によって微細化、及び溶融線近傍の溶接熱影響部のマルテンサイト・オーステナイト混合相の生成量低減を同時に制御することが開示されている。それによって、溶接熱影響部のＣＴＯＤ特性に優れた４６０ＭＰａ以上の降伏強度を有する鋼板が開示されている。特許文献２に開示された鋼板は、具体的には、特定の化学組成を有し、さらに、Ｍｇ－Ａｌ系酸化物を内包する特定のＴｉＮおよび酸化物と硫化物が複合した形態でＭｎを含有する特定の粒子が特定量存在する。 In Patent Document 2, grain boundary ferrite and ferrite side plates formed along the γ grain boundary of the weld heat affected zone near the melting line are refined, and intragranular transformation within the γ grain of the weld heat affected zone near the melting line It is disclosed to simultaneously control the miniaturization by the formation of ferrite and the reduction of the amount of martensite-austenite mixed phase formation in the weld heat affected zone near the melting line. Thus, a steel plate having a yield strength of 460 MPa or more excellent in the CTOD characteristics of the weld heat affected zone is disclosed. Specifically, the steel plate disclosed in Patent Document 2 has a specific chemical composition, and further includes a specific TiN containing an Mg-Al oxide and a complex of sulfide and an oxide. There is a specific amount of specific particles contained.
　しかしながら、特許文献２に記載の技術は、継手ＣＴＯＤ特性には着目されているが、アレスト性については、特に着目されていなかった。また、溶製方法および鋳造方法については特に検討されておらず、さらに、製鋼方法も具体的に検討されていなかった。そして、本発明者らが特許文献２に開示される技術を検討した結果、特許文献２に開示される技術の鋼板にも、アレスト性と継手ＣＴＯＤ特性との両特性を高レベルで満足させるために、さらなる改善の余地があることが分かった。 However, although the technique described in Patent Document 2 focuses on the joint CTOD characteristics, it does not focus on the arrestability in particular. Further, the melting method and the casting method have not been particularly studied, and furthermore, the steel making method has not been specifically studied. Then, as a result of examining the technology disclosed in Patent Document 2 by the present inventors, the steel plate of the technology disclosed in Patent Document 2 also satisfies both the arrestability and joint CTOD characteristics at a high level. It was found that there is room for further improvement.
　特許文献３には、鋼板の中心偏析部の硬化を抑制、鋼中のＴｉＮを均一に微細分散、及びＣａの硫化物の晶出を利用することにより、降伏応力が４２０ＭＰａ以上で、多層溶接部の溶接熱影響部の低温靭性に優れる鋼板が開示されている。特許文献３に開示される鋼板は、具体的には、特定の化学組成を有し、ＨＶｍａｘ／ＨＶａｖｅ≦１．３５＋０．００６／［Ｃ］－ｔ／５００の関係を満足する。 Patent Document 3 suppresses the hardening of the center segregated portion of the steel plate, uniformly disperses TiN in the steel uniformly, and crystallizes Ca sulfide, so that the yield stress is 420 MPa or more, and the multilayer welded portion There is disclosed a steel plate which is excellent in the low temperature toughness of the weld heat affected zone. Specifically, the steel plate disclosed in Patent Document 3 has a specific chemical composition, and satisfies the relationship of H Vmax / H Vave ≦ 1.35 + 0.006 / [C] −t / 500.
　しかしながら、特許文献３に記載の技術は、継手ＣＴＯＤ特性には着目されているが、アレスト性については、特に着目されていなかった。また、溶製方法および鋳造方法については特に検討されていなかった。そして、本発明者らが特許文献３に開示される技術を検討した結果、特許文献３に開示される技術の鋼板もまた、アレスト性と継手ＣＴＯＤ特性との両特性を高レベルで満足させるために、改善の余地があることが分かった。 However, although the technology described in Patent Document 3 focuses on the joint CTOD characteristics, it does not focus on the arrestability in particular. In addition, the melting method and the casting method have not been particularly studied. Then, as a result of examining the technology disclosed in Patent Document 3 by the present inventors, the steel plate of the technology disclosed in Patent Document 3 also satisfies both the arrestability and the joint CTOD characteristics at a high level. It turned out that there is room for improvement.
　上記の特許文献１～３に開示される鋼板について、本発明者らが検討した結果、従来よりも厳格化された規格を満足する、アレスト性および継手ＣＴＯＤ特性の両特性を高レベルで併せ持ち、さらに高強度である鋼板とするには、さらなる改善の余地があることを知見した。
　このように、より厳格化された規格を満足するような、従来の鋼板よりもアレスト性と継手ＣＴＯＤ特性がともに優れ、かつ高強度である鋼板は、未だ確立されていなかったのが実情である。 The inventors of the present invention examined the steel plates disclosed in the above Patent Documents 1 to 3 and as a result, they have both high arrestability and joint CTOD characteristics at a high level that satisfy the more stringent standards than before, It was found that there is room for further improvement in order to obtain a steel plate with higher strength.
Thus, a steel plate which is superior in both arrestability and joint CTOD characteristics and has higher strength than conventional steel plates which satisfy the more stringent standards has not been established yet. .
　本開示は、このような実情に鑑みてなされたものであり、より厳格化された規格をも満足し得る、優れたアレスト性と優れた継手ＣＴＯＤ特性とを併せ持ち、かつ高強度である鋼板を提供するものである。 The present disclosure has been made in view of such circumstances, and has high strength and steel sheet that has both excellent arrestability and excellent joint CTOD characteristics that can satisfy even more stringent standards. It is provided.
　本開示は、以下の態様を含む。 The present disclosure includes the following aspects.
（１）
　質量％で、
Ｃ　　：０．０３％～０．１４％、
Ｓｉ　：０．０１％～０．５０％、
Ｍｎ　：１．２０％～２．５０％、
Ｐ　　：０．０３０％以下、
Ｓ　　：０．０２０％以下、
Ｎｂ　：０．００３％～０．０５０％、
Ｔｉ　：０．００３％～０．０５０％、
Ａｌ　：０．００１％～０．１００％、
Ｎ　　：０．００１０％～０．００８０％、
Ｏ　　：０．００５０％以下、
Ｃａ　：０％～０．０１００％、
Ｍｇ　：０％～０．０１００％、
ＲＥＭ：０％～０．０１００％、
Ｚｒ　：０％～０．０１００％、
Ｔｅ　：０％～０．０１００％、
Ｖ　　：０％～０．１５０％
Ｃｕ　：０％～１．００％、
Ｎｉ　：０％～２．００％、
Ｃｒ　：０％～１．００％、
Ｍｏ　：０％～０．５０％、
Ｂ　　：０％～０．００５０％、並びに、
残部　：Ｆｅ及び不純物からなり、
　下記式（１）で表される炭素当量Ｃｅｑが、０．３０％～０．５５％であり、
　板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置での金属組織が、面積分率で、フェライト相１０．０％～７５．０％、ベイナイト相１０．０％～９０．０％、パーライト相０％～１５．０％、及びマルテンサイト・オーステナイト混合相０％～１．０％から構成され、全相の電子線後方散乱回折法により測定される平均粒径（直径）が２０μｍ以下であり、
　前記板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置に含有する、円相当径（直径）が２μｍ以上である、酸化物、硫化物、及び酸硫化物の合計が５０個／ｍｍ２以下であり、
　前記板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置でのＭｎ濃度（Ｍｎ（１／４））に対する、前記板厚方向に沿った断面の、鋼板表面から板厚方向の１／２位置でのＭｎ濃度（Ｍｎ（１／２））の比（Ｍｎ（１／２）／Ｍｎ（１／４））が、０．９０～１．８０である鋼板。
式（１）　Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５
　（ただし、式（１）中のＣ、Ｍｎ、Ｃｒ、Ｍｏ、Ｖ、Ｃｕ、およびＮｉは、鋼板に含まれる各元素の含有量（質量％）を表す。） (1)
In mass%,
C: 0.03% to 0.14%,
Si: 0.01% to 0.50%,
Mn: 1.20% to 2.50%,
P: 0.030% or less,
S: 0.020% or less,
Nb: 0.003% to 0.050%,
Ti: 0.003% to 0.050%,
Al: 0.001% to 0.100%,
N: 0.0010% to 0.0080%,
O: not more than 0.0050%,
Ca: 0% to 0.0100%,
Mg: 0% to 0.0100%,
REM: 0% to 0.0100%,
Zr: 0% to 0.0100%,
Te: 0% to 0.0100%,
V: 0% to 0.150%
Cu: 0% to 1.00%,
Ni: 0% to 2.00%,
Cr: 0% to 1.00%,
Mo: 0% to 0.50%,
B: 0% to 0.0050%, and
Remainder: Consists of Fe and impurities,
The carbon equivalent Ceq represented by the following formula (1) is 0.30% to 0.55%,
The metallographic structure at a quarter position in the plate thickness direction from the surface of the steel plate in the cross section along the plate thickness direction is, by area fraction, 10.0% to 75.0% of ferrite phase, 10.0% to bainite phase Average particle size of the entire phase, which is composed of 90.0%, 0% to 15.0% of pearlite phase, and 0% to 1.0% of martensite / austenite mixed phase, as measured by electron backscattering diffraction of all phases ( Diameter) is less than 20 μm,
The total of oxides, sulfides, and oxysulfides having a circle equivalent diameter (diameter) of 2 μm or more, which is contained at a quarter position in the plate thickness direction from the surface of the steel plate, of the cross section along the plate thickness direction 50 pcs / mm 2 or less,
From the steel plate surface to the plate thickness of the cross section along the plate thickness direction with respect to the Mn concentration (Mn (1/4) ) at a quarter position from the steel plate surface to the plate thickness direction A steel sheet in which the ratio (Mn (1/2) / Mn (1/4) ) of the Mn concentration (Mn (1/2) ) at half of the direction is 0.90 to 1.80.
Formula (1) Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
(However, C, Mn, Cr, Mo, V, Cu, and Ni in Formula (1) represent the content (mass%) of each element contained in the steel plate.)
（２）
　（１）に記載の鋼板を製造する方法であって、
　溶存酸素量を５０ｐｐｍ以下に調整した溶鋼を、最終溶存酸素量が２０ｐｐｍ以下になるように脱酸元素を添加して脱酸処理を行う工程と、
　前記脱酸処理を経た溶鋼を、１分あたり０．８０トン～５．００トンのスループットで連続鋳造し、前記連続鋳造するときの鋳片の中心固相率が０．５～１．０の範囲であるとき、前記鋳片に対し、鋳造進行方向１ｍあたり１．０ｍｍ～１．５ｍｍの圧下勾配で、圧下量が１ｍｍ～１５ｍｍとなる軽圧下を行い鋼片を得る工程と、
　前記鋼片を９５０℃～１１５０℃の温度域で加熱し、加熱後の鋼片に粗圧延を行った後、粗圧延後の鋼板に、鋼板の表面温度がＡｒ３点－３０℃～再結晶温度Ｔｒｅｘの温度域で、累積圧下率が５０％～７５％の仕上圧延を行う工程と、
　仕上圧延後の鋼板を、鋼板の板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置での温度が０℃～６００℃の温度域になるまで、前記１／４位置での平均冷却速度が１℃／秒～２０℃／秒で冷却する工程と、
　を有する鋼板の製造方法。 (2)
It is a method of manufacturing the steel plate as described in (1),
A step of deoxidizing the molten steel whose dissolved oxygen content has been adjusted to 50 ppm or less by adding a deoxidizing element so that the final dissolved oxygen content is 20 ppm or less;
The deoxidized molten steel is continuously cast at a throughput of 0.80 tons to 5.00 tons per minute, and the central solid phase ratio of the slab when the continuous casting is 0.5 to 1.0 When it is in the range, the slab is subjected to a light reduction with a reduction amount of 1 mm to 15 mm with a reduction gradient of 1.0 mm to 1.5 mm per 1 m of the casting advancing direction to obtain a steel piece;
The steel slab is heated in a temperature range of 950 ° C. to 1150 ° C., and after rough rolling is performed on the steel slab after heating, the surface temperature of the steel plate is Ar 3 point −30 ° C. to recrystallization on the steel plate after rough rolling Performing finish rolling with a cumulative rolling reduction of 50% to 75% in a temperature range of a temperature T rex ,
The steel sheet after finish rolling has a cross section along the thickness direction of the steel sheet until the temperature at a quarter position from the surface of the steel sheet in the thickness direction reaches a temperature range of 0 ° C. to 600 ° C. Cooling at an average cooling rate of 1 ° C./s to 20 ° C./s at
The manufacturing method of the steel plate which has.
（３）
　さらに、冷却工程後の鋼板を、３５０℃～６５０℃の温度域で焼戻しを行う工程を有する（２）に記載の鋼板の製造方法。 (3)
Furthermore, the method for producing a steel plate according to (2), further comprising the step of tempering the steel plate after the cooling step in a temperature range of 350 ° C. to 650 ° C.
　本開示によれば、より厳格化された規格にも満足し得る、優れたアレスト性と優れた継手ＣＴＯＤ特性とを併せ持ち、かつ高強度である鋼板を提供できる。 According to the present disclosure, it is possible to provide a steel plate which has both excellent arrestability and excellent joint CTOD characteristics which can satisfy even more stringent standards, and which has high strength.
　前述のように、従来の鋼板よりも、アレスト性及び継手ＣＴＯＤ特性がともに優れ、より厳格化された規格を満足する鋼板であって、高強度である鋼板が求められている。
　本発明者らは、このような要求に対し、鋭意検討を行った結果、ミクロ組織（組織形態、組織の粒径、介在物の量、及び中心偏析）を制御することで、より厳格化された規格にも満足し得る、優れたアレスト性と優れた継手ＣＴＯＤ特性とを併せ持ち、かつ、高強度である鋼板が得られることを見出し、本開示に係る鋼板を完成させた。また、本発明者らは、ミクロ組織（組織形態、組織の粒径、介在物の量、及び中心偏析）を制御するために、鋼板の製造方法についても詳細に検討した。その結果、溶鋼から鋼板に至るまでの過程（溶製、鋳造、及び製鋼）を、それぞれ制御することで、より厳格化された規格にも満足し得る、優れたアレスト性と優れた継手ＣＴＯＤ特性とを併せ持ち、かつ、高強度である鋼板が得られる製造方法を見出し、本開示に係る鋼板の製造方法を完成させるに至った。 As described above, a steel plate that is superior in both arrestability and joint CTOD characteristics and satisfies more stringent standards than conventional steel plates and that has high strength is required.
As a result of intensive investigations against these requirements, the present inventors further tightened by controlling the microstructure (tissue morphology, tissue particle size, amount of inclusions, and center segregation). It was found that a steel plate having both excellent arrestability and excellent joint CTOD characteristics that can satisfy even the above standards and having high strength can be obtained, and the steel plate according to the present disclosure is completed. The present inventors also examined in detail the method of manufacturing a steel plate in order to control the microstructure (the structure morphology, the particle size of the structure, the amount of inclusions, and the center segregation). As a result, by controlling the processes from molten steel to steel plate (melting, casting, and steel making) respectively, excellent arrestability and excellent joint CTOD characteristics that can meet even more stringent standards. In addition, the present inventors have found a manufacturing method that can obtain a steel plate having high strength, and complete the method of manufacturing a steel plate according to the present disclosure.
　以下、本開示に係る鋼板及び鋼板の製造方法について、好ましい実施形態の一例について詳細に説明する。 Hereinafter, an example of a preferred embodiment is explained in detail about a steel plate concerning the present indication, and a manufacturing method of a steel plate.
　なお、本開示において、「～」を用いて表される数値範囲は、「～」の前後に記載される数値を下限値及び上限値として含む範囲を意味する。
　本開示において、成分（元素）の含有量を示す「％」は、「質量％」を意味する。
　本開示において、Ｃ（炭素）の含有量を、「Ｃ量」と表記することがある。他の元素の含有量についても同様に表記することがある。
　本開示において、「工程」との用語は、独立した工程だけではなく、他の工程と明確に区別できない場合であってもその工程の所期の目的が達成されるのであれば、本用語に含まれる。 In the present disclosure, a numerical range represented using “to” means a range including numerical values described before and after “to” as the lower limit value and the upper limit value.
In the present disclosure, “%” indicating the content of a component (element) means “mass%”.
In the present disclosure, the content of C (carbon) may be denoted as “C amount”. The contents of other elements may be similarly described.
In the present disclosure, the term "step" is not limited to an independent step, and if the intended purpose of the step is achieved even if it can not be clearly distinguished from other steps, this term included.
　以下、本開示に係る鋼板の好ましい態様の一例について説明する。
　本開示に係る鋼板は、質量％で、
　Ｃ　：０．０３％～０．１４％、Ｓｉ：０．０１％～０．５０％、Ｍｎ：１．２０％～２．５０％、Ｐ　：０．０３０％以下、Ｓ　：０．０２０％以下、Ｎｂ：０．００３％～０．０５０％、Ｔｉ：０．００３％～０．０５０％、Ａｌ：０．００１％～０．１００％、Ｎ　：０．００１０％～０．００８０％、Ｏ（酸素）：０．００５０％以下、Ｃａ：０％～０．０１００％、Ｍｇ：０％～０．０１００％、ＲＥＭ：０％～０．０１００％、Ｚｒ：０％～０．０１００％、Ｔｅ：０％～０．０１００％、Ｖ：０％～０．１５０％、Ｃｕ：０％～１．００％、Ｎｉ：０％～２．００％、Ｃｒ：０％～１．００％、Ｍｏ：０％～０．５０％、Ｂ：０％～０．００５０％、並びに、残部：Ｆｅ及び不純物からなる化学組成を有する。 Hereinafter, an example of a desirable mode of a steel plate concerning the present disclosure is explained.
The steel plate according to the present disclosure is in mass%,
C: 0.03% to 0.14%, Si: 0.01% to 0.50%, Mn: 1.20% to 2.50%, P: 0.030% or less, S: 0.020% Nb: 0.003% to 0.050%, Ti: 0.003% to 0.050%, Al: 0.001% to 0.100%, N: 0.0010% to 0.0080%, O (oxygen): 0.0050% or less, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, REM: 0% to 0.0100%, Zr: 0% to 0.0100% , Te: 0% to 0.0100%, V: 0% to 0.150%, Cu: 0% to 1.00%, Ni: 0% to 2.00%, Cr: 0% to 1.00% Mo: 0% to 0.50%, B: 0% to 0.0050%, and the balance: Fe and impurities have a chemical composition.
　本開示に係る鋼板は、Ｃａ、Ｍｇ、ＲＥＭ、Ｚｒ、Ｔｅ、Ｖ、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、及びＢからなる群から選択される少なくとも１種を含んでいてもよく、含んでいなくてもよい。そのため、これらの元素の下限値は０％である。これら元素を含有する場合における下限は０％超であってもよい。 The steel plate according to the present disclosure may or may not include at least one selected from the group consisting of Ca, Mg, REM, Zr, Te, V, Cu, Ni, Cr, Mo, and B. May be Therefore, the lower limit value of these elements is 0%. When these elements are contained, the lower limit may be more than 0%.
　まず、本開示に係る鋼板の化学組成の限定理由を述べる。
　以下の説明において、各元素の説明における「％」は「質量％」を意味する。 First, the reasons for limitation of the chemical composition of the steel plate according to the present disclosure will be described.
In the following description, “%” in the description of each element means “mass%”.
（Ｃ：０．０３％～０．１４％）
　Ｃは、強度を確保するために必要な元素である。Ｃ量が０．０３％未満では、必要とする鋼板（以下、「母材」とも称する）の強度を確保することができない。しかし、Ｃ量が０．１４％を超えると、溶接性、及び要求される継手ＣＴＯＤ特性の確保が困難となる。そのため、Ｃ量は、０．０３％～０．１４％とする。Ｃ量の好ましい下限は０．０４％以上、より好ましくは０．０５％以上である。Ｃ量の好ましい上限は０．１２％以下、より好ましくは０．１０％以下である。 (C: 0.03% to 0.14%)
C is an element necessary to secure the strength. If the amount of C is less than 0.03%, the required strength of the steel plate (hereinafter, also referred to as “base material”) can not be secured. However, if the C content exceeds 0.14%, it will be difficult to ensure weldability and the required joint CTOD characteristics. Therefore, the amount of C is set to 0.03% to 0.14%. The preferable lower limit of the amount of C is 0.04% or more, more preferably 0.05% or more. The preferable upper limit of the amount of C is 0.12% or less, more preferably 0.10% or less.
（Ｓｉ：０．０１％～０．５０％）
　Ｓｉは、脱酸元素であり、固溶強化に有効な元素である。Ｓｉ量が０．０１％未満では、Ｓｉを含有する効果が得られないため、Ｓｉ量の下限は０．０１％以上とする。一方、Ｓｉ量が０．５０％を超えると溶接性、及び要求される継手ＣＴＯＤ特性が劣位となる。そのため、Ｓｉ量は、０．０１％～０．５０％とする。Ｓｉ量の好ましい下限は０．０５％以上、より好ましくは０．１０％以上である。Ｓｉ量の好ましい上限は０．４０％以下、より好ましくは０．３０％以下である。 (Si: 0.01% to 0.50%)
Si is a deoxidizing element and is an element effective for solid solution strengthening. If the amount of Si is less than 0.01%, the effect of containing Si can not be obtained, so the lower limit of the amount of Si is made 0.01% or more. On the other hand, if the Si content exceeds 0.50%, the weldability and the required joint CTOD characteristics become inferior. Therefore, the amount of Si is set to 0.01% to 0.50%. The preferable lower limit of the amount of Si is 0.05% or more, more preferably 0.10% or more. The upper limit of the amount of Si is preferably 0.40% or less, more preferably 0.30% or less.
（Ｍｎ：１．２０％～２．５０％）
　Ｍｎは、母材の強度及び靭性を向上させる有効な元素であり、Ｍｎの含有量は１．２０％以上とする。一方、Ｍｎを過剰に含有すると、継手靭性および溶接割れ性が劣位となるため２．５０％を上限とする。そのため、Ｍｎ量は、１．２０％～２．５０％とする。Ｍｎ量の好ましい下限は１．４０％以上、より好ましくは１．６０％以上である。Ｍｎ量の好ましい上限は２．２０％以下、より好ましくは２．００％以下である。 (Mn: 1.20% to 2.50%)
Mn is an effective element for improving the strength and toughness of the base material, and the content of Mn is 1.20% or more. On the other hand, when the Mn content is excessive, the joint toughness and the weld cracking resistance become inferior, and the upper limit is 2.50%. Therefore, the amount of Mn is set to 1.20% to 2.50%. The preferable lower limit of the amount of Mn is 1.40% or more, more preferably 1.60% or more. The preferable upper limit of the amount of Mn is 2.20% or less, more preferably 2.00% or less.
（Ｐ：０．０３０％以下）
　Ｐは、不純物として鋼板に存在する。しかし、Ｐ量が過剰になると、継手ＣＴＯＤ特性および母材の靭性を低下させる。そのため、Ｐ量の上限を０．０３０％以下とする。Ｐ量の好ましい上限は０．０２０％以下、より好ましくは０．０１０％以下である。Ｐ量は少ないほど好ましいため下限は特に規定しない。ただし、製造コストの観点から、０．００１％以上であってもよい。 (P: 0.030% or less)
P is present in the steel sheet as an impurity. However, when the amount of P is excessive, the joint CTOD characteristics and the toughness of the base material are reduced. Therefore, the upper limit of the amount of P is set to 0.030% or less. The upper limit of the amount of P is preferably 0.020% or less, more preferably 0.010% or less. The lower the amount of P, the lower the amount. However, from the viewpoint of the manufacturing cost, it may be 0.001% or more.
（Ｓ：０．０２０％以下）
　Ｓは、不純物として鋼板に存在する。しかし、Ｓ量が過剰になると、継手ＣＴＯＤ特性が劣位となる。このためＳ量の上限を０．０２０％以下とする。Ｓ量の好ましい上限は０．０１５％以下、より好ましい上限は０．０１０％以下、さらに好ましい上限は０．００５％以下、さらに好ましい上限は０．００２％以下である。Ｓ量は少ないほど好ましいため下限は特に規定しない。ただし、製造コストの観点から、０．００１％以上であってもよい。 (S: 0.020% or less)
S is present in the steel sheet as an impurity. However, when the amount of S becomes excessive, the joint CTOD characteristics become inferior. Therefore, the upper limit of the amount of S is made 0.020% or less. The upper limit of the amount of S is preferably 0.015% or less, more preferably 0.010% or less, still more preferably 0.005% or less, and still more preferably 0.002% or less. The lower the amount of S, the lower the amount. However, from the viewpoint of the manufacturing cost, it may be 0.001% or more.
（Ｎｂ：０．００３％～０．０５０％）
　Ｎｂは、再結晶温度を抑制し、微量の添加により組織微細化に寄与し、母材強度確保に有効な元素である。一方、０．０５０％を超えてＮｂを含有させると、溶接部を硬化させて著しく靭性を劣化させ、継手ＣＴＯＤ特性が劣位となる。そのため、Ｎｂ量は、０．００３％～０．０５０％とする。Ｎｂ量の好ましい下限は０．００６％以上、より好ましくは０．０１０％以上である。Ｎｂ量の好ましい上限は０．０４０％以下、より好ましくは０．０３０％以下である。 (Nb: 0.003% to 0.050%)
Nb suppresses the recrystallization temperature, contributes to the refinement of the structure by the addition of a small amount, and is an element effective for securing the strength of the base material. On the other hand, when Nb is contained in excess of 0.050%, the weld portion is hardened to deteriorate the toughness remarkably, and the joint CTOD characteristics become inferior. Therefore, the Nb content is set to 0.003% to 0.050%. The preferable lower limit of the amount of Nb is 0.006% or more, more preferably 0.010% or more. The upper limit of the Nb content is preferably 0.040% or less, more preferably 0.030% or less.
（Ｔｉ：０．００３％～０．０５０％）
　Ｔｉは、微量の添加により母材と溶接部の組織微細化を通じて靭性向上に寄与する元素である。また、Ｔｉは脱酸元素としても機能する。一方、Ｔｉを過剰に添加すると溶接部を硬化させ著しく靭性を劣化させ、継手ＣＴＯＤ特性が劣位となる。そのため、Ｔｉ量は０．００３％～０．０５０％とする。Ｔｉ量の好ましい下限は０．００６％以上、より好ましくは０．０１０％以上である。Ｔｉ量の好ましい上限は０．０３５％以下、より好ましくは０．０２０％以下である。 (Ti: 0.003% to 0.050%)
Ti is an element that contributes to the improvement of toughness through the refinement of the structure of the base material and the weld by the addition of a small amount. In addition, Ti also functions as a deoxidizing element. On the other hand, excessive addition of Ti hardens the weld and significantly deteriorates toughness, resulting in inferior joint CTOD characteristics. Therefore, the amount of Ti is set to 0.003% to 0.050%. The preferable lower limit of the amount of Ti is 0.006% or more, more preferably 0.010% or more. The preferable upper limit of the amount of Ti is 0.035% or less, more preferably 0.020% or less.
（Ａｌ：０．００１％～０．１００％）
　Ａｌは、脱酸元素であるため、Ａｌ量は０．００１％以上とする。一方、Ａｌを過剰に添加すると鋼片の表面品位を損ない、靭性に有害な介在物を形成するため、Ａｌ量の上限は０．１００％とする。そのため、Ａｌ量は０．００１％～０．１００％とする。Ａｌ量の好ましい下限は０．０１０％以上、より好ましくは０．０２０％以上である。Ａｌ量の好ましい上限は０．０６０％以下、より好ましくは０．０４０％以下である。 (Al: 0.001% to 0.100%)
Since Al is a deoxidizing element, the amount of Al is made 0.001% or more. On the other hand, the excessive addition of Al impairs the surface quality of the steel slab and forms an inclusion harmful to toughness, so the upper limit of the amount of Al is 0.100%. Therefore, the Al content is set to 0.001% to 0.100%. The preferable lower limit of the amount of Al is 0.010% or more, more preferably 0.020% or more. The upper limit of the amount of Al is preferably 0.060% or less, more preferably 0.040% or less.
（Ｎ：０．００１０％～０．００８０％）
　Ｎは、Ａｌと共に窒化物を形成し継手靭性を向上させるため、Ｎ量の下限を０．００１０％以上とする。しかし、Ｎの含有量が過剰であると、固溶Ｎによる脆化および伸びの低下が生じるため、Ｎ量の上限を０．００８０％以下とする。そのため、Ｎ量は０．００１０％～０．００８０％とする。Ｎ量の好ましい下限は０．００１５％以上、より好ましくは０．００２０％以上である。Ｎ量の好ましい上限は０．００６０％以下、より好ましくは０．００４０％以下である。 (N: 0.0010% to 0.0080%)
N forms a nitride together with Al to improve the joint toughness, so the lower limit of the N amount is made 0.0010% or more. However, if the content of N is excessive, embrittlement and elongation decrease due to solid solution N occur, so the upper limit of the amount of N is made 0.0080% or less. Therefore, the N content is set to 0.0010% to 0.0080%. The preferable lower limit of the N amount is 0.0015% or more, more preferably 0.0020% or more. The preferable upper limit of the amount of N is 0.0060% or less, more preferably 0.0040% or less.
（Ｏ：０．００５０％以下）
　Ｏ（酸素）は、酸化物を形成する。Ｏ量が０．００５０％を超えると、酸化物が粗大化して継手ＣＴＯＤ特性および母材の靭性が低下するため、Ｏ量の上限は０．００５０％以下とする。一方、Ｏは少ないほど好ましいため、下限は特に規定しない。ただし、Ｏを減らすには、例えば、ＲＨ真空脱ガス装置での還流作業が長時間となり現実的ではないため、Ｏ量の下限は０．０００５％以上であってもよい。Ｏ量の好ましい下限は０．０００７％以上であってもよく、０．００１０％以上であってもよい。Ｏ量の好ましい上限は０．００４０％以下、より好ましくは０．００３０％以下である。 (O: less than 0.0050%)
O (oxygen) forms an oxide. If the amount of O exceeds 0.0050%, the oxides are coarsened to reduce the joint CTOD characteristics and the toughness of the base material, so the upper limit of the amount of O is made 0.0050% or less. On the other hand, the lower limit of O is not particularly defined because the amount of O is preferably as small as possible. However, in order to reduce O, for example, since the reflux operation in the RH vacuum degassing apparatus takes a long time and is not realistic, the lower limit of the amount of O may be 0.0005% or more. The preferable lower limit of the amount of O may be 0.0007% or more, and may be 0.0010% or more. The upper limit of the amount of O is preferably 0.0040% or less, more preferably 0.0030% or less.
（Ｃａ：０％～０．０１００％）
　Ｃａは、脱酸元素であり、硫化物を形成することで粗大な介在物の生成を抑制し、微細な酸化物を形成して、有害な介在物の生成を抑制する元素である。したがって、Ｃａを含有させてもよい。しかしながら、Ｃａ量が０．０１００％を超えると、粗大な酸化物、硫化物、及び酸硫化物が形成され、継手ＣＴＯＤ特性が劣位となる。したがって、Ｃａ量は０％～０．０１００％とする。Ｃａ量の好ましい上限は０．００７０％以下、より好ましくは０．００４０％以下である。上記作用による効果をより確実に得るには、Ｃａ量の下限は０．０００１％以上とすることが好ましい。さらに好ましくは０．００１０％以上、特に好ましくは０．００２０％以上である。 (Ca: 0% to 0.0100%)
Ca is a deoxidizing element, and is an element that suppresses the formation of coarse inclusions by forming sulfides, forms a fine oxide, and suppresses the formation of harmful inclusions. Therefore, Ca may be contained. However, when the amount of Ca exceeds 0.0100%, coarse oxides, sulfides and acid sulfides are formed, and the joint CTOD characteristics become inferior. Therefore, the amount of Ca is set to 0% to 0.0100%. The upper limit of the amount of Ca is preferably 0.0070% or less, more preferably 0.0040% or less. In order to obtain the effect of the above action more reliably, the lower limit of the amount of Ca is preferably made 0.0001% or more. More preferably, it is 0.0010% or more, particularly preferably 0.0020% or more.
（Ｍｇ：０％～０．０１００％）
　Ｍｇは、脱酸元素であり、硫化物を形成することで粗大な介在物の生成を抑制し、微細な酸化物を形成して、有害な介在物の生成を抑制する元素である。したがって、Ｍｇを含有させてもよい。しかしながら、Ｍｇ量が０．０１００％を超えると、粗大な酸化物、硫化物、及び酸硫化物が形成され、継手ＣＴＯＤ特性が劣位となる。したがって、Ｍｇ量は０％～０．０１００％とする。Ｍｇ量の好ましい上限は０．００７０％以下、より好ましくは０．００４０％以下である。上記作用による効果をより確実に得るには、Ｍｇ量の下限は０．０００１％以上とすることが好ましい。さらに好ましくは０．００１０％以上、特に好ましくは０．００２０％以上である。 (Mg: 0% to 0.0100%)
Mg is a deoxidizing element, and is an element that suppresses the formation of coarse inclusions by forming sulfides, forms a fine oxide, and suppresses the formation of harmful inclusions. Therefore, Mg may be contained. However, when the Mg content exceeds 0.0100%, coarse oxides, sulfides and acid sulfides are formed, and the joint CTOD characteristics become inferior. Therefore, the amount of Mg is set to 0% to 0.0100%. The preferable upper limit of the amount of Mg is 0.0070% or less, more preferably 0.0040% or less. In order to obtain the effect by the above-mentioned effect more certainly, it is preferable to make the lower limit of the amount of Mg into 0.0001% or more. More preferably, it is 0.0010% or more, particularly preferably 0.0020% or more.
（ＲＥＭ：０％～０．０１００％）
　ＲＥＭは、脱酸元素であり、硫化物を形成することで粗大な介在物の生成を抑制し、微細な酸化物を形成して、有害な介在物の生成を抑制する元素である。したがって、ＲＥＭを含有させてもよい。しかしながら、ＲＥＭ量が０．０１００％を超えると、粗大な酸化物、硫化物、及び酸硫化物が形成され、継手ＣＴＯＤ特性が劣位となる。したがって、ＲＥＭ量は０％～０．０１００％とする。ＲＥＭ量の好ましい上限は０．００７０％以下、より好ましくは０．００４０％以下である。上記作用による効果をより確実に得るには、ＲＥＭ量の下限は０．０００１％以上とすることが好ましい。さらに好ましくは０．００１０％以上、特に好ましくは０．００２０％以上である。
　ここで、「ＲＥＭ」とはＳｃ、Ｙ、及びランタノイドの合計１７元素の総称である。ＲＥＭとしては、合計１７元素のうちの１種または２種以上の元素を含んでいればよい。ＲＥＭの含有量はこれら元素の合計含有量を指す。 (REM: 0% to 0.0100%)
REM is a deoxidizing element, is an element that suppresses the formation of coarse inclusions by forming sulfides, forms a fine oxide, and suppresses the formation of harmful inclusions. Therefore, REM may be contained. However, when the REM content exceeds 0.0100%, coarse oxides, sulfides and acid sulfides are formed, and the joint CTOD characteristics become inferior. Therefore, the REM amount is 0% to 0.0100%. The upper limit of the REM amount is preferably 0.0070% or less, more preferably 0.0040% or less. In order to obtain the effect by the above action more reliably, the lower limit of the amount of REM is preferably made 0.0001% or more. More preferably, it is 0.0010% or more, particularly preferably 0.0020% or more.
Here, "REM" is a general term for Sc, Y, and a total of 17 elements of lanthanoids. As REM, one or more elements out of 17 elements in total may be included. The content of REM refers to the total content of these elements.
（Ｚｒ：０％～０．０１００％）
　Ｚｒは、微量の添加により母材と溶接部の組織微細化を通じて靭性向上に寄与する元素である。また、Ｚｒは脱酸元素としても機能する。したがって、Ｚｒを含有させてもよい。しかしながら、Ｚｒ量が０．０１００％を超えると、溶接部を硬化させ、著しく靭性を劣化させ、継手ＣＴＯＤ特性が劣位となる。したがって、Ｚｒ量は０％～０．０１００％とする。Ｚｒ量の好ましい上限は０．００７０％以下、より好ましくは０．００４０％以下である。上記作用による効果をより確実に得るには、Ｚｒ量の下限は０．０００１％以上とすることが好ましい。さらに好ましくは０．００１０％以上、特に好ましくは０．００２０％以上である。 (Zr: 0% to 0.0100%)
Zr is an element that contributes to the improvement of toughness through the refinement of the structure of the base material and the weld by the addition of a small amount. In addition, Zr also functions as a deoxidizing element. Therefore, Zr may be contained. However, if the amount of Zr exceeds 0.0100%, the weld is hardened, the toughness is significantly deteriorated, and the joint CTOD characteristics become inferior. Therefore, the amount of Zr is set to 0% to 0.0100%. The preferable upper limit of the amount of Zr is 0.0070% or less, more preferably 0.0040% or less. In order to obtain the effect of the above action more reliably, the lower limit of the amount of Zr is preferably made 0.0001% or more. More preferably, it is 0.0010% or more, particularly preferably 0.0020% or more.
（Ｔｅ：０％～０．０１００％）
　Ｔｅは、細粒化により靭性の向上に寄与する元素である。したがって、Ｔｅを含有させてもよい。しかしながら、Ｔｅ量が０．０１００％を超えても、効果は飽和する。したがって、Ｔｅ量は０％～０．０１００％とする。Ｔｅ量の好ましい上限は０．００７０％以下、より好ましくは０．００４０％以下である。上記作用による効果をより確実に得るには、Ｔｅ量の下限は０．０００１％以上とすることが好ましい。さらに好ましくは０．００１０％以上、特に好ましくは０．００２０％以上である。 (Te: 0% to 0.0100%)
Te is an element that contributes to the improvement of toughness by grain refinement. Therefore, Te may be contained. However, the effect is saturated even if the amount of Te exceeds 0.0100%. Therefore, the Te content is set to 0% to 0.0100%. The upper limit of the amount of Te is preferably 0.0070% or less, more preferably 0.0040% or less. The lower limit of the amount of Te is preferably set to 0.0001% or more in order to obtain the effect by the above action more reliably. More preferably, it is 0.0010% or more, particularly preferably 0.0020% or more.
（Ｖ：０％～０．１５０％）
　Ｖは、析出強化により母材の強度上昇に寄与する元素である。したがって、Ｖを含有させてもよい。しかしながら、Ｖ量が０．１５０％を超えると、継手靭性を損なう。したがって、Ｖ量は０％～０．１５０％とする。Ｖ量の好ましい上限は０．０８０％以下、より好ましくは０．０６０％以下である。上記作用による効果をより確実に得るには、Ｖ量の下限は０．００１％以上とすることが好ましい。さらに好ましくは０．０１０％以上、特に好ましくは０．０２０％以上である。 (V: 0% to 0.150%)
V is an element that contributes to the increase in strength of the base material by precipitation strengthening. Therefore, V may be contained. However, if the V content exceeds 0.150%, the joint toughness is impaired. Therefore, the V amount is set to 0% to 0.150%. The preferable upper limit of the amount of V is 0.080% or less, more preferably 0.060% or less. The lower limit of the amount of V is preferably set to 0.001% or more in order to obtain the effect by the above action more reliably. More preferably, it is 0.010% or more, and particularly preferably 0.020% or more.
（Ｃｕ：０％～１．００％）
　Ｃｕは、焼入れ性を向上させ、母材の高強度化に有効な元素である。したがって、Ｃｕを含有させてもよい。しかしながら、Ｃｕ量が１．００％を超えると、継手の硬さの上昇に伴う靭性の低下が著しくなる。したがって、Ｃｕ量は０％～１．００％とする。Ｃｕ量の好ましい上限は０．８０％以下、より好ましくは０．６０％以下である。上記作用による効果をより確実に得るには、Ｃｕ量の下限は０．０１％以上とすることが好ましい。さらに好ましくは０．０５％以上、特に好ましくは０．１０％以上である。 (Cu: 0% to 1.00%)
Cu is an element which improves the hardenability and is effective for strengthening the base material. Therefore, Cu may be contained. However, if the amount of Cu exceeds 1.00%, the decrease in toughness accompanying the increase in hardness of the joint becomes remarkable. Therefore, the amount of Cu is set to 0% to 1.00%. The upper limit of the amount of Cu is preferably 0.80% or less, more preferably 0.60% or less. The lower limit of the amount of Cu is preferably 0.01% or more in order to obtain the effect by the above action more reliably. More preferably, it is 0.05% or more, particularly preferably 0.10% or more.
（Ｎｉ：０％～２．００％）
　Ｎｉは、母材の強度確保および靭性向上に有効な元素である。したがって、Ｎｉを含有させてもよい。しかしながら、Ｎｉ量が２．００％を超えても、Ｎｉを含有する効果は飽和し、コストが上昇する。したがって、Ｎｉ量は０％～２．００％とする。Ｎｉ量の好ましい上限は１．５０％以下、より好ましくは１．００％以下である。上記作用による効果をより確実に得るには、Ｎｉ量の下限は０．０１％以上とすることが好ましい。さらに好ましくは０．１０％以上、特に好ましくは０．２０％以上である。 (Ni: 0% to 2.00%)
Ni is an element effective for securing the strength and improving the toughness of the base material. Therefore, Ni may be contained. However, even if the amount of Ni exceeds 2.00%, the effect of containing Ni saturates and the cost increases. Therefore, the amount of Ni is set to 0% to 2.00%. The upper limit of the amount of Ni is preferably 1.50% or less, more preferably 1.00% or less. In order to obtain the effect of the above action more reliably, the lower limit of the amount of Ni is preferably 0.01% or more. More preferably, it is 0.10% or more, particularly preferably 0.20% or more.
（Ｃｒ：０％～１．００％）
　Ｃｒは、焼入れ性を向上させ、母材の高強度化に有効な元素である。したがって、Ｃｒを含有させてもよい。しかしながら、Ｃｒ量が１．００％を超えると、継手の硬さの上昇に伴って靭性が低下する。したがって、Ｃｒ量は０％～１．００％とする。Ｃｒ量の好ましい上限は０．８０％以下、より好ましくは０．６０％以下である。上記作用による効果をより確実に得るには、Ｃｒ量の下限は０．０１％以上とすることが好ましい。さらに好ましくは０．０５％以上、特に好ましくは０．１０％以上である。 (Cr: 0% to 1.00%)
Cr is an element which improves the hardenability and is effective in increasing the strength of the base material. Therefore, Cr may be contained. However, if the amount of Cr exceeds 1.00%, the toughness decreases with the increase in the hardness of the joint. Therefore, the amount of Cr is set to 0% to 1.00%. The upper limit of the amount of Cr is preferably 0.80% or less, more preferably 0.60% or less. In order to obtain the effect of the above action more reliably, the lower limit of the amount of Cr is preferably 0.01% or more. More preferably, it is 0.05% or more, particularly preferably 0.10% or more.
（Ｍｏ：０％～０．５０％）
　Ｍｏは、焼入れ性を向上させ、母材の高強度化に有効な元素である。したがって、Ｍｏを含有させてもよい。しかしながら、Ｍｏ量が０．５０％を超えると、継手の硬さ上昇に伴って靭性が低下する。したがって、Ｍｏ量は０％～０．５０％とする。Ｍｏ量の好ましい上限は０．４０％以下、より好ましくは０．３０％以下である。上記作用による効果をより確実に得るには、Ｍｏ量の下限は０．０１％以上とすることが好ましい。さらに好ましくは０．０５％以上、特に好ましくは０．１０％以上である。 (Mo: 0% to 0.50%)
Mo is an element that improves the hardenability and is effective in increasing the strength of the base material. Therefore, Mo may be contained. However, when the Mo content exceeds 0.50%, the toughness decreases with the increase in hardness of the joint. Therefore, the Mo content is 0% to 0.50%. The preferable upper limit of the Mo amount is 0.40% or less, more preferably 0.30% or less. In order to obtain the effect by the above-mentioned effect more certainly, it is preferable to make the lower limit of Mo amount into 0.01% or more. More preferably, it is 0.05% or more, particularly preferably 0.10% or more.
（Ｂ：０％～０．００５０％）
　Ｂは、微量添加により焼き入れ性を高め母材強度向上に寄与する元素である。したがって、Ｂを含有させてもよい。しかしながら、Ｂ量が０．００５０％を超えると、継手ＣＴＯＤ特性が劣位となる。したがって、Ｂ量は０％～０．００５０％とする。Ｂ量の好ましい上限は０．００４０％以下、より好ましくは０．００３０％以下である。上記作用による効果をより確実に得るには、Ｂ量の下限は０．０００１％以上とすることが好ましい。さらに好ましくは０．０００５％以上、特に好ましくは０．００１０％以上である。 (B: 0% to 0.0050%)
B is an element contributing to the improvement of the base material strength by enhancing the hardenability by the addition of a small amount. Therefore, B may be contained. However, when the B content exceeds 0.0050%, the joint CTOD characteristics become inferior. Therefore, the B content is 0% to 0.0050%. The upper limit of the amount of B is preferably 0.0040% or less, more preferably 0.0030% or less. In order to obtain the effect by the above action more reliably, the lower limit of the amount of B is preferably made 0.0001% or more. More preferably, it is 0.0005% or more, particularly preferably 0.0010% or more.
（残部）
　残部はＦｅおよび不純物である。不純物とは、原材料に含まれる成分、または、製造の過程で混入する成分であって、意図的に鋼板に含有させたものではない成分を指す。 (The rest)
The balance is Fe and impurities. The impurity refers to a component contained in the raw material or a component mixed in the process of production, which is not intentionally contained in the steel sheet.
（炭素当量Ｃｅｑ：０．３０％～０．５５％）
　本開示に係る鋼板は、下記式（１）により求められる炭素当量Ｃｅｑが、０．３０％～０．５５％である。
　式（１）　Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５
　ただし、式（１）中のＣ、Ｍｎ、Ｃｒ、Ｍｏ、Ｖ、Ｃｕ、およびＮｉは、鋼板に含まれる各元素の含有量（質量％）を表す。
　なお、含有量が０質量％の元素がある場合には、式（１）中の該当する元素の含有量として０質量％を代入して計算する。 (Carbon equivalent Ceq: 0.30% to 0.55%)
The steel plate according to the present disclosure has a carbon equivalent Ceq of 0.30% to 0.55% determined by the following equation (1).
Formula (1) Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
However, C, Mn, Cr, Mo, V, Cu, and Ni in Formula (1) represent the content (mass%) of each element contained in a steel plate.
In addition, when there exists an element whose content is 0 mass%, 0 mass% is substituted and calculated as content of the applicable element in Formula (1).
　炭素当量が０．３０％未満になると、母材となる鋼板に要求される強度特性（引張強さ、降伏応力）を満足し難くなる。炭素当量が０．５５％を超えると、要求されるアレスト性、継手ＣＴＯＤ特性を満足し難くなる。炭素当量の下限値は、好ましくは０．３５％以上、より好ましくは０．４０％以上である。炭素当量の上限値は、好ましくは０．５２％以下、より好ましくは０．４９％以下である。 When the carbon equivalent is less than 0.30%, it becomes difficult to satisfy the strength characteristics (tensile strength, yield stress) required for the steel plate as the base material. When the carbon equivalent exceeds 0.55%, it becomes difficult to satisfy the required arrestability and joint CTOD characteristics. The lower limit value of the carbon equivalent is preferably 0.35% or more, more preferably 0.40% or more. The upper limit of the carbon equivalent is preferably 0.52% or less, more preferably 0.49% or less.
　本開示に係る鋼板は、板厚方向に沿った断面の鋼板表面から板厚方向の１／４位置（以下、「板厚１／４部」と称する場合がある。）での金属組織（ミクロ組織）が、面積分率で、フェライト相１０．０％～７５．０％、ベイナイト相１０．０％～９０．０％、パーライト相０％～１５．０％、及びマルテンサイト・オーステナイト混合相（以下、「ＭＡ相」と称する場合がある。）０％～１．０％の組織形態から構成される。そして、全相（フェライト相、ベイナイト相、パーライト相、及びＭＡ相）の電子線後方散乱回折法により測定される平均粒径（直径）が２０μｍ以下である。
　なお、フェライト相、ベイナイト相、パーライト相、及びＭＡ相の面積分率の合計は、１００％である。
　以下、本開示に係る鋼板のミクロ組織（組織形態、組織の平均粒径、介在物の量、及び中心偏析）の限定理由について説明する。
　ここで、本開示において、板厚方向に沿った断面の鋼板表面から板厚方向の１／４位置は、１／４位置を中心として、２ｍｍ角の範囲を表す。 The steel plate according to the present disclosure has a metallographic structure (micro-structure at a quarter position in the plate thickness direction from the steel plate surface of the cross section along the plate thickness direction (hereinafter may be referred to as "quarter plate thickness"). Structure, area fraction, 10.0% to 75.0% of ferrite phase, 10.0% to 90.0% of bainite phase, 0% to 15.0% of pearlite phase, and martensite / austenite mixed phase (Hereinafter, it may be referred to as "MA phase".) It is composed of 0% to 1.0% tissue morphology. And the average particle diameter (diameter) measured by the electron beam backscattering diffraction method of all the phases (ferrite phase, bainite phase, pearlite phase, and MA phase) is 20 micrometers or less.
The sum of the area fractions of the ferrite phase, the bainite phase, the pearlite phase, and the MA phase is 100%.
Hereinafter, the reasons for limitation of the microstructure (the morphology of the steel sheet, the average particle size of the structure, the amount of inclusions, and the center segregation) of the steel plate according to the present disclosure will be described.
Here, in the present disclosure, the 1⁄4 position in the thickness direction from the surface of the steel plate of the cross section along the thickness direction represents a 2 mm square range around the 1⁄4 position.
（フェライト相：１０．０％～７５．０％）
　フェライト相は、母材の強度およびアレスト性に寄与する。面積分率が７５．０％を超えると、母材の強度が劣位となる。一方、フェライト相の面積分率が１０．０％未満であると、アレスト性が劣位となる。そのため、フェライト相の面積分率は、１０．０％～７５．０％とする。フェライト相の面積分率の好ましい範囲は、２０．０％～５０．０％である。 (Ferrite phase: 10.0% to 75.0%)
The ferrite phase contributes to the strength and arrestability of the base material. When the area fraction exceeds 75.0%, the strength of the base material becomes inferior. On the other hand, if the area fraction of the ferrite phase is less than 10.0%, the arrestability becomes inferior. Therefore, the area fraction of the ferrite phase is set to 10.0% to 75.0%. The preferred range of the area fraction of the ferrite phase is 20.0% to 50.0%.
（ベイナイト相：１０．０％～９０．０％）
　ベイナイト相は、主に母材の強度に寄与する。ベイナイト相の面積分率が１０．０％未満であると、母材の強度が劣位となる。一方、ベイナイト相の面積分率が９０．０％を超えると、アレスト性が劣位となる。そのため、ベイナイト相の面積分率は、１０．０％～９０．０％とする。ベイナイト相の面積分率の好ましい範囲は、５０．０％～８０．０％である。 (Bainite phase: 10.0% to 90.0%)
The bainite phase mainly contributes to the strength of the base material. If the area fraction of the bainite phase is less than 10.0%, the strength of the base material becomes inferior. On the other hand, when the area fraction of the bainite phase exceeds 90.0%, arrestability becomes inferior. Therefore, the area fraction of the bainite phase is 10.0% to 90.0%. The preferred range of the area fraction of the bainite phase is 50.0% to 80.0%.
（パーライト相：０％～１５．０％）
　パーライト相は、ミクロ組織中に含有してもよい。パーライト相の面積分率が過剰になると、母材の強度が劣位となる。そのため、パーライト相を含む場合、パーライト相の面積分率は１５．０％以下とする。パーライト相の面積分率の好ましい上限は、１０．０％以下である。なお、パーライト相は含有していなくてもよい。すなわち、パーライト相の下限値は０％である。 (Perlite phase: 0% to 15.0%)
The perlite phase may be contained in the microstructure. When the area fraction of the pearlite phase becomes excessive, the strength of the base material becomes inferior. Therefore, when the pearlite phase is included, the area fraction of the pearlite phase is 15.0% or less. The preferable upper limit of the area fraction of the pearlite phase is 10.0% or less. The pearlite phase may not be contained. That is, the lower limit value of the pearlite phase is 0%.
（ＭＡ相：０％～１．０％）
　ＭＡ相は、継手ＣＴＯＤ特性を低下させる為、ＭＡ相の面積分率が過剰になると、継手ＣＴＯＤ特性が劣位となる。そのため、ＭＡ相を含む場合、ＭＡ相の面積分率は１．０％以下とする。ＭＡ相は少ないほど好ましいため下限は特に規定しない。なお、ＭＡ相は含んでいない場合があってもよい。 (MA phase: 0% to 1.0%)
Since the MA phase lowers the joint CTOD characteristics, when the area fraction of the MA phase becomes excessive, the joint CTOD characteristics become inferior. Therefore, when the MA phase is included, the area fraction of the MA phase is 1.0% or less. The lower limit is not particularly defined as the amount of MA phase is as small as possible. The MA phase may not be included.
　フェライト相、ベイナイト相、パーライト相、及びＭＡ相の面積分率は、以下のようにして測定する。
　板厚１／４部の試料について、光学顕微鏡により、鋼板の圧延方向と垂直な方向の断面（いわゆるＣ方向断面）と、鋼板の幅方向と垂直な断面（いわゆるＬ方向断面）との金属組織を写真撮影して画像解析することによって求める。
　具体的には、まず、鋼板の圧延方向と垂直な方向の断面（Ｃ方向断面）、及び鋼板の幅方向と垂直な方向の断面（Ｌ方向断面）の、鋼板表面から板厚方向の１／４位置であって、鋼板の幅方向端面から１／４位置から、Ｌ方向断面観察用試料およびＣ方向断面観察用試料を採取する。
　次に、採取した試料をナイタールエッチングし、エッチング後に、光学顕微鏡を用いて、Ｌ方向断面（４視野）およびＣ方向断面（４視野）の合計で８視野を５００倍で撮影する。そして、得られた組織写真に対し、画像解析ソフトにより二値化処理を行い、画像解析を行う。白色に見える相をフェライト相、黒色に見える相をパーライト相、灰色に見える相をベイナイト相、又はＭＡ相（マルテンサイト・オーステナイト混合相）として、それぞれの面積率を求める。次に、ナイタールエッチングした部分をレペラエッチングし、ナイタールエッチングで灰色に見えた部分について画像解析を行い、白色に見えるものをＭＡ相（マルテンサイト・オーステナイト混合相）とし面積率を求める。
　そして、ナイタールエッチングして灰色に見えた面積率から、上記のＭＡ相（マルテンサイト・オーステナイト混合相）の面積率を引いたものを、ベイナイト相の面積率とする。 The area fractions of the ferrite phase, the bainite phase, the pearlite phase, and the MA phase are measured as follows.
The metallographic structure of a cross section in the direction perpendicular to the rolling direction of the steel plate (so-called C direction cross section) and a cross section perpendicular to the width direction of the steel plate (so-called L direction cross section) It is determined by taking a picture and analyzing the image.
Specifically, first, a cross section in the direction perpendicular to the rolling direction of the steel plate (C direction cross section) and a cross section in the direction perpendicular to the width direction of the steel plate (L direction cross section) The L-direction cross-section observation sample and the C-direction cross-section observation sample are collected from the 1⁄4 position from the width direction end face of the steel plate at four positions.
Next, the collected sample is subjected to nital etching, and after etching, eight fields of view are taken at a magnification of 500 with a total of L direction cross section (four fields of view) and C direction cross section (four fields of view) using an optical microscope. Then, binarization processing is performed on the obtained tissue photograph using image analysis software, and image analysis is performed. The area ratio is determined using the phase that appears white as a ferrite phase, the phase that appears black as a pearlite phase, and the phase that appears gray as a bainite phase or an MA phase (martensitic / austenitic mixed phase). Next, the nital-etched portion is repeller-etched, and image analysis is performed on a portion that appears gray by nital etching to determine what is white as the MA phase (martensitic-austenitic mixed phase) and the area ratio.
Then, the area ratio of the above-mentioned MA phase (the martensite / austenite mixed phase) is subtracted from the area ratio which appeared gray by nital etching, and this is defined as the area ratio of the bainite phase.
（平均粒径（直径）：２０μｍ以下）
　フェライト相、ベイナイト相、パーライト相、及びＭＡ相の全相の細粒化はアレスト性の向上に寄与する。これら全相の平均粒径（直径）が２０μｍを超えるとアレスト性が劣位となる。そのため、全相の平均粒径は２０μｍ以下（好ましくは、１５μｍ以下、より好ましくは１０μｍ以下）とする。全相の平均粒径は小さいほどアレスト性の向上に優位となる。全相の平均粒径の下限値は特に限定されず、例えば、１μｍ以上が挙げられ、５μｍ以上が挙げられる。 (Average particle size (diameter): 20 μm or less)
The grain refinement of all phases of the ferrite phase, the bainite phase, the pearlite phase, and the MA phase contributes to the improvement of the arrestability. If the average particle size (diameter) of all the phases exceeds 20 μm, arrestability becomes inferior. Therefore, the average particle diameter of all the phases is set to 20 μm or less (preferably 15 μm or less, more preferably 10 μm or less). The smaller the average particle size of all the phases, the better the improvement in arrestability. The lower limit of the average particle diameter of all phases is not particularly limited, and, for example, 1 μm or more can be mentioned, and 5 μm or more can be mentioned.
　全相の平均粒径は、結晶方位の情報を広い視野で精度良く測定できる電子線後方散乱回折法（ＥＢＳＰ：Ｅｌｅｃｔｒｏｎ　Ｂａｃｋ　Ｓｃａｔｔｅｒｉｎｇ　Ｐａｔｔｅｒｎ）により測定する。ＥＢＳＰ法を用いれば、ベイナイトのような複雑な組織の結晶粒径の測定も可能である。具体的には、以下の方法によって測定する。
　ＥＢＳＰ法により、板厚１／４部の５００μｍ×５００μｍの領域について、測定位置を１μｍずつ動かしながら繰り返し測定する。
　ここで、隣接粒との結晶方位差が１５°以上の境界を結晶粒界と定義し、この結晶粒界に囲まれた円相当径（直径）の面積分率加重平均値を求め、これを平均粒径とする。 The average grain size of all phases is measured by the electron back scattering pattern (EBSP) which can measure the crystal orientation information with a wide field of view with high accuracy. The EBSP method can also measure the grain size of complex structures such as bainite. Specifically, it is measured by the following method.
The EBSP method is repeated while moving the measurement position by 1 μm for a 500 μm × 500 μm area of 1/4 part of plate thickness.
Here, a boundary where the crystal orientation difference between adjacent grains is 15 ° or more is defined as a grain boundary, and the area fraction weighted average value of the equivalent circle diameter (diameter) surrounded by the grain boundaries is determined, Average particle size.
（円相当径が２μｍ以上の酸化物、硫化物、及び酸硫化物の合計が５０個／ｍｍ２以下）
　本開示に係る鋼板は、板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置に、円相当径が２μｍ以上である、酸化物、硫化物、及び酸硫化物のうちの少なくとも１種を含有している。そして、円相当径が２μｍ以上である、酸化物、硫化物、及び酸硫化物の含有量の合計は、個数密度として、５０個／ｍｍ２以下である。
　なお、酸硫化物は、酸化物と硫化物との複合物を表す。
　これら、板厚１／４部に含まれる、酸化物、硫化物、及び酸硫化物は、介在物（以下、酸化物、硫化物、及び酸硫化物をまとめて、単に「介在物」と称する場合がある。）である。
　以下、介在物の限定理由について説明する。 (The total of oxides, sulfides and acid sulfides with a circle equivalent diameter of 2 μm or more is 50 pieces / mm 2 or less)
The steel plate according to the present disclosure is an oxide, a sulfide, and an oxysulfide having a circle equivalent diameter of 2 μm or more at a quarter position in the plate thickness direction from the surface of the steel plate in a cross section along the plate thickness direction. Of at least one of And the sum total of content of an oxide, a sulfide, and an acid sulfide whose circle equivalent diameter is 2 micrometers or more is 50 pieces / mm < 2 > or less as a number density.
In addition, an acid sulfide represents the complex of an oxide and a sulfide.
These oxides, sulfides and acid sulfides contained in 1⁄4 part of plate thickness mean inclusions (hereinafter, oxides, sulfides and acid sulfides are collectively referred to simply as “inclusions”. There is a case.
Hereinafter, the reasons for limitation of inclusions will be described.
　板厚１／４部に含まれる、円相当径が２μｍ以上の粗大な介在物である、酸化物、硫化物、及び酸硫化物は、脆性破壊の起点となり、継手ＣＴＯＤ特性を低下させる。そのため、円相当径が２μｍ以上である、酸化物、硫化物、及び酸硫化物の合計を５０個／ｍｍ２以下（好ましくは４０個／ｍｍ２以下、より好ましくは３０個／ｍｍ２以下）とする。板厚１／４部での上記介在物の含有量は、少ないほうが継手ＣＴＯＤ特性の向上に優位となる。板厚１／４部に含まれる、上記介在物の含有量の下限値は特に限定されず、例えば、１０個／ｍｍ２以上が挙げられる。 Oxides, sulfides, and oxysulfides, which are coarse inclusions having a circle equivalent diameter of 2 μm or more, which are contained in 1⁄4 part of the plate thickness, become the starting point of brittle fracture and lower the joint CTOD characteristics. Therefore, the sum of oxides, sulfides and oxysulfides having a circle equivalent diameter of 2 μm or more is 50 pieces / mm 2 or less (preferably 40 pieces / mm 2 or less, more preferably 30 pieces / mm 2 or less) I assume. The smaller the content of the above inclusions in the 1⁄4 part of the plate thickness, the more advantageous for improving the joint CTOD characteristics. The lower limit value of the content of the above-mentioned inclusions contained in 1/4 part of board thickness is not particularly limited, and, for example, 10 / mm 2 or more can be mentioned.
　鋼板の板厚１／４部に含まれる酸化物、硫化物、及び酸硫化物の円相当径および含有量（個数密度）の測定は、走査型電子顕微鏡（ＳＥＭ：Ｓｃａｎｎｉｎｇ　Ｅｌｅｃｔｒｏｎ　Ｍｉｃｒｏｓｃｏｐｅ）を用いて、広い視野で測定することが好ましい。具体的には、以下の方法によって測定する。
　具体的には、鋼板の圧延方向と垂直な方向の断面（Ｃ方向断面）及び鋼板の幅方向と垂直な方向の断面（Ｌ方向断面）の、鋼板表面から板厚方向の１／４位置から試料を採取する。そして、鋼板の圧延方向と垂直な断面（いわゆるＣ方向断面）を研磨し観察する。また、鋼板の幅方向と垂直な断面（いわゆるＬ方向断面）を研磨し観察する。
　ＳＥＭ／ＷＤＸ（波長分散型Ｘ線分析装置（ＷＤＸ：Ｗａｖｅｌｅｎｇｔｈ　Ｄｉｓｐｅｒｓｉｖｅ　Ｘ－ｒａｙ　Ｓｐｅｃｔｒｏｍｅｔｅｒ）を備える走査型電子顕微鏡（日本電子社製、ＪＸＡ－８５３０Ｆ）を用いて、上記介在物について、ＳＥＭ／ＷＤＸ解析による測定を行う。ＳＥＭ／ＷＤＸ解析は、加速電圧１５ｋＶ、電流を８９μＡ～９１μＡとし、Ｌ方向断面およびＣ方向断面の合計で観察視野面積を１８０ｍｍ２～２００ｍｍ２とする。なお、Ｌ方向断面およびＣ方向断面のそれぞれの観察視野面積は、９０ｍｍ２～１００ｍｍ２とする。
　ＳＥＭ／ＷＤＸ解析は、まず、観察視野内に認められる円相当径（直径）が２μｍ以上の粒子を識別する。次に、ＷＤＸで各粒子の組成を分析し、下記定義にしたがって、酸化物、硫化物、及び酸硫化物を識別する。さらに、粒子のＳＥＭ粒子像を撮影し、画像解析によって各粒子の個数を数えることで、上記介在物の個数密度を求める。なお、介在物の解析は、ＳＥＭに付属のソフトを用いて、自動で連続的に測定してもよい。 The equivalent circle diameter and the content (number density) of oxides, sulfides and oxysulfides contained in 1/4 part of plate thickness of steel sheet are measured using a scanning electron microscope (SEM: Scanning Electron Microscope) It is preferable to measure with a wide visual field. Specifically, it is measured by the following method.
Specifically, from the surface of the steel plate to the thickness direction 1⁄4 of the cross section in the direction perpendicular to the rolling direction of the steel plate (C direction cross section) and the cross section in the direction perpendicular to the width direction of the steel plate (L direction cross section) Take a sample. Then, a cross section perpendicular to the rolling direction of the steel plate (so-called C direction cross section) is polished and observed. In addition, a cross section perpendicular to the width direction of the steel plate (so-called L direction cross section) is polished and observed.
SEM / WDX analysis of the above inclusions using a scanning electron microscope (JXA-8530F, manufactured by JEOL Ltd.) equipped with SEM / WDX (Wavelength Dispersive X-ray Spectrometer). In SEM / WDX analysis, the acceleration voltage is 15 kV, the current is 89 μA to 91 μA, and the L-direction cross section and the C direction cross-section make the observation field area 180 mm 2 to 200 mm 2 . Each observation visual field area of the cross section in the C direction is 90 mm 2 to 100 mm 2 .
The SEM / WDX analysis first identifies particles having a circle equivalent diameter (diameter) of 2 μm or more which is observed in the observation field of view. The composition of each particle is then analyzed by WDX to identify oxides, sulfides and acid sulfides according to the following definition. Further, the particle density of the inclusions is determined by taking a SEM particle image of the particles and counting the number of each particle by image analysis. The analysis of inclusions may be automatically and continuously measured using software attached to the SEM.
　ここで、本開示において、上記方法で観察した介在物のうち、下記条件に該当する介在物を酸化物、硫化物、及び酸硫化物と判断する。
　すなわち、介在物として、本開示に係る鋼板に含有するＦｅ以外の元素のすべてを解析対象の元素とし、分析対象ごとの質量％を算出する。分析対象すべての元素の合計は１００質量％となる。このとき、Ｏ（酸素）の含有量が１０質量％以上である介在物を酸化物、Ｓの含有量が３質量％以上である介在物を硫化物、Ｏ（酸素）の含有量が１０質量％以上、かつ、Ｓの含有量が３質量％以上である介在物を酸硫化物とする。 Here, in the present disclosure, among the inclusions observed by the above method, inclusions corresponding to the following conditions are determined as an oxide, a sulfide, and an acid sulfide.
That is, as inclusions, all elements other than Fe contained in the steel plate according to the present disclosure are elements to be analyzed, and the mass% for each object to be analyzed is calculated. The total of all elements to be analyzed is 100% by mass. At this time, inclusions with an O (oxygen) content of 10% by mass or more as oxides, inclusions with an S content of 3% by mass or more as sulfides, and O (oxygen) content as 10% Inclusions having a% or more content and a S content of 3% by mass or more are referred to as an acid sulfide.
　本開示に係る鋼板は、中心偏析が適度な範囲に制御されていることが重要である。
　本開示に係る鋼板は、中心偏析を表す指標として、以下の指標を採用する。
　本開示に係る鋼板は、板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置でのＭｎ濃度（Ｍｎ（１／４））に対する、板厚方向に沿った断面の、鋼板表面から板厚方向の１／２位置（以下、「板厚１／２部」と称する場合がある）でのＭｎ濃度（Ｍｎ（１／２））の比（Ｍｎ（１／２）／Ｍｎ（１／４））が、０．９０～１．８０である。
　ここで、本開示において、板厚方向に沿った断面の鋼板表面から板厚方向の１／２位置は、１／２位置を中心として、２ｍｍ角の範囲を表す。また、板厚方向に沿った断面の鋼板表面から板厚方向の１／４位置は、１／４位置を中心として、２ｍｍ角の範囲を表す。
　以下、中心偏析の限定理由について説明する。 In the steel plate according to the present disclosure, it is important that central segregation be controlled to a suitable range.
The steel plate according to the present disclosure adopts the following index as an index representing central segregation.
The steel plate according to the present disclosure has a cross section along the thickness direction, with respect to a Mn concentration (Mn (1/4) ) at a 1/4 position from the surface of the steel plate to the thickness direction, The ratio (Mn (1/2) / (Mn (1/2) ) ratio of Mn concentration (Mn (1/2) ) at half position in the plate thickness direction from the surface of the steel sheet (hereinafter sometimes referred to as "plate thickness 1/2 part" ) Mn (1/4) is 0.90 to 1.80.
Here, in the present disclosure, the 1⁄2 position in the thickness direction from the surface of the steel plate of the cross section along the thickness direction represents a 2 mm square range around the 1⁄2 position. In addition, the 1⁄4 position in the thickness direction from the surface of the steel plate of the cross section along the thickness direction represents a 2 mm square range around the 1⁄4 position.
Hereinafter, the reasons for limitation of center segregation will be described.
（Ｍｎ（１／２）／Ｍｎ（１／４）：０．９０～１．８０）
　Ｍｎは連続鋳造時に中心偏析して、板厚中心部に脆化域を形成するため、継手ＣＴＯＤ特性に影響を及ぼす。そのため、板厚１／４部のＭｎの濃度（Ｍｎ（１／４））に対する板厚１／２部のＭｎの濃度（Ｍｎ（１／２））の比（Ｍｎ（１／２）／Ｍｎ（１／４））を０．９０～１．８０とする。（Ｍｎ（１／２）／Ｍｎ（１／４））が１．８０を超えると板厚中心部のＭｎ濃度が濃くなりすぎるため、継手ＣＴＯＤ特性が低下する。Ｍｎ（１／２）／Ｍｎ（１／４）が０．９０未満の鋼板を安定的に得ることは、現実的に難しい。Ｍｎ（１／２）／Ｍｎ（１／４）の好ましい範囲は、０．９５～１．７０であり、より好ましい範囲は１．００～１．６０である。 (Mn (1/2) / Mn (1/4) : 0.90 to 1.80)
Mn is centrally segregated during continuous casting to form an embrittled zone at the center of the plate thickness, which affects the joint CTOD characteristics. Therefore, the ratio (Mn (1/2) / Mn) of the concentration (Mn (1/2) ) of Mn in the plate thickness 1/2 part to the concentration (Mn (1/4) ) of Mn in the plate thickness 1/4 part Let (1/4) ) be 0.90 to 1.80. If (Mn (1/2) / Mn (1/4) ) exceeds 1.80, the Mn concentration at the center of the plate thickness becomes too high, so the joint CTOD characteristics deteriorate. It is practically difficult to stably obtain a steel sheet with Mn (1/2) / Mn (1/4) less than 0.90. The preferred range of Mn (1/2) / Mn (1/4) is 0.95 to 1.70, and the more preferred range is 1.00 to 1.60.
　Ｍｎ（１／２）／Ｍｎ（１／４）は、以下のようにして求める。
　まず、板厚１／４部から採取したＭｎ濃度測定用試料および板厚１／２部から採取したＭｎ濃度測定用試料を準備する。次に、それぞれの試料について、試料の２ｍｍ角の範囲を測定範囲として、電子プローブ微小分析器（ＥＰＭＡ：Ｅｌｅｃｔｒｏｎ　Ｐｒｏｂｅ　ＭｉｃｒｏＡｎａｌｙｓｅｒ、測定条件；加速電圧：１５ｋＶ、ビーム径：２０μｍ、照射時間：２０ｍｓ、および測定ピッチ：２０μｍ）により、上記２ｍｍ角の測定範囲を測定したときのＭｎの濃度の最大値を測定する。そして、それぞれの試料から得られた最大濃度を、それぞれ、Ｍｎ（１／２）およびＭｎ（１／４）として、Ｍｎ（１／２）／Ｍｎ（１／４）を算出する。それぞれの試料は、Ｃ方向断面及びＬ方向断面から採取し、Ｃ方向断面及びＬ方向断面について測定を行う。 Mn (1/2) / Mn (1/4) is determined as follows.
First, a sample for measurement of Mn concentration collected from 1/4 part of plate thickness and a sample for measurement of Mn concentration collected from 1/2 part of plate thickness are prepared. Next, for each sample, an electron probe microanalyzer (EPMA: Electron Probe MicroAnalyzer, measurement conditions; acceleration voltage: 15 kV, beam diameter: 20 μm, irradiation time: 20 ms, taking a 2 mm square range of the sample as a measurement range) The maximum value of the concentration of Mn when the measurement range of 2 mm square is measured is measured by the measurement pitch: 20 μm. Then, Mn (1/2) / Mn (1/4) is calculated with the maximum concentrations obtained from the respective samples as Mn (1/2) and Mn (1/4) , respectively. Each sample is taken from the C direction cross section and the L direction cross section, and the measurement is performed on the C direction cross section and the L direction cross section.
　本開示に係る鋼板の板厚としては、特に限定されず、例えば、５０ｍｍ以上であることが挙げられ、さらに、５０ｍｍ～１００ｍｍであることが挙げられる。 The thickness of the steel plate according to the present disclosure is not particularly limited, and may be, for example, 50 mm or more, and may further be 50 mm to 100 mm.
　本開示に係る鋼板の引張強さ（ＴＳ）は、特に限定されず、高強度とする点で、５１０ＭＰａ以上（好ましくは５１０ＭＰａ～７２０ＭＰａ、より好ましくは５７０ＭＰａ～７２０ＭＰａ）であることがよい。また、降伏応力（ＹＰ）は、３９０ＭＰａ以上（好ましくは３９０ＭＰａ～６５０ＭＰａ、より好ましくは４６０ＭＰａ～６５０ＭＰａ）であることがよい。
　ここで、本開示に係る鋼板の引張強さ（ＴＳ）は、ＪＩＳ　Ｚ　２２４１（２０１１）の１Ｂ号引張試験片を用いて測定する。また、降伏応力（ＹＰ）はＪＩＳ　Ｚ２２４１（２０１１）の永久伸び０．２％時の永久伸び法の耐力を意味する。 The tensile strength (TS) of the steel plate according to the present disclosure is not particularly limited, and is preferably 510 MPa or more (preferably 510 MPa to 720 MPa, more preferably 570 MPa to 720 MPa) from the viewpoint of achieving high strength. In addition, the yield stress (YP) is preferably 390 MPa or more (preferably 390 MPa to 650 MPa, more preferably 460 MPa to 650 MPa).
Here, the tensile strength (TS) of the steel plate according to the present disclosure is measured using a No. 1B tensile test piece of JIS Z 2241 (2011). Moreover, the yield stress (YP) means the proof stress of the permanent elongation method at the time of permanent elongation 0.2% of JIS Z2241 (2011).
　また、本開示に係る鋼板は、温度勾配型ＥＳＳＯ試験における試験温度マイナス１０℃での脆性亀裂伝播停止靭性値Ｋｃａ（以下、「アレスト靱性値Ｋｃａ－１０℃」と称する場合がある。）が６０００Ｎ／ｍｍ１．５以上（好ましくは、８０００Ｎ／ｍｍ１．５以上）であることがよい。この特性を満足することで、鋼板は優れたアレスト性を有する。
　アレスト靱性値Ｋｃａ－１０℃は、ＮＫ船級協会　鋼船規則検査要領　Ｋ編　付属書　Ｋ３．１２．２－１．（２０１６年）の「温度勾配型ＥＳＳＯ試験及び温度勾配型二重引張試験に関する検査要領」に準拠して測定を行う。 Further, the steel sheet according to the present disclosure, brittle crack propagation stopping toughness value at test temperature minus 10 ° C. in a temperature gradient type ESSO test K ca (hereinafter also referred to as "arrest toughness value K ca-10 ° C.".) There 6000 N / mm 1.5 or more (preferably, 8000 N / mm 1.5 or more) may be a. By satisfying this property, the steel sheet has excellent arrestability.
Arrest toughness value K ca -10 ° C is defined by the NK Classification Society Steel Ship Regulations Inspection Guideline K, Appendix K3.12.2-1. Measure according to (2016) “Testing Procedure for Thermal Gradient ESSO Test and Thermal Gradient Double Tensile Test”.
　さらに、本開示に係る鋼板は、溶接部の亀裂先端開口変位試験（以下、「溶接部のＣＴＯＤ試験」と称する場合がある。）における試験温度マイナス１０℃での開口変位δｃ（以下、「開口変位δｃ－１０℃」と称する場合がある）が０．１０ｍｍ以上（好ましくは、０．２０ｍｍ以上）であることがよい。この特性を満足することで、鋼板は優れた継手ＣＴＯＤ特性を有する。
　開口変位δｃ－１０℃は、ＢＳ７４４８規格（Ｂｒｉｔｉｓｈ　Ｓｔａｎｄａｒｄ）Ｐａｒｔ１（１９９１）、及びＢＳ７４４８規格（Ｂｒｉｔｉｓｈ　Ｓｔａｎｄａｒｄ）Ｐａｒｔ２（１９９７）に準拠して測定を行う。具体的には、レ形開先の加工した鋼板突き合わせ部に、入熱量５ｋＪ／ｍｍでサブマージアーク溶接（ＳＡＷ溶接）を実施し、溶接部のＣＴＯＤ試験片の疲労ノッチが、レ形開先の垂直側の溶接線となるよう加工し、ＣＴＯＤ試験を－１０℃として実施する。 Furthermore, the steel plate according to the present disclosure has an opening displacement δc (hereinafter referred to as “opening” at a test temperature minus 10 ° C. in a crack tip opening displacement test of the welded part (hereinafter may be referred to as “CTOD test of welded part”). The displacement δc −10 ° C. ”may be referred to as“ 0.10 mm or more (preferably 0.20 mm or more) ”. By satisfying this characteristic, the steel plate has excellent joint CTOD characteristics.
The opening displacement δc −10 ° C. is measured in accordance with BS 7448 standard (British Standard) Part 1 (1991) and BS 7448 standard (British Standard) Part 2 (1997). Specifically, submerged arc welding (SAW welding) is carried out at a heat input of 5 kJ / mm at the butt-welded portion of a machined groove, and the fatigue notch of the CTOD test piece of the welded portion Process to the welding line on the vertical side and perform CTOD test at -10 ° C.
　なお、本開示に係る鋼板は、金属組織の形態、金属組織の各相の平均粒径、介在物の量、及び中心偏析が、前述の条件を満足するように制御されていることで、例えば、板厚が５０ｍｍ以上（例えば、５０ｍｍ～１００ｍｍ）であっても、引張強さ（ＴＳ）、降伏応力（ＹＰ）、アレスト靱性値Ｋｃａ－１０℃、及び開口変位δｃ－１０℃が全て、上記範囲の値を満足し得る。 In the steel plate according to the present disclosure, for example, the form of the metal structure, the average particle diameter of each phase of the metal structure, the amount of inclusions, and the center segregation are controlled to satisfy the above-mentioned conditions. , Tensile strength (TS), yield stress (YP), arrest toughness value K ca -10 ° C , and opening displacement δc -10 ° C , even when the plate thickness is 50 mm or more (for example, 50 mm to 100 mm) The value of the above range can be satisfied.
　次に、本開示に係る鋼板の製造方法における好ましい態様の一例について説明する。 Next, an example of a desirable mode in a method of manufacturing a steel plate according to the present disclosure will be described.
　本開示に係る鋼板は、例えば、下記の各工程を有する製造方法によって得ることができる。
　溶存酸素量を５０ｐｐｍ以下に調整した溶鋼を、最終溶存酸素量が２０ｐｐｍ以下になるように脱酸元素を添加して脱酸処理を行う工程（脱酸処理工程）、
　脱酸処理を経た溶鋼を、１分あたり０．８０トン～５．００トン（０．８０ｔｏｎ／ｍｉｎ～５．００ｔｏｎ／ｍｉｎ）のスループットで連続鋳造し、連続鋳造するときの鋳片の中心固相率が０．５～１．０の範囲であるとき、鋳片に対し、鋳造進行方向１ｍあたり１ｍｍ～１．５ｍｍ（１ｍｍ／ｍ～１．５ｍｍ／ｍ）の圧下勾配で、圧下量が１ｍｍ～１５ｍｍとなる軽圧下を行い、鋼片を得る工程（鋳造工程）、
　鋼片を９５０℃～１１５０℃の温度域で加熱し、加熱後の鋼片に粗圧延を行った後、粗圧延後の鋼板に、鋼板の表面温度がＡｒ３点－３０℃～再結晶温度Ｔｒｅｘの温度域で、累積圧下率が５０％～７５％の仕上圧延を行う工程（圧延工程）、
　仕上圧延後の鋼板を、鋼板の板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置での温度が０℃～６００℃の温度域になるまで、前記１／４位置での平均冷却速度が１℃／秒～２０℃／秒で冷却する工程（冷却工程）。
　以下、各工程について説明する。 The steel plate according to the present disclosure can be obtained, for example, by a manufacturing method having the following steps.
A step of performing deoxidation treatment by adding a deoxidizing element so that the final dissolved oxygen amount becomes 20 ppm or less in a molten steel whose dissolved oxygen amount has been adjusted to 50 ppm or less (deoxidation treatment step);
Continuous casting of the deoxidized molten steel at a throughput of 0.80 to 5.00 tons (0.80 ton / 5.00 to 5.00 ton / min) per minute, and center solidification of the slab during continuous casting When the phase ratio is in the range of 0.5 to 1.0, the amount of reduction is 1 to 1.5 mm (1 mm / m to 1.5 mm / m) with a reduction gradient of 1 mm to 1 mm in the casting advancing direction with respect to the slab. A step of obtaining a billet by applying a light pressure of 1 mm to 15 mm (casting step),
The billet is heated in a temperature range of 950 ° C. to 1150 ° C. and rough rolling is performed on the heated billet, and then the surface temperature of the steel plate is Ar 3 point −30 ° C. to recrystallization temperature on the steel plate after rough rolling A step (finishing step) of performing finish rolling with a cumulative rolling reduction of 50% to 75% in a temperature range of T rex
The steel sheet after finish rolling has a cross section along the thickness direction of the steel sheet until the temperature at a quarter position from the surface of the steel sheet in the thickness direction reaches a temperature range of 0 ° C. to 600 ° C. Cooling at an average cooling rate of 1 ° C./s to 20 ° C./s (cooling step).
Each step will be described below.
　（脱酸処理工程）
　脱酸元素を添加する前の溶鋼中の溶存酸素量を所定の量になるように調整した後、脱酸元素を添加して、溶鋼中の最終溶存酸素量を所定の量以下に調整する。この工程によって、介在物の個数密度が調整される。この工程は、継手ＣＴＯＤ特性に影響を及ぼす。 (Deacidification process)
After adjusting the amount of dissolved oxygen in the molten steel before adding the deoxidizing element to a predetermined amount, the final amount of dissolved oxygen in the molten steel is adjusted to a predetermined amount or less by adding the deoxidizing element. By this process, the number density of inclusions is adjusted. This process affects the joint CTOD characteristics.
　まず、溶鋼から炭素を除く１次精錬を行った後、溶鋼の成分調整をするにあたり、脱酸元素を添加する前の溶鋼中の溶存酸素量の調整を行う。溶鋼中の溶存酸素量が５０ｐｐｍを超えた状態では、後述の脱酸元素を添加して、溶鋼中の最終溶存酸素量を２０ｐｐｍ以下に調整することは難しくなる。そのため、脱酸元素を添加する前の溶鋼中の溶存酸素量の調整は、５０ｐｐｍ以下（好ましくは４０ｐｐｍ以下、より好ましくは３０ｐｐｍ以下）とする。脱酸元素を添加する前の溶鋼中の溶存酸素量の下限は特に限定されず、例えば、２０ｐｐｍを超える程度にまで調整してもよい。なお、ｐｐｍは質量基準である。 First, after performing primary refining which removes carbon from molten steel, when adjusting the composition of molten steel, adjustment of the amount of dissolved oxygen in molten steel before adding deoxidizing element is performed. When the amount of dissolved oxygen in the molten steel exceeds 50 ppm, it is difficult to adjust the final amount of dissolved oxygen in the molten steel to 20 ppm or less by adding a deoxidizing element described later. Therefore, adjustment of the amount of dissolved oxygen in molten steel before adding a deoxidizing element is 50 ppm or less (preferably 40 ppm or less, more preferably 30 ppm or less). The lower limit of the amount of dissolved oxygen in the molten steel before adding the deoxidizing element is not particularly limited, and may be adjusted to, for example, more than 20 ppm. Note that ppm is on a mass basis.
　溶鋼の溶存酸素量を５０ｐｐｍ以下に調整する方法としては、特に限定されず、例えば、ＲＨ（Ｒｕｈｒｓｔａｈｌ－Ｈｅｒａｅｕｓ）真空脱ガス装置を用いて行うことが挙げられる。ＲＨ真空脱ガス装置による溶存酸素量の調整は、例えば、真空度が１ｔｏｒｒ～５ｔｏｒｒ（≒１３３Ｐａ～６６７Ｐａ）で、溶鋼を１分～３分還流することがよい。 The method for adjusting the dissolved oxygen content of the molten steel to 50 ppm or less is not particularly limited, and examples thereof include performing using a RH (Ruhrstahl-Heraeus) vacuum degassing apparatus. In adjusting the amount of dissolved oxygen by the RH vacuum degassing apparatus, for example, it is preferable to reflux molten steel at a degree of vacuum of 1 to 5 torr (≒ 133 Pa to 667 Pa) for 1 to 3 minutes.
　次に、溶鋼の溶存酸素量が５０ｐｐｍ以下となった後の溶鋼に対し、脱酸元素を添加して、脱酸元素添加後の溶鋼中の最終溶存酸素量を２０ｐｐｍ以下に調整する。脱酸元素添加後の溶鋼中の最終溶存酸素量が２０ｐｐｍを超えている場合は、酸化物、硫化物、及び酸硫化物の介在物の個数密度を所定量以下に制御することが難しくなる。
　脱酸元素添加後の溶鋼中の最終溶存酸素量は少ないほどよい。脱酸元素添加後の溶鋼中の最終溶存酸素量の上限としては、例えば、１５ｐｐｍ以下が好ましく、１０ｐｐｍ以下がより好ましい。脱酸元素添加後の溶鋼中の最終溶存酸素量の下限としては、特に限定されず、例えば、５ｐｐｍ以上とすることがよい。 Next, a deoxidizing element is added to the molten steel after the dissolved oxygen content of the molten steel becomes 50 ppm or less, and the final dissolved oxygen content in the molten steel after addition of the deoxidizing element is adjusted to 20 ppm or less. When the final amount of dissolved oxygen in the molten steel after addition of the deoxidizing element exceeds 20 ppm, it becomes difficult to control the number density of inclusions of oxides, sulfides and acid sulfides to a predetermined amount or less.
The final amount of dissolved oxygen in the molten steel after addition of the deoxidizing element should be as low as possible. As an upper limit of the amount of final dissolved oxygen in the molten steel after deoxidation element addition, 15 ppm or less is preferable, for example, and 10 ppm or less is more preferable. The lower limit of the final amount of dissolved oxygen in the molten steel after addition of the deoxidizing element is not particularly limited, and may be, for example, 5 ppm or more.
　ここで、脱酸元素としては、Ａｌ、Ｃａ、Ｓｉ、Ｔｉ、Ｍｇ、Ｚｒ、およびＲＥＭからなる群から選択される少なくとも１種を用いる。これら脱酸元素を２種以上の複数の元素を用いる場合、脱酸元素の添加順序は、特に限定されない。例えば、ＡｌとＡｌ以外の脱酸元素とを同時に添加してもよく、Ａｌを添加した後、Ａｌ以外の脱酸元素を添加してもよい。 Here, as the deoxidizing element, at least one selected from the group consisting of Al, Ca, Si, Ti, Mg, Zr, and REM is used. When two or more of these deoxidizing elements are used, the addition order of the deoxidizing elements is not particularly limited. For example, Al and a deoxidizing element other than Al may be simultaneously added, or after adding Al, a deoxidizing element other than Al may be added.
　溶鋼の最終溶存酸素量を２０ｐｐｍ以下に調整する方法としては、特に限定されず、例えば、脱酸元素添加後の溶鋼に対し、ＲＨ真空脱ガス装置を用いて行うことが挙げられる。ＲＨ真空脱ガス装置による脱酸元素添加後の溶鋼の最終溶存酸素量の調整は、例えば、真空度が１ｔｏｒｒ～５ｔｏｒｒ（≒１３３Ｐａ～６６７Ｐａ）で、溶鋼を１０分～６０分還流することがよい。 It does not specifically limit as method to adjust the final dissolved oxygen content of molten steel to 20 ppm or less, For example, it performs performing using RH vacuum degassing apparatus with respect to the molten steel after deoxidation element addition. For adjustment of the final dissolved oxygen content of the molten steel after adding the deoxidizing element by the RH vacuum degassing apparatus, for example, it is preferable to reflux the molten steel for 10 to 60 minutes at a degree of vacuum of 1 to 5 torr (≒ 133 Pa to 667 Pa). .
　そして、各元素が所望の含有量となるように、歩留まりを考慮して化学組成を調整する。各元素の添加方法については、化学組成が上記条件を満足するように鋼板に含有できる方法であれば、特に限定されない。 And a chemical composition is adjusted in consideration of a yield so that each element may become desired content. The method of adding each element is not particularly limited as long as it can be contained in the steel sheet so that the chemical composition satisfies the above conditions.
　なお、脱酸元素を添加する前での溶存酸素量、及び脱酸元素添加後の最終溶存酸素量の測定は、他の化合物と化合して酸化物となっていない、酸素単体として存在するフリー状態の酸素を測定する。
　具体的には、脱酸元素を添加する前における溶存酸素量、及び脱酸元素添加後における最終溶存酸素量は、公知の起電力測定法によって求める（例えば、「起電力測定による転炉炉中溶鋼酸素の迅速分析」（井樋田ら、鉄と鋼、社団法人日本鉄鋼協会、１９７２年、第１０号、Ｐ１２５～Ｐ１３２）を参照。）。 In addition, the measurement of the amount of dissolved oxygen before adding the deoxidizing element and the final amount of dissolved oxygen after adding the deoxidizing element are free from existing oxygen as a simple substance which is not oxidized as a compound with other compounds. Measure the state oxygen.
Specifically, the amount of dissolved oxygen before adding the deoxidizing element and the final amount of dissolved oxygen after adding the deoxidizing element are determined by a known method of electromotive force measurement (for example, “in converter furnace by electromotive force measurement "Rapid analysis of molten steel oxygen" (see Ikuta et al., Iron and Steel, Japan Iron and Steel Institute, 1972, No. 10, pp. 125-132).
（鋳造工程）
　脱酸処理を経た溶鋼を、所定のスループットで連続鋳造し、連続鋳造するときの鋳片の中心固相率が所定の範囲であるとき、所定の軽圧下を行って鋼片を得る工程である。この工程によって、介在物の個数密度が調整されるとともに、中心偏析が抑制される。この工程は、継手ＣＴＯＤ特性に影響を与える工程となる。 (Casting process)
This is a process to obtain steel slabs by performing constant light pressure reduction when the solid phase ratio of slabs when casting continuously casting and continuously casting molten steel that has undergone deoxidation treatment is within a predetermined range. . By this process, the number density of inclusions is adjusted, and central segregation is suppressed. This process is a process of affecting the joint CTOD characteristics.
　まず、脱酸処理を経た、所定の化学組成を有する溶鋼を、０．８０ｔｏｎ／ｍｉｎ～５．００ｔｏｎ／ｍｉｎのスループットで連続鋳造する。溶鋼のスループットが５．００ｔｏｎ／ｍｉｎを超えると、酸化物、硫化物、及び酸硫化物の介在物の浮上分離効果が低下しやすくなるため、鋳片中に介在物が存在しやすくなる。その結果、得られた鋼板における鋼板中の介在物の個数密度を所定量以下に制御することが難しくなる。一方、溶鋼のスループットが０．８０ｔｏｎ／ｍｉｎ未満の場合は生産性が低下する。上記介在物の個数密度をより制御する点で、溶鋼のスループットは、３．００ｔｏｎ／ｍｉｎ～４．５０ｔｏｎ／ｍｉｎが好ましい。 First, molten steel having a predetermined chemical composition, which has undergone deoxidation treatment, is continuously cast at a throughput of 0.80 ton / min to 5.00 ton / min. When the throughput of the molten steel exceeds 5.00 ton / min, the floating separation effect of the oxide, sulfide, and acid sulfide inclusions tends to be reduced, so that the inclusions are easily present in the slab. As a result, it becomes difficult to control the number density of inclusions in the steel plate in the obtained steel plate to a predetermined amount or less. On the other hand, when the throughput of molten steel is less than 0.80 ton / min, productivity is reduced. The throughput of the molten steel is preferably 3.00 ton / min to 4.50 ton / min in that the number density of inclusions is further controlled.
　なお、本開示において、「スループット」は、１分間あたりに生産される鋳片の製造速度（ｔｏｎ／ｍｉｎ）を表す。スループットは、下記式で求められる。
　スループット（ｔｏｎ／ｍｉｎ）＝鋳造幅（ｍｍ）×鋳造厚（ｍｍ）×鋳造速度（ｍｍ／ｍｉｎ）×溶鋼密度（ｔｏｎ／ｍｍ３）
　ここで、溶鋼密度は、溶鋼の金属種によって決まるが、本開示では、溶鋼密度は、７．８５×１０－９（ｔｏｎ／ｍｍ３）とする。 In the present disclosure, “throughput” refers to the production rate (ton / min) of slabs produced per minute. The throughput can be obtained by the following equation.
Throughput (ton / min) = casting width (mm) × casting thickness (mm) × casting speed (mm / min) × molten steel density (ton / mm 3 )
Here, the molten steel density is determined by the metal type of the molten steel, but in the present disclosure, the molten steel density is 7.85 × 10 −9 (ton / mm 3 ).
　次に、所定のスループットで連続鋳造するときの鋳片を所定の条件で軽圧下を行う。軽圧下は、鋳片の凝固末期である鋳片の中心固相率が０．５～１．０の範囲であるとき、鋳片に対し、１．０ｍｍ／ｍ～１．５ｍｍ／ｍ（好ましくは１．１ｍｍ／ｍ～１．４ｍｍ／ｍ、より好ましくは１．２ｍｍ／ｍ～１．３ｍｍ／ｍ）の圧下勾配で、圧下量が１ｍｍ～１５ｍｍ（好ましくは３ｍｍ～１２ｍｍ、より好ましくは５ｍｍ～１０ｍｍ）となるように行う。この条件で軽圧下を行うことで、Ｍｎ等の濃化溶鋼を鋳造方向の上流側に排出させ、Ｍｎの中心偏析が低減する。 Next, light reduction is performed under a predetermined condition on a cast piece for continuous casting at a predetermined throughput. The light reduction is 1.0 mm / m to 1.5 mm / m (preferably) to the slab when the central solid phase ratio of the slab at the end of solidification of the slab is in the range of 0.5 to 1.0. Is a reduction gradient of 1.1 mm / m to 1.4 mm / m, more preferably 1.2 mm / m to 1.3 mm / m), and a reduction amount is 1 mm to 15 mm (preferably 3 mm to 12 mm, more preferably 5 mm) Make it to be 10mm). By performing light reduction under these conditions, concentrated molten steel such as Mn is discharged upstream in the casting direction, and central segregation of Mn is reduced.
　なお、軽圧下は、中心固相率が０．５～１．０の範囲である間、常に、１．０ｍｍ／ｍ～１．５ｍｍ／ｍの圧下勾配で、圧下量が１ｍｍ～１５ｍｍとなる軽圧下を行う必要はない。軽圧下は、中心固相率が０．５～１．０の範囲となる間のいずれかのタイミングにおいて、１．０ｍｍ／ｍ～１．５ｍｍ／ｍの圧下勾配で、圧下量が１ｍｍ～１５ｍｍとなる軽圧下を行えばよい。例えば、中心固相率が０．５～０．８の範囲となる間、０．６～０．８の範囲となる間、０．６～０．７の範囲となる間など、いずれかのタイミングであるときに、圧下勾配を１．０ｍｍ／ｍ～１．５ｍｍ／ｍの範囲とし、圧下量が１ｍｍ～１５ｍｍとなるように、軽圧下を行えばよい。 For light reduction, while the central solid phase ratio is in the range of 0.5 to 1.0, the reduction is always 1 mm to 15 mm with a reduction gradient of 1.0 mm / m to 1.5 mm / m. There is no need to carry out a light reduction. The light reduction is 1 mm to 15 mm at a reduction gradient of 1.0 mm / m to 1.5 mm / m at any timing during which the central solid phase ratio is in the range of 0.5 to 1.0. The light reduction to be For example, while the central solid phase ratio is in the range of 0.5 to 0.8, or in the range of 0.6 to 0.8, or in the range of 0.6 to 0.7, At the timing, the pressure reduction gradient may be in the range of 1.0 mm / m to 1.5 mm / m, and the light reduction may be performed so that the amount of pressure reduction is 1 mm to 15 mm.
　ここで、本開示において、鋳造進行方向１ｍあたりの１．０ｍｍ～１．５ｍｍ圧下勾配とは、鋳造進行方向１ｍにつき、鋳造ロールの間隙を１．０ｍｍ～１．５ｍｍに狭めて軽圧下することを表す。
　また、中心固相率とは、鋳片厚み方向の中心部で、かつ、鋳片幅方向の溶融部分の固相率と定義できる。中心固相率は、伝熱・凝固計算によって求めることができる。伝熱・凝固計算としては、エンタルピー法、等価比熱法などが広く知られており、いずれの方法を用いてもよい。中心固相率は、簡易的には、下記式が知られており、下記式を用いて求めてもよい。
　中心固相率＝（溶鋼の液相線温度－鋼片の中心部温度）／（溶鋼の液相線温度－溶鋼の固相線温度） Here, in the present disclosure, with a 1.0 mm to 1.5 mm reduction gradient per 1 m of casting progressing direction, the gap between the casting rolls is narrowed to 1.0 mm to 1.5 mm and light reduction is performed per 1 m of casting progressing direction. Represents
Further, the central solid phase ratio can be defined as the solid phase ratio of the molten portion in the central portion of the cast slab thickness direction and in the cast slab width direction. The central solid phase rate can be determined by heat transfer and solidification calculation. As heat transfer and solidification calculation, enthalpy method, equivalent specific heat method, etc. are widely known, and any method may be used. The central solid phase ratio is simply known by the following formula, and may be determined using the following formula.
Solid phase ratio = (liquidus temperature of molten steel-core temperature of billet) / (liquidus temperature of molten steel-solidus temperature of molten steel)
（圧延工程）
　次に、鋳造工程を経て得られた鋼片を所定の温度で加熱し、加熱後の鋼片に粗圧延を行う。そして、粗圧延後の鋼板に、鋼板の表面温度が所定の温度で、所定の累積圧下率となるように仕上圧延を行う。この工程は、鋼片の加熱により、オーステナイト粒を細粒化し、所定の仕上圧延により、オーステナイトに効率的に歪を蓄積して、ミクロ組織の細粒化に寄与する工程である。そして、この工程は、アレスト性に影響を及ぼす。 (Rolling process)
Next, the steel slab obtained through the casting process is heated at a predetermined temperature, and rough rolling is performed on the heated steel slab. Then, finish rolling is performed on the steel plate after rough rolling so that the surface temperature of the steel plate becomes a predetermined cumulative rolling reduction at a predetermined temperature. This process is a process of refining austenite grains by heating a steel piece, efficiently accumulating strain in austenite by predetermined finish rolling, and contributing to the grain refinement of a microstructure. And, this process affects the arrestability.
　まず、鋼片を、９５０℃～１１５０℃（好ましくは１０００℃～１１００℃）の温度域で加熱する。鋼片の加熱温度を９５０℃以上とすることで、オーステナイト化が十分となり、オーステナイト粒が微細化される。一方、鋼片の加熱温度を１１５０℃以下とすることで、オーステナイトの粗大化が抑制され、オーステナイト粒が微細化される。そして、この温度範囲とすることで、優れたアレスト性が得られる。
　なお、加熱温度は、鋼片の全厚の平均温度を表す。 First, the billet is heated in a temperature range of 950 ° C. to 1150 ° C. (preferably 1000 ° C. to 1100 ° C.). By setting the heating temperature of the billet to 950 ° C. or more, austenitization becomes sufficient and austenite grains are refined. On the other hand, by setting the heating temperature of the steel piece to 1150 ° C. or less, coarsening of austenite is suppressed and austenite grains are refined. And, by setting this temperature range, excellent arrestability can be obtained.
In addition, heating temperature represents the average temperature of the total thickness of a billet.
　次に、加熱後の鋼片を粗圧延し、粗圧延後の鋼板に仕上圧延を施す。仕上圧延は、鋼板の表面温度がＡｒ３点－３０℃～再結晶温度Ｔｒｅｘの温度域で、累積圧下率が５０％～７５％となるように行う。
　なお、仕上圧延の温度域は、鋼板の表面温度を表し、鋼板の表面温度がＡｒ３点－３０℃～再結晶温度Ｔｒｅｘに達してから圧延を開始する。 Next, the steel slab after heating is roughly rolled, and the steel plate after rough rolling is subjected to finish rolling. The finish rolling is performed such that the cumulative rolling reduction is 50% to 75% in the temperature range where the surface temperature of the steel sheet is Ar 3 point −30 ° C. to the recrystallization temperature T rex .
The temperature range of the finish rolling represents the surface temperature of the steel plate, and the rolling is started after the surface temperature of the steel plate reaches Ar 3 point −30 ° C. to the recrystallization temperature T rex .
　鋼板の表面温度がＡｒ３点－３０℃～再結晶温度Ｔｒｅｘの温度域であることで、ミクロ組織の各相の粗大化が抑制される。特に、フェライトが細粒化され、良好なアレスト性が得られる。仕上圧延の開始温度がＡｒ３－３０℃以上であると、二相域圧延（フェライト＋オーステナイトの二相が存在する温度域での圧延）とならないため、粗大な加工フェライト（圧延中生成したフェライト）の生成が抑制される。一方、再結晶温度Ｔｒｅｘ温度以下であると、未再結晶域圧延となり、フェライトの粗大化が抑制される。 When the surface temperature of the steel sheet is in a temperature range of Ar 3 point −30 ° C. to the recrystallization temperature T rex , the coarsening of each phase of the microstructure is suppressed. In particular, ferrite is finely divided to obtain good arrestability. If the start temperature of finish rolling is Ar 3 -30 ° C. or higher, two-phase zone rolling (rolling in a temperature range where ferrite + austenite two phase exists) will not be performed, so coarse worked ferrite (ferrite formed during rolling) Generation is suppressed. On the other hand, if the recrystallization temperature is equal to or lower than the Rex temperature, rolling in the non-recrystallization region is performed, and coarsening of ferrite is suppressed.
　また、累積圧下率が５０％以上とすることで、オーステナイトに効率的に歪を蓄積することができ、ミクロ組織の各相の細粒化に寄与する。特に、オーステナイト中のフェライト核生成サイトが増え、フェライトが細粒化される。一方、累積圧下率が７５％を超えると、設備能力により圧延が困難であったり、圧延温度調整時間等により生産性が低下する。そのため、累積圧下率は５０％～７５％（好ましくは５５％～６５％）とする。 Further, by setting the cumulative rolling reduction to 50% or more, strain can be efficiently accumulated in austenite, which contributes to the grain refinement of each phase of the microstructure. In particular, ferrite nucleation sites in austenite are increased and ferrite is refined. On the other hand, if the cumulative rolling reduction exceeds 75%, rolling is difficult due to the facility capacity, and productivity decreases due to the rolling temperature adjustment time or the like. Therefore, the cumulative rolling reduction is set to 50% to 75% (preferably 55% to 65%).
　ここで、Ａｒ３は、下記式（２）、及び再結晶温度Ｔｒｅｘは、下記式（３）で表される。
式（２）　Ａｒ３＝９１０－３１０［Ｃ］＋６５［Ｓｉ］－８０［Ｍｎ］－２０［Ｃｕ］
　　　　　　　　　－５５［Ｎｉ］－１５［Ｃｒ］－８０［Ｍｏ］
式（３）　Ｔｒｅｘ＝－９１９００［Ｎｂ＊］２＋９４００［Ｎｂ＊］＋７７０
（ただし、式（３）中、［Ｎｂ＊］は、下記式（４）で表される。式（４）中、Ｔは鋼片の加熱温度、単位は摂氏温度（℃）を表す。
式（４）　Ｓｏｌ．Ｎｂ＝（１０（－６７７０／（Ｔ＋２７３）＋２．２６））／（Ｃ＋１２／１４×Ｎ）
　また、［Ｎｂ］≧［Ｓｏｌ．Ｎｂ］の場合は、［Ｎｂ＊］＝［Ｓｏｌ．Ｎｂ］、
及び［Ｎｂ］＜［Ｓｏｌ．Ｎｂ］の場合は、［Ｎｂ＊］＝［Ｎｂ］の関係を満たす。
　なお、［Ｎｂ］は、Ｎｂ含有量（質量％）を表し、［Ｓｏｌ．Ｎｂ］は式（４）で求めたＳｏｌ．Ｎｂ（固溶Ｎｂ）（質量％）を表す。） Here, Ar 3 is represented by the following formula (2), and the recrystallization temperature T rex is represented by the following formula (3).
Equation (2) Ar 3 = 910-310 [ C] +65 [Si] -80 [Mn] -20 [Cu]
-55 [Ni] -15 [Cr] -80 [Mo]
Formula (3) T rex = -91900 [Nb *] 2 + 9400 [Nb *] + 770
(However, in Formula (3), [Nb *] is represented by following formula (4). In Formula (4), T represents the heating temperature of a billet, and a unit represents celsius temperature (degreeC).
Formula (4) Sol. Nb = (10 ( -6770 / (T + 273) +2.26 )) / (C + 12/14 × N)
Also, [Nb]] [Sol. [Nb *] = [Sol. Nb],
And [Nb] <[Sol. In the case of Nb], the relationship of [Nb *] = [Nb] is satisfied.
[Nb] represents the Nb content (% by mass), and [Sol. Nb] is Sol. Nb (solid solution Nb) (mass%) is represented. )
　ここで、本開示において、仕上圧延の累積圧下率とは、下記式で表される。
　仕上圧延の累積圧下率＝（（粗圧延後の板厚－仕上圧延後の板厚）／粗圧延後の板厚）×１００
　なお、粗圧延後の板厚は、仕上圧延を開始するときの板厚と同じである。 Here, in the present disclosure, the cumulative rolling reduction of finish rolling is represented by the following equation.
Cumulative rolling reduction of finish rolling = ((plate thickness after rough rolling-plate thickness after finish rolling) / plate thickness after rough rolling) × 100
In addition, the plate thickness after rough rolling is the same as the plate thickness at the time of starting finish rolling.
（冷却工程）
　次に、仕上圧延を施した後の鋼板を、所定の冷却速度で、所定の温度になるまで冷却する。この条件での冷却により、ミクロ組織が細粒化されるとともに、組織形態（前述の全相の面積分率）が制御される。この工程は、アレスト性および強度に影響を及ぼす。 (Cooling process)
Next, the steel plate after finish rolling is cooled to a predetermined temperature at a predetermined cooling rate. Cooling under these conditions refines the microstructure and controls the morphology (area fraction of all phases described above). This process affects arrestability and strength.
　仕上圧延後の鋼板を、鋼板の板厚１／４部での冷却速度が１℃／秒～２０℃／秒で、鋼板の板厚１／４部での温度が０℃～６００℃の温度域になるまで冷却する。つまり、冷却停止温度が、鋼板の板厚１／４部での温度として０℃～６００℃の温度域である。冷却方法は、特に限定されず、例えば、水冷等の方法が挙げられる。板厚１／４部での冷却速度を１℃／秒～２０℃／秒で、鋼板の板厚１／４部での温度が０℃～６００℃の温度域になるまで冷却することで、所定量のフェライト相及び所定量のベイナイト相が得られる。
　なお、冷却速度および冷却停止温度は、板厚１／４部における計算冷却速度および計算冷却停止温度である。また、冷却速度は、冷却開始から冷却停止までの平均冷却速度である。 The steel plate after finish rolling has a cooling rate of 1 ° C / s to 20 ° C / s at a plate thickness of 1/4 of the steel plate, and a temperature of 0 ° C to 600 ° C at a plate thickness of 1/4 of a steel plate Cool down to area. That is, the cooling stop temperature is a temperature range of 0 ° C. to 600 ° C. as a temperature at a thickness of 1/4 of the steel plate. The cooling method is not particularly limited, and examples thereof include methods such as water cooling. By cooling at a cooling rate of 1 ° C / sec to 20 ° C / sec at a plate thickness of 1⁄4 part and cooling to a temperature range of 0 ° C to 600 ° C at a plate thickness of 1⁄4 part A predetermined amount of ferrite phase and a predetermined amount of bainite phase are obtained.
The cooling rate and the cooling stop temperature are the calculated cooling rate and the calculated cooling stop temperature at a quarter of the plate thickness. Moreover, a cooling rate is an average cooling rate from a cooling start to a cooling stop.
　本開示に係る鋼板を得るための好ましい製造方法は、必要に応じて、さらに、冷却工程後の鋼板を、３５０℃～６５０℃の温度域で焼戻しを行う工程（焼戻し工程）を有していてもよい。 The preferable manufacturing method for obtaining the steel plate according to the present disclosure further includes a step (tempering step) of tempering the steel plate after the cooling step in a temperature range of 350 ° C. to 650 ° C., if necessary. It is also good.
（焼戻し工程）
　鋼板の冷却後に、必要に応じて、３５０℃～６５０℃（好ましくは４５０℃～５５０℃）の温度域で焼戻して熱処理を行い、鋼板の強度と靭性を調整してもよい。焼き戻しを行う場合、焼戻しの温度が３５０℃以上であると、ひずみ除去による靭性改善効果が高まる。一方、焼戻しの温度が６５０℃以下とすると、強度低下を抑制することができる。 (Tempering process)
After cooling the steel plate, if necessary, it may be tempered by heat treatment in a temperature range of 350 ° C. to 650 ° C. (preferably 450 ° C. to 550 ° C.) to adjust the strength and toughness of the steel plate. When tempering is performed, if the tempering temperature is 350 ° C. or more, the effect of improving the toughness by strain removal is enhanced. On the other hand, when the tempering temperature is set to 650 ° C. or less, strength reduction can be suppressed.
　なお、本開示に係る鋼板の製造方法は、上述の製造方法に限定されない。鋼板の製造方法が上述以外の製造方法であっても、その鋼板が規定範囲内にあれば、その鋼板は、本開示に係る鋼板の範囲に包含されると見なされる。 In addition, the manufacturing method of the steel plate which concerns on this indication is not limited to the above-mentioned manufacturing method. Even if the manufacturing method of a steel plate is a manufacturing method other than the above-mentioned, if the steel plate is within the specified range, the steel plate is considered to be included in the range of the steel plate according to the present disclosure.
　以下、本開示に係る鋼板及び鋼板の製造方法における好ましい実施形態の一例について、実施例によってさらに詳細に説明する。ただし、下記実施例は本開示に係る鋼板及び鋼板の製造方法を限定する性質のものではない。また、本開示に係る鋼板及び鋼板の製造方法は、前記または後記した趣旨に適合し得る範囲で適用に変更して実施することも可能であり、それらはいずれも本開示の技術範囲に含まれる。 Hereinafter, an example demonstrates in more detail an example of a desirable embodiment in a manufacturing method of a steel plate concerning this indication, and a steel plate. However, the following example is not a thing of the property which limits the manufacturing method of the steel plate which concerns on this indication, and a steel plate. Further, the steel sheet and the method for producing the steel sheet according to the present disclosure can be modified and implemented as long as they can be applied to the purpose described above or later, and all of them are included in the technical scope of the present disclosure. .
＜実施例１＞
　表１に鋼板の化学成分を示す。まず、表２に示す値となるように、脱酸元素添加前の溶存酸素量、及び脱酸元素添加後の溶鋼の最終溶存酸素量を調整し、鋼板の化学成分が表１に示す値となるように、溶鋼の化学成分を調整した。次に、調整後の溶鋼を、表２に示すスループット、圧下勾配、及び圧下量となるように、連続鋳造して鋼片を得た。その後、鋼片に対して、表２に示す加熱条件で加熱し、表２に示す仕上圧延条件および冷却条件で、仕上圧延及び冷却を行い、鋼板を得た。そして、得られた鋼板の一部について、表２に示す温度で焼き戻しを行った。得られた各鋼板の板厚を表３に示す。 Example 1
Table 1 shows the chemical composition of the steel plate. First, the dissolved oxygen amount before the addition of the deoxidizing element and the final dissolved oxygen amount of the molten steel after the addition of the deoxidizing element are adjusted so as to obtain the values shown in Table 2; The chemical composition of the molten steel was adjusted so that Next, the prepared molten steel was continuously cast to obtain a billet so as to have the throughput, the reduction gradient, and the reduction amount shown in Table 2. Thereafter, the steel slab was heated under the heating conditions shown in Table 2, and subjected to finish rolling and cooling under the finish rolling conditions and cooling conditions shown in Table 2 to obtain steel plates. And about the temperature shown in Table 2, it tempered about a part of obtained steel plate. The plate thicknesses of the obtained steel plates are shown in Table 3.
　また、得られた各鋼板について、フェライト相、ベイナイト相、パーライト相、及びマルテンサイト・オーステナイト混合相の面積率、これらの全相の平均粒径（直径）、並びに円相当径（直径）が２μｍ以上の介在物（酸化物、硫化物、及び酸硫化物）の個数密度を、既述の方法に従って測定した。
　さらに、前記板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置でのＭｎ濃度（Ｍｎ（１／４））に対する、前記板厚方向に沿った断面の、鋼板表面から板厚方向の１／２位置でのＭｎ濃度（Ｍｎ（１／２））の比（Ｍｎ（１／２）／Ｍｎ（１／４））を、既述の方法に従って測定した。
　そして、鋼板の引張り強さ（ＴＳ）、降伏応力（ＹＰ）、温度勾配型ＥＳＳＯ試験における試験温度マイナス１０℃での脆性亀裂伝播停止靭性値Ｋｃａ（アレスト靱性値Ｋｃａ－１０℃）、及び溶接部の亀裂先端開口変位試験における試験温度マイナス１０℃での開口変位δｃ（開口変位δｃ－１０℃）を、既述の方法に従って測定した。
　これら特性について測定した結果を表３に示す。 In addition, with respect to each steel plate obtained, the area ratio of ferrite phase, bainite phase, pearlite phase, and martensite-austenite mixed phase, average particle diameter (diameter) of all phases, and equivalent circle diameter (diameter) of 2 μm. The number density of the above inclusions (oxide, sulfide, and acid sulfide) was measured according to the method described above.
Furthermore, from the surface of the steel sheet in the cross section along the thickness direction, the Mn concentration (Mn (1/4) ) at a 1/4 position from the surface of the steel plate to the cross section along the thickness direction The ratio (Mn (1/2) / Mn (1/4) 2) of the Mn concentration (Mn (1/2) ) at the 1/2 position in the thickness direction was measured according to the method described above.
And tensile strength (TS) of steel sheet, yield stress (YP), brittle crack propagation arrest toughness value K ca (arrest toughness value K ca -10 ° C. ) at test temperature minus 10 ° C. in temperature gradient type ESSO test, and The opening displacement δc (opening displacement δc −10 ° C. ) at a test temperature of −10 ° C. in the crack tip opening displacement test of the weld was measured according to the method described above.
The results of measurement of these characteristics are shown in Table 3.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
　表１中、「－」は、意図的に添加していないことを表す。 In Table 1, "-" represents that it was not intentionally added.
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　表２中、焼き戻し欄の「－」は、焼き戻し無しであることを、冷却欄のうち、冷却速度欄の「－」は、冷却速度が１℃／秒（１℃／ｓ）未満であることを、それぞれ表す。
　なお、冷却速度が１℃／秒未満の冷却は、冷却水等を用いない空冷による冷却である。
　冷却水等を用いない空冷であるので、冷却停止温度欄も「－」として表記した。
　詳細には、空冷での冷却の場合（冷却速度欄が「－」の場合）、冷却速度が１℃／秒未満の冷却で、鋼板は最終的には常温（室温：２５℃）まで冷却される。このように、冷却欄は、冷却速度が１℃／秒以上であるもののみ、冷却停止温度欄に停止温度を表記している。 In Table 2, “-” in the tempering column indicates that tempering is not performed, and “-” in the cooling speed column in the cooling column indicates that the cooling rate is less than 1 ° C./s (1 ° C./s) Each represents something.
Note that cooling with a cooling rate of less than 1 ° C./sec is air cooling without using cooling water or the like.
Since it is air cooling which does not use cooling water etc., the cooling stop temperature column was also described as "-".
Specifically, in the case of air cooling (when the cooling rate column is “-”), the steel plate is finally cooled to room temperature (room temperature: 25 ° C.) with a cooling rate of less than 1 ° C./sec. Ru. Thus, in the cooling column, only the cooling rate of 1 ° C./sec or more indicates the stop temperature in the cooling stop temperature column.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　表３中、「α分率」はフェライト相の面積分率を、「Ｂ分率」はベイナイト相の面積分率を、「Ｐ分率」はパーライト相の面積分率を、「ＭＡ分率」はマルテンサイト・オーステナイト混合相の面積分率を、それぞれ表す。
　「平均粒径」は、フェライト相、ベイナイト相、パーライト相、及びマルテンサイト・オーステナイト混合相の全相の平均粒径（直径）を表す。
　「２μｍ以上介在物個数」は、円相当径（直径）が２μｍ以上である、酸化物、硫化物、及び酸硫化物の合計の個数密度を表す。
　「ｔ／２Ｍｎ÷ｔ／４Ｍｎ」は、板厚１／４部でのＭｎ濃度（Ｍｎ（１／４））に対する、板厚１／２部でのＭｎ濃度（Ｍｎ（１／２））の比（Ｍｎ（１／２）／Ｍｎ（１／４））を表す。 In Table 3, “α fraction” indicates the area fraction of ferrite phase, “B fraction” indicates the area fraction of bainite phase, “P fraction” indicates the area fraction of pearlite phase, and “MA fraction” Represents the area fraction of the martensite / austenite mixed phase, respectively.
"Average particle size" represents the average particle size (diameter) of all phases of the ferrite phase, the bainite phase, the pearlite phase, and the martensite-austenite mixed phase.
“Number of inclusions of 2 μm or more” represents the total number density of oxides, sulfides, and oxysulfides having a circle equivalent diameter (diameter) of 2 μm or more.
“T / 2 Mn ÷ t / 4 Mn” is the Mn concentration (Mn (1/2) ) at the plate thickness 1/2 part to the Mn concentration (Mn ( 1/4) ) at the plate thickness 1/4 . It represents the ratio (Mn (1/2) / Mn (1/4) ).
　表３から明らかなように、番号１～番号２１は、板厚が５０ｍｍ～１００ｍｍであり、アレスト靱性値Ｋｃａ－１０℃が、６０００Ｎ／ｍｍ１．５以上であり、かつ、開口変位δｃ－１０℃が、０．１０ｍｍ以上であることが分かる。さらに、番号１～番号２１は、引張強さが５１０ＭＰａ～７２０ＭＰａ、および降伏応力が３９０ＭＰａ～６５０ＭＰａであることがわかる。すなわち、本開示に係る鋼板で規定される範囲内である番号１～番号２１は、板厚が５０ｍｍ～１００ｍｍであっても、優れたアレスト性と優れた継手ＣＴＯＤ特性とを併せ持ち、高強度である鋼板であることがわかる。 Table 3 As is apparent, the number 1 to number 21, the plate thickness is 50 mm-100 mm, arresting toughness value K ca-10 ° C. is, it is 6000 N / mm 1.5 or more, and, the opening displacement .delta.c - It is understood that 10 ° C. is 0.10 mm or more. Further, it can be seen that Nos. 1 to 21 have a tensile strength of 510 MPa to 720 MPa and a yield stress of 390 MPa to 650 MPa. That is, Nos. 1 to 21 within the range defined by the steel plate according to the present disclosure have both excellent arrestability and excellent joint CTOD characteristics even with a plate thickness of 50 mm to 100 mm, and have high strength. It turns out that it is a certain steel plate.
　一方、本開示に係る鋼板で規定される成分範囲を外れる番号３３～番号３９は、いずれの鋼板も、特に継手ＣＴＯＤ特性が劣位であった。また、番号２２、番号２４～番号２７、及び番号２９～番号３２は、本開示に係る鋼板で規定される成分範囲内ではあったが、ミクロ組織が規定範囲を外れる組織であったため、アレスト性および継手ＣＴＯＤ特性の少なくとも一方が劣位であった。さらに、番号２３および番号２８は、本開示に係る鋼板で規定される成分範囲内ではあったが、番号２８は、フェライト相が規定範囲外であり、番号２３はベイナイト相の面積率が０％である。そのため、優れたアレスト性と優れた継手ＣＴＯＤ特性とを併せ持ってはいるが、鋼板の強度（引張強さ及び降伏応力の少なくとも一方）が劣位であった。 On the other hand, for the numbers 33 to 39 outside the component range defined in the steel plate according to the present disclosure, the joint CTOD characteristics were particularly inferior in all steel plates. Moreover, although No. 22, No. 24 to No. 27, and No. 29 to No. 32 were within the component range defined by the steel plate according to the present disclosure, the microstructure was a structure outside the defined range, and therefore arrested And at least one of the joint CTOD characteristics was inferior. Furthermore, although No. 23 and No. 28 were within the component range defined in the steel plate according to the present disclosure, No. 28 is outside the prescribed range of the ferrite phase, and No. 23 is 0% of the area ratio of the bainite phase It is. Therefore, the steel sheet has inferior strength (at least one of tensile strength and yield stress) although it has excellent arrestability and excellent joint CTOD characteristics.
　以上より、本開示に係る鋼板は、より厳格化された規格にも満足し得る、優れたアレスト性と優れた継手ＣＴＯＤ特性とを併せ持ち、高強度である鋼板である。そのため、本開示に係る鋼板によれば、特に、コンテナ船の重要部材（例えば、Ｈａｔｃｈ　Ｓｉｄｅ　Ｃｏａｍｉｎｇ、Ｕｐｐｅｒ　Ｄｅｃｋ）にも、好適に適用され得る。 As described above, the steel plate according to the present disclosure is a steel plate having high strength that has both excellent arrestability and excellent joint CTOD characteristics that can satisfy even more stringent standards. Therefore, according to the steel plate according to the present disclosure, in particular, it can be suitably applied to important members of a container ship (for example, Hatch Side Coating, Upper Deck).

Claims (3)

  1. 　質量％で、
    Ｃ　　：０．０３％～０．１４％、
    Ｓｉ　：０．０１％～０．５０％、
    Ｍｎ　：１．２０％～２．５０％、
    Ｐ　　：０．０３０％以下、
    Ｓ　　：０．０２０％以下、
    Ｎｂ　：０．００３％～０．０５０％、
    Ｔｉ　：０．００３％～０．０５０％、
    Ａｌ　：０．００１％～０．１００％、
    Ｎ　　：０．００１０％～０．００８０％、
    Ｏ　　：０．００５０％以下、
    Ｃａ　：０％～０．０１００％、
    Ｍｇ　：０％～０．０１００％、
    ＲＥＭ：０％～０．０１００％、
    Ｚｒ　：０％～０．０１００％、
    Ｔｅ　：０％～０．０１００％、
    Ｖ　　：０％～０．１５０％
    Ｃｕ　：０％～１．００％、
    Ｎｉ　：０％～２．００％、
    Ｃｒ　：０％～１．００％、
    Ｍｏ　：０％～０．５０％、
    Ｂ　　：０％～０．００５０％、並びに、
    残部　：Ｆｅ及び不純物からなり、
    　下記式（１）で表される炭素当量Ｃｅｑが、０．３０％～０．５５％であり、
    　板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置での金属組織が、面積分率で、フェライト相１０．０％～７５．０％、ベイナイト相１０．０％～９０．０％、パーライト相０％～１５．０％、及びマルテンサイト・オーステナイト混合相０％～１．０％から構成され、全相の電子線後方散乱回折法により測定される平均粒径（直径）が２０μｍ以下であり、
    　前記板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置に含有する、円相当径（直径）が２μｍ以上である、酸化物、硫化物、及び酸硫化物の合計が５０個／ｍｍ２以下であり、
    　前記板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置でのＭｎ濃度（Ｍｎ（１／４））に対する、前記板厚方向に沿った断面の、鋼板表面から板厚方向の１／２位置でのＭｎ濃度（Ｍｎ（１／２））の比（Ｍｎ（１／２）／Ｍｎ（１／４））が、０．９０～１．８０である鋼板。
    式（１）　Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５
    　（ただし、式（１）中のＣ、Ｍｎ、Ｃｒ、Ｍｏ、Ｖ、Ｃｕ、およびＮｉは、鋼板に含まれる各元素の含有量（質量％）を表す。）     In mass%,
    C: 0.03% to 0.14%,
    Si: 0.01% to 0.50%,
    Mn: 1.20% to 2.50%,
    P: 0.030% or less,
    S: 0.020% or less,
    Nb: 0.003% to 0.050%,
    Ti: 0.003% to 0.050%,
    Al: 0.001% to 0.100%,
    N: 0.0010% to 0.0080%,
    O: not more than 0.0050%,
    Ca: 0% to 0.0100%,
    Mg: 0% to 0.0100%,
    REM: 0% to 0.0100%,
    Zr: 0% to 0.0100%,
    Te: 0% to 0.0100%,
    V: 0% to 0.150%
    Cu: 0% to 1.00%,
    Ni: 0% to 2.00%,
    Cr: 0% to 1.00%,
    Mo: 0% to 0.50%,
    B: 0% to 0.0050%, and
    Remainder: It consists of Fe and impurities,
    The carbon equivalent Ceq represented by the following formula (1) is 0.30% to 0.55%,
    The metallographic structure at a quarter position in the plate thickness direction from the surface of the steel plate in the cross section along the plate thickness direction is, by area fraction, 10.0% to 75.0% of ferrite phase, 10.0% to bainite phase Average particle size of the entire phase, which is composed of 90.0%, 0% to 15.0% of pearlite phase, and 0% to 1.0% of martensite / austenite mixed phase, as measured by electron backscattering diffraction of all phases ( Diameter) is less than 20 μm,
    The total of oxides, sulfides, and oxysulfides having a circle equivalent diameter (diameter) of 2 μm or more, which is contained at a quarter position in the plate thickness direction from the surface of the steel plate, of the cross section along the plate thickness direction 50 pcs / mm 2 or less,
    From the steel plate surface to the plate thickness of the cross section along the plate thickness direction with respect to the Mn concentration (Mn (1/4) ) at a quarter position from the steel plate surface to the plate thickness direction A steel sheet in which the ratio (Mn (1/2) / Mn (1/4) ) of the Mn concentration (Mn (1/2) ) at half of the direction is 0.90 to 1.80.
    Formula (1) Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
    (However, C, Mn, Cr, Mo, V, Cu, and Ni in Formula (1) represent the content (mass%) of each element contained in the steel plate.)
  2. 　請求項１に記載の鋼板を製造する方法であって、
    　溶存酸素量を５０ｐｐｍ以下に調整した溶鋼を、最終溶存酸素量が２０ｐｐｍ以下になるように脱酸元素を添加して脱酸処理を行う工程と、
    　前記脱酸処理を経た溶鋼を、１分あたり０．８０トン～５．００トンのスループットで連続鋳造し、前記連続鋳造するときの鋳片の中心固相率が０．５～１．０の範囲であるとき、前記鋳片に対し、鋳造進行方向１ｍあたり１．０ｍｍ～１．５ｍｍの圧下勾配で、圧下量が１ｍｍ～１５ｍｍとなる軽圧下を行い鋼片を得る工程と、
    　前記鋼片を９５０℃～１１５０℃の温度域で加熱し、加熱後の鋼片に粗圧延を行った後、粗圧延後の鋼板に、鋼板の表面温度がＡｒ３点－３０℃～再結晶温度Ｔｒｅｘの温度域で、累積圧下率が５０％～７５％の仕上圧延を行う工程と、
    　仕上圧延後の鋼板を、鋼板の板厚方向に沿った断面の、鋼板表面から板厚方向の１／４位置での温度が０℃～６００℃の温度域になるまで、前記１／４位置での平均冷却速度が１℃／秒～２０℃／秒で冷却する工程と、
    　を有する鋼板の製造方法。     It is a method of manufacturing the steel plate according to claim 1, wherein
    A step of deoxidizing the molten steel whose dissolved oxygen content has been adjusted to 50 ppm or less by adding a deoxidizing element so that the final dissolved oxygen content is 20 ppm or less;
    The deoxidized molten steel is continuously cast at a throughput of 0.80 tons to 5.00 tons per minute, and the central solid phase ratio of the slab when the continuous casting is 0.5 to 1.0 When it is in the range, the slab is subjected to a light reduction with a reduction amount of 1 mm to 15 mm with a reduction gradient of 1.0 mm to 1.5 mm per 1 m of the casting advancing direction to obtain a steel piece;
    The steel slab is heated in a temperature range of 950 ° C. to 1150 ° C., and after rough rolling is performed on the steel slab after heating, the surface temperature of the steel plate is Ar 3 point −30 ° C. to recrystallization on the steel plate after rough rolling Performing finish rolling with a cumulative rolling reduction of 50% to 75% in a temperature range of a temperature T rex ,
    The steel sheet after finish rolling has a cross section along the thickness direction of the steel sheet until the temperature at a quarter position from the surface of the steel sheet in the thickness direction reaches a temperature range of 0 ° C. to 600 ° C. Cooling at an average cooling rate of 1 ° C./s to 20 ° C./s at
    The manufacturing method of the steel plate which has.
  3. 　さらに、冷却工程後の鋼板を、３５０℃～６５０℃の温度域で焼戻しを行う工程を有する請求項２に記載の鋼板の製造方法。 The method for producing a steel plate according to claim 2, further comprising the step of tempering the steel plate after the cooling step in a temperature range of 350 ° C to 650 ° C.
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