WO2022045351A1 - Tôle d'acier, et procédé de fabrication de celle-ci - Google Patents

Tôle d'acier, et procédé de fabrication de celle-ci Download PDF

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Publication number
WO2022045351A1
WO2022045351A1 PCT/JP2021/031920 JP2021031920W WO2022045351A1 WO 2022045351 A1 WO2022045351 A1 WO 2022045351A1 JP 2021031920 W JP2021031920 W JP 2021031920W WO 2022045351 A1 WO2022045351 A1 WO 2022045351A1
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less
steel sheet
rolling
steel
temperature
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PCT/JP2021/031920
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English (en)
Japanese (ja)
Inventor
啓介 中井
大貴 今城
真吾 中村
祥晃 新宅
清孝 中島
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日本製鉄株式会社
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Application filed by 日本製鉄株式会社 filed Critical 日本製鉄株式会社
Priority to JP2022505647A priority Critical patent/JP7127752B2/ja
Priority to CN202180025270.6A priority patent/CN115362276B/zh
Priority to KR1020227033731A priority patent/KR20220147125A/ko
Publication of WO2022045351A1 publication Critical patent/WO2022045351A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B37/00—Control devices or methods specially adapted for metal-rolling mills or the work produced thereby
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite

Definitions

  • the present invention relates to a steel sheet and a method for manufacturing the same.
  • the welded structure is required to have a brittle crack propagation stopping property (hereinafter referred to as "arrest property") in which the brittle crack is stopped by the base metal even if a brittle crack is generated at the welded joint. Be done.
  • arrest property a brittle crack propagation stopping property
  • An object of the present invention is to solve the above-mentioned problems and to provide a steel sheet having high strength and excellent low temperature toughness, fracture toughness and arrest property, and a method for producing the same.
  • the gist of the present invention is the following steel sheet and its manufacturing method.
  • the chemical composition of the steel sheet is mass%.
  • the metallographic structure at a position 1/4 t from the surface of the steel sheet is formed.
  • In% area it contains more than 80% bainite and The average length of the bainite ferrite constituting the bainite in the major axis direction is 10 ⁇ m or less.
  • the average length of the former austenite grains at a position 1 / 4t from the surface of the steel sheet in the thickness direction is 20 ⁇ m or less, and the average aspect ratio is 2. .5 or more.
  • the area ratio of the region where the ⁇ 110 ⁇ surface forms an angle within 15 ° with respect to the vertical surface is 30.
  • the area ratio of the region where the ⁇ 100 ⁇ plane forms an angle within 15 ° with respect to the vertical plane is 10 to 40%.
  • the area ratio of the region where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the vertical plane is 40 to 70%.
  • the chemical composition is, instead of a part of the Fe, by mass%.
  • Ti 0.050% or less, Cu: 1.50% or less, Ni: 2.50% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 0.150% or less, and B: 0.0050% or less, It contains at least one selected from the group consisting of The steel sheet according to (1) above.
  • the chemical composition is, instead of a part of the Fe, by mass%.
  • Mg 0.0100% or less
  • Ca 0.0100% or less
  • REM 0.0100% or less
  • It contains at least one selected from the group consisting of The steel sheet according to (1) or (2) above.
  • the chemical composition is, instead of a part of the Fe, by mass%.
  • the chemical composition is, instead of a part of the Fe, by mass%.
  • W 1.00% or less
  • Sn 0.50% or less, It contains at least one selected from the group consisting of The steel sheet according to any one of (1) to (4) above.
  • a heating step, a rough rolling step, a primary accelerated cooling step, a finish rolling step, and a secondary accelerated cooling step are sequentially performed on a steel piece having the chemical composition according to any one of (1) to (5) above.
  • the steel pieces are heated to a heating temperature of 950 to 1080 ° C.
  • the rough rolling step was carried out in a range where the surface temperature of the steel pieces was Trex or more and 1050 ° C. or less.
  • the cumulative rolling reduction in the rough rolling step is 10 to 75%.
  • cooling is started in the range where the surface temperature of the steel piece is Ar 3 or higher, cooling is stopped in the range of 500 ° C. or higher and Ar 3-30 ° C. or lower, and the average cooling rate during that period is 35. Water-cooled under the condition of ⁇ 100 ° C / sec.
  • the finish rolling step is carried out in a range where the surface temperature of the steel piece is less than Trex and the temperature at the center of the thickness of the steel piece is Ar 3 or more and less than Trex .
  • the number of rolling passes n in the finish rolling step is 4 to 15, the average value of the rolling shape ratio mj obtained by the following formula ( i ) is 0.5 to 1.0, and the cumulative rolling reduction rate is 65 to 90%.
  • the time between passes should be 15 seconds or less.
  • the time from the completion of the finish rolling step to the start of cooling in the secondary accelerated cooling step is set to 50 seconds or less.
  • cooling is performed at 0 to 550 ° C. under the conditions that the cooling start temperature is Trex -10 ° C. or lower and the average cooling rate from the cooling start to the cooling end is 5 to 50 ° C./sec. Water cool to stop temperature, Steel sheet manufacturing method.
  • m j 2 (R (H j-1 -H j )) 1/2 / (H j-1 + H j ) ...
  • j in the above formula is a natural number from 1 to n (where n is the number of rolling passes), m j is the rolling shape ratio of the jth pass, R is the roll radius (mm), and H j-1 is j.
  • the plate thickness (mm) after -1 pass and H j represent the plate thickness (mm) after j pass.
  • Ar 3 is obtained by the following formula (ii)
  • Trex is obtained by the following formula (iii).
  • the element symbol in the following formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.
  • T in the above formula represents the heating temperature (° C.) of the steel piece in the heating step.
  • a tempering step of heating to a temperature range of 350 to 650 ° C. is further performed.
  • the present inventors first investigated a method for achieving both high strength and improvement of low temperature toughness and fracture toughness. As a result, the strength is increased by using bainite as the main component of the metal structure, and in addition to the miniaturization and flattening of the bainite structure, the bainite ferrite constituting the bainite is refined not only to have low temperature toughness. It was found that the decrease in fracture toughness can be suppressed.
  • C 0.040 to 0.160% C is contained in an amount of 0.040% or more in order to secure the strength of the steel sheet.
  • the C content is 0.040% or more, preferably 0.050% or more or more than 0.050%, more preferably 0.060% or more or more than 0.075%.
  • the C content is 0.160% or less, preferably 0.140% or less, and more preferably 0.120% or less.
  • Si 0.01-0.50% Since Si is effective as a deoxidizing element and a strengthening element, it is contained in an amount of 0.01% or more. On the other hand, if the Si content exceeds 0.50%, the low temperature toughness and the fracture toughness are significantly deteriorated, so the Si content is set to 0.50% or less. Therefore, the Si content is 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. The Si content is 0.50% or less, preferably 0.40% or less, more preferably 0.35% or less, still more preferably 0.30% or less.
  • Mn 0.70 to 2.50% Mn is contained in an amount of 0.70% or more in order to economically secure the strength of the steel sheet.
  • the Mn content is set to 2.50% or less. .. Therefore, the Mn content is 0.70% or more, preferably 0.90% or more, and more preferably 1.20% or more.
  • the Mn content is 2.50% or less, preferably 2.00% or less, more preferably 1.80% or less, still more preferably 1.60% or less.
  • P 0.030% or less
  • P is an element present in steel as an impurity.
  • the content of P is 0.030% or less. It is preferably 0.020% or less, more preferably 0.015% or less.
  • the lower limit is 0%, but the P content may be 0.0001% or more in consideration of the cost for reducing the P content.
  • S 0.020% or less S is an element present in steel as an impurity.
  • the S content exceeds 0.020%, a large amount of MnS stretched in the central segregation portion is generated, and the low temperature toughness, fracture toughness and ductility deteriorate. Therefore, the S content is set to 0.020% or less. It is preferably 0.010% or less. The lower the S content is, the more preferable it is, so the lower limit is not particularly specified, but the S content may be 0.0001% or more from the viewpoint of manufacturing cost.
  • Al 0.001 to 0.100%
  • Al is generally an element positively contained as a deoxidizing element, and the Al content is 0.001% or more.
  • the Al content is 0.100% or less, preferably 0.050% or less.
  • N 0.0010 to 0.0080% Since N has the effect of forming a Ti nitride and suppressing an increase in the austenite particle size when the steel piece is heated, it is contained in an amount of 0.0010% or more. However, if the N content exceeds 0.0080%, the steel sheet becomes embrittlement, so the N content is set to 0.0080% or less. Therefore, the N content is 0.0010% or more, preferably 0.0015% or more, and more preferably 0.0020% or more. The N content is 0.0080% or less, preferably 0.0065% or less, and more preferably 0.0060% or less.
  • Nb 0.003 to 0.050% Nb can improve the strength and toughness of the steel sheet. Further, in order to obtain a predetermined microstructure, rolling in the unrecrystallized austenite region is required, but Nb is an effective element for expanding the unrecrystallized temperature region, and raises the rolling temperature. It also contributes to productivity improvement. In order to obtain this effect, it is contained in an amount of 0.003% or more. However, if the Nb content exceeds 0.050%, the low temperature toughness, fracture toughness and weldability deteriorate, so the Nb content is set to 0.050% or less. Therefore, the Nb content is 0.003% or more, preferably 0.005% or more, and more preferably 0.008% or more. The Nb content is 0.050% or less, preferably 0.025% or less, and more preferably 0.018% or less.
  • At least one selected from the group consisting of Ti, Cu, Ni, Cr, Mo, V and B for the purpose of improving the strength is further selected. It may be contained in the range shown below. The reason for limiting each element will be described.
  • Ti 0.050% or less Ti has an effect of improving the strength and toughness of the steel sheet, and may be contained if necessary. However, if Ti is excessively contained, the welded portion is hardened and the toughness is significantly deteriorated. Therefore, the Ti content is 0.050% or less, preferably 0.035% or less, and more preferably 0.020% or less. When the above effect is to be obtained more reliably, the Ti content is preferably 0.003% or more, more preferably 0.006% or more, still more preferably 0.010% or more.
  • Cu 1.50% or less Cu has the effect of improving the strength and toughness of the steel sheet, and may be contained as necessary. However, if Cu is contained in an excessive amount, the performance is not improved in proportion to the increase in alloy cost, but rather it may cause surface cracking. Therefore, the Cu content is 1.50% or less, preferably 1.20% or less, and more preferably 1.00% or less. When the above effect is to be obtained more reliably, the Cu content is preferably 0.005% or more, more preferably 0.010% or more, still more preferably 0.050% or more.
  • Ni 2.50% or less
  • Ni is an element having an effect of improving the strength of the steel sheet, and may be contained as necessary. Further, Ni is an element having an effect of increasing the toughness of the steel matrix (fabric) in the solid solution state. However, if Ni is excessively contained, the low temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the Ni content is 2.50% or less, preferably 1.00% or less, more preferably 0.50% or less, still more preferably 0.30% or less. When the above effect is to be obtained more reliably, the Ni content is preferably 0.005% or more, more preferably 0.010% or more, still more preferably 0.050% or more.
  • Cr 1.00% or less Cr is an element having an effect of improving the strength of the steel sheet, and may be contained as necessary. However, if Cr is excessively contained, low temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the Cr content is 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less, still more preferably 0.30% or less. When the above effect is to be obtained more reliably, the Cr content is preferably 0.005% or more, more preferably 0.010% or more, still more preferably 0.050% or more.
  • Mo 1.00% or less Mo is an element having an effect of improving the strength of the steel sheet, and may be contained as necessary. However, if Mo is contained in an excessive amount, low temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the Mo content is 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less, still more preferably 0.30% or less. When the above effect is to be obtained more reliably, the Mo content is preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.010% or more.
  • V 0.150% or less Since V is an element having an effect of improving the strength of the steel sheet, it may be contained if necessary. However, if V is excessively contained, low temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the V content is 0.150% or less, preferably 0.100% or less, more preferably 0.070% or less, still more preferably 0.050% or less. When the above effect is to be obtained more reliably, the V content is preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.010% or more.
  • B 0.0050% or less
  • B is an element that enhances hardenability and contributes to improving the strength of the steel sheet, and may be contained as necessary. However, if B is contained in an excessive amount, the low temperature toughness and the fracture toughness are lowered. Therefore, the B content is 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less. When the above effect is to be obtained more reliably, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.
  • At least one selected from the group consisting of Mg, Ca and REM is further contained in the range shown below for the purpose of controlling inclusions. You may. The reason for limiting each element will be described.
  • Mg 0.0100% or less
  • Mg is a deoxidizing element, which suppresses the formation of coarse inclusions by forming sulfides and suppresses the formation of harmful inclusions by forming fine oxides. It is an element that does. Therefore, it may be contained as needed. However, if Mg is excessively contained, coarse oxides, sulfides, and acid sulfides are likely to be formed, and low temperature toughness and fracture toughness are deteriorated. Therefore, the Mg content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. When the above effect is to be obtained more reliably, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.
  • Ca 0.0100% or less
  • Ca is a deoxidizing element, which suppresses the formation of coarse inclusions by forming sulfides and suppresses the formation of harmful inclusions by forming fine oxides. It is an element to be used. Therefore, it may be contained as needed. However, if Ca is excessively contained, coarse oxides, sulfides, and acid sulfides are likely to be formed, and low temperature toughness and fracture toughness are deteriorated. Therefore, the Ca content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. When the above effect is to be obtained more reliably, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.
  • REM 0.0100% or less REM is a deoxidizing element, which suppresses the formation of coarse inclusions by forming sulfides and suppresses the formation of harmful inclusions by forming fine oxides. It is an element that does. Therefore, it may be contained as needed. However, if REM is excessively contained, coarse oxides, sulfides, and acid sulfides are likely to be formed, and low temperature toughness and fracture toughness are deteriorated. Therefore, the REM content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. When the above effect is desired to be obtained more reliably, the REM content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.
  • REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the REM means the total content of these elements.
  • Lanthanoids are industrially added in the form of misch metal.
  • At least one selected from the group consisting of Zr and Te is further contained in the range shown below for the purpose of miniaturizing the metal structure. May be good. The reason for limiting each element will be described.
  • Zr 0.0100% or less
  • Zr is an element that contributes to the improvement of toughness by miniaturizing the structure of the steel sheet.
  • Zr also functions as a deoxidizing element. Therefore, it may be contained as needed.
  • excessive Zr content reduces low temperature toughness and fracture toughness. Therefore, the Zr content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less.
  • the Zr content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.
  • Te 0.0100% or less Te is an element that contributes to the improvement of toughness by refining the structure of the steel sheet, and may be contained as necessary. However, even if Te is excessively contained, the above effect is saturated. Therefore, the Te content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. When the above effect is to be obtained more reliably, the Te content is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.
  • At least one selected from the group consisting of W and Sn may be contained in the range shown below for the purpose of improving corrosion resistance. .. The reason for limiting each element will be described.
  • W 1.00% or less W is an element that dissolves and adsorbs to rust in the form of oxygen acid ion WO 4- , suppresses the permeation of chloride ions in the rust layer, and improves corrosion resistance, so it is necessary. It may be contained according to the above. However, even if W is excessively contained, not only the above effect is saturated, but also low temperature toughness and fracture toughness may be lowered. Therefore, the W content is 1.00% or less, preferably 0.75% or less. When the above effect is to be obtained more reliably, the W content is preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.010% or more.
  • Sn 0.50% or less
  • Sn is an element that dissolves as Sn 2+ and has an action of suppressing corrosion by an inhibitory action in an acidic chloride solution.
  • Sn has an effect of suppressing the anode melting reaction of steel and improving corrosion resistance. Therefore, it may be contained as needed.
  • the Sn content is 0.50% or less, preferably 0.30% or less.
  • the Sn content is preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.010% or more.
  • the balance is Fe and impurities.
  • impurity is a component mixed with raw materials such as ore and scrap and various factors in the manufacturing process when the steel sheet is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something. O can also be mixed in the steel sheet as an impurity, but it is permissible if the O content is 0.0040% or less.
  • the metal structure is mainly bainite. Specifically, by setting the area ratio of bainite at the 1 / 4t position on the C cross section to 80% or more, it is possible to secure the strength of the steel sheet.
  • the area ratio of bainite is preferably 90% or more. It is not necessary to set an upper limit on the area ratio of bainite, that is, it may be bainite single phase.
  • Ferrite, pearlite, and martensite / austenite mixed phase may be mixed as the residual structure, but it is permissible if the total area ratio of these is 20% or less.
  • the total area ratio is preferably 10% or less. It is preferable that the total area ratio of these is small, and the lower limit is not particularly limited.
  • the total area ratio may be 0%. Further, it may be more than 0% or 1% or more.
  • the bainite structure is finely and flattened, and the bainite ferrite is further refined to achieve both the strength of the steel sheet, low temperature toughness, and fracture toughness. It is possible to improve the toughness. Specifically, the bainite organization must meet the following provisions.
  • Average length of bainitic ferrite 10 ⁇ m or less At the 1 / 4t position in the C cross section, the average length of bainite ferrite constituting bainite in the major axis direction shall be 10 ⁇ m or less.
  • the average length of bainitic ferrite is preferably 8 ⁇ m or less.
  • Average length in the thickness direction of the old austenite grains 20 ⁇ m or less
  • Average aspect ratio of the old austenite grains 2.5 or more
  • the miniaturization of the bainite structure controls the heating temperature before hot rolling to a low level and does not recrystallize. This can be achieved by performing finish rolling at a high-pressure reduction ratio in the region. That is, the old austenite grains of bainite have a shape elongated in the rolling direction. Therefore, at the 1 / 4t position in the L cross section, the average length of the old austenite grains in the thickness direction is 20 ⁇ m or less, and the average aspect ratio is 2.5 or more.
  • the average length of the old austenite grains in the thickness direction is preferably 15 ⁇ m or less. Further, the average aspect ratio of the old austenite grains is preferably more than 2.5, more preferably 4.0 or more.
  • the area ratio of the metal structure is calculated as follows. First, a sample is taken from the steel plate so that the 1 / 4t position on the C cross section is the observation surface. Then, the observation surface is night-game-etched, and after etching, eight fields of view are photographed at a magnification of 500 using an optical microscope. Then, image analysis is performed on the obtained tissue photograph, and the area ratio of each is obtained by using ferrite as the one that looks white and pearlite as the one that looks black.
  • the night-game-etched part is repeller-etched, the part that looks gray by night-game etching is image-analyzed, and the area ratio is obtained with the part that looks white as the MA phase.
  • the average length of bainite ferrite and the area ratio of bainite are calculated by KAM (Kernel Average Missionation) analysis using EBSD (Electron Back Scatter Diffraction).
  • KAM Kernel Average Missionation
  • EBSD Electro Back Scatter Diffraction
  • the region where the local orientation difference exceeds 1.0 ° is bainitic ferrite.
  • bainitic ferrite having a length in the major axis direction of 1 ⁇ m or more is targeted.
  • the area ratio of bainite is the sum of the area ratios of bainite ferrite.
  • the average length and aspect ratio of the old austenite grains in the thickness direction are measured according to JIS G 0551: 2013.
  • a sample is taken from the steel plate so that the 1 / 4t position on the L cross section is the observation surface.
  • the observation surface is mirror-polished, it is corroded by the Behcet-Beaujard method using a saturated aqueous solution of picric acid.
  • the grains that appear black due to corrosion are called old austenite grains.
  • the observation surface on which the old austenite grains are exposed is observed with an optical microscope, and a field of view having an area of 0.05 mm 2 or more is photographed with 8 fields or more (total 0.40 mm 2 or more). Then, the thickness of the old austenite grains is measured by a cutting method based on the tissue photograph taken by an optical microscope, and the average value thereof is taken as the average length in the thickness direction of the old austenite grains. In the measurement, the old austenite grains having a length of 1 ⁇ m or more in the thickness direction are targeted.
  • the maximum length in the major axis direction and the maximum length in the minor axis direction orthogonal to the major axis direction are measured for each old austenite grain, and the ratio (maximum length / short axis in the major axis direction) is measured. Axis maximum length) is calculated. Then, the average value is taken as the average aspect ratio of the old austenite grains.
  • finish rolling is performed in the unrecrystallized region at a high pressure reduction rate, the old austenite grains show a shape extended in the rolling direction, so the major axis direction is the rolling direction and the minor axis direction is the plate thickness direction ( So-called ND direction).
  • External stress is the stress applied externally to the steel structure. Brittle cracks often occur and propagate in the direction perpendicular to the highest external stress. Therefore, here, the highest stress applied externally to the steel structure is defined as the external stress. Generally, the external stress is applied almost parallel to the rolling direction of the steel sheet. Therefore, a surface perpendicular to the external stress can be treated as a surface perpendicular to the rolling direction of the steel sheet.
  • the area ratio of the region where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section at the 1 / 2t position of the C cross section is set to 40 to 70%, the brittleness near the 1 / 2t position will be reduced.
  • the driving force of crack propagation can be reduced because the crack propagates at an angle instead of propagating straight.
  • the area ratio of the region where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section at the 1 / 2t position of the C cross section is less than 40%, the effect of inclining and propagating the crack cannot be obtained. ..
  • the area ratio exceeds 70%, the arrest property is deteriorated by propagating while the crack is inclined without receiving the resistance at the 1 / 4t position described later.
  • the area ratio is preferably 45% or more, preferably 65% or less, and more preferably 60% or less.
  • the cracks will propagate while being inclined, and the effect of improving the arrest property cannot be sufficiently exhibited. Therefore, at the 1 / 4t position of the C cross section, in order to propagate the crack straight, the area where the ⁇ 100 ⁇ plane forms an angle within 15 ° with respect to the C cross section is 10 to 40% in area ratio. To. Thereby, it is possible to suppress the propagation of the inclined crack propagation at the 1 / 2t position to the plate thickness portion other than the 1 / 2t position.
  • the area ratio of the region where the ⁇ 100 ⁇ plane forms an angle within 15 ° with respect to the C cross section at the 1 / 4t position of the C cross section is less than 10%, the effect of propagating the crack straight cannot be obtained.
  • the area ratio exceeds 40%, the crack propagation at the 1 / 4t position becomes dominant rather than the 1 / 2t position, and the crack propagates straight, so that the arrest property is lowered.
  • the area ratio is preferably 13% or more, and more preferably 15% or more.
  • the area ratio is preferably 37% or less, more preferably 35% or less.
  • the area where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section at the 1 / 10t position of the C cross section is the area ratio. If it is set to 30 to 60%, it is possible to suppress the propagation of straight cracks at the 1 / 4t position to the vicinity of the surface layer.
  • the area ratio of the region where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section at the 1 / 10t position of the C cross section is less than 30%, the effect of inclining and propagating the crack cannot be obtained.
  • the area ratio exceeds 60%, the arrest property is deteriorated by propagating while the crack is inclined without receiving resistance at the 1 / 4t position.
  • the area ratio is preferably 35% or more, preferably 55% or less, and more preferably 50% or less.
  • the texture is measured by the electron backscatter diffraction (EBSD) method.
  • EBSD electron backscatter diffraction
  • the EBSD method measures crystal orientation information at 1 ⁇ m pitch for a total of 100,000 points at the 1 / 2t position and 1 / 10t position of the C cross section. From this, the ⁇ 110 ⁇ plane of each measurement point is known, and the number of measurement points where the angle formed by the ⁇ 110 ⁇ plane of each measurement point and the C cross section, that is, the angle formed by the normal of each plane is within 15 ° is obtained. , The area ratio is obtained by dividing by the total number of measurements (100,000 points) measured by the EBSD method.
  • crystal orientation information is measured at a pitch of 1 ⁇ m at a position of 1 / 4t of the C cross section for a total of 100,000 points. From this, the ⁇ 100 ⁇ plane of each measurement point is known, and the number of measurement points where the angle formed by the ⁇ 100 ⁇ plane of each measurement point and the C cross section, that is, the angle formed by the normal of each plane is within 15 ° is obtained. , The area ratio is obtained by dividing by the total number of measurements (100,000 points) measured by the EBSD method.
  • the mechanical properties of the steel sheet according to the present invention are not particularly limited, but the steel sheet according to the present invention has high strength and is excellent in low temperature toughness, fracture toughness and arrest property. Specifically, it is preferable that the yield stress (YS) is 460 to 860 MPa and the tensile strength (TS) is 570 to 980 MPa. Further, it is preferable that the fracture surface transition temperature (vTrs), which is an index of low temperature toughness, is ⁇ 60 ° C. or lower. Further, it is preferable that the Crack Tip Opening Displacement (CTOD) value at ⁇ 10 ° C., which is an index of fracture toughness, is 0.50 mm or more.
  • YS yield stress
  • TS tensile strength
  • vTrs fracture surface transition temperature
  • CTOD Crack Tip Opening Displacement
  • the tensile strength (TS) and yield stress (YS) are measured using a No. 1B tensile test piece collected from the center of the plate thickness in the direction perpendicular to the rolling direction based on JIS Z 2241: 2011. Specifically, the yield stress (YS) is the proof stress of the permanent elongation method at 0.2% permanent elongation.
  • the evaluation of the fracture surface transition temperature (vTrs) is based on JIS Z 2242: 2005, and the test piece is a V-notch test piece and is collected so as to include the 1 / 4t position of the steel plate. Further, according to ISO 15653: 2018, a CTOD test piece having the total thickness in the plate thickness direction of the base metal as the notch position of 3-point bending is collected, and the CTOD value at ⁇ 10 ° C. is measured.
  • the brittle crack propagation stop toughness value Kca (hereinafter referred to as “arest toughness value Kca -10 ° C ”) at a test temperature of ⁇ 10 ° C. is 6000 N / mm 1.5 or more. It is preferably 8000 N / mm 1.5 or more, and more preferably 8000 N / mm 1.5 or more. By satisfying this characteristic, the steel sheet has excellent arrest property.
  • Arrest toughness value Kca -10 ° C is NK Ship Class Association Steel Ship Regulation Inspection Procedure K Edition Annex K3.12.2-1. Measurements are performed in accordance with (2016) "Inspection Guidelines for Temperature Gradient ESSO Test and Temperature Gradient Double Tensile Test".
  • the non-ductile transition temperature (hereinafter referred to as “NDT temperature”) in the NRL drop test is preferably ⁇ 100 ° C. or lower, and more preferably ⁇ 110 ° C. or lower. By satisfying this characteristic, the steel sheet has excellent arrest property.
  • the NDT temperature is determined by conducting a test in accordance with the NRL drop weight test method specified in ASTM E208-06.
  • the NRL drop test method will be described in detail.
  • a type P3 test piece specified in ASTM E208 is collected so as to include the outermost surface of the steel plate.
  • the type P3 test piece is a test piece having a length of 130 mm, a width of 50 mm, and a thickness of 16 mm. At this time, the sample is collected so that the thickness direction of the test piece coincides with the plate thickness direction of the steel sheet and the longitudinal direction of the test piece coincides with the rolling direction of the steel sheet.
  • a weld bead extending in a direction parallel to the longitudinal direction of the test piece is formed on the outermost surface of the steel plate perpendicular to the thickness direction of the test piece.
  • the welding material having low toughness specified in ASTM E208 is used.
  • the length of the weld bead is adjusted to be in the range of 60 to 70 mm and the width is adjusted to be in the range of 12 to 16 mm.
  • a notch parallel to the width direction of the test piece is formed on the weld bead. At this time, the width of the notch is set to 1.5 mm or less, and the distance between the groove bottom of the notch and the test piece is adjusted to be in the range of 1.8 to 2.0 mm.
  • the impact bending load due to the drop weight is applied to the surface opposite to the surface on which the weld bead is formed.
  • Break with crack propagation
  • No Break without crack propagation
  • the above drop test is performed using two test pieces, for example, starting from the condition of -100 ° C and changing the test temperature at 5 ° C intervals (in the case of No Break, the temperature drops by 5 ° C, Break's (In the case of an increase of 5 ° C.), the temperature 5 ° C. lower than the lowest test temperature at which No Break was obtained for both of the two test pieces is defined as the non-ductile transition temperature.
  • the thickness of the steel plate according to the present invention is not particularly limited, but when used as a welded structure, the thickness is preferably 10 to 70 mm, preferably 20 to 60 mm. Is more preferable. Further, the effect of improving the low temperature toughness and the fracture toughness in the present invention is remarkably exhibited when the thickness is less than 50 mm.
  • (E) Method for manufacturing a steel sheet The manufacturing conditions for the steel sheet according to the present invention are not particularly limited. For example, for a steel piece having the above-mentioned chemical composition, a heating step, a rough rolling step, and a primary acceleration are performed under the following conditions. It can be manufactured by sequentially performing a cooling step, a finish rolling step, and a secondary accelerated cooling step. Each process will be described.
  • the heating step is a step that contributes to the microstructure control of the austenite phase by heating the steel pieces.
  • the above steel pieces are heated to a heating temperature of 950 to 1080 ° C.
  • the heating step may be performed in a heating furnace.
  • heating the steel pieces to 950 to 1080 ° C. means heating the steel pieces so that the average temperature of the total thickness of the steel pieces when extracted from the heating furnace is in the range of 950 to 1080 ° C., and is described in the present specification.
  • the average temperature of the total thickness of the steel pieces is referred to as the heating temperature of the steel pieces. Further, the total thickness average temperature can be calculated from the temperature in the heating furnace, the heating time, and the surface temperature of the steel piece.
  • the heating temperature is less than 950 ° C., austeniticization becomes insufficient and hardenability is lowered due to the miniaturization of austenite grains, so that it is difficult to obtain a thick steel sheet and high strength steel sheet. Further, the miniaturization of the austenite grains promotes recrystallization during finish rolling, so that the aspect ratio of the old austenite grains is lowered. Further, when the heating temperature exceeds 1080 ° C., the austenite grains become coarse and it becomes difficult to make the bainite structure finer in the final structure.
  • the preferred heating temperature range is 1000-1050 ° C.
  • the rough rolling step is carried out in the range where the surface temperature of the steel pieces is Trex or more and 1050 ° C. or less. That is, the rough rolling is started when the surface temperature of the steel pieces is Trex or more and 1050 ° C. or less, and the rough rolling is finished when the surface temperature of the steel pieces is Trex or more and 1050 ° C. or less.
  • the rough rolling step is carried out in the range of 1050 ° C. or lower.
  • the austenite becomes coarse and the surface layer is excessively quenched by the primary acceleration cooling step described later and becomes excessively hard. Further, in the finish rolling process described later, sufficient strain cannot be applied to the surface layer, while the strain is concentrated inside and introduced. As a result, at the 1 / 10t position of the C cross section, the area ratio of the region where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section is less than 30%.
  • the surface temperature at the end of rough rolling may be higher than the surface temperature at the start of rough rolling. It is considered that this is due to the effect of processing heat generation due to rough rolling and the effect of heat transfer in the plate thickness direction of the steel piece due to the internal temperature being higher than the surface temperature.
  • the cumulative rolling reduction in rough rolling shall be in the range of 10 to 75%.
  • the cumulative rolling reduction in rough rolling is a value obtained by subtracting the plate thickness after the end of rough rolling from the plate thickness at the start of rough rolling and dividing by the plate thickness at the start of rough rolling. If the cumulative rolling reduction during rough rolling is less than 10%, it is difficult to make the austenite finer by recrystallization, and porosity may remain to cause internal cracking, resulting in deterioration of ductility and toughness. In addition, when the cumulative rolling reduction rate exceeds 75%, the austenite grains become excessively fine, and recrystallization during finish rolling is promoted, so that the aspect ratio of the old austenite grains decreases and the number of passes increases. As a result, productivity decreases.
  • the preferred cumulative reduction rate is 30-60%.
  • the steel piece after rough rolling is referred to as a steel plate.
  • (C) Primary accelerated cooling step In the primary accelerated cooling step, the steel sheet after rough rolling is water-cooled. In the primary accelerated cooling step, cooling is started in the range where the surface temperature of the steel sheet is Ar 3 or higher, cooling is stopped in the range of 500 ° C or higher and Ar 3-30 ° C or lower, and the average cooling rate during that period is 35 to 100 ° C. Cool with water under the condition of / sec.
  • the area ratio of the region where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section becomes excessive. Therefore, at the 1 / 4t position of the C cross section, the area ratio of the region where the ⁇ 100 ⁇ plane forms an angle within 15 ° with respect to the C cross section is less than 10%.
  • the internal temperature becomes Ar 3 or more, and it is possible to prevent ferrite transformation to the inside of the steel sheet.
  • the area ratio of the region where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section at the 1 / 10t position of the C cross section is 60% or less. Can be.
  • the finish rolling step is carried out in the range where the surface temperature of the steel sheet is less than Trex and the temperature at the center of the thickness of the steel sheet is Ar 3 or more and less than Trex . That is, after the completion of the primary accelerated cooling step, finish rolling is started in a state where the surface temperature of the steel sheet is less than Trex and the temperature at the center of the thickness is Ar 3 or more and less than Trex , and the surface temperature of the steel sheet is high. Is less than Trex , and the temperature at the center of the thickness is Ar 3 or more and less than Trex , and the finish rolling is finished.
  • the temperature at the center of the thickness can be calculated by considering the ambient temperature, time, specific heat of the steel sheet, density, thermal conductivity, processing calorific value, transformation calorific value, and heat removal from contact with the roll. Is.
  • the finish rolling By performing the finish rolling in the range of less than Trex , it becomes possible to impart strain to the austenite grains without recrystallization. This makes it possible to miniaturize bainite in the final structure.
  • the finishing temperature is set in the range where the surface temperature is Trex or higher, recrystallization is promoted and the aspect ratio of the old austenite grains is lowered.
  • the area ratio of the surface where the processed ferrite is not generated at the 1 / 10t position and the ⁇ 110 ⁇ surface forms an angle within 15 ° with respect to the surface perpendicular to the rolling direction of the steel sheet can be set to 30% or more. It disappears.
  • the finish rolling is performed in the range where the temperature at the center of the thickness is less than Ar 3 , processed ferrite is generated, and not only the structure mainly composed of bainite cannot be obtained in the final structure, but also the ⁇ 110 ⁇ surface.
  • the area ratio of the surface forming an angle within 15 ° with respect to the C cross section cannot be set to 40% or more.
  • the processed ferrite since the processed ferrite may be generated in the surface layer portion, it is not necessary to set a lower limit for the surface temperature in the finish rolling.
  • the number of rolling passes n in finish rolling is 4 to 15, and the average value of the rolling shape ratio mj in each pass obtained by the following formula ( i ) is 0.5 to 1.0.
  • m j 2 (R (H j-1 -H j )) 1/2 / (H j-1 + H j ) ... (i)
  • j in the above formula is a natural number from 1 to n (where n is the number of rolling passes), m j is the rolling shape ratio of the jth pass, R is the roll radius (mm), and H j-1 is j.
  • the plate thickness (mm) after -1 pass and H j represent the plate thickness (mm) after j pass.
  • the number of rolling passes n is less than 4, it is difficult to set the average value of the rolling shape ratio m j to 1.0 or less. On the other hand, when the number of rolling passes n exceeds 15, the productivity decreases.
  • the preferred number of rolling passes n is 5 to 13 passes.
  • the rolled shape ratio is an index showing what kind of strain component is applied to the steel sheet by rolling.
  • the rolling shape ratio is small, a large amount of shear strain component is applied, and when it is large, a large amount of compressive strain component is applied. That is, the strain component changes by changing the rolled shape ratio.
  • the change in the strain component has a great influence on the formation of the texture at the 1 / 4t position. Therefore, the average value of the rolled shape ratio mj is set to 0.5 to 1.0.
  • the average value of the rolling shape ratio mj is less than 0.5, the shear strain of rolling becomes dominant at the 1 / 4t position. As a result, the ⁇ 100 ⁇ texture develops, and it becomes difficult to reduce the area ratio of the surface whose ⁇ 100 ⁇ surface forms an angle within 15 ° with respect to the C cross section to 40% or less.
  • the average value of the rolling shape ratio mj exceeds 1.0, the compressive strain of rolling becomes dominant at the 1 / 4t position.
  • the range of the average value of the preferable rolled shape ratio mj is 0.6 to 0.9.
  • the cumulative rolling reduction in finish rolling shall be in the range of 65 to 90%.
  • the cumulative rolling reduction in finish rolling is a value obtained by subtracting the plate thickness after the end of finish rolling from the plate thickness at the start of finish rolling (after the end of rough rolling) and dividing by the plate thickness at the start of finish rolling.
  • the area ratio of the region where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section at the 1 / 10t position of the C cross section becomes more than 60%. Further, when the cumulative reduction rate exceeds 90%, recrystallization is promoted, the aspect ratio of the old austenite grains is lowered, the number of passes is increased, and the productivity is lowered. Further, the area ratio of the region where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section at the 1 / 10t position and the 1 / 2t position of the C cross section decreases.
  • the preferred cumulative reduction rate is 70-80%.
  • the time between passes in finish rolling shall be 15 seconds or less.
  • the inter-pass time exceeds 15 seconds, the strain applied by the processing is recovered, bainite in the final structure cannot be sufficiently refined, recrystallization is promoted, and the aspect ratio of the old austenite grains is lowered.
  • the shorter the inter-pass time the more preferable it is. Therefore, it is not necessary to set a lower limit, but it is preferably 3 seconds or more from the viewpoint of operability.
  • finish rolling is performed by reverse rolling.
  • the time between passes in finish rolling means that the steel sheet is rolled by a rolling roll while moving forward, the rear end of the steel sheet comes out of the rolling roll, the traveling direction of the steel sheet is reversed backward, and the rear end of the steel sheet is again. Means the time it takes for the roll to be bitten into the rolling roll.
  • the time from the completion of finish rolling to the start of cooling in the accelerated cooling process described later is set to 50 seconds or less.
  • the time from the completion of finish rolling to the start of cooling exceeds 50 seconds, the strain applied by the processing is recovered, bainite in the final structure cannot be sufficiently refined, recrystallization is promoted, and the old austenite grains are promoted.
  • the aspect ratio of is reduced.
  • the time from the completion of finish rolling to the start of cooling means the time from when the tip of the steel sheet traveling forward passes through the rolling roll in the final pass to the start of water cooling.
  • Ar 3 means the transformation start temperature at which the transformation from the austenite grains to the ferrite grains starts in the temperature lowering process, and is obtained by the following equation (ii).
  • Trex means the recrystallization temperature which is the lowest temperature at which equiaxial recrystallized grains can be generated and grown, and is obtained by the following equation (iii).
  • the element symbol in the following formula represents the content (mass%) of each element contained in the steel sheet, and if it is not contained, 0 is substituted.
  • T in the above formula represents the heating temperature (° C.) of the steel piece in the heating step.
  • (E) Secondary accelerated cooling step In the secondary accelerated cooling step, the steel sheet that has been finished rolled is water-cooled. At this time, water cooling is performed to a cooling stop temperature of 0 to 550 ° C. under the condition that the cooling start temperature is Trex -10 ° C. or lower and the average cooling rate from the cooling start to the cooling end is 5 to 50 ° C./sec. ..
  • the final structure can be made mainly bainite by water cooling to a cooling stop temperature of 0 to 550 ° C at an average cooling rate of 5 to 50 ° C / sec.
  • the average cooling rate and the cooling stop temperature are adjusted according to the value of Ceq in the chemical composition of the steel sheet, and are set to conditions under which martensitic transformation does not occur.
  • (F) Tempering step After the secondary accelerated cooling step, a tempering step of heating to a temperature range of 350 to 650 ° C. may be further provided. By performing the tempering step, it is possible to reduce the dislocation density that has become excessively high due to cooling. When the cooling shutdown temperature in the secondary accelerated cooling step is high, the self-tempering effect can be obtained, so that the tempering step does not have to be performed. On the other hand, in the secondary accelerated cooling step, for example, when the cooling is performed to about room temperature, it is preferable to perform a tempering step.
  • the metallographic structure of the obtained steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, a sample was taken from the steel plate so that the 1 / 4t position on the C cross section was the observation surface. Then, the observation surface is nital-etched, and after etching, eight fields of view are photographed at a magnification of 500 using an optical microscope, and image analysis is performed on the obtained microstructure photograph. Was taken as pearl light, and the area ratio of each was calculated.
  • the part that had been etched by night game was subjected to repera etching, and the image analysis was performed on the part that looked gray by night game etching, and the area ratio was calculated with the part that looked white as the MA phase.
  • the average length of bainitic ferrite and the area ratio of bainite were calculated by KAM analysis using EBSD.
  • the region where the local orientation difference exceeds 1.0 ° was defined as bainitic ferrite.
  • bainitic ferrite having a length in the major axis direction of 1 ⁇ m or more was targeted.
  • the area ratio of bainite is the sum of the area ratios of bainite ferrite.
  • the average length and the average aspect ratio of the old austenite grains in the thickness direction were measured according to JIS G 0551: 2013.
  • a sample was taken from the steel plate so that the 1 / 4t position on the L cross section was the observation surface.
  • the observation surface was mirror-polished, it was corroded by the Behcet-Beaujard method using a saturated aqueous solution of picric acid to reveal old austenite grains.
  • the observation surface on which the old austenite grains appeared was observed with an optical microscope, and a field of view with an area of 0.05 mm 2 or more was photographed for 8 fields or more (total 0.40 mm 2 or more). Then, the thickness of the old austenite grains was measured by a cutting method based on the tissue photograph taken by an optical microscope, and the average value was taken as the average length in the thickness direction of the old austenite grains. In the measurement, old austenite grains having a length of 1 ⁇ m or more in the thickness direction were targeted.
  • the maximum length in the major axis direction and the maximum length in the minor axis direction orthogonal to the major axis direction were measured for each old austenite grain, and the ratio (maximum length / short axis) was measured. The maximum axis length) was calculated, and the average value was taken as the average aspect ratio of the old austenite grains.
  • the texture was measured by the EBSD method. Specifically, according to the EBSD method, at the 1 / 2t position and the 1 / 10t position, the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section, and at the 1 / 4t position, the ⁇ 100 ⁇ plane. Maps of regions forming an angle within 15 ° with respect to the C cross section were created, and their area ratios were obtained by dividing the total area by the measured area.
  • the EBSD method was used to measure crystal orientation information at 1 ⁇ m pitch for a total of 100,000 points at the 1 / 2t position and 1 / 10t position of the C cross section. From this, the ⁇ 110 ⁇ plane of each measurement point is known, and the number of measurement points where the angle between the ⁇ 110 ⁇ plane of each measurement point and the C cross section, that is, the angle formed by the normal of each plane is within 15 ° is obtained. , The area ratio was obtained by dividing by the total number of measurements (100,000 points) measured by the EBSD method.
  • crystal orientation information was measured at a pitch of 1 ⁇ m at a position of 1 / 4t of the C cross section for a total of 100,000 points. From this, the ⁇ 100 ⁇ plane of each measurement point is known, and the number of measurement points where the angle formed by the ⁇ 100 ⁇ plane of each measurement point and the C cross section, that is, the angle formed by the normal of each plane is within 15 ° is obtained. , The area ratio was obtained by dividing by the total number of measurements (100,000 points) measured by the EBSD method.
  • Table 4 shows the results of these measurements.
  • the ferrite area ratio is "F fraction”
  • the pearlite area ratio is “P fraction”
  • the bainite area ratio is “B fraction”
  • the MA phase area ratio is "MA fraction”.
  • the average length of bainitic ferrite in the major axis direction is referred to as "BF length”.
  • the area ratio of the region where the ⁇ 110 ⁇ plane forms an angle within 15 ° with respect to the C cross section is " ⁇ 110 ⁇ area ratio”
  • the area ratio of the region where the ⁇ 100 ⁇ plane forms an angle within 15 ° with respect to the C cross section is expressed as " ⁇ 100 ⁇ area ratio”.
  • TS tensile strength
  • YS yield stress
  • the test piece was measured using a No. 1B tensile test piece collected with the direction perpendicular to the rolling direction (width direction) from the center of the plate thickness as the longitudinal direction.
  • the yield stress (YS) was the proof stress of the permanent elongation method when the permanent elongation was 0.2%.
  • those having a YS of 460 MPa or more and a TS of 570 MPa or more are considered to have high strength.
  • V-notch test pieces were collected so as to include the 1 / 4t position of the steel plate, and the fracture surface transition temperature (vTrs) was evaluated in accordance with JIS Z 2242: 2005. At this time, two V-notch test pieces were taken so that the longitudinal direction of the test pieces coincided with the rolling direction and the width direction of the steel sheet. In this example, the two test pieces having vTrs of ⁇ 60 ° C. or lower were considered to have excellent low temperature toughness.
  • CTOD test pieces having the total thickness in the plate thickness direction of the base metal as the notch position of 3-point bending were collected, and the CTOD value at ⁇ 10 ° C. was measured.
  • the test was performed 3 times and the minimum values are shown in the table. In this example, those having a minimum CTOD value of 0.50 mm or more at ⁇ 10 ° C. are considered to have excellent fracture toughness.
  • Test No. 27 the C content was excessive, so that the low temperature toughness and the fracture toughness deteriorated.
  • Test No. 28 had a low C content, did not have a bainite-based structure, had insufficient strength, and deteriorated low-temperature toughness and fracture toughness.
  • Test No. 29 the low temperature toughness and the fracture toughness deteriorated due to the excessive Si content.
  • Test No. 30 the low temperature toughness and fracture toughness deteriorated due to the excessive Mn content.
  • Test No. 31 had a low Mn content and insufficient strength.
  • Test number 32 had an excessive content of P and S, test number 33 had an excessive content of Al, and test number 34 had an excessive content of N, so that low temperature toughness and fracture toughness deteriorated.
  • Test No. 35 the N content was low and the old austenite grains became coarse, so that the low temperature toughness and the fracture toughness deteriorated.
  • Test number 36 had an excessive Nb content, resulting in deterioration of low temperature toughness and fracture toughness.
  • Test No. 37 the Nb content was low, the BF length was excessive, and the aspect ratio of the old austenite grains was small, so that the low temperature toughness and the fracture toughness deteriorated.
  • Test No. 38 the heating temperature in the heating step was high, the BF length and the old austenite granules were coarsened, and the low temperature toughness, fracture toughness and arrest property were deteriorated.
  • Test No. 39 the heating temperature was low, the bainite area ratio was low, and the aspect ratio of the old austenite grains was low, so that the strength was insufficient and the low temperature toughness and fracture toughness were deteriorated.
  • Test No. 40 since the end temperature of rough rolling was less than Trex , the BF length and the old austenite grains were coarsened, and the low temperature toughness, fracture toughness and arrest property were deteriorated.
  • test number 41 since the cumulative rolling reduction rate of rough rolling was high, the aspect ratio of the old austenite grains decreased, and the low temperature toughness, fracture toughness and arrest property deteriorated.
  • Test No. 42 since the cumulative reduction rate was low, the old austenite grains were coarsened, and the low temperature toughness, fracture toughness and arrest property were deteriorated.
  • test number 43 since the start temperature of rough rolling was high, the texture of the surface layer could not be controlled, and the arrest property deteriorated.
  • test number 44 since the cooling start temperature in the primary accelerated cooling step is low, the finish rolling end temperature is also less than Ar 3 , and as a result, the bainite area ratio becomes low, the BF length and the old austenite grains become coarse, and further. , The desired texture was not obtained. Therefore, the strength is insufficient, and the low temperature toughness, fracture toughness and arrest property are deteriorated.
  • test number 45 since the cooling shutdown temperature in the primary accelerated cooling process was high, sufficient processed ferrite could not be obtained on the surface layer portion, and the arrest property deteriorated.
  • the cooling stop temperature was low, the structure was not mainly bainite, and the desired texture was not obtained. Therefore, the strength was insufficient and the low temperature toughness, fracture toughness and arrest property were deteriorated.
  • test number 47 since the cooling rate in the primary accelerated cooling step was high, the texture of the surface layer could not be controlled, and the arrest property deteriorated.
  • Test No. 48 since the cooling rate in the primary accelerated cooling step was low, the bainite area ratio was low, the desired texture was not obtained, the strength was insufficient, and the low temperature toughness, fracture toughness and arrest property were deteriorated.
  • test number 49 since the start temperature of finish rolling was Trex or higher, the BF length became coarse, the aspect ratio of the old austenite grains decreased, and the texture of the surface layer could not be controlled, resulting in low temperature toughness. Fracture toughness and arrestability deteriorated.
  • Test No. 50 since the end temperature at the central portion of the thickness of the finish rolling was less than Ar 3 , processed ferrite was excessively generated, the strength became insufficient, and the low temperature toughness, fracture toughness and arrest property deteriorated.
  • test number 51 the number of rolling passes in finish rolling was small and the average value of the rolling shape ratio was high, so that the desired texture could not be obtained and the arrest property deteriorated.
  • Test No. 52 the average value of the rolled shape ratio was high, so that the desired texture could not be obtained and the arrest property was deteriorated.
  • Test No. 53 since the average value of the rolled shape ratio was low, a desired texture could not be obtained, and the arrest property was deteriorated.
  • test number 54 the cumulative rolling reduction rate of finish rolling was high, the aspect ratio of the old austenite grains was lowered, and the texture of the surface layer could not be controlled, so that the low temperature toughness, fracture toughness and arrest property deteriorated.
  • Test No. 55 the cumulative reduction rate was low, so that the BF length was coarsened, the aspect ratio of the old austenite grains was lowered, and the desired texture was not obtained, resulting in low temperature toughness, fracture toughness, and arrestability. Deteriorated.
  • test number 56 the time between passes is long, and in test number 57, the time from the completion of finish rolling to the start of cooling is long, so that the BF length becomes coarse and the aspect ratio of the old austenite grains decreases, and the low temperature toughness and fracture toughness Has deteriorated.
  • Test No. 58 since the cooling rate in the accelerated cooling step was high, the MA phase was excessively generated, so that the low temperature toughness, the fracture toughness and the arrest property deteriorated.
  • Test No. 59 had a low cooling rate, did not have a bainite-based structure, had insufficient strength, and deteriorated low temperature toughness and fracture toughness. Since the cooling stop temperature of the test number 60 was high, the structure was not mainly composed of bainite, the strength was insufficient, and the low temperature toughness and the fracture toughness deteriorated.
  • Test No. 61 the cooling start temperature exceeded Trex -10 ° C. and the BF length became coarse, so that the low temperature toughness was good, but the fracture toughness deteriorated.
  • the steel plate according to the present invention can be suitably used as a material for welded structures such as ships, high-rise buildings, other buildings, bridges, marine structures, LNG storage tanks and other large tanks, and line pipes. ..

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

L'invention concerne une tôle d'acier dont la composition chimique est telle qu'en % en masse, C vaut 0, 040 à 0,160％, Si vaut 0,01 à 0,50％, Mn vaut 0,70 à 2,50％, P vaut 0,030％ ou moins, S vaut 0,020％ ou moins, Al vaut 0,001 à 0,100％, N vaut 0,0010 à 0,0080％ et Nb vaut 0,003 à 0,050％, le reste étant constitué de Fe et d'impuretés. La structure métallique en une position à 1/4t au niveau d'un plan transversal C, contient 80% en surface ou plus d'une bainite. La longueur moyenne dans une direction axe long d'une ferrite bainitique configurant la bainite, est inférieure ou égale à 10μｍ. La longueur moyenne dans une direction épaisseur de grains d'austénite antérieur en position à 1/4t au niveau d'un plan transversal L, est inférieure ou égale à 20μm, et la moyenne de leur rapport d'aspect est supérieure ou égale à 2,5. Le rapport surfacique de {110} en une position à 1/10t est compris entre 30 et 60%, le rapport surfacique de {100} en position à 1/4t est compris entre 10 et 40%, et le rapport surfacique de {110} en une position à 1/2t est compris entre 40 et 70％.
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WO2009072559A1 (fr) * 2007-12-06 2009-06-11 Nippon Steel Corporation Procédé de production d'une plaque épaisse d'acier à haute résistance mécanique qui présente d'excellentes propriétés de blocage des fractures fragiles et une excellente ténacité de la partie affectée par la chaleur en soudage par grand apport de chaleur, et plaque épaisse d'acier à haute résistance mécanique qui présente d'excellentes propriétés de blocage des fractures fragiles et une excellente ténacité de la zone affectée par la chaleur en soudage par grand apport de chaleur
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WO2009072559A1 (fr) * 2007-12-06 2009-06-11 Nippon Steel Corporation Procédé de production d'une plaque épaisse d'acier à haute résistance mécanique qui présente d'excellentes propriétés de blocage des fractures fragiles et une excellente ténacité de la partie affectée par la chaleur en soudage par grand apport de chaleur, et plaque épaisse d'acier à haute résistance mécanique qui présente d'excellentes propriétés de blocage des fractures fragiles et une excellente ténacité de la zone affectée par la chaleur en soudage par grand apport de chaleur
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WO2024116737A1 (fr) * 2022-11-29 2024-06-06 Ｊｆｅスチール株式会社 Plaque d'acier ultra-épaisse à haute résistance et son procédé de production
JP7508029B1 (ja) 2022-11-29 2024-07-01 Ｊｆｅスチール株式会社 高強度極厚鋼板およびその製造方法

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