WO2018030186A1 - 高強度厚鋼板およびその製造方法 - Google Patents

高強度厚鋼板およびその製造方法 Download PDF

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WO2018030186A1
WO2018030186A1 PCT/JP2017/027513 JP2017027513W WO2018030186A1 WO 2018030186 A1 WO2018030186 A1 WO 2018030186A1 JP 2017027513 W JP2017027513 W JP 2017027513W WO 2018030186 A1 WO2018030186 A1 WO 2018030186A1
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Prior art keywords
steel plate
strength
thickness
thick steel
less
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PCT/JP2017/027513
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English (en)
French (fr)
Japanese (ja)
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佳子 竹内
勇樹 田路
亮 荒尾
克行 一宮
長谷　和邦
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Ｊｆｅスチール株式会社
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Priority to CN201780049245.5A priority Critical patent/CN109563591A/zh
Priority to KR1020197006739A priority patent/KR102239631B1/ko
Priority to JP2018532938A priority patent/JP6593541B2/ja
Publication of WO2018030186A1 publication Critical patent/WO2018030186A1/ja

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    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous 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
    • 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
    • C22C38/14—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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the present invention relates to a high-strength thick steel plate, and more particularly, to a high-strength thick steel plate excellent in brittle crack propagation stopping characteristics used for large structures such as ships, marine structures, low-temperature storage tanks, and construction / civil engineering structures. . Moreover, this invention relates to the manufacturing method of the said high intensity
  • Ni steel As a means for improving the brittle crack propagation stopping property of steel, a method of increasing the Ni content in steel is known. For example, 9% Ni steel is used on a commercial scale in LNG tanks. However, since an increase in the amount of Ni in the steel causes a significant increase in manufacturing cost, it is difficult to use the 9% Ni steel for applications other than the LNG storage tank.
  • relatively thin steel materials having a plate thickness of less than 50 mm are used for applications such as ships and line pipes that do not reach cryogenic temperatures such as LNG.
  • an excellent brittle crack propagation stopping characteristic can be realized by reducing the particle size by TMCP (Thermo-Mechanical Control Process) and improving low temperature toughness.
  • Patent Document 1 proposes a steel material in which the structure of the surface layer portion is made ultrafine in order to improve the brittle crack propagation stopping characteristics.
  • the structure of the surface layer portion is made ultrafine in order to improve the brittle crack propagation stopping characteristics.
  • Patent Document 2 in order to improve the brittle crack propagation stop property in a steel material having a ferrite-pearlite-based microstructure, the ferrite grain size, the ferrite grain aspect ratio, and the standard deviation of the ferrite grain size in the steel sheet surface layer are controlled. It has been proposed to do.
  • Patent Document 3 proposes a steel sheet that has improved brittle crack propagation stopping characteristics by refining ferrite crystal grains and controlling subgrains formed in ferrite crystal grains.
  • the resistance to brittle fracture is increased by causing separation on the fracture surface of the steel material in a direction parallel to the plate surface and relaxing the stress at the brittle crack tip.
  • Patent Document 4 discloses that the (110) plane X-ray intensity ratio is set to 2 or more by controlled rolling, and the area ratio of coarse crystal grains having an equivalent circle diameter of 20 ⁇ m or more is set to 10% or less, thereby causing brittle fracture resistance. It is described that the characteristics are improved.
  • Patent Document 5 proposes a welded structural steel excellent in brittle crack propagation stopping characteristics of the joint part, in which the (100) plane X-ray plane strength ratio in the rolled surface inside the plate thickness is 1.5 or more. Yes.
  • the steel for welded structure by developing the texture as described above, a shift occurs between the stress load direction and the crack propagation direction, and as a result, the brittle crack propagation stop characteristic is improved.
  • Japanese Patent Publication No. 7-100814 JP 2002-256375 A Japanese Patent No. 3467767 Japanese Patent No. 3548349 Japanese Patent No. 2659661
  • the conventional techniques as described above are all intended for a steel plate having a thickness of about 50 mm, and whether or not necessary characteristics can be obtained when applied to a thick material having a thickness of about 70 mm. It is unknown. In particular, the crack propagation characteristics in the plate thickness direction required in the hull structure are also unclear.
  • the present invention advantageously solves the above-mentioned problems, and has excellent brittle crack propagation stopping characteristics even when the plate thickness exceeds 50 mm, and can be manufactured by an industrially very simple process.
  • An object is to provide a thick steel plate.
  • an object of this invention is to provide the manufacturing method of the high strength thick steel plate which can manufacture the said high strength thick steel plate stably by an industrially simple process.
  • the temperature difference in the plate thickness direction can be reduced as shown in FIG. 2 by heating the front and back surfaces of the steel plate during rolling, and the temperature is lower than ever. I thought that I could roll it stably. Thereby, when hot rolling is performed under the same conditions as before, even higher brittle crack propagation stopping characteristics can be obtained. In addition, the rolling conditions necessary for obtaining the same level of brittle crack propagation stopping characteristics can be relaxed as compared with the conventional ones.
  • the present invention has been completed. That is, the gist configuration of the present invention is as follows.
  • Ceq [C] + [Mn] / 6 + [Cu] / 15 + [Ni] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5 (1) Ceq ⁇ 0.40 (2)
  • the parenthesis in the formula (1) represents the content (% by mass) of the element in the parenthesis in the high-strength thick steel plate, and is 0 when the element is not contained.
  • the plate thickness is 50-100mm, Kca ( ⁇ 10 ° C.) is 7000 N / mm 3/2 or more, 2.
  • Kca ⁇ 10 ° C.
  • the component composition is mass%, Nb: 0.005 to 0.05%, Cu: 0.01 to 0.5%, 4.
  • the component composition is mass%, Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: 0.0030% or less,
  • a method for producing a high-strength thick steel plate according to any one of 1 to 6 above A heating step of heating the steel having the component composition according to any one of 1, 4, and 5 to a heating temperature of 1000 to 1200 ° C .; A hot rolling step in which the heated steel is hot rolled into a hot rolled steel sheet, The hot rolling step, Hot rolling in which the plate thickness 1/2 position is in the austenite recrystallization temperature range; The plate thickness 1/2 position includes hot rolling in the austenite non-recrystallization temperature range, A method for producing a high-strength thick steel plate, wherein the steel is heated from both front and back surfaces during the hot rolling step.
  • the texture at both the plate thickness 1/2 position and the steel plate surface is appropriately controlled, so that even when the plate thickness exceeds 50 mm, the brittle crack propagation stopping property is excellent.
  • a strong thick steel plate can be obtained.
  • the high-strength thick steel plate of the present invention largely contributes to improving the safety of ships by applying it to deck members joined to hatch side combing in the strong deck structure of container ships and bulk carriers. It is extremely useful in industry.
  • the present invention will be specifically described.
  • the component composition and texture are defined as described above.
  • C 0.03 to 0.20%
  • C is an element that improves the strength of steel, and in the present invention, it is necessary to contain 0.03% or more in order to ensure a desired strength. However, if the C content exceeds 0.20%, the weldability is deteriorated and the toughness is also adversely affected. Therefore, the C content is 0.03 to 0.20%.
  • the C content is preferably 0.05 to 0.15%.
  • Si 0.03-0.5%
  • Si is effective as a deoxidizing element and as a steel strengthening element. However, if the content is less than 0.03%, the effect is not obtained. On the other hand, when the Si content exceeds 0.5%, not only the surface properties of the steel are impaired, but also the toughness is extremely deteriorated. Therefore, the Si content is 0.03 to 0.5%.
  • the Si content is preferably 0.04 to 0.40%.
  • Mn 0.5 to 2.2% Mn is contained as a strengthening element. If the Mn content is less than 0.5%, the effect is not sufficient. On the other hand, if the Mn content exceeds 2.2%, the weldability deteriorates and the steel material cost also increases. Therefore, the Mn content is 0.5 to 2.2%.
  • P 0.02% or less
  • P is an inevitable impurity in steel, and as the content increases, the toughness deteriorates. Therefore, in order to maintain good toughness even in a thick steel plate having a thickness exceeding 50 mm, the P content is set to 0.02% or less.
  • the P content is preferably 0.01% or less, and more preferably 0.006% or less.
  • the lower limit is not limited, and may be 0%, but industrially it exceeds 0%.
  • S 0.01% or less S is an unavoidable impurity in steel, and as the content increases, the toughness deteriorates. Therefore, in order to maintain good toughness even in a thick steel plate having a plate thickness exceeding 50 mm, the S content is set to 0.01% or less.
  • the S content is preferably 0.005% or less, and more preferably 0.003% or less.
  • the lower limit is not limited, and may be 0%, but industrially it exceeds 0%.
  • Ti 0.005 to 0.03%
  • Ti has the effect of forming nitrides, carbides, or carbonitrides by adding a trace amount, and making the crystal grains finer to improve the base material toughness. The effect can be obtained when the Ti content is 0.005% or more.
  • the Ti content is set to 0.005 to 0.03%.
  • the Ti content is preferably 0.006 to 0.028%.
  • Al 0.005 to 0.080%
  • Al is an element added as a deoxidizing material, and 0.005% or more must be added to obtain the effect.
  • the Al content is set to 0.005 to 0.080%.
  • the Al content is preferably 0.020 to 0.040%.
  • N 0.0050% or less
  • N is an element that combines with Al in the steel, adjusts the crystal grain size during rolling, and strengthens the steel. However, if the N content exceeds 0.0050%, the toughness deteriorates, so the N content is set to 0.0050% or less.
  • the lower limit of the N content is not particularly limited, but is preferably 0.0010% or more, and more preferably 0.0015% or more.
  • the component composition of the high-strength thick steel plate in one embodiment of the present invention is composed of the above elements, the remaining Fe and inevitable impurities.
  • the component composition further optionally contains one or more selected from the group consisting of Nb, Cu, Ni, and Cr. Is possible.
  • Nb 0.005 to 0.05% Nb precipitates as NbC during ferrite transformation or reheating, and contributes to increasing the strength.
  • Nb has an effect of expanding the non-recrystallized region in rolling in the austenite region, and contributes to the refinement of ferrite, so it is also effective in improving toughness. The effect is exhibited when the content is 0.005% or more. However, when the content exceeds 0.05%, coarse NbC precipitates, which causes a decrease in toughness. Therefore, when Nb is contained, the Nb content is set to 0.005 to 0.05%.
  • Cu 0.01 to 0.5%
  • Cu is an element that enhances the hardenability of steel, and contributes to improving functions such as toughness, high-temperature strength, and weather resistance in addition to improving strength after rolling. These effects are exhibited by the content of 0.01% or more, but excessive content deteriorates by changing toughness and weldability. Therefore, the Cu content is set to 0.01 to 0.5%.
  • Ni 0.01 to 1.5%
  • Ni is an element that enhances the hardenability of steel, and contributes to functional improvements such as toughness, high-temperature strength, and weather resistance, in addition to improving strength after rolling. These effects are exhibited when the content is 0.01% or more. On the other hand, excessive inclusion causes deterioration of the toughness and weldability, and increases the cost of the alloy. Therefore, the Ni content is set to 0.01 to 1.5%.
  • Cr 0.01 to 0.5% Cr, like Cu, is an element that enhances the hardenability of steel, and contributes to functional improvements such as toughness, high-temperature strength, and weather resistance in addition to improving strength after rolling. These effects are exhibited by the content of 0.01% or more, but excessive content deteriorates by changing toughness and weldability. Therefore, the Cr content is set to 0.01 to 0.5%.
  • the said component composition further contains 1 or 2 or more selected from the group which consists of Mo, V, B, Ca, and REM arbitrarily further Is possible.
  • Mo 0.01 to 0.5% Mo, like Cu and Cr, is an element that enhances the hardenability of steel, and contributes to functional improvements such as toughness, high-temperature strength, and weather resistance in addition to improving strength after rolling. These effects are exhibited by the content of 0.01% or more, but excessive content deteriorates by changing toughness and weldability. Therefore, the Mo content is set to 0.01 to 0.5%.
  • V 0.001 to 0.10%
  • V is an element that improves the strength of the steel by precipitation strengthening that precipitates as V (CN). This effect is exhibited by containing V by 0.001% or more. On the other hand, when V is contained exceeding 0.10%, toughness is rather lowered. Therefore, when V is contained, the V content is set to 0.001 to 0.10%.
  • B 0.0030% or less
  • B is an element having an effect of enhancing the hardenability of steel in a small amount and can be arbitrarily contained. However, if the B content exceeds 0.0030%, the toughness of the welded portion decreases. Therefore, the B content is 0.0030% or less.
  • the lower limit of the B content is not particularly limited. However, when B is contained, the B content is preferably 0.0006% or more from the viewpoint of obtaining good hardenability.
  • Ca 0.0050% or less
  • Ca is an element having the effect of improving the toughness by refining the structure of the weld heat affected zone, and the effect of the present invention is not impaired as long as it is contained in an appropriate amount. Therefore, Ca can be contained as necessary. However, when Ca is contained excessively, coarse inclusions are formed and the toughness of the base material is deteriorated. Therefore, when Ca is contained, the Ca content is set to 0.0050% or less. On the other hand, the lower limit of the Ca content is not particularly limited. However, when Ca is added, the Ca content is preferably 0.0001% or more in order to sufficiently obtain the addition effect.
  • REM 0.0100% or less REM (rare earth metal) is an element that has the effect of improving the toughness by refining the structure of the weld heat affected zone in the same way as Ca. Will not be damaged. Therefore, REM can optionally be contained. However, if REM is excessively contained, coarse inclusions are formed and the toughness of the base material is deteriorated. Therefore, when REM is contained, the REM content is set to 0.0100% or less. On the other hand, the lower limit of the REM content is not particularly limited. However, when REM is added, the REM content is preferably 0.0005% or more in order to sufficiently obtain the effect of addition.
  • the carbon equivalent Ceq defined by the following formula (1) satisfies the condition of the following formula (2).
  • Ceq [C] + [Mn] / 6 + [Cu] / 15 + [Ni] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5 (1)
  • the parenthesis in the above formula (1) represents the content (% by mass) of the element in the parenthesis in the high-strength thick steel plate, and is 0 when the element is not contained.
  • the strength and texture strength can be maintained even in a thick steel plate having a plate thickness exceeding 50 mm.
  • the upper limit of Ceq is not particularly limited, but is preferably 0.55 or less, more preferably 0.53 or less, and even more preferably 0.50 or less.
  • the texture is such that the ⁇ 113 ⁇ ⁇ 110> orientation strength at the plate thickness 1/2 position is 4.0 or more and the ⁇ 113 ⁇ ⁇ 110> orientation strength at the steel plate surface is 1.7 or more.
  • the texture is such that the ⁇ 113 ⁇ ⁇ 110> orientation strength at the plate thickness 1/2 position is 4.0 or more and the ⁇ 113 ⁇ ⁇ 110> orientation strength at the steel plate surface is 1.7 or more.
  • the ⁇ 113 ⁇ ⁇ 110> orientation strength at the plate thickness 1/2 position is 4.1 or more, and the ⁇ 113 ⁇ ⁇ 110> orientation strength at the steel plate surface is It is preferable to set it as 1.9 or more.
  • the upper limit of the ⁇ 113 ⁇ ⁇ 110> azimuth strength at the plate thickness 1/2 position is not particularly limited, and the higher the better, but generally it may be 7.0 or less. Moreover, it does not specifically limit about the upper limit of ⁇ 113 ⁇ ⁇ 110> azimuth
  • the ⁇ 113 ⁇ ⁇ 110> azimuth strength at the plate thickness 1/2 position and the ⁇ 113 ⁇ ⁇ 110> azimuth strength at the steel plate surface can be determined as random strength ratios by the X-ray pole figure method, respectively. Specifically, it can be measured by the method described in the examples. In the measurement, a positional error of several percent is allowed.
  • the area fraction of bainite is more preferably 90% or more.
  • the upper limit of the area fraction of the bainite is not particularly limited, and may be 100%.
  • the remainder other than bainite is not particularly limited and can be an arbitrary structure.
  • the total area fraction of the remaining tissue is preferably 15% or less. The area fraction can be measured by the method described in Examples.
  • vE Charpy absorbed energy at ⁇ 40 ° C. at a position of 1 ⁇ 4 ° C .: vE ( ⁇ 40 ° C.) is preferably 250 J or more, more preferably 280 J or more, and 300 J or more. It is more preferable.
  • the upper limit of the vE ( ⁇ 40 ° C.) is not particularly limited, but is generally 420 J or less, and may be 400 J or less.
  • the vE ( ⁇ 40 ° C.) at a position (depth) 5 mm from the surface of the high-strength thick steel plate is preferably 250 J or more, more preferably 280 J or more, and more preferably 300 J or more.
  • the upper limit of the vE ( ⁇ 40 ° C.) is not particularly limited, but is generally 420 J or less, and may be 400 J or less.
  • both the vE ( ⁇ 40 ° C.) at the position of 5 mm from the surface of the steel sheet and the position of the thickness 1/2 are obtained by heating the steel from both the front and back sides during the hot rolling process.
  • the Charpy fracture surface transition temperature at a 1 ⁇ 4 thickness position is ⁇ 40 ° C. or lower.
  • the lower limit of the Charpy fracture surface transition temperature is not particularly limited, but may generally be ⁇ 130 ° C. or higher.
  • Kca ( ⁇ 10 ° C.) is 7000 N / mm 3/2 or more. it can.
  • Kca (-10 ° C.) is preferably in the 7500N / mm 3/2 or more, more preferably, to 8000 N / mm 3/2 or more, still more preferably 9000 N / mm 3/2 or more.
  • the upper limit is not particularly limited, but in general, it may be 13000 N / mm 3/2 or less.
  • the value of Kca ( ⁇ 10 ° C.) can be measured by a temperature gradient type ESSO test, and specifically can be obtained by the method described in the examples.
  • the tensile strength (TS) of the high-strength thick steel plate of the present invention is not particularly limited, but the tensile strength TS at the 1/4 thickness position is preferably 570 MPa or more, more preferably 580 MPa or more, More preferably, it is 590 MPa or more.
  • the upper limit of TS is not particularly limited, but generally, the tensile strength TS at the 1/4 thickness position may be 700 MPa or less.
  • the plate thickness of the high-strength thick steel plate of the present invention is not particularly limited, and can be an arbitrary value. However, since the effect of the present invention becomes more significant as the plate thickness increases, the plate thickness is preferably 50 mm or more, more preferably more than 50 mm, further preferably 60 mm or more, and 70 mm or more. More preferably. On the other hand, the upper limit of the plate thickness is not particularly limited, but may generally be 100 mm or less.
  • the high-strength thick steel plate of the present invention can be produced by hot rolling steel having the above-described component composition under specific conditions. Specifically, the following steps (1) and (2) are sequentially performed. (1) A heating step of heating steel to a heating temperature of 1000 to 1200 ° C. (2) A hot rolling process in which the heated steel is hot rolled to form a hot rolled steel sheet. In the (2) hot rolling step, the following steps (2-1) and (2-2) are sequentially performed. (2-1) Hot rolling (recrystallization zone rolling) in which the plate thickness 1 ⁇ 2 position is in the austenite recrystallization temperature range. (2-2) Hot rolling (non-recrystallized zone rolling) in which the plate thickness 1/2 position is in the austenite non-recrystallized temperature range.
  • step (3) A cooling step of cooling the hot-rolled steel sheet to a cooling stop temperature of 500 ° C. or lower at a cooling rate of 3 ° C./s or higher.
  • step (4) may be optionally performed.
  • Heating temperature 1000-1200 ° C
  • the steel having the above composition is heated. At that time, if the heating temperature is less than 1000 ° C., sufficient rolling time in the austenite recrystallization temperature region cannot be ensured. On the other hand, when the heating temperature exceeds 1200 ° C., the austenite grains become coarse, leading to a decrease in toughness, and an oxidation loss becomes remarkable, resulting in a decrease in yield. Therefore, the heating temperature is set to 1000 to 1200 ° C. From the viewpoint of improving the toughness of the steel sheet, the heating temperature is preferably 1000 to 1170 ° C, and more preferably 1050 to 1170 ° C.
  • the steel used for the heating step is not particularly limited and can be manufactured by an arbitrary method.
  • a steel piece (slab) obtained by melting a molten steel having the above component composition in a converter or the like and obtaining it by continuous casting can be used.
  • hot rolling is performed.
  • (2-1) hot rolling (recrystallization zone rolling) is performed in the austenite recrystallization temperature range at a position where the plate thickness is 1/2, and then (2-2) plate thickness 1 / 2 position performs hot rolling (non-recrystallization zone rolling) in the austenite non-recrystallization temperature range.
  • the rolling end temperature in the hot rolling step is not particularly limited, but is preferably Ar 3 point or higher.
  • the cumulative rolling reduction in the above hot rolling is not particularly limited.
  • the upper limit of the recrystallization temperature range cumulative rolling reduction is not particularly limited, but is preferably 60% or less from the viewpoint of rolling load.
  • the cumulative reduction ratio (non-recrystallized area cumulative reduction ratio) in hot rolling in the (2-2) plate thickness 1/2 position in the austenite non-recrystallization temperature range be 45% or more.
  • the upper limit of the unrecrystallized temperature range cumulative rolling reduction is not particularly limited, but is preferably 90% or less from the viewpoint of rolling load. From the viewpoint of texture control, it is preferable to control the cumulative rolling reduction in this way.
  • the steel is heated from both the front and back sides during the (2) hot rolling step.
  • the temperature distribution in the plate thickness direction can be controlled, and the temperature difference between the steel plate surface and the inside can be reduced.
  • vE ⁇ 40 ° C.
  • a position of 5 mm from the surface of the steel sheet and a position of a thickness of 1/2 can be set to 250 J or more.
  • the heating from both the front and back surfaces is performed such that the temperature difference between the steel surface and the plate thickness 1/2 at the time when the heating is completed is 30 ° C. or less. Thereby, it can suppress that a ferrite produces
  • the temperature difference is more preferably 20 ° C. or less, and further preferably 10 ° C. or less.
  • the lower limit is not particularly limited, and may be 0 ° C. or higher.
  • the timing which performs the said heating is not specifically limited, What is necessary is just in the middle of a hot rolling process.
  • heating is performed before the start of non-recrystallization zone rolling, from the viewpoint of temperature control, it is preferable to start the non-recrystallization zone rolling within 30 seconds after the completion of the heating.
  • the heating in the hot rolling step is not particularly limited and can be performed by any method such as induction heating or furnace heating.
  • Cooling rate 3 ° C./s or more Cooling stop temperature: 500 ° C. or less
  • the steel sheet that has been rolled is 500 ° C. or less at a cooling rate of 3 ° C./s or more from the viewpoint of maintaining the texture developed during rolling. It is preferable to cool to a cooling stop temperature.
  • the upper limit of a cooling rate is not specifically limited, It is preferable to set it as 10 degrees C / s or less.
  • the lower limit of the cooling stop temperature is not particularly limited, but is preferably 0 ° C. or higher.
  • the cooling start temperature in the cooling step it is preferable that the Ar 3 point or more.
  • Tempering temperature When performing tempering treatment after the Ac 1 point below the cooling step, it is preferable to perform the tempering under the following tempering temperature point Ac. This is because if the tempering temperature is higher than the Ac 1 point, the texture developed during rolling may be lost.
  • the lower limit of the tempering temperature is not particularly limited, but is preferably 400 ° C. or higher.
  • the temperature at the plate thickness 1/2 position is obtained by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer or calculation based on the center temperature measured in advance. Moreover, the temperature in the cooling conditions after rolling is the temperature at the plate thickness 1/2 position.
  • the steel was heated from both the front and back surfaces except for some comparative examples.
  • the heating was performed after completion of recrystallization zone rolling and before starting non-recrystallization zone rolling. At that time, the non-recrystallization zone rolling was started within 30 seconds from the end of heating.
  • the heating was performed by furnace heating using an atmospheric furnace and induction heating by high frequency.
  • the test piece used for measuring vE ( ⁇ 40 ° C.) at a position 5 mm from the surface of the steel sheet was taken from the surface of the steel sheet after removing the scale (black skin) formed on the surface of the steel sheet. did. Since the thickness of the test piece is 10 mm, the measurement position on the test piece is the center position in the thickness direction of the test piece, that is, the position of 5 mm from the steel plate surface to the plate thickness direction.
  • the X-ray-diffraction measurement was implemented using the X-ray-diffraction apparatus using Mo ray source, and the (200), (110), and (211) positive electrode dot diagram was calculated
  • the ratio of ⁇ 113 ⁇ ⁇ 110> orientation strength to random strength was calculated by calculating a three-dimensional crystal orientation density function from the obtained positive electrode dot diagram.
  • the high-strength thick steel plate that satisfies the conditions of the present invention has a vE ( ⁇ 40 ° C.) at a position of 5 mm from the steel plate surface, a thickness of 1/2, and a thickness of 1/4.
  • Kca ( ⁇ 10 ° C.) was 7000 N / mm 3/2 or more and had excellent brittle crack propagation stopping characteristics.
  • the high strength thick steel plate of the comparative example not satisfying the conditions of the present invention was inferior in at least one of vE ( ⁇ 40 ° C.) and Kca ( ⁇ 10 ° C.) at the 1/4 thickness position.

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WO2013099318A1 (ja) * 2011-12-27 2013-07-04 Ｊｆｅスチール株式会社 脆性亀裂伝播停止特性に優れた構造用高強度厚鋼板およびその製造方法
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