WO2020153085A1 - Tôle d'acier épaisse, et procédé de fabrication de celle-ci - Google Patents

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

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WO2020153085A1
WO2020153085A1 PCT/JP2019/050700 JP2019050700W WO2020153085A1 WO 2020153085 A1 WO2020153085 A1 WO 2020153085A1 JP 2019050700 W JP2019050700 W JP 2019050700W WO 2020153085 A1 WO2020153085 A1 WO 2020153085A1
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thick steel
steel plate
cooling
steel sheet
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PCT/JP2019/050700
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English (en)
Japanese (ja)
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義浩 兵藤
仁 末吉
横田　智之
聡 伊木
和田　裕
羽鳥　聡
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Ｊｆｅスチール株式会社
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Priority to KR1020217022156A priority Critical patent/KR102557520B1/ko
Priority to CN201980089824.1A priority patent/CN113330125A/zh
Priority to JP2020518831A priority patent/JP6981546B2/ja
Priority to CN202311433530.0A priority patent/CN117568718A/zh
Publication of WO2020153085A1 publication Critical patent/WO2020153085A1/fr

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    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to a thick steel plate, and particularly to a thick steel plate excellent in both total elongation and fatigue crack propagation resistance.
  • INDUSTRIAL APPLICABILITY The thick steel plate of the present invention can be suitably used for welded structures, such as ships, marine structures, bridges, buildings, and tanks, which are strongly required to have structural safety.
  • the present invention also relates to a method for manufacturing the thick steel plate.
  • Heavy steel plates are widely used in structures such as ships, offshore structures, bridges, buildings, and tanks.
  • the thick steel plate is required to have excellent fatigue properties as well as mechanical properties such as strength and toughness and weldability.
  • the thick steel plate is required to have fatigue characteristics capable of ensuring the safety of the structure even under such a repeated load. In particular, it is effective to improve the fatigue crack propagation resistance of the thick steel plate in order to prevent ultimate fracture such as breakage of the member.
  • Patent Document 1 proposes a steel plate for a tanker, which has excellent fatigue crack propagation resistance in a wet hydrogen sulfide environment.
  • the steel sheet has a mixed structure of ferrite as the first phase and bainite and/or pearlite as the second phase.
  • the average grain size of ferrite is set to 20 ⁇ m or less.
  • Patent Document 2 proposes a steel sheet having excellent fatigue crack propagation resistance.
  • the steel sheet has a microstructure composed of a hard part and a soft part, and a hardness difference between the hard part and the soft part is 150 or more in Vickers hardness.
  • Patent Document 3 proposes a duplex stainless steel having a microstructure composed of bainite and ferrite having an area ratio of 38 to 52%.
  • the fatigue crack propagation resistance is improved by controlling the Vickers hardness of the ferrite phase portion and the density of the boundary between the ferrite phase and the bainite phase.
  • Patent Documents 1 to 3 have the following problems (1) to (3).
  • the thick steel plate has excellent fatigue crack propagation resistance not only in one direction but also in the plate thickness direction, rolling direction, and width direction. Is required.
  • the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thick steel plate having the following excellent features (1) to (3).
  • (1) It has both excellent fatigue crack propagation resistance and total elongation.
  • (2) Excellent fatigue crack propagation resistance in the plate thickness direction, rolling direction, and width direction.
  • (3) It can be manufactured without requiring a high degree of cooling control in the two-phase region.
  • this invention aims at providing the manufacturing method of the said thick steel plate.
  • the present inventors have obtained the following findings as a result of investigations to solve the above problems.
  • a bainite as a soft phase and a pearlite as a hard phase are both contained in a specific area fraction, and a bainite and a parlor each have a microstructure having a crystal grain size within a specific range.
  • a thick steel sheet having both excellent fatigue crack propagation resistance and total elongation can be obtained.
  • the thick steel sheet having the microstructure can be manufactured by controlling the manufacturing conditions, particularly the conditions in hot rolling and subsequent accelerated cooling. Since the thick steel sheet has bainite as the first phase, it is more suitable for production by an online process than conventional steel sheets.
  • the present invention was made based on the above findings, and has the following gist.
  • composition of the components is% by mass, Cr: 0.01-1.00%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Mo: 0.01 to 1.00%, Nb: 0.005 to 0.050%, V: 0.005 to 0.050%, Ti: 0.005 to 0.050%, B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.020%, 2.
  • the thick steel sheet according to 1 above which contains one or more selected from the group consisting of Mg: 0.0001 to 0.020% and REM: 0.0001 to 0.020%.
  • Hot rolling the heated steel material into a hot rolled steel sheet The hot rolled steel sheet is acceleratedly cooled under the conditions of a cooling start temperature: Ar 3 points or higher, a cooling stop temperature: 450 to 700° C., and an average cooling rate on the surface of the steel sheet from the start of cooling to a stop of cooling: 20 to 60° C./s.
  • a method of manufacturing a thick steel plate In the hot rolling, a cumulative reduction ratio in a temperature range of 950° C. or higher is 80% or higher, and a cumulative reduction ratio in a temperature range of lower than 950° C. and Ar 3 points or higher is 50% or higher. Manufacturing method.
  • the thick steel plate of the present invention has both excellent fatigue crack propagation resistance and total elongation, and is also excellent in fatigue crack propagation resistance in all of the plate thickness direction, rolling direction, and width direction. Further, the thick steel plate of the present invention can be stably manufactured without requiring a high degree of cooling control in the two-phase region. Therefore, the thick steel sheet of the present invention greatly contributes to the improvement of reliability of the steel structure and the reduction of life cycle cost.
  • C 0.01 to 0.16%
  • C is an element having an effect of improving strength. Further, C has an effect of promoting generation of a pearlite phase which is advantageous in fatigue resistance. If the C content is less than 0.01%, the desired strength and fatigue crack propagation resistance cannot be obtained. Therefore, the C content is set to 0.01% or more. On the other hand, if the C content exceeds 0.16%, the total elongation and weldability deteriorate. Therefore, the C content is 0.16% or less, preferably 0.15% or less, more preferably 0.10% or less.
  • Si 1.00% or less
  • Si is an element that has a deoxidizing effect and also has an effect of further improving strength. Si also has an effect of suppressing excessive cementite formation. However, if the Si content exceeds 1.00%, not only the weldability and toughness deteriorate, but also the formation of a pearlite phase advantageous for fatigue resistance is suppressed. Therefore, the Si content is 1.00% or less, preferably 0.50% or less.
  • the lower limit of the Si content is not particularly limited, but from the viewpoint of enhancing the effect of adding Si, the Si content is preferably 0.01% or more, and more preferably 0.10% or more.
  • Mn 0.50-2.00%
  • Mn is an element having the effect of enhancing the hardenability and, as a result, improving the strength and toughness of the thick steel plate.
  • the Mn content is set to 0.50% or more, preferably 0.80% or more.
  • the Mn content exceeds 2.00%, the hardenability becomes too high, and as a result, the formation of the pearlite phase, which is advantageous in fatigue resistance, is suppressed.
  • the Mn content exceeds 2.00%, the total elongation and toughness decrease. Therefore, the Mn content is 2.00% or less, preferably 1.65% or less.
  • P 0.030% or less
  • P is an element contained in the thick steel plate as an impurity and deteriorates toughness and total elongation. Therefore, the P content is set to 0.030% or less.
  • the lower the P content the better. Therefore, the lower limit of the P content is not particularly limited, and the P content may be 0% or more, or may be more than 0%. However, since excessive reduction increases the manufacturing cost, from the viewpoint of manufacturing cost, the P content is preferably 0.0005% or more, and more preferably 0.001% or more.
  • S 0.020% or less S is an element contained in the thick steel plate as an impurity and deteriorates toughness. Therefore, the S content is 0.020% or less, preferably 0.010% or less. On the other hand, since the lower the S content, the better, the lower limit of the S content is not particularly limited, and the S content may be 0% or more, or may be more than 0%. However, since excessive reduction increases the manufacturing cost, from the viewpoint of manufacturing cost, the S content is preferably 0.0005% or more, and more preferably 0.001% or more.
  • Al 0.06% or less
  • Al is an element that acts as a deoxidizing agent and is generally used in the molten steel deoxidizing process. Further, Al fixes N in the steel as AlN and contributes to the improvement of the toughness of the base material. However, when the Al content exceeds 0.06%, the toughness and total elongation of the base material (thick steel plate) are reduced, and Al is mixed in the weld metal portion during welding, and the toughness of the weld portion is deteriorated. Therefore, the Al content is 0.06% or less, preferably 0.05% or less.
  • the lower limit of the Al content is not particularly limited, but the Al content is preferably 0.01% or more from the viewpoint of enhancing the effect of adding Al.
  • N 0.0060% or less
  • N is an element that combines with Al in steel to form AlN and contributes to the improvement of strength through refinement of crystal grains during hot rolling.
  • the N content is 0.0060% or less, preferably 0.0050% or less.
  • the lower limit of the N content is not particularly limited, but the N content is preferably 0.0020% or more from the viewpoint of enhancing the effect of adding N.
  • the thick steel plate according to one embodiment of the present invention may have a component composition containing the above elements and the balance Fe and inevitable impurities.
  • the component composition of the thick steel plate according to another embodiment of the present invention may further contain at least one of the following elements.
  • properties such as strength, toughness, weldability, and weather resistance of the thick steel plate can be further improved.
  • Cr 0.01-1.00%
  • Cr is an element having an effect of further improving strength and weather resistance. Further, Cr is an element that promotes the production of cementite, and promotes the production of a pearlite phase that is advantageous in fatigue resistance.
  • the Cr content is set to 0.01% or more, preferably 0.10% or more in order to obtain the above effect. On the other hand, if the Cr content exceeds 1.00%, the weldability and toughness are impaired. Therefore, the Cr content is 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.
  • Cu 0.01-1.00%
  • Cu is an element having the effects of further increasing strength by solid solution and improving weather resistance.
  • the Cu content is set to 0.01% or more in order to obtain the above effect.
  • the Cu content exceeds 1.00%, the weldability is impaired, and defects are likely to occur during the production of thick steel plates. Therefore, the Cu content is 1.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.
  • Ni 0.01-1.00%
  • Ni is an element having an effect of improving low temperature toughness and weather resistance, and Ni improves hot embrittlement when Cu is added.
  • the Ni content is set to 0.01% or more in order to obtain the above effect.
  • the Ni content is 1.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.
  • Mo 0.01-1.00%
  • Mo is an element having an effect of further improving strength.
  • the Mo content is 0.01% or more in order to obtain the above effect.
  • the Mo content exceeds 1.00%, the weldability and toughness are impaired. Therefore, the Mo content is 1.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.
  • Nb 0.005 to 0.050%
  • Nb is an element that has an effect of suppressing recrystallization of austenite during hot rolling and refining the finally obtained crystal grains. Further, Nb precipitates during air cooling after accelerated cooling, and further improves the strength.
  • the Nb content is 0.005% or more in order to obtain the above effect.
  • the Nb content exceeds 0.050%, the hardenability becomes excessive and martensite is formed, so that the desired structure cannot be obtained and the toughness decreases. Therefore, the Nb content is 0.050% or less, preferably 0.040% or less.
  • V 0.005 to 0.050%
  • V is an element that precipitates during air cooling after accelerated cooling and has the effect of further improving strength.
  • the V content is set to 0.005% or more in order to obtain the above effect.
  • the V content exceeds 0.050%, the weldability and toughness deteriorate. Therefore, the V content is 0.050% or less, preferably 0.030% or less.
  • Ti 0.005 to 0.050%
  • Ti is an element which has the effect of further increasing the strength and improving the toughness of the weld.
  • the Ti content is 0.005% or more in order to obtain the above effect.
  • the Ti content exceeds 0.050%, the cost increase becomes remarkable. Therefore, the Ti content is 0.050% or less, preferably 0.030% or less, more preferably 0.020% or less.
  • B 0.0001 to 0.0050%
  • B is an element having the effect of enhancing the hardenability and, as a result, further improving the strength.
  • the B content is 0.0001% or more in order to obtain the above effect.
  • the B content is 0.0050% or less, preferably 0.0030% or less.
  • Ca 0.0001 to 0.020%
  • Ca is an element that controls the morphology of sulfides and, as a result, has the effect of further improving toughness.
  • the Ca content is 0.0001% or more in order to obtain the above effect.
  • the Ca content exceeds 0.020%, the effect is saturated. Therefore, the Ca content is 0.020% or less.
  • Mg 0.0001 to 0.020%
  • Mg is an element that has the effect of improving toughness through the refinement of crystal grains.
  • the Mg content is 0.0001% or more in order to obtain the above effect.
  • the Mg content exceeds 0.02%, the effect is saturated. Therefore, the Mg content is 0.020% or less.
  • REM 0.0001 to 0.020% REM (rare earth metal) is an element having an effect of improving toughness.
  • the REM content is 0.0001% or more in order to obtain the above effect.
  • the REM content exceeds 0.020%, the effect is saturated. Therefore, the REM content is 0.020% or less.
  • the thick steel plate according to one embodiment of the present invention includes, by area fraction, 75 to 97% bainite and 3 to 25% pearlite, and the bainite crystal grain size is 18 ⁇ m or less in terms of an average circle equivalent diameter. It has a microstructure having a particle diameter of 10 ⁇ m or less in terms of an average circle equivalent diameter.
  • the microstructure in the present invention refers to the microstructure at the 1/4 position (1/4 t position) of the plate thickness t of the thick steel plate.
  • the area fraction and crystal grain size of each structure can be measured by observing a cross section parallel to the rolling direction at a depth of 1/4 from the surface of the thick steel plate and observing it. More specifically, the area fraction and the crystal grain size can be determined by the method described in the examples.
  • bainite is the first phase in the microstructure and functions as a soft phase.
  • Ferrite is a typical soft phase contained in steel materials, but bainite is more effective in suppressing crack growth than ferrite. Therefore, by setting the area fraction of bainite to 75% or more, the growth of fatigue cracks can be suppressed. If the area fraction of bainite is less than 75%, the desired fatigue crack propagation resistance cannot be obtained.
  • the area fraction of bainite is preferably 80% or more. On the other hand, when the area fraction of bainite exceeds 97%, pearlite becomes insufficient, and as a result, fatigue crack propagation cannot be suppressed. Therefore, the area fraction of bainite is set to 97% or less.
  • Bainite crystal grain size 18 ⁇ m or less
  • the bainite crystal grain size is 18 ⁇ m or less in terms of average circle equivalent diameter.
  • By refining bainite desired toughness and total elongation characteristics can be obtained. If the bainite crystal grain size exceeds 18 ⁇ m in terms of average circle equivalent diameter, desired toughness and total elongation cannot be obtained.
  • the lower limit of the grain size of bainite is not particularly limited, excessive grain refining makes production difficult. Therefore, in actual production, the grain size of bainite is preferably 5 ⁇ m or more.
  • the bainite in the present invention includes upper bainite, acicular ferrite, and granular bainite.
  • pearlite is the second phase in the microstructure and functions as a hard phase.
  • the area fraction of pearlite is set to 3% or more, preferably 5% or more.
  • the area fraction of pearlite is 25% or less, preferably 20% or less.
  • Crystal grain size of pearlite 10 ⁇ m or less
  • the crystal grain size of pearlite is 10 ⁇ m or less in terms of average circle equivalent diameter.
  • desired toughness and total elongation characteristics can be obtained. If the crystal grain size of pearlite is more than 10 ⁇ m in average circle equivalent diameter, desired toughness and total elongation cannot be obtained.
  • the lower limit of the crystal grain size of pearlite is not particularly limited, but may be 1 ⁇ m or more and may be 2 ⁇ m or more.
  • the perlite in the present invention includes perlite and pseudo perlite.
  • the thick steel plate according to the embodiment of the present invention may have a microstructure composed of bainite and pearlite.
  • the microstructure may further optionally include other tissues.
  • the other structure may be, for example, one or both of martensite and ferrite.
  • the martensite includes island martensite, lath martensite, and lens martensite.
  • the area fraction (total area fraction) of the other organizations is not particularly limited.
  • the area fraction of the other organizations is preferably 5% or less.
  • the thick steel plate according to the embodiment of the present invention 75-97% bainite, It may have a microstructure consisting of 3 to 25% pearlite, and 0 to 5% bainite and a structure other than pearlite.
  • a steel plate having a plate thickness of 6 mm or more is defined as a “thick steel plate” according to the usual definition.
  • the upper limit of the plate thickness is not particularly limited, but the present invention is particularly suitably applied to a relatively thin thick steel plate. Therefore, the plate thickness of the thick steel plate in the present invention is preferably 25 mm or less, and more preferably less than 20 mm.
  • the thick steel plate of the present invention can have excellent tensile strength (TS) as a result of having the above-described composition and microstructure.
  • TS tensile strength
  • the value of TS is not particularly limited, it is preferably 500 MPa or more, more preferably 530 MPa or more, still more preferably 550 MPa or more.
  • the upper limit of TS is not limited, but may be, for example, 720 MPa or less, 700 MPa or less, 640 MPa or less, or 620 MPa or less.
  • the yield stress (YS) of the thick steel plate of the present invention is not particularly limited, but may be 420 MPa or higher, 430 MPa or higher, and 440 MPa or higher. Further, YS may be 560 MPa or less, 530 MPa or less, and 520 MPa or less.
  • the thick steel plate of the present invention has excellent toughness as a result of having the above-described composition and microstructure.
  • the toughness of the thick steel sheet of the present invention is not particularly limited, but the Charpy absorbed energy vE 0 at 0° C., which is one of the indicators of toughness, is preferably 100 J or more, more preferably 130 J or more, and 150 J or more. More preferably, and most preferably 200 J or more.
  • the upper limit of vE 0 is not limited, but may be, for example, 400 J or less, 300 J or less, or 270 J or less. Note that vE 0 can be measured by the method described in the examples.
  • the thick steel plate of the present invention has excellent total elongation (EL) as a result of having the above-described composition and microstructure.
  • EL total elongation
  • the value of EL is not particularly limited, it is preferably 15% or more, more preferably 16% or more, further preferably 17% or more, and most preferably 20% or more.
  • the upper limit of EL is also not particularly limited, but may be 30% or less. EL can be measured by the method described in the examples.
  • the difference between the Vickers hardness at a position 1 mm deep from the surface of the thick steel plate and the Vickers hardness at the plate thickness center portion of the thick steel plate (hereinafter referred to as “hardness difference”) is 40 HV or less.
  • the lower limit of the hardness difference may be 0HV.
  • the hardness difference may be, for example, 10 HV or more. The hardness difference can be measured by the method described in the examples.
  • the thick steel plate of the present invention can have excellent fatigue crack propagation resistance in all of the plate thickness direction, rolling direction, and width direction.
  • the fatigue crack propagation resistance (da/dN) can be used as an index of fatigue crack propagation resistance.
  • the value of the fatigue crack propagation velocity is not particularly limited.
  • the fatigue crack propagation speed in the plate thickness direction (Z direction) preferably satisfies the following conditions (a) and (b).
  • either one of the fatigue crack propagation speed in the rolling direction (L direction) and the fatigue crack propagation speed in the width direction (C direction) satisfies the following conditions (c) and (d): It is more preferable that both satisfy the conditions of (c) and (d).
  • C Fatigue crack propagation velocity under the condition of stress intensity factor range ⁇ K: 15 MPa/m 1/2 is 1.75 ⁇ 10 ⁇ 8 (m/cycle) or less
  • D Fatigue crack propagation velocity under the condition of stress intensity factor range ⁇ K: 25 MPa/m 1/2 is 8.50 ⁇ 10 ⁇ 8 (m/cycle) or less
  • the thick steel plate in one embodiment of the present invention can be manufactured by sequentially performing the following steps (1) to (3) on a steel material having the above-described composition. (1) Heating (2) Hot rolling (3) Accelerated cooling
  • the temperature refers to the surface temperature of the object to be treated (steel material or hot rolled steel sheet).
  • the cooling rate is the cooling rate at the average temperature in the thickness direction of the steel sheet.
  • Step material Any steel material may be used as long as it has the above-described composition.
  • the composition of the thick steel plate finally obtained is the same as that of the steel material used.
  • the steel material for example, a steel slab can be used.
  • Heating Heating temperature 1000 to 1250°C
  • the steel material is heated to a heating temperature of 1000° C. or higher and 1250° C. or lower. If the heating temperature is lower than 1000°C, the temperature required for the next hot rolling cannot be secured. On the other hand, when the heating temperature exceeds 1250° C., the crystal grains of steel become coarse and the toughness deteriorates.
  • the heated steel material is hot rolled into a hot rolled steel sheet.
  • the cumulative rolling reduction in the hot rolling needs to satisfy the following conditions.
  • Cumulative rolling reduction in the temperature range of 950° C. or higher 80% or more
  • the austenite grains are refined.
  • bainite produced by transformation during accelerated cooling and pearlite produced from untransformed austenite are refined. If the cumulative rolling reduction is less than 80%, the bainite and pearlite are not sufficiently refined, and the toughness is lowered, so that the total elongation is deteriorated.
  • the upper limit of the cumulative rolling reduction ratio in the temperature range of 950° C. or higher is not particularly limited, but may be 90% or lower, for example.
  • Cumulative rolling reduction in temperature range below 950°C, Ar3 point or higher 50% or more
  • austenite grains are refined and accelerated. Bainite produced by transformation during cooling and pearlite produced from untransformed austenite are refined.
  • the upper limit of the cumulative rolling reduction ratio in a temperature range of less than 950° C. and three or more points of Ar is not particularly limited, but may be 80% or less and 75% or less, for example.
  • Ar3 point can be obtained by the following formula.
  • Ar3(°C) 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
  • the element symbol in the above formula represents the content (mass %) of the element in the steel material, and is set to zero when the element is not contained in the steel material.
  • Cooling start temperature Ar 3 points or more If the cooling start temperature in the above accelerated cooling is less than Ar 3 points, ferrite and coarse pearlite excessively precipitate, and the strength and fatigue crack propagation resistance decrease. Therefore, the cooling start temperature is set to 3 Ar or higher.
  • the upper limit of the cooling start temperature is not particularly limited, but it is preferably 870° C. or lower from the viewpoint of ensuring the cumulative reduction ratio in the temperature range of Ar 3 points or higher.
  • the fact that the cooling start temperature is at or above the Ar 3 point means that the rolling end temperature is at or above the Ar 3 point. If the rolling end temperature is less than Ar3 points, the rolling will be in the two-phase region and the total elongation will be deteriorated. If the rolling end temperature is at or above the Ar3 point, rolling will be performed in the austenite single-phase region, so that the total elongation will be deteriorated. Can be prevented.
  • Cooling stop temperature 450-700°C
  • the cooling stop temperature in the accelerated cooling is set to 700°C or lower, preferably 650°C or lower.
  • the cooling stop temperature exceeds 700° C.
  • the cooling stop temperature is lower than 450° C.
  • the amount of martensite produced increases, so that a desired microstructure cannot be obtained and toughness and total elongation decrease.
  • the cooling stop temperature is 450° C. or higher, preferably 500° C. or higher, and more preferably 550° C. or higher.
  • Average cooling rate 20-60°C/s
  • the average cooling rate in the accelerated cooling is 20° C./s or more. If the average cooling rate is lower than 20° C./s, ferrite is generated and the desired microstructure is not formed, so that the fatigue crack propagation resistance decreases. Further, since the toughness decreases, the desired total elongation cannot be obtained. On the other hand, if the average cooling rate exceeds 60° C./s, residual stress due to cooling strain and excessive martensite occur, resulting in deterioration of total elongation. Therefore, the upper limit of the cooling rate is set to 60°C/s.
  • the average cooling rate is preferable to reduce the average cooling rate. Specifically, by setting the average cooling rate to 50° C./s or less, the hardness difference can be set to 40 HV or less.
  • the said average cooling rate shall point out the average cooling rate in the steel plate surface from the acceleration cooling start to the acceleration cooling stop.
  • the method for performing the accelerated cooling is not particularly limited, and any method can be used.
  • the accelerated cooling can be performed by intermittent cooling in which water cooling and air cooling are alternately repeated. After water cooling for a certain time from the start of cooling, by stopping the water cooling and air cooling, the heat retained by the central part of the steel sheet that has not yet fully cooled causes recuperation on the steel sheet surface side and the temperature distribution in the thickness direction. Becomes uniform. Then, accelerated cooling by water cooling is performed again from the reheated temperature range. By repeating the water cooling and the air cooling at least once, the average cooling rate in the vicinity of the surface can be controlled within a predetermined range and the formation of the hard phase can be suppressed.
  • the processing after the above accelerated cooling is not particularly limited.
  • the thick steel plate after the accelerated cooling can be left to cool in the atmosphere.
  • the temperature can be cooled to room temperature, for example.
  • the warp of the thick steel plate can be optionally corrected by a hot leveler.
  • the thick steel plate of the present invention is manufactured by an on-line process using equipment provided with a rolling device and an accelerated cooling device on the transportation line.
  • the thick steel plate was manufactured by the following procedure.
  • a steel slab (steel material) having the chemical composition shown in Table 1 was produced by the converter-continuous casting method.
  • the thickness of the steel slab was as shown in Table 2.
  • the steel slab was heated to the heating temperature shown in Table 2 and then hot-rolled at the cumulative rolling reduction shown in Table 2 to obtain a hot-rolled steel sheet.
  • Table 2 also shows the rolling end temperature in the hot rolling and the plate thickness (final plate thickness) of the obtained hot rolled steel plate. Then, the hot rolled steel sheet was accelerated and cooled under the conditions shown in Table 2 to obtain a thick steel sheet.
  • the plate thickness of the obtained thick steel plate is the same as the final plate thickness.
  • the length direction cross section refers to a cross section perpendicular to the width direction of the thick steel plate.
  • SEM scanning electron microscope
  • Crystal grain size of bainite was measured using the sample for microstructure observation.
  • the surface of the sample was mirror-polished, and the crystal orientation was measured from the electron backscattering diffraction image using an Electron Back-Scattering Pattern (EBSP) device attached to SEM.
  • EBSP Electron Back-Scattering Pattern
  • a region surrounded by 200 ⁇ m squares was measured at intervals of 0.3 ⁇ m, and a region surrounded by grain boundaries having a crystal orientation difference between adjacent crystal grains of 15° or more was defined as a crystal grain.
  • the average equivalent circle diameter was determined.
  • the obtained average equivalent circle diameter is regarded as the grain size of bainite.
  • a Charpy impact test piece was sampled from the center of the plate thickness of the thick steel plate in parallel with the rolling direction (L direction), and a Charpy impact test was performed at 0° C. according to JIS Z 2202 to obtain an absorbed energy vE 0 . It was measured.
  • the difference (hardness difference) between the Vickers hardness at the position 1 mm deep from the surface of the thick steel plate and the Vickers hardness at the central portion of the thickness of the thick steel plate was measured by the following procedure. After the cross section of the sample used for the observation of the microstructure was mirror-polished, the Vickers hardness was measured according to JIS Z 2244. The measurement was carried out at 3 points each at a position 1 mm deep from the surface of the thick steel plate and at the center of the plate thickness, and an average value was obtained. The load during measurement was 10 kgf. The obtained average value was used to calculate the difference (hardness difference) between the Vickers hardness at the position 1 mm deep from the surface of the thick steel plate and the Vickers hardness at the plate thickness center portion of the thick steel plate.
  • the fatigue crack propagation speed (da/dN) in the plate thickness direction (Z direction), rolling direction (L direction), and width direction (direction perpendicular to rolling direction, C direction). was measured under two conditions of stress intensity factor range ⁇ K: 15 MPa/m 1/2 and 25 MPa/m 1/2 .
  • the fatigue crack propagation speed in the rolling direction was measured using a test piece taken from a thick steel plate such that the load direction was the rolling direction.
  • the fatigue crack propagation speed in the width direction was measured using a test piece taken from a thick steel plate such that the load direction was the width direction.
  • the test piece was a compact tension test piece according to ASTM E647. In the above measurement, a fatigue crack propagation test was carried out based on the crack gauge method, and the fatigue crack propagation rate was obtained.
  • the one-side notched simple tensile fatigue test piece shown in Fig. 1 was used.
  • the test piece was sampled from a thick steel plate, and the fatigue crack propagation speed when the crack propagated in the plate thickness direction was measured.
  • the thick steel plate satisfying the conditions of the present invention had extremely excellent properties satisfying all the following conditions. In particular, it had both excellent fatigue crack propagation resistance and total elongation, and was also excellent in fatigue crack propagation resistance in all of the plate thickness direction, rolling direction, and width direction.
  • the thick steel plate of the comparative example that did not satisfy the conditions of the present invention did not satisfy at least one of the following conditions.
  • ⁇ TS 500 MPa or more
  • ⁇ EL 15% or more (when using JIS No. 1A test piece)
  • EL 19% or more (when using JIS No.

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Abstract

L'invention fournit une tôle d'acier épaisse qui, tout en combinant une excellente résistance à la propagation de fissures dues à la fatigue et un excellent allongement total, présente une excellente résistance à la propagation de fissures dues à la fatigue à la fois dans une direction épaisseur, dans une direction laminage et dans une direction largeur.　Plus précisément, l'invention concerne une tôle d'acier épaisse qui possède une composition comprenant, en % en masse, ０,０１ à ０,１６％ de C, １,００％ ou moins de Si, ０,５０ à ２,００％ de Mn, ０,０３０％ ou moins de P, ０,０２０％ ou moins de S, ０,０６％ ou moins de Al, et ０,００６０％ ou moins de N, le reste étant constitué de Fe et des impuretés inévitables, et qui possède une structure contenant en pourcentage surfacique ７５ à ９７％ d'une bainite, et ３ à ２５％ d'une perlite, le diamètre de grains cristallins de la bainite étant inférieur ou égal à １８μｍ en termes de diamètre moyen de cercle équivalent, et le diamètre de grains cristallins de la perlite étant inférieur ou égal à １0μｍ en termes de diamètre moyen de cercle équivalent.
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