Classifications

• • 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
  • B21B1/24—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 in a continuous or semi-continuous process
  • B21B1/26—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 in a continuous or semi-continuous process by hot-rolling, e.g. Steckel hot mill
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/12—Accessories for subsequent treating or working cast stock in situ
  • B22D11/128—Accessories for subsequent treating or working cast stock in situ for removing
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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 steel plate suitable for use in steel structures such as liquefied gas storage tanks, ships, and marine structures, and a method for manufacturing the same.
• CCS Carbon Capture and Storage
• CO2 Carbon Capture and Storage
• 9% Ni steel which has good toughness even at low temperatures, has traditionally been used.
• storage tanks are manufactured by bending steel plates. Generally, steel plates are strained during processing, and their toughness deteriorates with subsequent aging. Therefore, ensuring low-temperature toughness after strain aging is important from the perspective of ensuring the tank's fracture safety.
• the Charpy test has been used primarily to evaluate the toughness of steel, but in recent years, the Crack Tip Opening Displacement Test (hereinafter referred to as the "CTOD test”) has been increasingly applied to thick steel plates used in steel structures as a method for evaluating fracture resistance with greater precision.
• COD test Crack Tip Opening Displacement Test
• the toughness evaluation area is an extremely small area, and if a localized embrittlement zone is present, the CTOD test may show low toughness even if good toughness is obtained in the Charpy impact test.
• Ni content is known as a way to improve the low-temperature toughness of steel, and 9% Ni steel is used on a commercial scale for liquefied natural gas (LNG) storage tanks and the like.
• LNG liquefied natural gas
• the TMCP method is used to refine the crystal grains, thereby improving low-temperature toughness.
• Patent Documents 1 and 2 propose techniques for improving low-temperature toughness by optimizing the heating temperature and hot rolling conditions to refine the crystal grain size.
• Patent Document 3 proposes a technique for improving low-temperature toughness by adding 5.0% or more Ni.
• Liquefied CO2 storage tanks that are loaded onto ships for long-distance transport of CO2 from emission sites to storage sites require steel plates with a yield strength of 320 MPa or more, excellent CTOD properties, and low-temperature toughness at the center of the plate thickness after strain aging.
• Patent Documents 1 and 2 propose technologies for improving the low-temperature toughness of steel sheets before processing, but no consideration is given to CTOD properties or toughness after strain aging, and it cannot be said that the toughness of the center of the sheet thickness after embrittlement due to CTOD properties or strain aging is sufficiently ensured.
• Patent Document 3 proposes a technology for ensuring low-temperature toughness by adding 5.0% or more Ni, but since an increase in the Ni content necessitates a significant increase in alloy costs, it is difficult to apply this to steel materials for CO2 storage tanks, which require low-cost transportation. Furthermore, no consideration has been given to the CTOD characteristics and toughness after strain aging.
• Patent Documents 1 to 3 did not consider CTOD properties and low-temperature toughness in the center of the plate thickness after strain aging, and it was not possible to achieve both high strength, CTOD properties, and toughness in the center of the plate thickness after strain aging while suppressing alloy costs.
• the present invention was made in consideration of the above-mentioned problems with conventional technology, and its purpose is to provide a steel plate that has high strength, excellent CTOD characteristics, and excellent low-temperature toughness in the center of the plate thickness after strain aging, along with a manufacturing method thereof.
• high strength in the present invention refers to a yield strength of 320 MPa or more in a tensile test at the center of the plate thickness.
• Excellent CTOD characteristics refers to a crack tip opening displacement of 0.10 mm or more in a CTOD test at a test temperature of -55°C.
• Excellent low-temperature toughness refers to an average absorbed energy vE -60 of 50 J or more in a Charpy test at -60°C at the center of the plate thickness.
• the center of the plate thickness refers to a region having a thickness of 10% of the plate thickness from the center of the plate thickness in both surface directions of the steel plate.
• austenite recrystallization can be promoted by rolling at a rolling reduction/pass average value of 3.5% or more in the recrystallization temperature range of 950°C or higher, with a cumulative reduction of 40% or more.
• Sufficient strain can then be introduced into the center of the thickness by rolling at a rolling reduction/pass average value of 3.5% or more between the Ar3 point and 950°C, with a cumulative reduction of 40% or more. It was found that this allows the average effective grain size in the center of the thickness to be 20 ⁇ m or less.
• the central segregation area is harder than the surrounding area, it becomes the starting point of fracture, reducing CTOD characteristics and low-temperature toughness.
• Low-temperature toughness also decreases after strain aging, and the amount of this decrease increases the higher the content of impurity elements such as P. Therefore, improving central segregation is important in order to ensure low-temperature toughness after strain aging.
• center segregation can be reduced, hardening of the center segregated area can be inhibited, and the number of segregated grains can be reduced. This can improve CTOD characteristics and low-temperature toughness after strain aging.
• the inventors discovered that the combination of (1) and (2) above not only provides excellent strength and toughness to the base material, but also excellent toughness in the center of the plate thickness after strain aging; specifically, it combines high strength, CTOD characteristics, and low-temperature toughness after strain aging.
• the present invention has been completed based on the above findings and further investigations. That is, the gist of the present invention is as follows.
• the component composition is, in mass%, C: 0.03-0.15%, Si: 0.50% or less, Mn: 0.3 to 2.5%, P: 0.030% or less, S: 0.0050% or less, Ni: 0.01 to 5.00%, Al: 0.005-0.100%, N: 0.0100% or less, and O: 0.0100% or less, and Ceq defined by formula (1) satisfies formula (2), with the remainder being Fe and inevitable impurities;
• the hardness of the center segregation part of the steel plate satisfies formula (3), the number density of segregated grains having a circle equivalent diameter of 100 ⁇ m or more is 2.0 grains/ mm2 or less, A steel sheet having an average effective grain size of 20 ⁇ m or less in the center portion of the sheet thickness.
• Hv max is the maximum value of the Vickers hardness of the central segregation portion
• Hv ave is the average value of the Vickers hardness at the 1/8 to 3/8 positions and the 5/8 to 7/8 positions in the plate thickness
• t is the plate thickness (mm) of the steel plate.
• C 0.03-0.15%
• C is an element that improves hardenability and improves the strength of steel, and a content of 0.03% or more is required to achieve these effects.
• the C content is set to 0.03% or more, preferably 0.05% or more.
• the C content is set to 0.15% or less, preferably 0.12% or less.
• Si 0.50% or less Si is also used as a deoxidizer and is an element that is inevitably contained as an impurity, but if the Si content exceeds 0.50%, not only will the surface properties of the steel be impaired, but the CTOD properties and low-temperature toughness will also decrease. Therefore, the upper limit of the Si content is limited to 0.50%, and preferably 0.40% or less. The lower limit of the Si content is not particularly limited, but is preferably 0.04% or more.
• Mn 0.3-2.5%
• Mn is an element that has the effect of improving the hardenability and strength of steel.
• the Mn content is set to 0.3% or more, preferably 0.8% or more.
• the Mn content is set to 2.5% or less, preferably 2.3% or less.
• P 0.030% or less
• P is an element that has a large effect of embrittling grain boundaries, and if contained in large amounts, it reduces CTOD properties and low-temperature toughness, so the P content is limited to 0.030% or less.
• the P content is preferably 0.020% or less. It is desirable to reduce the P content as much as possible, and there is no particular lower limit for the P content, but excessively low P content leads to increased refining time and costs, so the P content is preferably 0.001% or more.
• S 0.0050% or less
• S is an element that reduces CTOD characteristics and low-temperature toughness, so the upper limit of the S content is limited to 0.0050%.
• the S content is preferably 0.0030% or less. It is desirable to reduce the S content as much as possible, and there is no lower limit for the S content. However, excessively low S content increases refining time and costs, so the S content is preferably 0.0001% or more.
• Ni 0.01-5.00%
• Ni is an element effective in improving the strength, CTOD characteristics, and low-temperature toughness of the base metal, and must be contained in an amount of 0.01% or more.
• the Ni content is set to 5.00% or less, preferably 4.80% or less, and more preferably 3.50% or less.
• the Ni content is preferably 0.10% or more.
• Al 0.005-0.100%
• Al is an element added to deoxidize molten steel, and for this purpose, an Al content of 0.005% or more is required. However, if the Al content exceeds 0.100%, the CTOD properties and low-temperature toughness deteriorate. Therefore, the Al content is set to 0.100% or less, and preferably 0.080% or less. The Al content is preferably 0.010% or more.
• N 0.0100% or less
• N is an element that reduces CTOD characteristics and low-temperature toughness, so the upper limit of the N content is limited to 0.0100%.
• the N content is preferably 0.0090% or less. It is desirable to reduce the N content as much as possible, and there is no lower limit for the N content. However, excessive reduction in N content increases refining time and costs, so the N content is preferably 0.0005% or more, and more preferably 0.0010% or more.
• O 0.0100% or less Since O is an element that reduces CTOD characteristics and low-temperature toughness, the upper limit of the O content is limited to 0.0100%.
• the O content is preferably 0.0090% or less. It is desirable to reduce the O content as much as possible, and there is no lower limit for the O content, but excessive reduction in O content leads to an increase in refining time and an increase in costs, so the O content is preferably 0.0005% or more, and more preferably 0.0010% or more.
• the basic composition of the steel plate of the present invention (hereinafter sometimes simply referred to as "thick steel plate") consists of the above elements, with the balance being Fe and unavoidable impurities. With this basic composition, the steel plate of the present invention can achieve the desired properties.
• one or more elements selected from the group consisting of Cu, Cr, Mo, V, Ti, Nb, B, Ca, W, REM, and Mg can be optionally contained in the amounts shown below. Note that, as each of the components Cu, Cr, Mo, V, Ti, Nb, B, Ca, W, REM, and Mg shown below can be contained as needed, their contents may be 0%.
• Cu 1.50% or less Cu may be added as needed.
• Cu is an element that can increase the strength of steel plates without significantly deteriorating low-temperature toughness, but if the Cu content exceeds 1.50%, surface cracking due to a Cu-enriched layer formed directly below the scale becomes a problem. Therefore, when Cu is contained, it is preferable to limit the Cu content to 1.50% or less.
• the Cu content is more preferably 1.30% or less.
• the Cu content is preferably 0.05% or more in order to fully obtain its effects.
• Cr 1.50% or less Cr may be added as needed. Cr is an element that improves the hardenability and strength of steel, but if the Cr content exceeds 1.50%, the CTOD characteristics and low-temperature toughness will deteriorate. Therefore, when Cr is contained, the Cr content is preferably 1.50% or less. The Cr content is more preferably 1.30% or less. When Cr is contained, in order to fully obtain its effects, the Cr content is preferably 0.01% or more, more preferably 0.10% or more.
• Mo 1.50% or less Mo may be added as needed. Mo is an element that improves the hardenability and strength of steel, but if the Mo content exceeds 1.50%, the CTOD characteristics and low-temperature toughness decrease. Therefore, when Mo is contained, the Mo content is preferably 1.50% or less. The Mo content is more preferably 1.30% or less. When Mo is contained, the Mo content is preferably 0.10% or more in order to fully obtain its effects.
• V 0.20% or less V may be added as needed.
• V is an element that improves the strength of the base metal, but if the V content exceeds 0.20%, the CTOD characteristics and low-temperature toughness decrease. Therefore, when V is contained, the V content is preferably limited to 0.20% or less. The V content is more preferably 0.15% or less. When V is contained, the V content is preferably 0.01% or more in order to fully obtain its effects.
• Ti 0.100% or less Ti may be added as needed. Ti precipitates in steel as TiN. The precipitated TiN has the effect of suppressing coarsening of austenite grains, and by refining the crystal grain size, improves the CTOD properties and low-temperature toughness. On the other hand, if the Ti content exceeds 0.100%, the precipitation of solute Ti and coarse TiC will actually deteriorate the CTOD properties and low-temperature toughness. Therefore, when Ti is contained, it is preferable to limit the Ti content to 0.100% or less. The Ti content is more preferably 0.080% or less. When Ti is contained, the Ti content is preferably 0.005% or more in order to fully obtain its effects.
• Nb 0.100% or less Nb may be added as needed.
• Nb is an element that is effective in improving the strength of the base material, but a Nb content exceeding 0.100% reduces the CTOD characteristics and low-temperature toughness. Therefore, when Nb is contained, the Nb content is preferably limited to 0.100% or less. The Nb content is more preferably 0.050% or less. When Nb is contained, the Nb content is preferably 0.005% or more in order to fully obtain its effects.
• B 0.0050% or less B may be added as needed.
• B is an element that can improve hardenability even with a very small amount of B, thereby improving the strength of the steel sheet.
• the B content is preferably 0.0050% or less, and more preferably 0.0030% or less.
• the B content is preferably 0.0005% or more in order to fully obtain its effects.
• Ca 0.0100% or less
• Ca is an element that improves the toughness of the weld heat-affected zone by forming oxysulfides that are highly stable at high temperatures. Since the effects of the present invention are not impaired even when Ca is contained, it may be added as needed. However, if Ca is contained in an amount exceeding 0.0100%, coarse inclusions are formed, deteriorating the CTOD characteristics and low-temperature toughness. Therefore, when Ca is contained, the Ca content is preferably limited to 0.0100% or less. The Ca content is more preferably 0.0080% or less. When Ca is contained, in order to fully obtain its effects, the Ca content is preferably 0.0002% or more, more preferably 0.0015% or more.
• W 0.50% or less W may be added as needed.
• W is an element that improves the strength of the base metal, but if the W content exceeds 0.50%, weldability decreases. Therefore, when W is contained, it is preferable to limit the W content to 0.50% or less.
• the W content is more preferably 0.40% or less.
• the W content is preferably 0.02% or more, more preferably 0.10% or more.
• REM 0.025% or less REM may be added as needed.
• REM rare earth metal
• the REM content is more preferably 0.020% or less.
• the REM content is preferably 0.001% or more, more preferably 0.010% or more.
• REM is a collective term for 17 elements, including 15 lanthanoid elements, Y, and Sc, and these elements can be contained alone or in combination. Therefore, the content of REM means the total content of these elements.
• Mg 0.0150% or less Mg may be added as needed.
• Mg is an element that improves weldability by forming oxysulfide inclusions that are highly stable at high temperatures.
• the Mg content exceeds 0.0150%, the effect of adding Mg becomes saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Mg is contained, it is preferable to limit the Mg content to 0.0150% or less.
• the Mg content is more preferably 0.0100% or less.
• the Mg content is preferably 0.0002% or more in order to fully obtain its effect.
• Ceq 0.300% or more and 0.550% or less
• appropriate elements are required to ensure high strength and good low-temperature toughness in the center of the plate thickness.
• the brackets [ ] in formula (1) represent the content (mass %) of the element shown in the brackets, and if the element is not contained, the content is set to zero.
• Hv max /Hv ave is a dimensionless parameter that represents the hardness of the center segregation. If this value is higher than the value calculated by (1.35 + 0.006/[C] - t/500), stress concentration in the center segregation, which is the hardened part, increases, making fracture more likely. As a result, the CTOD value and low-temperature toughness decrease, so it is set to (1.35 + 0.006/[C] - t/500) or less. Hv max /Hv ave is preferably set to (1.25 + 0.006/[C] - t/500) or less.
• Hv max is the maximum value of the hardness values measured using a Vickers hardness tester in a region having a thickness of 10% of the plate thickness from the center of the plate thickness to both surfaces of the steel plate, including the central segregation, at 0.5 mm intervals in the plate thickness direction of the steel plate.
• Hv ave is the average value of the hardness values measured using a Vickers hardness tester in the region from 1/8 to 3/8 and from 5/8 to 7/8 of the plate thickness in the plate thickness direction at 1 mm intervals in the plate thickness direction of the steel plate. In both cases, the load of the Vickers hardness tester is 10 kgf.
• the central segregation present in the steel sheet becomes the fracture initiation point, thereby deteriorating the CTOD characteristics and low-temperature toughness after strain aging.
• the number of segregated grains having a circle-equivalent diameter of 100 ⁇ m or more per mm2 exceeds 2.0 grains/ mm2 , the crack tip opening displacement ( ⁇ ) in the CTOD test and the low-temperature toughness after strain aging are likely to be insufficient.
• the segregated grains in the present invention are portions where [Mn] EPMA /[Mn] ⁇ 1.33 or more, where [Mn] EPMA is the Mn concentration (mass%) at the measurement position, and [Mn] is the Mn content (mass%) in the entire steel sheet.
• the frequency with which the number density of such segregated grains is measured needs only to measure one or two cross sections of any one steel plate among steel plates that have the same slab melting conditions and rolling conditions. As long as the slab melting method and rolling conditions are not changed, the number density of segregated grains can be produced with good reproducibility, so the measurement results at the above measurement frequency can be said to be representative of the whole.
• the "number density of segregated grains" can be measured by the method described in the Examples. Because segregated grains tend to be present in large numbers in the central segregation region, the number density of segregated grains will be small when measured across the entire thickness. Since it is important to control the number density of segregated grains in the center of the plate thickness in order to achieve the desired properties, the number density of segregated grains is evaluated in a region that includes the central segregation region, extending 1.5 mm from the center of the plate thickness toward both surfaces of the steel plate, for a total thickness of 3 mm.
• Average effective grain size at the center of the sheet thickness 20 ⁇ m or less
• the average effective grain size at the center of the sheet thickness is set to 20 ⁇ m or less. Refining the grain size at the center of the sheet thickness can improve strength, CTOD characteristics, and low-temperature toughness after strain aging. On the other hand, if the average effective grain size is larger than 20 ⁇ m, coarse grains become fracture origins, so even if center segregation is reduced, the desired CTOD characteristics and low-temperature toughness after strain aging cannot be obtained.
• the average effective grain size is preferably 18 ⁇ m or less. Since a smaller average effective grain size is more advantageous, no lower limit is particularly specified, but excessive grain refinement increases the manufacturing load, so it is preferable to set it to 1 ⁇ m or more.
• effective grain size is defined as the circle-equivalent diameter of a grain surrounded by a grain boundary whose orientation with respect to adjacent grains is 15° or more, i.e., a high-angle grain boundary.
• the average effective grain size can be measured by the method described in the Examples.
• temperatures refer to temperatures at the center of the plate thickness unless otherwise specified.
• the temperature at the center of the plate thickness can be measured, but in an actual production line, it may also be determined by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer.
• a steel slab satisfying the above-mentioned composition is produced by continuous casting, in which soft reduction is performed two or more times upstream of the final solidification position at a reduction rate of 0.3 mm/min to 2.5 mm/min. If the reduction rate is less than 0.3 mm/min, the reduction amount per unit time is insufficient, the flow of concentrated molten steel cannot be suppressed, and center segregation cannot be reduced.
• the reduction rate exceeds 2.5 mm/min, the reduction amount per unit time becomes too large, pushing the concentrated molten steel in the center of the slab upstream in the casting direction, resulting in negative segregation in the center of the slab due to reduced solute elements.
• the reduction rate is preferably 0.4 mm/min or more and preferably 2.3 mm/min or less.
• the number of times soft reduction is performed at a reduction rate of 0.3 mm/min to 2.5 mm/min is one or less, the effect of sweeping the molten steel in the unsolidified portion upstream becomes insufficient, and the effect of reducing segregation by soft reduction becomes insufficient.
• the number of times soft reduction is performed, but from the viewpoint of cost-effectiveness of installing soft reduction rolls, it is preferably 30 times or less, and more preferably 10 times or less.
• Heating conditions for steel slabs before rolling 990°C or higher and 1250°C or lower
• steel slabs are heated to a heating temperature of 990°C or higher and 1250°C or lower. If the heating temperature is lower than 990°C, deformation resistance during rolling increases, which increases the load on the rolling mill and increases the number of passes, resulting in a decrease in production efficiency.
• the heating temperature is preferably 1000°C or higher.
• the heating temperature should be set to 1250°C or below, and preferably 1230°C or below.
• Hot rolling conditions The purpose of hot rolling in this invention is to appropriately introduce strain into the center of the sheet thickness to obtain the desired fine grain structure. To achieve this, it is important to control both rolling in the recrystallization temperature range of 950°C or higher and rolling at the Ar3 point or higher but lower than 950°C.
• Hot rolling in the recrystallization temperature range The heated steel slab is hot rolled at a temperature of 950° C. or higher. If the hot rolling temperature is less than 950° C., recrystallization does not occur easily, and the austenite grains are not sufficiently refined.
• rolling with an average reduction rate per pass of less than 3.5% fails to introduce sufficient strain into the center of the plate thickness. Even if the average reduction rate per pass is 3.5% or more, if the cumulative reduction rate is less than 40%, recrystallization will not progress sufficiently, resulting in insufficient austenite grain refinement. For this reason, rolling is performed at 950°C or higher with an average reduction rate per pass of 3.5% or more and a cumulative reduction rate of 40% or more.
• the cumulative reduction rate is also preferably 45% or more.
• the average reduction rate per pass is preferably 12.0% or less. Furthermore, because an excessive increase in the cumulative reduction rate makes it impossible to achieve the desired plate thickness, the cumulative reduction rate is preferably 95% or less.
• Hot rolling in the non-recrystallization temperature range After hot rolling in the recrystallization temperature range, further rolling is performed at a temperature between the Ar3 point and less than 950°C, with an average reduction/pass value of 3.5% or more and a cumulative reduction of 40% or more. Because recrystallization is difficult to occur during rolling in the non-recrystallization temperature range, strain introduced by rolling accumulates without being consumed by recrystallization and functions as a transformation nucleus in the subsequent cooling process. As a result, the structure of the final steel plate can be refined. However, rolling with an average reduction/pass value of less than 3.5% fails to introduce sufficient strain into the center of the plate thickness, and rolling with a cumulative reduction rate of less than 40% results in insufficient grain refinement.
• the average reduction/pass value is preferably 4.0% or more.
• the cumulative rolling reduction is preferably 45% or more.
• brackets [ ] represent the content (mass%) of the element shown in the brackets, and if the element is not contained, the content is set to zero.
• Rolling end temperature Ar3 point (°C) or higher
• the heated steel slab is hot rolled at a temperature of Ar3 point or higher.
• the rolling end temperature of hot rolling is Ar3 point or higher. If the rolling end temperature is lower than Ar3 point, the structure before the start of cooling will be a two-phase structure of austenite and ferrite, which will result in poor uniformity of the steel structure and significantly reduced manufacturing stability. Therefore, the rolling end temperature is set to Ar3 point or higher, preferably ( Ar3 point + 10°C) or higher.
• the hot-rolled steel sheet is cooled.
• This cooling can be performed by any method, for example, water cooling, as long as the following conditions are met.
• Average cooling rate 3.0°C/s or more If the average cooling rate at the center of the plate thickness is less than 3.0°C/s, a coarse ferrite phase will form in the structure of the steel plate, deteriorating the strength, CTOD characteristics, and low-temperature toughness. Therefore, the average cooling rate at the center of the plate thickness is set to 3.0°C/s or more.
• the average cooling rate is preferably 4.0°C/s or more and preferably 100°C/s or less.
• the average cooling rate refers to the average value of the cooling rate from 800°C to 500°C.
• the average cooling rate refers to the average value of the cooling rate from 800°C to the cooling stop temperature.
• the rolling end temperature is 800°C or lower
• the average cooling rate refers to the average value of the cooling rate from the rolling end temperature to 500°C.
• the average cooling rate refers to the average value of the cooling rate from the rolling end temperature to the cooling stop temperature.
• Cooling stop temperature 600°C or less
• the hot-rolled steel sheet is cooled to a cooling stop temperature of 600°C or less at the center of the sheet thickness. If the cooling stop temperature is higher than 600°C, the structure after transformation becomes coarse, the base material strength becomes insufficient, and the CTOD characteristics and low-temperature toughness deteriorate. For this reason, the cooling stop temperature is set to 600°C or less.
• the cooling stop temperature is preferably 570°C or less.
• Tempering temperature 700°C or less After cooling is stopped, further tempering can be performed as desired. Tempering can improve the toughness of the base material. On the other hand, if the tempering temperature exceeds 700°C, various carbonitrides precipitate in the steel, and the microstructure obtained by transformation disappears, resulting in a decrease in strength and toughness. Therefore, the tempering temperature is preferably 700°C or less. The tempering temperature is more preferably 650°C or less. The tempering temperature is preferably 300°C or more.
• the number density of segregated grains, Vickers hardness, average effective grain size, yield strength, tensile strength, low-temperature toughness after 5% strain aging, and CTOD value were measured using the following methods.
• the measured Mn concentration is defined as [Mn] EPMA
• the portion where [Mn] EPMA /[Mn] ⁇ 1.33 is defined as a segregated grain.
• the number density of segregated grains was calculated by dividing the number of segregated grains having a circle-equivalent diameter of 100 ⁇ m or more by the measured area.
• a thickness cross section parallel to the rolling direction of the steel plate (i.e., a cross section perpendicular to the width direction) was taken from the width center of the steel plate and mirror-polished.
• the regions at 1/8 to 3/8 and 5/8 to 7/8 of the plate thickness were measured in the thickness direction of the steel plate at 1 mm intervals in the plate thickness direction using a Vickers hardness tester, and the average value of the values was taken as Hv ave .
• the load of the Vickers hardness tester was 10 kgf in both cases.
• CTOD test Full-thickness CTOD test specimens were taken so that the longitudinal direction of the test specimen was perpendicular to the rolling direction, and the crack tip opening displacement was measured.
• CTOD value ( ⁇ ) was evaluated at a test temperature of -55°C.
• the test was conducted using three test specimens, and the minimum measured value was taken as ⁇ .
• specimens satisfying ⁇ 0.10 mm were evaluated as having good CTOD properties.
• Evaluation results are also shown in Table 3.
• the invention examples met the component composition and manufacturing conditions of the present invention, and also met the conditions for hardness of the central segregation region, number density of segregated grains, and average effective crystal grain size, and exhibited high strength and excellent CTOD properties and low-temperature toughness after strain aging.
• the steel plates (comparative examples) that did not meet the conditions of the present invention were poor in one or more of strength, CTOD properties, and low-temperature toughness after strain aging, and were inferior in properties to the invention examples.

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