WO2025192080A1 - Tôle d'acier hautement résistante, et procédé de fabrication de celle-ci - Google Patents

Tôle d'acier hautement résistante, et procédé de fabrication de celle-ci

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Publication number
WO2025192080A1
WO2025192080A1 PCT/JP2025/003147 JP2025003147W WO2025192080A1 WO 2025192080 A1 WO2025192080 A1 WO 2025192080A1 JP 2025003147 W JP2025003147 W JP 2025003147W WO 2025192080 A1 WO2025192080 A1 WO 2025192080A1
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steel sheet
strength
temperature
rolling
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PCT/JP2025/003147
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English (en)
Japanese (ja)
Inventor
重宏 ▲高▼城
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JFE Steel Corp
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JFE Steel Corp
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Priority to JP2025530769A priority Critical patent/JP7758255B1/ja
Publication of WO2025192080A1 publication Critical patent/WO2025192080A1/fr
Pending legal-status Critical Current
Anticipated expiration legal-status Critical

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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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to high-strength steel sheets with a tensile strength (TS) of 980 MPa or more, suitable primarily for use in structural parts of automobile bodies and electrical parts, and to a method for manufacturing such steel sheets.
  • TS tensile strength
  • solid solution strengthening, precipitation strengthening, and dislocation strengthening are known methods for strengthening steel.
  • transformation strengthening To increase the strength of steel plate with a tensile strength of over 980 MPa, it is necessary to increase the proportion of hard structures such as martensite and bainite through transformation strengthening.
  • the steel plate contains more of these hard phases, the steel plate's workability decreases. As a result, it cannot be used in parts that require complex processing, and the range of applicable parts is limited.
  • a major challenge in the development of steel materials for automobiles has been achieving both high strength and high workability while still containing hard phases.
  • Patent Documents 1 and 2 disclose a technique in which, in the final annealing process, the sheet is cooled to a temperature below the Ms point, then reheated, and the martensite formed during cooling is tempered to achieve high stretch flangeability.
  • Patent Document 3 discloses a method in which the steel sheet is tempered after being plated.
  • Patent Document 4 discloses a method for producing steel sheets with good bendability, in which the steel sheet is plated, cooled to below 200°C, and then further tempered in a temperature range of 100°C to 600°C.
  • Patent No. 5463685 International Publication No. 2009/054539 Japanese Patent Application Publication No. 06-108152 Japanese Patent Application Laid-Open No. 2017-48412 Patent No. 6787525 Patent No. 6705561
  • Patent Document 4 discloses an invention that achieves good elongation properties by forming a surface decarburized layer, explaining the effect of soft ferrite on the surface layer to improve bendability.
  • the formation of soft ferrite in the surface layer not only reduces the strength level of the base material, but also increases the number of interfaces between structures with different mechanical properties, such as ferrite and martensite, which promotes the formation of fine cracks and voids during processing.
  • materials with a tensile strength of over 980 MPa are susceptible to hydrogen embrittlement, leading to fracture.
  • the inventors have investigated a manufacturing method for improving the strength-ductility balance and hydrogen embrittlement resistance of high-strength steel plate that contains 70% or more primarily tempered martensite, with the remainder consisting primarily of austenite, and has a tensile strength (TS) of 980 MPa or more.
  • TS tensile strength
  • the present invention was made in consideration of the above circumstances. Specifically, the objective is to provide a high-strength steel sheet that maintains a TS of 980 MPa or more, has excellent hydrogen embrittlement resistance in the bent portion, and has an improved strength-ductility balance without utilizing a softened surface layer, and a manufacturing method for the same.
  • the bent portion is one that has undergone bending processing under particularly strict forming conditions, for example, with a bend angle of more than 30 degrees.
  • the inventors conducted extensive research to resolve the above issues.
  • the inventors also focused on the upstream manufacturing processes, primarily revising the heat treatment method beyond simple temperature and time adjustments.
  • the inventors further conducted the following experiments.
  • the steel sheet was then pickled and cold-rolled to a thickness of 1.0 mm to obtain a cold-rolled steel sheet.
  • the cold-rolled steel sheet was then heat-treated in a nitrogen atmosphere at 675°C for 60 minutes, cooled, and pickled to remove any minor scale formed on the surface.
  • the cold-rolled steel sheet was then soaked at 880°C for 60 seconds in a nitrogen atmosphere, gas-cooled to 150°C, reheated and held at 300°C for 45 seconds, and gas-cooled to room temperature.
  • the obtained steel sheet was pickled, and then a JIS No. 5 test piece (gauge length 50 mm, parallel portion width 25 mm) was cut out in a direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z2241 (2022).
  • the inventors discovered that the strength-ductility balance can be improved by performing a two-stage heat treatment, once before cold rolling and once after cold rolling. As will be described in detail below, they also discovered that hydrogen embrittlement resistance can be improved by combining this with previously unconventional optimization of the addition of Cu, Sb, and Sn, leading to the completion of this invention.
  • the inventors have found that by controlling the structure before cold rolling, performing ultra-strong rolling in cold rolling, and performing slow heating or two-stage annealing (heat treatment) in final annealing, it is possible to highly control the crystal orientation of the steel sheet without utilizing the soft ferrite phase.
  • the steel has a chemical composition comprising at least two of the following in a total amount: 0.01% or more and 1.0% or less; Ti: 0.200% or less, Nb: 0.200% or less, and V: 0.200% or less in a total amount of 0.01% or more; and the balance consisting of Fe and unavoidable impurities.
  • the present invention makes it possible to achieve a better balance of strength and ductility than ever before.
  • the range of applications for automobile body parts can be expanded, and it can also be applied to parts that require complex press working, making a significant contribution to reducing the weight of automobile bodies.
  • C is one of the important basic components of steel, and in the present invention, it is an important element that affects the fractions of martensite, ferrite, and retained austenite. If the C content is less than 0.090%, the fraction of martensite decreases, making it difficult to achieve the desired tensile strength. Furthermore, the reduced amount of carbides reduces the number of ⁇ 100 ⁇ grains, resulting in significant cracking during bending tests. On the other hand, if the C content exceeds 0.300%, martensite becomes embrittled, making it difficult to achieve the desired elongation. Therefore, the C content is set to 0.090% or more and 0.300% or less.
  • the preferred lower limit is 0.120% or more, more preferably 0.150% or more.
  • the preferred upper limit is 0.280% or less, and even more preferably 0.240% or less.
  • Silicon (Si) is one of the important basic components of steel.
  • silicon suppresses carbide formation during continuous annealing and promotes the formation of retained austenite, thereby affecting the hardness of martensite and the fraction of retained austenite. If the Si content is less than 0.40%, the fraction of retained austenite decreases, making it difficult to achieve the desired El. On the other hand, if the Si content exceeds 2.50%, the carbon concentration in the retained austenite increases excessively, reducing local ductility and bending strength. Therefore, the Si content is set to 0.40% or more and 2.50% or less.
  • the preferred lower limit is 0.60% or more, more preferably 0.80% or more.
  • the preferred upper limit is 2.00% or less, more preferably 1.80% or less.
  • Mn is one of the important basic components of steel, and in the present invention, it is an important element that affects the fraction of martensite. If the Mn content is less than 1.8%, the fraction of martensite decreases, making it difficult to achieve a TS of 980 MPa or more. On the other hand, if the Mn content exceeds 4.0%, the fraction of tempered martensite decreases, reducing local ductility. Bendability also decreases. Therefore, the Mn content is set to 1.8% or more and 4.0% or less.
  • the preferred lower limit is 2.0% or more, more preferably 2.2% or more.
  • the preferred upper limit is 3.8% or less, more preferably 3.6% or less.
  • P 0.100% or less
  • P segregates at prior austenite grain boundaries and embrittles the grain boundaries, reducing the local ductility of the steel sheet and decreasing elongation. It also reduces bendability. Therefore, the P content must be 0.100% or less. While there is no particular lower limit for the P content, it is preferable to set it to 0.001% or more because P is a solid-solution strengthening element and can increase the strength of the steel sheet. Therefore, the P content is set to 0.100% or less. Preferably, it is set to 0.070% or less.
  • S 0.0200% or less
  • S exists as sulfides and reduces the local ductility of the steel sheet, thereby reducing elongation. It also reduces bendability. Therefore, the S content must be 0.0200% or less. There is no particular lower limit for the S content, but due to production technology constraints, it is preferable to set it to 0.0001% or more. Therefore, the S content is set to 0.0200% or less, preferably 0.0050% or less.
  • Al 0.200% or less
  • Al raises the A3 transformation point and causes the microstructure to contain a large amount of ferrite, preventing high strength achieved by utilizing the martensite structure. Therefore, the Al content must be 0.200% or less.
  • the Al content is preferably 0.001% or more because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite.
  • the preferred Al content is 0.150% or less.
  • N 0.0200% or less
  • N exists as a nitride and reduces the local ductility of the steel sheet, thereby reducing elongation. It also reduces bendability. Therefore, the N content must be 0.0200% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more. Therefore, the N content is 0.0200% or less, preferably 0.0100% or less.
  • O exists as an oxide and reduces the local ductility of the steel sheet, thereby reducing elongation. It also reduces bendability. Therefore, the O content must be 0.0100% or less. While there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is set to 0.0100% or less, and preferably 0.0050% or less.
  • Ti 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, at least one of which is 0.01% or more]
  • Ti, Nb, and V combine with carbon and nitrogen to refine the crystal structure, thereby increasing the strength of the material. Therefore, it is necessary to contain at least one of them in an amount of 0.01% or more.
  • the amount is more preferably 0.015% or more, and even more preferably 0.04% or more. When two or more elements are contained, the total content thereof must be 0.01% or more.
  • the Ti and Nb contents are preferably each 0.200% or less. While there are no specific lower limits for the Ti and Nb contents, it is more preferable that the Ti and Nb contents be 0.001% or more, since they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, if Ti and Nb are contained, their contents should each be 0.200% or less. More preferably, the Ti and Nb contents should each be 0.001% or more. Even more preferably, the Ti and Nb contents should each be 0.100% or less.
  • V content be 0.200% or less. While there is no particular lower limit for the V content, it is more preferable that the V content be 0.001% or more, because V increases the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, if V is contained, its content should be 0.200% or less. A more preferable V content is 0.001% or more. A more preferable V content is 0.100% or less.
  • the Cu and Sn contents are preferably set to 0.5% or less. While there is no particular lower limit for the Cu and Sn contents, because these elements improve hardenability, the contents are preferably set to 0.005% or more. More preferably, they are set to 0.01% or more. Furthermore, they are preferably set to 0.30% or less.
  • the Sb content is preferably 0.07% or less. While there is no specific lower limit for the Sb content, a content of 0.001% or more is more preferable in order to achieve the effect of suppressing surface decarburization. More preferably, it is 0.004% or more.
  • Cu, Sn, and Sb are features of the present invention, and at least two of these elements must be contained, with a total content of 0.01% or more.
  • These elements are known as surface segregation elements, and adding a certain amount can suppress surface decarburization during the annealing process, thereby suppressing the decrease in strength of the base material due to the uneven structure in the thickness direction and the softening of the surface layer through the thickness. More preferably, at least two of Cu, Sn, and Sb are contained in a total content of 0.015% or more.
  • a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance including Fe and unavoidable impurities.
  • a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance consisting of Fe and unavoidable impurities.
  • unavoidable impurities include H, Zn, Pb, As, Se, Ge, Sr, and Cs. A total content of 0.100% or less of these impurities is acceptable.
  • the high-strength steel plate of the present invention may further contain, by mass%, at least one element selected from Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 1.00% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination.
  • Ta and W if each is 0.10% or less, do not form large amounts of coarse precipitates or inclusions, and do not reduce the local ductility of the steel sheet, resulting in no reduction in elongation. Furthermore, bendability is not reduced. Therefore, it is preferable that the Ta and W contents each be 0.10% or less. While there is no specific lower limit for the Ta and W contents, they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, it is more preferable that the Ta and W contents each be 0.01% or more. Therefore, if Ta and W are contained, their contents should each be 0.10% or less. More preferably, the Ta and W contents are 0.01% or more. An even more preferable Ta and W content is 0.08% or less.
  • the Cr and Mo contents be 1.00% or less. While there are no specific lower limits for the Cr and Mo contents, it is more preferable that the Cr and Mo contents be 0.01% or more, as these elements improve hardenability. Therefore, if Cr and Mo are contained, their contents should each be 1.00% or less. More preferably, the Cr and Mo contents are 0.01% or more. An even more preferable Cr and Mo content is 0.80% or less.
  • Co is 0.010% or less, coarse precipitates and inclusions do not increase, and the local ductility of the steel sheet is not reduced, so elongation does not decrease. Furthermore, bendability does not decrease. Therefore, it is preferable that the Co content be 0.010% or less. While there is no particular lower limit for the Co content, because Co is an element that improves hardenability, it is more preferable that the Co content be 0.001% or more. Therefore, if Co is contained, its content should be 0.010% or less. A more preferable Co content is 0.001% or more. An even more preferable Co content is 0.008% or less.
  • Ni is 1.00% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the local ductility of the steel sheet will not be reduced, so elongation will not be reduced. Furthermore, bendability will not be reduced. Therefore, it is preferable that the Ni content be 1.00% or less. While there is no specific lower limit for the Ni content, since Ni is an element that improves hardenability (generally an element that improves corrosion resistance), it is more preferable that the Ni content be 0.001% or more. Therefore, if Ni is contained, its content should be 1.00% or less, and more preferably 0.001% or more. An even more preferable Ni content is 0.500% or less.
  • Ca, Mg, and REM if each is 0.0100% or less, do not increase coarse precipitates or inclusions, do not reduce the local ductility of the steel sheet, and therefore do not reduce elongation. Furthermore, bendability does not decrease. Therefore, it is preferable that the contents of Ca, Mg, and REM are each 0.0100% or less. While there are no specific lower limits for the contents of Ca, Mg, and REM, because these elements spheroidize the shape of nitrides and sulfides and improve the local ductility of the steel sheet, it is more preferable that the contents of Ca, Mg, and REM are each 0.0005% or more.
  • REM rare earth elements
  • Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to the total content of these elements.
  • Zr and Te if each is 0.100% or less, do not increase coarse precipitates or inclusions, and do not reduce the local ductility of the steel sheet, resulting in no reduction in elongation. Furthermore, bendability is not reduced. Therefore, it is preferable that the Zr and Te contents be 0.100% or less. While there are no specific lower limits for the Zr and Te contents, it is more preferable that the Zr and Te contents be 0.001% or more, as these elements spheroidize the shape of nitrides and sulfides and improve the local ductility of the steel sheet. Therefore, if Zr and Te are contained, their contents should be 0.100% or less. A more preferable Zr and Te content is 0.001% or more. An even more preferable Zr and Te content is 0.080% or less.
  • Hf is 0.10% or less, the amount of coarse precipitates and inclusions will not increase, and the local ductility of the steel sheet will not decrease, so elongation will not decrease. Furthermore, bendability will not decrease. Therefore, it is preferable that the Hf content be 0.10% or less. While there is no specific lower limit for the Hf content, it is more preferable that the Hf content be 0.01% or more, as it is an element that spheroidizes the shape of nitrides and sulfides and improves the local ductility of the steel sheet. Therefore, if Hf is contained, its content should be 0.10% or less. A more preferable Hf content is 0.01% or more. An even more preferable Hf content is 0.08% or less.
  • the Bi content be 0.200% or less. While there is no specific lower limit for the Bi content, because Bi is an element that reduces segregation, it is more preferable that the Bi content be 0.001% or more. Therefore, if Bi is contained, its content should be 0.200% or less. A more preferable Bi content is 0.001% or more. An even more preferable Bi content is 0.100% or less.
  • the B content is preferably 0.0100% or less. While there is no specific lower limit for the B content, because B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, it is more preferable that the B content be 0.0003% or more. Therefore, if B is contained, its content should be 0.0100% or less. A more preferable B content is 0.0003% or more. An even more preferable B content is 0.0080% or less.
  • Ti, Nb, and B are available at relatively low cost and are therefore actively used in high-strength steel plates.
  • a common feature of these elements is the effect of delaying the recrystallization of the steel structure. This effect is particularly pronounced when Ti, Nb, and B are added simultaneously.
  • the present invention is a technology that suppresses the austenite transformation from the elongated unrecrystallized structure during final annealing, and therefore is particularly effective in steel types to which Ti, Nb, and B are added simultaneously.
  • the ranges for Ti, Nb, and B are Ti: 0.010% to 0.200%, Nb: 0.005% to 0.200%, and B: 0.0005% to 0.0100%.
  • the locations that define the structure are the 1/4 position in the plate thickness direction and the outermost layer.
  • the structure at the 1/4 position in the plate thickness direction which is generally considered to be the location with the average properties and structure of a steel plate.
  • ferrite and bainite are 0% or more and 20% or less. Electron microscope observation field area ratio
  • ferrite is a soft structure and is effective in improving workability, it has a strong effect of reducing the strength of the 980 MPa-class high-strength steel sheet targeted by the present invention.
  • ferrite may be polygonal ferrite, pseudo-polygonal ferrite, or granular bainitic ferrite.
  • Bainite like martensite, is an aggregate of lath-shaped crystal grains, but is softer than martensite.
  • bainite includes upper bainite and lower bainite. To achieve high strength, the total content of ferrite and bainite is set to 20% or less.
  • the total content of ferrite and bainite is set to 15% or less. Even more preferably, the total content of ferrite and bainite is less than 2% and 10% or less. Since ferrite and bainite are not necessarily present, the lower limit is set to 0% or more.
  • the ferrite area ratio is determined using the following method. First, a thickness cross section (L cross section) parallel to the rolling direction of the steel sheet is polished and then etched with 3 vol. % nital. Ten observations are made at a 1/4 thickness position using a scanning electron microscope (SEM) at 2000x magnification. The 1/4 thickness position refers to the position corresponding to the 1/4 position of the thickness in the depth direction from the steel sheet surface. Furthermore, in the case of zinc plating, the steel sheet surface refers to the interface between the zinc plating layer and the base steel sheet. Next, using the obtained structural images, the structural (ferrite) area ratio is calculated for each of the 10 fields using Media Cybernetics' Image-Pro. The average of the area ratios for these 10 fields is defined as the "ferrite area ratio.” In the structural images above, ferrite appears as a gray structure (base structure). Retained austenite and martensite appear as white structures.
  • Retained austenite contributes to the ductility of the steel sheet through the TRIP effect and is an essential structure in the present invention.
  • the lower limit is set to 4% or more.
  • retained austenite is contained in excess, it may transform into a hard martensite structure when applied to and formed into automotive parts, etc., and may impair processing characteristics.
  • the upper limit is set to 20% or less.
  • a preferred lower limit is 6% or more.
  • a preferred upper limit is less than 16%. Note that the volume fraction of retained austenite can be converted into an area fraction.
  • the method for measuring the volume fraction of retained austenite is as follows: The steel plate is mechanically ground in the thickness direction (depth direction) to 1/4 of the plate thickness, and then chemically polished with oxalic acid to create an observation surface. This observation surface is observed using X-ray diffraction. A Co K ⁇ radiation source is used as the incident X-ray, and the intensity of the diffraction peaks of the ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ planes of fcc iron (austenite) is measured relative to the diffraction intensity of the ⁇ 200 ⁇ , ⁇ 211 ⁇ , and ⁇ 220 ⁇ planes of bcc iron.
  • Tempered martensite is 70% or more. Electron microscope observation field area ratio
  • Tempered martensite is characterized by an aggregate of lath-shaped crystal grains containing iron-based carbides. It has higher ductility than fresh martensite and is also characterized by its tendency to easily obtain the high yield strength required to ensure the crashworthiness of automotive components. To achieve these effects, the area fraction of tempered martensite must be 70% or more. A more preferred area fraction of tempered martensite is 75% or more, and even more preferably 80% or more. While there is no particular upper limit for the area fraction of tempered martensite, a tempered martensite area fraction of 94% or less is preferred to ensure the area fraction of retained austenite. The area fraction of tempered martensite can be measured by the method described below.
  • Fresh martensite is martensite that does not contain iron-based carbides. Steel plates containing fresh martensite have high strength but poor processing characteristics, so the area fraction of fresh martensite in the steel plate according to this embodiment is limited to 20% or less. On the other hand, even if the area fraction of fresh martensite is 0%, the steel plate according to this embodiment has sufficient strength. Therefore, the steel plate according to this embodiment does not need to contain fresh martensite, so the lower limit of the area fraction of fresh martensite is 0%. A more preferable range is 10% or less.
  • the effects of the present invention are not impaired even if carbides such as pearlite and cementite and other known steel plate structures are contained, as long as their total area ratio is within a range of 8% or less.
  • other steel plate structures can be confirmed and determined, for example, by SEM observation.
  • the area ratios of tempered martensite, fresh martensite, pearlite, and bainite are measured as follows: A sample of annealed steel sheet is cut, and the cross section of the sheet thickness parallel to the rolling direction is polished. It is then corroded with 3 vol. % nital. Images are taken of three fields of view at 1/4 of the sheet thickness (a position corresponding to 1/4 of the sheet thickness in the depth direction from the surface of the steel sheet) using a SEM (scanning electron microscope) at 1500x magnification. Observations at higher magnifications may be made to confirm carbides in detail. Note that when zinc plating is present, the steel sheet surface refers to the interface between the zinc plating layer and the steel sheet.
  • the obtained image data is used to determine the area ratio of each structure using Image-Pro manufactured by Media Cybernetics, and the average area ratio of each structure within the field of view is taken as the area ratio of each structure.
  • fresh martensite is distinguished as a white or light gray region
  • tempered martensite as a gray or dark gray region containing misoriented carbides
  • pearlite as a black and white lamellar structure.
  • the gray or dark gray region containing aligned carbides is bainite.
  • this classification can be difficult in actual observation. For example, even tempered martensite may be observed to have aligned carbide orientation.
  • gray or dark gray regions containing aligned carbides and in which the structure interfaces extend linearly are classified as bainite, while gray or dark gray regions containing other carbides are classified as tempered martensite. Furthermore, because fresh martensite is difficult to distinguish from retained austenite, which also appears in white or light gray regions, the area fraction of the white or light gray regions is determined, and the volume fraction of retained austenite determined by the above method is then subtracted from the area fraction to determine the area fraction.
  • the outermost layer was selected as a region that does not include unsteady parts such as scale at the very outermost layer.
  • the outermost layer refers to a position 20 ⁇ m into the sheet thickness from the surface of the steel sheet. As with the 1/4 sheet thickness position, if a zinc-plated layer is present, the interface between the zinc-plated layer and the base steel sheet is considered to be the steel sheet surface.
  • the structure at the outermost layer also includes a structure fraction within the range specified at 1/4 sheet thickness above. However, since the structure is more preferably uniform throughout the entire sheet thickness, the following is used.
  • a feature of the present invention is that the structure at the outermost layer is not significantly different from the structure at the 1/4 thickness position. Therefore, it is preferable that each of the above structures is within a range of plus or minus 5% of the value at the 1/4 thickness position. Because finite-field observation involves a certain amount of variation, it will not be exactly equivalent to the value at the 1/4 thickness position, but it is more preferable that it is within a range of plus or minus 3% of the value at the 1/4 thickness position.
  • Patent Document 4 discloses an invention that achieves good elongation properties by forming a surface decarburized layer. The present invention is characterized by suppressing this type of non-steady surface structure that was previously formed intentionally or unavoidably during annealing, and is therefore distinct from such inventions.
  • the tensile strength level of the steel sheet targeted by the present invention is 980 MPa or higher.
  • the strength level is not limited to 980 MPa, but includes high strengths up to about 1.5 GPa, such as 1180 MPa, 1300 MPa, and 1470 MPa.
  • the manufacturing method shown in the present invention can be applied to various materials regardless of the strength level of the steel sheet.
  • the rolling direction thickness cross section was evaluated by SEM-EBSD, and the texture of the tempered martensite was evaluated.
  • ODF orientation distribution function
  • ODF measurements were performed using the SEM-EBSD method described above, using a TSL OIM Analysis 8.0.
  • the surface layer measurement range was from the surface to a position 20 ⁇ m in the sheet thickness direction. While EBSD was used to measure ODF here, other known methods, such as XRD, may also be used.
  • XRD XRD
  • the above-mentioned high-strength steel sheets may be coated with an electrolytic galvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer. These coating layers can be formed using conventional methods.
  • Steel material having the aforementioned chemical composition is melted using a conventional refining process, and then made into a steel slab using a conventional ingot-blooming and rolling method or continuous casting method.
  • thin steel slabs with a thickness of 100 mm or less can be produced using a direct casting method.
  • the steel slab is heated and held at a temperature of 1080°C or higher and 1300°C or lower, and then subjected to hot rolling.
  • a hot-rolled sheet is produced through rough rolling and finish rolling, and then wound into a coil. After finish rolling, the sheet is cooled to 720°C or lower within 2 seconds at a cooling rate of 70°C or higher per second.
  • the thickness of the hot-rolled sheet is preferably 0.8 mm or higher and 4.0 mm or lower.
  • the lower limit is 1080°C or higher, and more preferably 1120°C or higher. Furthermore, if the temperature is higher than 1300°C, the slab becomes more likely to deform, increasing the frequency of defects such as scabs. Therefore, the upper limit is set to 1300°C or lower, and more preferably 1280°C or lower.
  • Hot rolling must be completed in the austenite single phase region in order to improve the strength and ductility balance of the material by homogenizing the structure within the steel sheet, so the finish rolling finish temperature is set to 850°C or higher. Furthermore, if the finish rolling finish temperature exceeds 1000°C, the hot-rolled structure becomes coarse, and accordingly, the structure after annealing also becomes coarse, resulting in a decrease in the strength of the steel sheet.
  • the preferred lower limit of the finish rolling finish temperature is 875°C or higher.
  • the preferred upper limit of the finish rolling finish temperature is 950°C or lower.
  • the average cooling rate to 720°C is set to 70°C or higher per second, more preferably 90°C or higher.
  • cooling refers to water cooling, oil cooling, or gas cooling performed for the purpose of lowering the steel sheet temperature.
  • the average cooling rate to 720°C refers to the average cooling rate from the start of cooling until the steel sheet temperature reaches 720°C.
  • skin-pass rolling may be performed before annealing the hot-rolled sheet. Skin-pass rolling can correct the shape of the steel sheet.
  • the obtained hot-rolled sheet is annealed before cold rolling to coarsen the carbides in the steel while suppressing surface decarburization.
  • This step is an important process for sharpening the crystal orientation after final annealing, i.e., for obtaining the above-mentioned ODF.
  • coiling is performed under the coiling temperature conditions of the present invention, a large amount of hard structure is formed, and if left as is, the cold rolling load is high, so this step is also useful from the viewpoint of reducing the rolling load.
  • the annealing temperature is set to the Ac1 point or lower.
  • the lower limit is set to 400°C, and more preferably, 450°C or higher.
  • the annealing time is set to 48 hours or more.
  • the reason for this limitation is that a long annealing time is necessary to sufficiently spheroidize the carbides that precipitated relatively finely during coiling. If the annealing time is less than 48 hours, the spheroidization of the carbides does not progress sufficiently, making it difficult to obtain ⁇ 111 ⁇ grains, which are advantageous for local ductility, in the subsequent rolling annealing process.
  • a more preferable annealing time is 60 hours or more. Furthermore, decarburization from the surface layer of the steel sheet during annealing is generally promoted as the temperature increases.
  • the annealing temperature must be set to Ac1 or less, and promotion of decarburization due to high temperatures is suppressed. Even so, although decarburization tends to progress with long annealing, decarburization is suppressed due to the addition of predetermined amounts of Cu, Sb, and Sn.
  • the annealing time it is preferably 300 hours or less from the viewpoints of productivity and cost. In this case, as long as the annealing temperature is within the range of 400°C or higher and Ac1 point or lower, the effect obtained will not change whether the annealing temperature is maintained at a constant temperature or not. As an example of not maintaining a constant temperature, it may take 48 hours or more to raise the temperature from 400°C or higher to Ac1 point or lower.
  • Cold rolling The steel sheet is then preferably pickled and then cold rolled to a desired thickness.
  • the cold rolling may be performed by either tandem rolling (unidirectional rolling) or reverse rolling, or by using a known warm rolling technique or interpass aging technique.
  • tandem rolling unidirectional rolling
  • reverse rolling or by using a known warm rolling technique or interpass aging technique.
  • the reasons for limiting the cold rolling conditions are as follows.
  • Total reduction 50% or more
  • the total rolling reduction is also important for obtaining the above-mentioned ODF. If the total rolling reduction is low, the driving force for recrystallization is low, so sufficient recrystallization does not occur in the subsequent annealing process, making it impossible to achieve both high strength and high ductility. Furthermore, since ⁇ 111 ⁇ grains, which contribute to improving local ductility, are more likely to be obtained when the cold rolling ratio is high, the total rolling reduction is set to 50% or more, more preferably 60% or more. Although there is no particular upper limit, it is preferably set to 95% or less from the viewpoint of the rolling load.
  • the obtained steel sheet is subjected to a heat treatment and, if necessary, a plating treatment.
  • the cold-rolled steel sheet is heated to 600°C or higher and lower than the Ac1 point.
  • the average heating rate is not particularly limited, but if it is too low, the crystal grains may become coarse and the strength may be impaired, so it is set to 5°C/s or higher. There is no particular upper limit, and there is no problem even if conditions exceeding 100°C/s are applied using an induction heating method or the like.
  • the heat treatment involves first heating to a temperature range of 600°C or higher but lower than the Ac1 point (first stage), holding that temperature range for at least 60 seconds, and then heating to a temperature range of 50°C or higher but lower than the Ac1 point (second stage).
  • the material is then cooled to a cooling stop temperature range of 50°C or higher but lower than 350°C, at an average cooling rate of at least 10°C/s, to at least 500°C.
  • the material is then heated to a temperature range above the cooling stop temperature but higher than 250°C but lower than 600°C, and held at that temperature range for at least 10 seconds.
  • This first-stage annealing plays an important role in the manufacturing method of the present invention and is a process for actively recrystallizing steel sheets at temperatures below the Ac1 point. It is known that steel sheets subjected to large strain during cold rolling form many ⁇ 111 ⁇ grains after recrystallization. Therefore, if recrystallization can be performed prior to the subsequent transformation of the structure by heating to the Ac1 point or higher, it is possible to form many ⁇ 111 ⁇ grains.
  • the annealing temperature is below 600°C, the rolled structure is not recrystallized, and a coarse structure elongated in the rolling direction remains.
  • the annealing temperature is higher than the Ac1 point, austenite grains form during annealing, which promotes element partitioning between ferrite and austenite grains, resulting in a non-uniform structure and impairing the ductility of the final annealed steel sheet. That is, in order to fully develop recrystallization, the temperature is set to 600°C or higher, more preferably 650°C or higher.
  • the effect obtained will not change whether the temperature is held at a constant value or not.
  • it may take 60 seconds or more to raise the temperature from 600°C or higher to lower than the Ac1 point.
  • the annealing time must be 60 seconds or more. There is no specific upper limit.
  • this first stage annealing is performed on a different production line from the subsequent second stage annealing.
  • the first stage annealing can be performed batchwise, followed by the second stage annealing using continuous annealing.
  • the annealing temperature is lower than the Ac1 point, a large amount of ferrite phase remains in the structure after annealing, and sufficient strength cannot be obtained. In order to obtain high strength, it is better to have as little ferrite phase as possible, and for this purpose, it is more preferable that the annealing temperature be the Ac3 point or higher. On the other hand, if the annealing temperature is higher than 950°C, the structure becomes excessively large, making it difficult to achieve both high strength and high ductility, and also reducing the ⁇ 111 ⁇ grains, so the annealing temperature is more preferably 920°C or lower.
  • the Ac1 point and Ac3 point shown in this specification can be calculated from the following formulas (A) and (B) described in "Leslie Steel Science” (Maruzen Co., Ltd., translated and supervised by Shigeyasu Koda, published May 31, 1985, page 273).
  • [ ] indicates the content (mass%) of each element, and the content of elements not contained in the steel sheet or elements below the lower detection limit of analysis may be calculated as 0 mass%.
  • Ac1 point (°C) 723-10.7 ⁇ [Mn]-16.9 ⁇ [Ni]+29.1 ⁇ [Si]+16.9 ⁇ [Cr]+290 ⁇ [As]+6.38 ⁇ [W]...(A)
  • Ac3 point (°C) 910-203 ⁇ [C]-15.2 ⁇ [Ni]+44.7 ⁇ [Si]+104 ⁇ [V]+31.5 ⁇ [Mo]+13.1 ⁇ [W ]-30 ⁇ [Mn]-11 ⁇ [Cr]-20 ⁇ [Cu]+700 ⁇ [P]+400 ⁇ [Al]+120 ⁇ [As]+400 ⁇ [Ti]...(B) [Average cooling rate to at least 500°C: 10°C/s or more] In the cooling step after annealing, the average cooling rate to at least 500°C is preferably high, and is set to 10°C/s or higher.
  • a more preferable rate is 15°C/s or higher. Meanwhile, no particular upper limit is specified.
  • the reason for specifying the average cooling rate to at least 500°C is that, within the range of the composition of the present invention, if the cooling rate is slow near this temperature range, structures that are undesirable for improving the strength-ductility balance, such as pearlite and bainite, are likely to form.
  • the fraction of fresh martensite which is useful for high strength but has poor ductility, increases, making it difficult to achieve both high strength and high ductility even if the subsequent heat treatment conditions are adjusted.
  • a preferred lower limit is 75°C or higher.
  • a preferred upper limit is 300°C or lower.
  • the cooling method used in the present invention may be any known method, such as gas cooling, oil cooling, mist cooling, or low-melting-point liquid metal cooling.
  • the steel is reheated to temper the martensite that has formed and to stabilize the retained austenite.
  • the steel may be reheated immediately, or may be reheated after maintaining a temperature within a range in which carbides do not precipitate significantly.
  • the temperature range in which carbides do not precipitate significantly is preferably 250°C or lower, and more preferably 200°C or lower.
  • the reheating temperature is too low, the carbon diffusion required to obtain the above-mentioned structure will not be sufficiently promoted, making it impossible to achieve both high strength and high ductility. Furthermore, if the reheating temperature is too high, coarse carbides will precipitate, causing softening. Therefore, after reheating to a temperature range above the cooling stop temperature and above 250°C and below 600°C, it is necessary to hold the material at that temperature range for 10 seconds or more. A more preferable range is above the cooling stop temperature and above 280°C and below 400°C. Furthermore, if the holding time is less than 10 seconds, the carbon diffusion required for austenite stabilization will not be sufficient, and ductility will not be sufficiently improved. A holding time of 30 seconds or more is more preferable, and even more preferable is 60 seconds or more.
  • hot-dip galvanizing may be performed within the above-mentioned range, i.e., within a temperature range of more than 250°C and not more than 600°C, to produce a hot-dip galvanized steel sheet, or after hot-dip galvanizing, alloying treatment may be performed in a temperature range of 470°C or more and 600°C or less to produce an alloyed hot-dip galvanized steel sheet.
  • the steel sheet of the present invention may be electroplated to produce an electroplated steel sheet.
  • temper rolling may be performed.
  • the preferred range for elongation is 0.02% to 2.0%.
  • the technology disclosed in this disclosure makes it possible to manufacture thin steel sheets that are both high-strength and highly ductile.
  • the above steel material with the chemical composition shown in Table 4 and the balance consisting of Fe and unavoidable impurities, was melted and bloomed into a steel slab, which was then hot-rolled under the conditions shown in Table 5, cooled, and coiled. It was then pickled and cold-rolled. It was then heat-treated once or twice. The number of heat treatments was one if the first-stage heating temperature was a continuous temperature rise, and two if the first-stage heating temperature was not a continuous temperature rise. Some of the steel was then subjected to hot-dip galvanizing, galvannealed hot-dip galvanizing, and electro-galvanizing processes after the heat treatment.
  • Test pieces JIS No. 5 test pieces (gauge length 50 mm, parallel portion width 25 mm) were cut from the obtained steel sheets in the direction perpendicular to the rolling direction, and tensile tests were performed in accordance with JIS Z2241. Test pieces with a strength of 980 MPa or more were considered to have high strength.
  • the elongation value was calculated by measuring the gauge length when the test pieces were butted together after tensile fracture, and calculating the change in elongation value before and after the tensile test. Here, a value of 10% or more was considered to be a high elongation value.
  • test pieces were embedded in carbon resin so that the rolling direction and the thickness direction were the observation surfaces, and the structure was evaluated.

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Abstract

L'invention concerne une tôle d'acier hautement résistante et un procédé de fabrication de celle-ci. La tôle d'acier hautement résistante de l'invention présente une structure d'acier en une position à 1/4 d'épaisseur de tôle et au niveau de la couche la plus superficielle qui est telle que (ferrite+bainite) représente 0 à 20% en rapport surfacique, un γ résiduel représente 4 à 20% en rapport volumique, une martensite revenue représente 70% ou plus en rapport surfacique, et une martensite fraîche représente 0 à 20% en rapport surfacique. Dans une fonction de distribution d'orientation dans laquelle la structure d'agrégat de ladite martensite revenue obtenue par évaluation d'un plan transversal d'épaisseur de tôle de direction laminage selon un procédé de microscopie électronique à balayage - diffraction d'électrons rétrodiffusés, est indiquée à l'intérieur d'un espace d'Euler tel que ０≦φ１≦９０°, φ２=４５° et ０≦Φ≦９０° présenté dans un procédé de Bunge, la valeur ODF maximale de grains ｛１１１｝ de valeur maximale de résistance pour φ１＝５×ｎ°, φ２=４５° et Φ＝５５° (n représentant un nombre entier de 0 à 18), est supérieure ou égale à 2,0 en une position à 1/4 d'épaisseur de tôle, et la valeur ODF maximale de grains ｛１００｝ de valeur maximale de résistance pour φ１＝５×ｎ°, φ２=４５° et Φ＝０° (n représentant un nombre entier de 0 à 18), est supérieure ou égale à 1,0 au niveau d'une couche superficielle de l'épaisseur. Enfin, la tôle d'acier hautement résistante de l'invention présente une résistance à la traction supérieure ou égale à ９８０ＭＰａ, et se révèle excellente en termes de caractéristiques de résistance à la fragilité à l'hydrogène d'une partie usinée en courbe.
PCT/JP2025/003147 2024-03-12 2025-01-31 Tôle d'acier hautement résistante, et procédé de fabrication de celle-ci Pending WO2025192080A1 (fr)

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Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2004106571A1 (fr) * 2003-05-27 2004-12-09 Nippon Steel Corporation Tole mince en acier a resistance elevee presentant une resistance excellente a la rupture differee apres formation et procede de preparation associe, et parties automobiles necessitant une certaine resistance fabriquees a partie de tole mince en acier a resistance elevee
KR20140083801A (ko) * 2012-12-26 2014-07-04 주식회사 포스코 도금성이 우수한 고강도 냉연강판 및 그의 제조방법
WO2020203159A1 (fr) * 2019-03-29 2020-10-08 日本製鉄株式会社 Tôle d'acier et son procédé de fabrication
WO2022190959A1 (fr) * 2021-03-10 2022-09-15 日本製鉄株式会社 Tôle d'acier laminée à froid et son procédé de fabrication
WO2023068369A1 (fr) * 2021-10-21 2023-04-27 日本製鉄株式会社 Tôle d'acier
WO2023218732A1 (fr) * 2022-05-11 2023-11-16 Ｊｆｅスチール株式会社 Tôle d'acier, élément et procédés de production correspondants
WO2023233036A1 (fr) * 2022-06-03 2023-12-07 Thyssenkrupp Steel Europe Ag Acier laminé à froid à haute résistance présentant une sensibilité réduite à la fragilisation par l'hydrogène et son procédé de fabrication

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2004106571A1 (fr) * 2003-05-27 2004-12-09 Nippon Steel Corporation Tole mince en acier a resistance elevee presentant une resistance excellente a la rupture differee apres formation et procede de preparation associe, et parties automobiles necessitant une certaine resistance fabriquees a partie de tole mince en acier a resistance elevee
KR20140083801A (ko) * 2012-12-26 2014-07-04 주식회사 포스코 도금성이 우수한 고강도 냉연강판 및 그의 제조방법
WO2020203159A1 (fr) * 2019-03-29 2020-10-08 日本製鉄株式会社 Tôle d'acier et son procédé de fabrication
WO2022190959A1 (fr) * 2021-03-10 2022-09-15 日本製鉄株式会社 Tôle d'acier laminée à froid et son procédé de fabrication
WO2023068369A1 (fr) * 2021-10-21 2023-04-27 日本製鉄株式会社 Tôle d'acier
WO2023218732A1 (fr) * 2022-05-11 2023-11-16 Ｊｆｅスチール株式会社 Tôle d'acier, élément et procédés de production correspondants
WO2023233036A1 (fr) * 2022-06-03 2023-12-07 Thyssenkrupp Steel Europe Ag Acier laminé à froid à haute résistance présentant une sensibilité réduite à la fragilisation par l'hydrogène et son procédé de fabrication

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