WO2024070889A1 - Tôle d'acier, élément, et procédés de fabrication associés - Google Patents

Tôle d'acier, élément, et procédés de fabrication associés Download PDF

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Publication number
WO2024070889A1
WO2024070889A1 PCT/JP2023/034299 JP2023034299W WO2024070889A1 WO 2024070889 A1 WO2024070889 A1 WO 2024070889A1 JP 2023034299 W JP2023034299 W JP 2023034299W WO 2024070889 A1 WO2024070889 A1 WO 2024070889A1
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temperature
steel sheet
content
annealing
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PCT/JP2023/034299
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English (en)
Japanese (ja)
Inventor
大洋 浅川
真平 吉岡
英之 木村
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Ｊｆｅスチール株式会社
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Publication of WO2024070889A1 publication Critical patent/WO2024070889A1/fr

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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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—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 steel sheets and components used in various applications such as automobiles and home appliances, and to methods for manufacturing them.
  • Patent Document 1 discloses a high-strength cold-rolled steel sheet with excellent workability and impact resistance, containing, by mass%, C: 0.05-0.3%, Si: 0.3-2.5%, Mn: 0.5-3.5%, P: 0.003-0.100%, S: 0.02% or less, Al: 0.010-0.5%, ferrite: 20% or more, tempered martensite: 10-60%, martensite: 0-10%, retained austenite: 3-15%, and having a steel structure in which the average crystal grain size of the low-temperature transformation phase consisting of martensite, tempered martensite, and retained austenite is 3 ⁇ m or less.
  • Patent Document 1 uses a process called Q&P (Quenching & Partitioning) in which the material is cooled to a temperature range between the martensite transformation start temperature (Ms) and the martensite transformation completion temperature (Mf) during the cooling process, and then reheated and held to stabilize the residual ⁇ .
  • Q&P Quenching & Partitioning
  • Ms martensite transformation start temperature
  • Mf martensite transformation completion temperature
  • Patent Document 2 discloses a high-strength steel plate with excellent workability, containing, by mass%, C: 0.05-0.5%, Si: 0.01-2.5%, Mn: 0.5-3.5%, P: 0.003-0.100%, S: 0.02% or less, and Al: 0.010-0.5%, with a steel structure containing, by area, 0-10% ferrite, 0-10% martensite, and 60-95% tempered martensite, and 5-20% retained austenite as determined by X-ray diffraction, with a tensile strength of 1200 MPa or more and a hole expansion ratio of 50% or more.
  • Patent Document 3 a steel plate containing, by mass%, C: 0.10% to 0.73%, Si: 3.0% or less, Mn: 0.5% to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, and N: 0.010% or less is heated to the austenite single phase region or the (austenite + ferrite) two-phase region, and then, using the martensitic transformation start temperature Ms as an index, the target cooling stop temperature is set to a temperature range below Ms and above (Ms - 150°C).
  • This method of manufacturing high-strength steel plate has excellent workability and tensile strength (TS) and excellent stability of mechanical properties, and in which the coldest part of the steel plate in the width direction is held in a temperature range from the target cooling stop temperature to (cooling stop temperature + 15°C) for 15 seconds to 100 seconds, is disclosed.
  • TS tensile strength
  • Patent Document 1 proposes a steel sheet with excellent strength, ductility and hole expandability, in which the product of tensile strength and total elongation (TS x El) is 22,000 MPa ⁇ % or more and ⁇ is 70% or more.
  • TS x El tensile strength and total elongation
  • Patent Document 2 proposes a steel sheet with excellent workability, in which TS is 1200 MPa or more, ⁇ is 50% or more, and El is 13% or more.
  • TS is 1200 MPa or more
  • ⁇ is 50% or more
  • El is 13% or more.
  • Patent Document 3 proposes a method for producing high-strength steel plate by heating to the austenite single-phase region or the (austenite + ferrite) two-phase region, followed by cooling with a target cooling stop temperature in a temperature range below Ms and above (Ms-150°C), in which the coldest portion of the steel plate in the plate width direction is held in a temperature range from the target cooling stop temperature to (cooling stop temperature + 15°C) for a time of 15 seconds to 100 seconds.
  • the present invention was made to solve these problems, and aims to provide steel plates, components, and methods for manufacturing them that are high-strength, have excellent ductility and hole expansion properties, and have excellent stability of mechanical properties in the plate width direction.
  • high strength refers to a tensile strength TS evaluated in accordance with JIS Z2241 (2011) of 1180 MPa or more.
  • Excellent ductility refers to a total elongation (EL) evaluated in accordance with JIS Z2241 (2011) of 11.0% or more.
  • Excellent stability of mechanical properties in the sheet width direction refers to a standard deviation of 0.9% or less in total elongation (EL) evaluated in accordance with JIS Z2241 (2011) using 20 pieces in total, including both ends of the sheet width, taken as JIS No. 5 tensile test pieces parallel to the rolling direction and equally spaced in the sheet width direction.
  • the inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that it is possible to homogenize the structure of the hot-rolled sheet in the sheet width direction by controlling the temperature during coiling, and to significantly reduce the variation in mechanical properties in the sheet width direction by slowly cooling the sheet from near the Ms point to the cooling stop temperature in the annealing process, thereby improving the stability of press forming.
  • the present invention provides the following: [1] In mass%, C: 0.08 to 0.35%, Si: 0.4 to 3.0%, Mn: 1.5 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol. Al: 1.0% or less, N: 0.015% or less;
  • the balance has a composition consisting of Fe and unavoidable impurities, Ferrite area ratio: 5% or less (including 0%), Total area ratio of tempered martensite and lower bainite: 70% or more; Volume fraction of retained austenite: 5 to 15%; The area ratio of fresh martensite is 10% or less (including 0%).
  • the component composition in mass%, B: 0.01% or less, Ti: 0.1% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1.5% or less, Mo: 1.0% or less, V: 0.5% or less, Nb: 0.1% or less, The steel plate according to [1], containing one or more selected from Zr: 0.2% or less and W: 0.2% or less.
  • the component composition in mass%, Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0040% or less, The steel sheet according to [1] or [2], containing one or more selected from Sb: 0.1% or less and Sn: 0.1% or less.
  • [4] The steel sheet according to any one of [1] to [3], having a plating layer on a surface of the steel sheet.
  • [5] A member made using the steel plate according to any one of [1] to [4].
  • [6] A steel slab having a component composition according to any one of [1] to [3], After holding the slab at a heating temperature of 1100°C or higher for 1800s or more, Finish hot rolling is performed at a finish rolling temperature of 850°C or higher, Cooling is performed in a temperature range from the finish rolling temperature to 650° C. at an average cooling rate of 40° C./s or more, a hot rolling process in which the coiling temperature is set to 600° C.
  • the maximum temperature difference between the coiling temperature and the temperature at the center of the sheet width in the sheet width direction during coiling is set to 50° C. or less to obtain a hot-rolled steel sheet;
  • the hot-rolled steel sheet A cold rolling process in which the steel sheet is cold-rolled at a rolling ratio of 30% or more to obtain a cold-rolled steel sheet;
  • the cold-rolled steel sheet After heating in the temperature range from 700° C.
  • Cooling is performed at an average cooling rate CR1 of 10 ° C./s or more in a temperature range from the annealing temperature to a slow cooling start temperature T1 which is equal to or higher than (Ms-30 ° C.) and equal to or lower than (Ms+30 ° C.),
  • the temperature range from the annealing start temperature T1 to the annealing stop temperature T2, which is equal to or higher than (Ms-220°C) and equal to or lower than (Ms-100°C) is cooled at an average cooling rate CR2 of 1 to 10°C/s
  • Heating is performed at an average heating rate HR2 of 2 ° C./s or more in a temperature range from the slow cooling stop temperature T2 to a reheating holding temperature T3 of 300 ° C.
  • the reheating holding temperature T3 is held for 20 s or more and 3000 s or less, and an annealing step of cooling the steel sheet in a temperature range from the reheating holding temperature T3 to 50°C at an average cooling rate CR3 of 0.1°C/s or more.
  • annealing step during cooling from the annealing temperature to the slow cooling start temperature T1, or during reheating and holding at the reheating holding temperature T3, hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment is performed.
  • a method for manufacturing a component comprising a step of subjecting the steel plate according to any one of [1] to [4] to at least one of forming and joining to form a component.
  • the present invention provides steel plates, components, and methods for manufacturing the same that are high strength, have excellent ductility and hole expansion properties, and have excellent stability of mechanical properties in the plate width direction.
  • the steel sheet of the present invention contains, by mass%, C: 0.08-0.35%, Si: 0.4-3.0%, Mn: 1.5-3.5%, P: 0.02% or less, S: 0.01% or less, sol. Al: 1.0% or less, N: 0.015% or less, with the remainder being Fe and unavoidable impurities. It has a steel structure with an area ratio of ferrite: 5% or less (including 0%), a combined area ratio of tempered martensite and lower bainite: 70% or more, a volume ratio of retained austenite: 5-15%, and an area ratio of fresh martensite: 10% or less (including 0%), and the standard deviation of the total elongation (EL) in the sheet width direction is 0.9% or less.
  • the chemical composition of the steel sheet of the present invention will be described.
  • the unit of content “%” means “mass %.”
  • “high strength” means that the tensile strength TS is 1180 MPa or more.
  • C (C: 0.08 to 0.35%) C is contained to increase the strength of tempered martensite or lower bainite and ensure a TS of 1180 MPa or more. If the C content is less than 0.08%, the desired TS cannot be stably obtained, so the C content is set to 0.08% or more.
  • the C content is preferably 0.10% or more, and more preferably 0.14% or more.
  • excessive addition of C leads to a decrease in hole expandability due to an increase in the number density of carbides, a decrease in ductility, and even a deterioration in the shape fixability of parts due to an excessive increase in YS. Therefore, the C content is set to 0.35% or less.
  • the C content is preferably 0.30% or less, and more preferably 0.25% or less.
  • Si improves the strength of the steel sheet by solid solution strengthening, and furthermore, by suppressing the coarsening of carbides, suppresses the decrease in strength due to tempering. If the Si content is less than 0.4%, the desired TS may not be stably obtained, and the desired ductility may not be obtained, so the Si content is set to 0.4% or more.
  • the Si content is preferably 1.0% or more, and more preferably 1.4% or more.
  • the Si content is set to 3.0% or less.
  • the Si content is preferably 2.5% or less, and more preferably 2.0% or less.
  • Mn is an element effective in improving hardenability. If the Mn content is less than 1.5%, ferrite or pearlite is excessively generated. As a result, tempered martensite and lower bainite are not sufficiently obtained, and the desired TS cannot be obtained. Therefore, the Mn content is set to 1.5% or more.
  • the Mn content is preferably 2.0% or more, and more preferably 2.4% or more.
  • excessive addition of Mn forms coarse MnS, which significantly reduces the hole expandability and bendability. Therefore, the Mn content is set to 3.5% or less, and preferably 3.0% or less.
  • P 0.02% or less
  • P is an effective element for strengthening steel, but excessive addition of P significantly reduces spot weldability. Therefore, the P content is set to 0.02% or less, and preferably 0.01% or less. Although there is no particular restriction on the lower limit of the P content, a large amount of cost is required to make the P content less than 0.002%, so the P content is preferably 0.002% or more.
  • S (S: 0.01% or less) S forms coarse sulfides with Mn, which reduces hole expandability and bendability. Therefore, the S content is set to 0.01% or less.
  • the S content is preferably 0.002% or less, and more preferably 0.001% or less. Although there is no particular restriction on the lower limit of the S content, a large amount of cost is required to make the S content less than 0.0002%, so the S content is preferably 0.0002% or more.
  • sol. Al is an element added as a deoxidizer in the steelmaking process. If the sol. Al content exceeds 1.0%, inclusions such as Al 2 O 3 and AlN increase, which reduces hole expandability and bendability. Therefore, the sol. Al content is set to 1.0% or less.
  • the sol. Al content is preferably 0.2% or less, and more preferably 0.05% or less. Although the lower limit of the sol. Al content is not particularly specified, in order to obtain a sufficient deoxidizing effect, the sol. Al content is preferably 0.001% or more, more preferably 0.010% or more, and further preferably 0.020% or more.
  • the N content is set to 0.015% or less.
  • the N content is preferably 0.008% or less, and more preferably 0.005% or less. Although there is no particular restriction on the lower limit of the N content, the production cost increases significantly when the N content is less than 0.001%, so the N content is preferably 0.001% or more.
  • the composition of the steel sheet of the present invention contains the above-mentioned components as basic components, and the balance other than the composition of the components contains iron (Fe) and inevitable impurities.
  • the balance has a composition consisting of Fe and inevitable impurities. Examples of unavoidable impurities include Zn, Co, etc., and in the present invention, even if these elements are contained within the range of a normal steel composition, the effect of the present invention is not impaired.
  • the component composition of the steel sheet of the present invention may appropriately contain the following (A) and/or (B) as optional elements.
  • the B content is 0.01% or less, when Ti is contained, the Ti content is 0.1% or less, when Cu is contained, the Cu content is 1% or less, when Ni is contained, the Ni content is 1% or less, when Cr is contained, the Cr content is 1.5% or less, when Mo is contained, the Mo content is 1.0% or less, when V is contained, the V content is 0.5% or less, when Nb is contained, the Nb content is 0.1% or less, when Zr is contained, the Zr content is 0.2% or less, and when W is contained, the W content is 0.2% or less.
  • the B content is preferably 0.0050% or less, more preferably 0.0030% or less, and more preferably 0.0003% or more.
  • the Ti content is preferably 0.080% or less, more preferably 0.050% or less.
  • the Ti content is preferably 0.001% or more, and more preferably 0.010% or more.
  • the Cu content is preferably 0.50% or less, more preferably 0.20% or less.
  • the Cu content is preferably 0.001% or more, and more preferably 0.030% or more.
  • the Ni content is preferably 0.50% or less, more preferably 0.20% or less.
  • the Ni content is preferably 0.001% or more, more preferably 0.030% or more.
  • the Cr content is preferably 1.2% or less, more preferably 1.0% or less.
  • the Cr content is preferably 0.001% or more, and more preferably 0.200% or more.
  • the Mo content is preferably 0.50% or less, and more preferably 0.20% or less.
  • the Mo content is preferably 0.001% or more, and more preferably 0.010% or more.
  • the V content is preferably 0.50% or less, more preferably 0.20% or less.
  • the V content is preferably 0.001% or more, more preferably 0.010% or more.
  • the Nb content is preferably 0.08% or less, more preferably 0.05% or less.
  • the Nb content is preferably 0.001% or more, and more preferably 0.010% or more.
  • the Zr content is preferably 0.1% or less, more preferably 0.05% or less.
  • the Zr content is preferably 0.001% or more, and more preferably 0.010% or more.
  • the W content is preferably 0.1% or less, more preferably 0.05% or less, and further preferably 0.03% or less.
  • the W content is preferably 0.001% or more, and more preferably 0.005% or more.
  • the amount of addition exceeds a certain amount, the effect is saturated, so when Ca is contained, the Ca content is 0.0040% or less, when Ce is contained, the Ce content is 0.0040% or less, when La is contained, the La content is 0.0040% or less, when Mg is contained, the Mg content is 0.0040% or less, when Sb is contained, the Sb content is 0.1% or less, and when Sn is contained, the Sn content is 0.1% or less.
  • the Ca content is preferably 0.0030% or less. Also, the Ca content is preferably 0.0003% or more.
  • the Ce content is preferably 0.0030% or less. Also, the Ce content is preferably 0.0003% or more.
  • the La content is preferably 0.0030% or less. Also, the La content is preferably 0.0003% or more.
  • the La content is further preferably 0.0010% or more.
  • the Mg content is preferably 0.0030% or less. Also, the Mg content is preferably 0.0003% or more.
  • the Sb content is preferably 0.05% or less, more preferably 0.02% or less, and more preferably 0.0003% or more.
  • the Sb content is further preferably 0.0020% or more.
  • the Sn content is preferably 0.05% or less, more preferably 0.02% or less, and more preferably 0.0003% or more.
  • the Sn content is further preferably 0.0020% or more.
  • the optional elements contained in amounts less than the lower limit do not impair the effects of the present invention. If the optional elements are contained in amounts less than the lower limit, the optional elements are considered to be contained as unavoidable impurities.
  • ferrite area ratio 5% or less (including 0%)
  • ferrite contributes to improving ductility, due to the difference in hardness between ferrite and hard phases such as tempered martensite, it becomes the origin of voids during punching or press forming, degrading press formability.
  • the area ratio of ferrite exceeds 5%, the deterioration of press formability becomes significant.
  • the area ratio of ferrite exceeds 5%, the desired TS cannot be obtained, and the desired stability of mechanical properties in the sheet width direction cannot be obtained. Therefore, the area ratio of ferrite is set to 5% or less, preferably 3% or less, and more preferably 0%.
  • Total area ratio of tempered martensite and lower bainite 70% or more
  • the total area ratio of tempered martensite and lower bainite is set to 70% or more, preferably 80% or more, and more preferably 85% or more.
  • tempered martensite and lower bainite have different transformation timings, they are low-temperature transformation products and have similar effects on mechanical properties, so they are evaluated based on the total area ratio.
  • the total area ratio of tempered martensite and lower bainite is preferably 95% or less, and more preferably 93% or less.
  • volume fraction of retained austenite (Volume fraction of retained austenite: 5 to 15%)
  • the retained austenite contributes to improving uniform elongation due to the TRIP effect.
  • the volume fraction of the retained austenite is set to 5% or more.
  • the volume fraction of the retained austenite is preferably 7% or more, and more preferably 9% or more.
  • the volume fraction of retained austenite is set to 15% or less.
  • Fresh martensite is very hard and can become the starting point of cracks during press forming. If the area ratio of fresh martensite exceeds 10%, the desired ductility cannot be obtained, and furthermore, the desired stability of mechanical properties in the sheet width direction cannot be obtained. Therefore, from the viewpoints of suppressing cracking, improving ductility, and furthermore, stability of mechanical properties in the sheet width direction, the area ratio of fresh martensite is set to 10% or less, preferably 5% or less, and more preferably 3% or less. Note that the area ratio of fresh martensite may be 0%.
  • one or more types of residual structures such as upper bainite and pearlite may be generated as a residual structure other than the above-mentioned ferrite, tempered martensite, lower bainite, retained austenite and fresh martensite, but the object of the present invention can be achieved as long as the above-mentioned ferrite, tempered martensite, lower bainite, retained austenite and fresh martensite are satisfied. It is preferable that the residual structures such as pearlite and upper bainite are 5% or less in total.
  • the steel sheet of the present invention may also have a plating layer on the surface of the steel sheet.
  • the type of plating layer is not particularly limited, but may be a zinc plating layer, such as an electrolytic zinc plating layer, a hot-dip zinc plating layer, or an alloyed hot-dip zinc plating layer.
  • the area ratios of ferrite, tempered martensite, lower bainite, and fresh martensite are measured by cutting out a plate width cross section parallel to the rolling direction, mirror-polishing it, and then etching it with 1 vol% nital. Using an SEM, 10 fields of view are observed at 5,000x magnification at the 1/4 thickness position, and the area ratios are measured using the point count method (in accordance with ASTM E562-83 (1988)).
  • ferrite is the area that appears the blackest under the SEM, and is an equiaxed area with almost no carbides inside.
  • Tempered martensite and lower bainite are areas that appear gray under the SEM, and are areas where lath-shaped substructures and carbide precipitation are observed.
  • Fresh martensite is the area that appears white and lumpy under the SEM, and is an area where no substructures are observed inside.
  • the volume fraction of retained austenite is determined by X-ray diffraction using steel plates that have been mechanically ground and polished with oxalic acid to a depth of 100 ⁇ m or more so that the measurement position is on 1/4 of the plate thickness.
  • a Co-K ⁇ source is used for the incident X-rays, and the volume fraction of retained austenite is calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.
  • the volume fraction of retained austenite determined by X-ray diffraction is equal to the area fraction.
  • the steel plate of the present invention has a tensile strength TS evaluated in accordance with JIS Z2241 (2011) of 1180 MPa or more, and has high strength.
  • the steel sheet of the present invention has a total elongation (EL) evaluated in accordance with JIS Z2241 (2011) of 11.0% or more, and is excellent in ductility.
  • the steel sheet of the present invention has a standard deviation of total elongation (EL) of 0.9% or less, as evaluated in accordance with JIS Z2241 (2011), using 20 pieces in total taken at equal intervals in the sheet width direction, including both ends of the sheet width, as JIS No. 5 tensile test pieces parallel to the rolling direction, and has excellent stability of mechanical properties in the sheet width direction.
  • the steel sheet of the present invention may have a standard deviation of the tensile strength TS in the sheet width direction of 15.0 MPa or less.
  • the temperatures when heating or cooling the steel slab (steel material), steel plate, etc. shown below refer to the surface temperatures of the steel slab (steel material), steel plate, etc., unless otherwise specified.
  • the method for producing a steel sheet of the present invention includes a hot rolling step in which a steel slab having the above-mentioned composition is held at a slab heating temperature of 1100°C or more for 1800s or more, followed by finish hot rolling at a finish rolling temperature of 850°C or more, cooling at an average cooling rate of 40°C/s or more in the temperature range from the finish rolling temperature to 650°C, setting the coiling temperature to 600°C or less, and coiling under conditions in which the maximum temperature difference with the temperature at the center of the sheet width in the sheet width direction during coiling is 50°C or less to obtain a hot rolled steel sheet, a cold rolling step in which the above hot rolled steel sheet is cold rolled at a rolling reduction rate of 30% or more to obtain a cold rolled steel sheet, and a heating step in which the above cold rolled steel sheet is heated at an average heating rate HR1 of 0.5°C/s or more in the temperature range from 700°C to (Ac 3 -10°C), followed by heating at an average heating
  • the steelmaking process can be carried out according to a conventional method.
  • the hot rolling process, the pickling process, the cold rolling process, and the annealing process will be described below.
  • Methods for hot rolling a steel slab include a method of reheating a steel slab cooled to room temperature and then rolling it, a method of directly rolling a steel slab after continuous casting without heating it, and a method of rolling a steel slab after continuous casting by subjecting it to a short-term heat treatment.
  • the steel slab is held at a slab heating temperature of 1100°C or more for 1800s or more by any of the above methods, and then finish hot rolling is performed at a finish rolling temperature of 850°C or more.
  • the steel slab is cooled at an average cooling rate of 40°C/s or more in a temperature range from the finish rolling temperature to 650°C, and the coiling temperature is set to 600°C or less, and the coiling is performed under conditions of a maximum temperature difference in the sheet width direction during coiling of 50°C or less, to obtain a hot-rolled steel sheet.
  • slab heating temperature 1100°C or higher
  • slab heating holding time 1800 s or more
  • the slab heating temperature is set to 1100° C. or higher.
  • the slab heating temperature is preferably 1180° C., and more preferably 1200° C. or higher.
  • the slab heating holding time is set to 1800 seconds or more.
  • the slab heating temperature is preferably 1,300° C. or less, and the slab heating holding time is preferably 3 hours or less.
  • the finish rolling temperature is set to 850° C. or higher. There is no particular upper limit, but it is preferably 950° C. or less.
  • the average cooling rate from the finish rolling temperature to 650°C is set to 40°C/s or more. This average cooling rate is preferably 60°C/s or more.
  • the average cooling rate here is "(finish rolling temperature (°C)-650°C)/cooling time from the finish rolling temperature to 650°C (seconds)".
  • the coiling temperature is set to 600°C or less.
  • the coiling temperature is preferably 550°C or less.
  • the coiling temperature is preferably 400°C or more.
  • the maximum temperature difference is the maximum value of the temperature difference between the temperature at the center of the sheet width and any position in the sheet width direction, i.e., the maximum temperature difference refers to the difference between the temperature at the center of the sheet width and the lowest temperature in the sheet width direction.
  • the maximum coiling temperature difference in the sheet width direction is set to 50°C or less.
  • the maximum coiling temperature difference in the sheet width direction is preferably 30°C or less, and more preferably 20°C or less.
  • the hot-rolled steel sheet can be subjected to a heat treatment as necessary in order to reduce the cold rolling load.
  • pickling After the hot rolling step, pickling may be carried out to remove scale from the surface layer of the hot-rolled sheet.
  • the pickling method is not particularly limited, and may be carried out according to a conventional method.
  • the cold rolling ratio (cumulative cold rolling ratio) is set to 30% or more from the viewpoint of controlling the recrystallization behavior in the subsequent annealing and stabilizing the material. Although there is no particular upper limit for the cold rolling ratio, if it exceeds 95%, the cold rolling load may increase excessively. Therefore, the cold rolling ratio is preferably 95% or less.
  • the average heating rate HR1 from 700°C to (Ac 3 -10°C) is preferably 1.0°C/s or more, more preferably 1.5°C/s or more.
  • the average heating rate HR1 is preferably 50° C./s or less, and more preferably 20° C./s or less.
  • the average heating rate HR1 is "(Ac 3 -10° C.) -700° C.)/heating time (seconds) from 700° C. to (Ac 3 -10° C.)".
  • the annealing temperature is set to (Ac 3 - 10°C) or higher.
  • the annealing temperature is preferably (Ac 3 + 50°C) or lower.
  • the holding time is less than 30 seconds, the carbides may remain undissolved, resulting in reduced hole expansion and bendability. Furthermore, if the holding time (annealing time) is less than 30 seconds, the desired stability of mechanical properties in the plate width direction cannot be obtained. Therefore, the holding time should be 30 seconds or more.
  • the holding time is preferably 60 seconds or more.
  • Ac3 is calculated by the following formula.
  • the [element symbol] means the content (mass%) of each element.
  • Ac3 (°C) 910 - 203 x [C] 1/2 - 15.2 x [Ni] + 44.7 x [Si] + 104 x [V] + 31.5 x [Mo] + 13.1 x [W] - (30 x [Mn] + 11 x [Cr] + 20 x [Cu] - 700 x [P] - 400 x [sol. Al] - 120 x [As] - 400 x [Ti])
  • CR1 Average cooling rate CR1 from annealing temperature to slow cooling start temperature T1: 10 ° C./s or more
  • Slow cooling start temperature T1 martensite transformation start temperature Ms ⁇ 30°C ((Ms - 30°C) or more, (Ms + 30°C) or less)
  • CR1 is set to 10°C/s or more.
  • CR1 is preferably 15°C/s or more. Although there is no upper limit for CR1, an excessive increase in the average cooling rate may promote uneven cooling in the sheet width direction, which may lead to a decrease in material uniformity in the sheet width direction. Therefore, CR1 is preferably 1000° C./s or less, and more preferably 100° C./s or less.
  • the average cooling rate CR1 is "(annealing temperature (°C) - slow cooling start temperature T1 (°C)) / cooling time (seconds) from the annealing temperature to the slow cooling start temperature T1".
  • T1 exceeds (Ms+30°C)
  • ferrite and pearlite are also excessively generated.
  • the desired tempered martensite and lower bainite are not obtained, and the desired strength may not be obtained.
  • T1 exceeds (Ms+30°C)
  • the area ratio of fresh martensite exceeds 10%, and the desired ductility is not obtained, and further, the desired stability of mechanical properties in the sheet width direction is not obtained. Therefore, T1 is set to (Ms+30°C) or less.
  • T1 is preferably (Ms+20°C) or less, and more preferably (Ms+10°C) or less.
  • T1 is set to be equal to or greater than (Ms-30° C.), preferably equal to or greater than (Ms-20° C.), and more preferably equal to or greater than (Ms-10° C.).
  • the martensitic transformation start temperature Ms can be determined by using a Formaster testing machine, using a cylindrical test piece (diameter 3 mm ⁇ height 10 mm), holding the test piece at an annealing temperature of (Ac -10 °C) or higher, and then quenching the test piece at a cooling rate of 30°C/s or higher using helium gas, and measuring the volume change.
  • CR2 is set to 10°C/s or less. If CR2 is less than 1°C/s, the line length increases and the production efficiency decreases, so CR2 is set to 1°C/s or more.
  • the average cooling rate CR2 is "(slow cooling start temperature T1 (°C)-slow cooling stop temperature T2 (°C))/cooling time (seconds) from the slow cooling start temperature T1 to the slow cooling stop temperature T2.”
  • T2 is set to be (Ms-220°C) or more.
  • T2 is preferably (Ms-200°C) or more, and more preferably (Ms-180°C) or more.
  • T2 exceeds (Ms-100°C)
  • C is not sufficiently distributed from martensite and lower bainite to austenite during slow cooling, so that decomposition of austenite may occur during the reheating and holding process, which causes variations in mechanical properties in the sheet width direction.
  • T2 exceeds (Ms-100°C)
  • the area ratio of fresh martensite exceeds 10%, and the desired ductility cannot be obtained, and further, the desired stability of mechanical properties in the sheet width direction cannot be obtained. Therefore, T2 is set to (Ms-100°C) or less.
  • the average heating rate HR2 is set to 2°C/s or more.
  • HR2 is preferably 5°C/s or more, and more preferably 10°C/s or more.
  • HR2 is preferably 50°C/s or less, and more preferably 20°C/s or less.
  • the average heating rate HR2 is "reheating holding temperature T3 (°C) - slow cooling stop temperature T2 (°C) / heating time (seconds) from the slow cooling stop temperature T2 to the reheating holding temperature T3.”
  • the reheating is performed to stabilize austenite by C distribution. If the reheating temperature T3 is less than 300°C, C distribution is not sufficient and the desired amount of retained austenite cannot be obtained, so there is a concern that the ductility may decrease. If the reheating temperature T3 is less than 300°C, the desired stability of mechanical properties in the sheet width direction cannot be obtained. Therefore, the reheating temperature T3 is set to 300°C or higher. T3 is preferably 330°C or higher, and more preferably 350°C or higher.
  • the reheating holding temperature T3 exceeds 450°C, transformation from austenite to pearlite occurs, and the desired amount of retained austenite cannot be obtained, which may result in a decrease in ductility. Moreover, if the reheating holding temperature T3 exceeds 450° C., the desired stability of mechanical properties in the sheet width direction cannot be obtained. Therefore, the reheating holding temperature T3 is set to 450° C. or less. T3 is preferably 420° C. or less.
  • the reheating holding time (holding time (residence time) at the reheating holding temperature T3) is less than 20 seconds, sufficient C distribution does not occur, and the desired amount of retained austenite cannot be obtained. Therefore, the reheating holding time is set to 20 seconds or more.
  • the reheating holding time is preferably 50 seconds or more, and more preferably 100 seconds or more. Since the effect of carbon distribution by reheating and holding is saturated at more than 3000 seconds, the reheating and holding time is set to 3000 seconds or less, preferably 1500 seconds or less, and more preferably 600 seconds or less.
  • the average cooling rate CR3 from the reheating holding temperature T3 to 50°C is set to 0.1°C/s or more.
  • CR3 is preferably 5°C/s or more, and more preferably 8°C/s or more.
  • CR3 is preferably 100° C./s or less, and more preferably 50° C./s or less.
  • the average cooling rate CR3 is "(reheating holding temperature T3 (°C) - 50°C) / cooling time (seconds) from reheating holding temperature T3 to 50°C.”
  • hot-dip galvanizing treatment in the annealing step, hot-dip galvanizing treatment can be performed during cooling from the annealing temperature to the slow-cooling start temperature T1, or during reheating and holding at the reheating and holding temperature T3.
  • the hot-dip galvanizing treatment may be hot-dip galvanizing treatment.
  • hot-dip galvanizing treatment it is preferable to immerse the steel sheet in a zinc plating bath at 440°C or more and 500°C or less, perform hot-dip galvanizing treatment, and then adjust the coating weight by gas wiping or the like.
  • a galvanizing alloying treatment can be performed after the hot dip galvanizing treatment.
  • a galvanizing alloying treatment it is preferable to perform the galvanizing alloying treatment in a temperature range of 480° C. or more and 600° C. or less after immersion in the galvanizing bath.
  • the steel sheet after annealing can be subjected to temper rolling.
  • the elongation rate is preferably 0.1% or more.
  • the elongation rate is preferably 0.5% or less.
  • the steel sheet after annealing may be subjected to leveller straightening.
  • the leveller straightening method is not particularly specified, and may be carried out according to a conventional method.
  • an electroplating treatment such as electrogalvanizing can be carried out as a surface treatment.
  • the thickness of the steel plate of the present invention thus obtained is preferably 0.5 mm or more, and more preferably 2.0 mm or less.
  • the plate width is preferably 600 mm or more, and 1700 mm or less.
  • the member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining processes.
  • the manufacturing method of the member of the present invention also includes a step of subjecting the steel plate of the present invention to at least one of forming and joining processes to form the member.
  • the steel plate of the present invention has a tensile strength of 1180 MPa or more, excellent ductility and hole expandability, and excellent stability of mechanical properties in the plate width direction. Therefore, members obtained using the steel plate of the present invention also have high strength, excellent ductility and hole expandability, and excellent stability of mechanical properties in the plate width direction. Furthermore, the use of the members of the present invention makes it possible to reduce weight. Therefore, the members of the present invention can be suitably used, for example, for vehicle body frame parts.
  • the members of the present invention also include welded joints.
  • general processing methods such as pressing can be used without restrictions.
  • general welding methods such as spot welding and arc welding, riveting, crimping, etc. can be used without restrictions.
  • a slab having the chemical composition shown in Table 1 was held at a slab heating temperature of 1230°C for 3000s, then hot rolled at a finishing rolling temperature of 870°C, cooled at an average cooling rate of 65°C/s in the temperature range from the finishing rolling temperature to 650°C, and coiled at the coiling temperature shown in Table 2 with the maximum temperature difference in the sheet width direction during coiling to produce a hot-rolled steel sheet with a thickness of 2.8 mm and a width of 1100 mm.
  • the hot-rolled steel sheet was cold rolled at a reduction rate of 50% to produce a cold-rolled steel sheet with a thickness of 1.4 mm and a width of 1100 mm.
  • the cold-rolled steel sheet was annealed under the conditions shown in Table 2.
  • the average heating rate HR1 in heating from 700° C. to (Ac 3 ⁇ 10° C.) was set to 2.0° C./s.
  • No. 11 had a steel sheet surface subjected to electrolytic galvanizing treatment (EG), and
  • No. 12 had a steel sheet surface subjected to hot-dip galvanizing treatment.
  • No. 12 had a steel sheet surface subjected to alloying treatment (GA) at 510° C. for 10 seconds in order to make the plated layer into an alloyed hot-dip galvanized layer.
  • TS Tensile strength
  • EL total elongation
  • Df is the hole diameter (mm) when a crack occurs
  • D0 is the initial hole diameter (mm).
  • the stability of the mechanical properties in the plate width direction was evaluated by taking a total of 20 JIS No. 5 tensile test pieces parallel to the rolling direction at equal intervals in the plate width direction, including both ends, and conducting the tensile test described above to determine the standard deviation of TS and EL. Pieces with a standard deviation of EL of 0.9% or less were determined to have excellent stability of the mechanical properties in the plate width direction. There was no particular regulation on the standard deviation of TS, but a standard deviation of TS of 15.0 MPa or less was determined to have excellent stability of the mechanical properties in the plate width direction.
  • the examples of the present invention shown in Tables 2 and 3 are excellent in strength, ductility, hole expansion property, and stability of mechanical properties, whereas the comparative examples are inferior in one or more of these.
  • the standard deviation of tensile strength TS in the examples of the present invention was able to be reduced to 15.0 MPa or less.
  • the components obtained by molding, the components obtained by joining, and the components obtained by further molding and joining have high strength and excellent ductility, hole expandability, and stability of mechanical properties in the plate width direction, similar to the steel plate of the present invention, because the steel plate of the present invention has high strength and excellent ductility, hole expandability, and stability of mechanical properties in the plate width direction.

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

Abstract

L'invention fournit une tôle d'acier, un élément et des procédés de fabrication associés, laquelle tôle d'acier se révèle hautement résistante, présente des propriétés d'expansion de trous et une endurance qui sont excellentes, et est dotée d'une excellente stabilité de caractéristiques mécaniques dans une direction largeur de tôle.　Plus précisément, l'invention concerne une tôle d'acier qui présente une composition comprenant, en % en masse, ０,０８ à ０,３５％ d'un C,０,４ à ３,０％ d'un Si, １,５ à ３,５％ d'un Mn, ０,０２％ ou moins d'un P, ０,０１％ ou moins d'un S, １,０％ ou moins d'un ｓｏｌ．Ａｌ et ０,０１５％ ou moins d'un N, le reste étant constitué de Fe et des impuretés inévitables. En outre, cette tôle d'acier présente une structure telle que le rapport surfacique d'une ferrite est inférieur ou égal à 5% (0% inclus), le rapport surfacique total d'une martensite revenue et d'une bainite inférieure est supérieur ou égal à 70%, le rapport volumique d'une austénite résiduelle est compris entre 5 et 15%, et le rapport surfacique d'une martensite fraîche est inférieur ou égal à １０％ (0% inclus). L'écart-type de d'allongement total (EL) dans la direction largeur de tôle est inférieur ou égal à ０,９％.
PCT/JP2023/034299 2022-09-30 2023-09-21 Tôle d'acier, élément, et procédés de fabrication associés WO2024070889A1 (fr)

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Citations (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2000212684A (ja) * 1999-01-20 2000-08-02 Kobe Steel Ltd 板幅方向における伸びのバラツキが改善された高強度高延性冷延鋼板、および高強度高延性冷延鋼板の製造方法
JP2002302734A (ja) * 2001-01-31 2002-10-18 Kobe Steel Ltd 加工性に優れた高強度鋼板およびその製造方法
JP2015224359A (ja) * 2014-05-27 2015-12-14 Ｊｆｅスチール株式会社 高強度鋼板の製造方法
WO2017138503A1 (fr) * 2016-02-10 2017-08-17 Ｊｆｅスチール株式会社 Tôle d'acier à haute résistance et procédé de fabrication de cette dernière
WO2017138504A1 (fr) * 2016-02-10 2017-08-17 Ｊｆｅスチール株式会社 Tôle d'acier à haute résistance et procédé de fabrication de cette dernière
WO2018055695A1 (fr) * 2016-09-21 2018-03-29 新日鐵住金株式会社 Tôle d'acier
WO2018147400A1 (fr) * 2017-02-13 2018-08-16 Ｊｆｅスチール株式会社 Plaque d'acier à haute résistance et son procédé de fabrication
WO2019212047A1 (fr) * 2018-05-01 2019-11-07 日本製鉄株式会社 Tôle en acier galvanisé et procédé de fabrication de celle-ci
WO2020262652A1 (fr) * 2019-06-28 2020-12-30 日本製鉄株式会社 Tôle d'acier
WO2021070951A1 (fr) * 2019-10-10 2021-04-15 日本製鉄株式会社 Feuille d'acier laminée à froid et son procédé de fabrication

Patent Citations (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2000212684A (ja) * 1999-01-20 2000-08-02 Kobe Steel Ltd 板幅方向における伸びのバラツキが改善された高強度高延性冷延鋼板、および高強度高延性冷延鋼板の製造方法
JP2002302734A (ja) * 2001-01-31 2002-10-18 Kobe Steel Ltd 加工性に優れた高強度鋼板およびその製造方法
JP2015224359A (ja) * 2014-05-27 2015-12-14 Ｊｆｅスチール株式会社 高強度鋼板の製造方法
WO2017138503A1 (fr) * 2016-02-10 2017-08-17 Ｊｆｅスチール株式会社 Tôle d'acier à haute résistance et procédé de fabrication de cette dernière
WO2017138504A1 (fr) * 2016-02-10 2017-08-17 Ｊｆｅスチール株式会社 Tôle d'acier à haute résistance et procédé de fabrication de cette dernière
WO2018055695A1 (fr) * 2016-09-21 2018-03-29 新日鐵住金株式会社 Tôle d'acier
WO2018147400A1 (fr) * 2017-02-13 2018-08-16 Ｊｆｅスチール株式会社 Plaque d'acier à haute résistance et son procédé de fabrication
WO2019212047A1 (fr) * 2018-05-01 2019-11-07 日本製鉄株式会社 Tôle en acier galvanisé et procédé de fabrication de celle-ci
WO2020262652A1 (fr) * 2019-06-28 2020-12-30 日本製鉄株式会社 Tôle d'acier
WO2021070951A1 (fr) * 2019-10-10 2021-04-15 日本製鉄株式会社 Feuille d'acier laminée à froid et son procédé de fabrication

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