WO2020170710A1 - Tôle d'acier à haute résistance, procédé de fabrication d'une tôle d'acier laminée à chaud, procédé de fabrication d'une tôle d'acier crue laminée à froid, et procédé de fabrication de tôle d'acier à haute résistance - Google Patents

Tôle d'acier à haute résistance, procédé de fabrication d'une tôle d'acier laminée à chaud, procédé de fabrication d'une tôle d'acier crue laminée à froid, et procédé de fabrication de tôle d'acier à haute résistance Download PDF

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WO2020170710A1
WO2020170710A1 PCT/JP2020/002392 JP2020002392W WO2020170710A1 WO 2020170710 A1 WO2020170710 A1 WO 2020170710A1 JP 2020002392 W JP2020002392 W JP 2020002392W WO 2020170710 A1 WO2020170710 A1 WO 2020170710A1
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steel sheet
martensite
average
temperature
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PCT/JP2020/002392
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English (en)
Japanese (ja)
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克利 ▲高▼島
由康 川崎
横田　毅
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Ｊｆｅスチール株式会社
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Priority to JP2020526464A priority Critical patent/JP6950826B2/ja
Publication of WO2020170710A1 publication Critical patent/WO2020170710A1/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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—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 a high-strength steel plate, a hot-rolled steel plate manufacturing method, a cold-rolled full-hard steel plate manufacturing method, and a high-strength steel plate manufacturing method.
  • High-strength steel sheets used for automobile structural members and reinforcing members are required to have excellent workability.
  • workability in processing a component having a complicated shape, not only individual characteristics such as elongation and hole expandability are excellent, but also all of them are required to be excellent.
  • press-formed parts are often combined by resistance welding (spot welding), and may be welded to hot-dip galvanized steel sheets.
  • spot welding spot welding
  • zinc may melt on the surface of the steel sheet during spot welding, and residual stress may be generated in the vicinity of the welded portion, which may cause liquid metal brittleness and cracks in the steel sheet.
  • the high-strength steel plate is a non-galvanized steel plate, if the other steel plate to be welded is a galvanized steel plate, the zinc will melt.
  • spot welding since stress is applied to the welded portion in order to maintain the rigidity of the entire automobile body, it is feared that hydrogen that enters from the use environment may cause delayed fracture.
  • the high-strength steel sheet has workability, cracking of the resistance weld portion due to liquid metal brittleness and delayed fracture does not easily occur, and it has excellent resistance to cracking of the resistance weld portion. Need to be present.
  • Patent Document 1 discloses a technique of improving surface crack resistance during resistance welding by controlling the contents of Si, Al, and Mn.
  • the present invention has been developed in view of the above circumstances, and an object of the present invention is to provide a high-strength steel sheet excellent in workability and crack resistance of resistance welded portions.
  • the high-strength steel sheet in the present invention is a steel sheet having a tensile strength (TS) of 780 MPa or more.
  • TS tensile strength
  • excellent workability means having both excellent elongation and excellent hole expandability.
  • excellent elongation and excellent hole expandability can mean those having an elongation of 14% or more and a hole expansion rate of 35% or more, respectively, in the tests described below.
  • the resistance to cracking in resistance welding means that the occurrence of cracking during resistance welding and the occurrence of cracking in the resistance welding portion due to hydrogen in the operating environment are both suppressed.
  • a predetermined angle for example, 6 degrees
  • cracks did not occur in the resistance welding part, and hydrogen was added to the welded body by cathodic electrolytic charging. It can mean that no rupture is seen after a certain time.
  • This invention is based on the above findings, and its gist is as follows.
  • the balance has a component composition consisting of Fe and inevitable impurities, Where Mo, N, Ti and B are expressed by the formula (1): [Mo]+2 ⁇ ([Ti]-3.4 ⁇ [N])+45 ⁇ [B] ⁇ 0.20 (Here, [Mo], [Ti], [N] and [B] are the contents (mass %) of Mo, Ti, N and B, respectively, and [N] is 0 mass %.
  • the ferrite has an average crystal grain size of 6 ⁇ m or less, and an average aspect ratio of 2.0 or less
  • the martensite has an average crystal grain size of 5 ⁇ m or less, and an average aspect ratio of 2.0 or less
  • the bainite has an average crystal grain size of 5 ⁇ m or less
  • the average free path of the martensite is 8.0 ⁇ m or less, and, among the martensite, the martensite grains in which 10 or more carbides having a grain size of 0.1 ⁇ m or more exist in the martensite grains have a volume relative to the total martensite.
  • the above component composition is mass% and further Cr: 0.80% or less, Cu: 0.50% or less, Ni: 0.50% or less,
  • [5] A method for producing cold-rolled full-hard steel sheet, in which the hot-rolled steel sheet obtained by the production method of [4] is pickled and cold-rolled.
  • the cold rolled full hard steel sheet obtained by the manufacturing method of [5] is heated to a soaking temperature of 750°C to 900°C at an average heating rate of 0.5°C/min to 5.0°C/min.
  • a method for producing a high-strength steel sheet which is maintained at the temperature for 1 hour or more and then cooled to room temperature at an average cooling rate of 1.0°C/h or more and 100°C/h or less.
  • a high-strength steel sheet having high tensile strength, excellent workability (excellent elongation and hole expandability), and excellent resistance weld cracking resistance is provided together with its manufacturing method.
  • the manufacturing method of a hot-rolled steel plate and the manufacturing method of a cold-rolled full hard steel plate which can bring about the said high strength steel plate are provided.
  • C 0.05% or more and 0.18% or less C is an element effective in strengthening the steel sheet, and contributes to the formation of the second phase (including pearlite, bainite, and martensite) other than the first phase, ferrite. It is an important element. If the C content is less than 0.05%, it is difficult to secure the necessary volume fraction of pearlite, bainite, and martensite, and it is difficult to obtain the desired strength. Therefore, the content is 0.05% or more. It is preferably 0.075% or more. When the content of C is excessive, the hardness after resistance welding becomes hard, the toughness at the time of resistance welding decreases, and the crack resistance of the resistance welding part deteriorates.
  • martensite with high hardness which is insufficient in the formation of carbides, becomes excessive, so that good hole expandability and delayed fracture properties cannot be obtained. Further, the volume fraction of martensite increases, making it difficult to secure the volume fraction of ferrite, and the elongation decreases. Therefore, its content should be 0.18% or less. It is preferably 0.16% or less.
  • Si 0.8% or less Si is an element that contributes to the strengthening by strengthening the solid solution of ferrite.
  • the content is excessive, the toughness during resistance welding decreases and the resistance to cracking of the resistance weld becomes poor. to degrade. Further, martensite in which a predetermined amount of carbide is formed cannot be obtained, and good hole expandability and delayed fracture properties cannot be obtained. Therefore, the content is 0.8% or less. It is preferably 0.50% or less. It is more preferably 0.30% or less. The lower limit is not particularly limited, but 0.005% or more is preferable because an extremely low Si content increases cost.
  • Mn 1.5% to 3.0%
  • Mn is an element that contributes to strengthening by promoting solid solution strengthening and formation of the second phase. It is also an element that stabilizes austenite and is necessary for controlling the volume fraction of the second phase. In order to obtain these effects, its content should be 1.5% or more. It is preferably 2.1% or more. More preferably, it is 2.2% or more. On the other hand, when the content is excessive, Mn bands are generated, so that the hole expandability and resistance weld cracking resistance deteriorate. Therefore, the content is 3.0% or less. It is preferably 2.6% or less.
  • P 0.05% or less
  • P is an element that contributes to strengthening by solid solution strengthening, but if the content is excessive, segregation to the grain boundaries becomes significant and the grain boundaries become brittle, so resistance welding Part cracking property decreases. Therefore, the content is set to 0.05% or less. It is preferably 0.04% or less.
  • the lower limit is not particularly limited, but an extremely low P content increases the steelmaking cost, so 0.0005% or more is preferable.
  • the content of S is 0.005% or less.
  • the content is 0.005% or less. It is preferably 0.0045% or less.
  • the lower limit is not particularly limited, but 0.0002% or more is preferable because an extremely low S content increases the steelmaking cost.
  • Al 0.01% or more and 0.10% or less
  • Al is an element necessary for deoxidation, and its content is 0.01% or more in order to obtain this effect. On the other hand, if it exceeds 0.10%, the effect is saturated, so the content is made 0.10% or less. It is preferably 0.06% or less.
  • N 0.010% or less
  • N is an element that forms coarse nitrides and deteriorates the hole expandability. This tendency becomes remarkable when N exceeds 0.010%. Therefore, its content is 0.010% or less. It is preferably 0.008% or less.
  • the lower limit is not particularly limited, but 0.0005% or more is preferable because extremely low N conversion increases cost.
  • Mo 0.05% or more and 0.50% or less
  • Mo is an element that contributes to strengthening by promoting the formation of the second phase. It is an element that stabilizes austenite during annealing and is necessary for controlling the volume fraction of the second phase. Further, it is an important element in the present invention because the hardenability can be secured even if the cooling rate during annealing is low. In order to obtain these effects, the content is 0.05% or more. It is preferably 0.08% or more. On the other hand, if the content is excessive, the second phase is excessively generated, and the elongation and the hole expandability deteriorate. Therefore, its content should be 0.50% or less. It is preferably 0.43% or less.
  • Ti 0.01% or more and 0.10% or less Ti is an element that contributes to strengthening by forming fine carbonitrides. It also contributes to suppressing the reaction of B, which is an essential element in the present invention, with N. Furthermore, it is an important element in the present invention because it can control the grain growth of fine carbonitrides during annealing and generate ferrite and martensite that are close to spherical. In order to obtain these effects, the content is 0.01% or more. It is preferably 0.03% or more. It is more preferably 0.04% or more. On the other hand, if Ti is added in a large amount, the elongation is significantly reduced, so the content is made 0.10% or less. It is preferably 0.09% or less.
  • B 0.0002% or more and 0.0100% or less B improves the hardenability and contributes to the strengthening by promoting the formation of the second phase. It is an element that does not lower the martensite transformation start point while ensuring the hardenability. is there. In addition, segregation at grain boundaries improves grain boundary strength, which is effective for delayed fracture resistance. In order to obtain these effects, the content is 0.0002% or more. It is preferably 0.0005% or more. On the other hand, if the content is excessive, toughness is deteriorated and resistance weld cracking resistance is deteriorated. Therefore, the content is 0.0100% or less. It is preferably 0.0050% or less.
  • [Mo], [Ti], [N] and [B] are the contents (mass %) of Mo, Ti, N and B, respectively, and even if [N] is 0 mass %. Good.
  • the above formula (1) secures a predetermined structure, and is an index in securing strength, workability and resistance weld cracking resistance, and when the value on the left side is less than 0.20, the predetermined structure is Otherwise, it may be difficult to combine strength, workability, and resistance weld cracking resistance. Therefore, the value on the left side is 0.20 or more. It is preferably 0.21 or more. The upper limit is 1.15, and since it is difficult to secure ductility with an increase in quenching elements, it is preferably 0.85 or less.
  • the high-strength steel sheet of the present invention may further contain one or more of the following components.
  • V 0.06% or less
  • V is an element that contributes to the strength increase by forming fine carbonitrides.
  • its content is preferably 0.01% or more in order to obtain this effect. It is more preferably 0.015% or more.
  • its content is 0.06% or less. It is preferably 0.055% or less.
  • Nb 0.05% or less
  • Nb is an element that contributes to strengthening by forming fine carbonitrides like V.
  • its content is preferably 0.005% or more in order to obtain this effect. It is more preferably 0.01% or more.
  • the content is set to 0.05% or less. It is preferably 0.035% or less.
  • Cr 0.80% or less Cr is an element that contributes to strengthening by promoting the formation of the second phase. Further, Cr is an element that stabilizes austenite during annealing, and is effective in controlling the volume fraction of the second phase.
  • its content is preferably 0.01% or more in order to obtain these effects. It is more preferably 0.05% or more.
  • the content is excessive, the second phase is excessively generated, which may adversely affect elongation and bending workability, and excessively generate a surface oxide, which deteriorates galvanizability and chemical conversion treatability. Therefore, its content is 0.80% or less. It is preferably 0.65% or less.
  • Cu 0.50% or less
  • Cu is an element that contributes to strengthening by solid solution strengthening and also contributes to strengthening by promoting generation of the second phase.
  • its content is preferably 0.05% or more in order to obtain these effects. It is more preferably 0.12% or more.
  • the content if the content exceeds 0.50%, the effect is saturated, and surface defects due to Cu are likely to occur. Therefore, its content should be 0.50% or less. It is preferably 0.33% or less.
  • Ni 0.50% or less
  • Ni is an element that contributes to strengthening by solid solution strengthening and also contributes to strengthening by promoting generation of the second phase.
  • its content is preferably 0.05% or more in order to obtain these effects. It is more preferably 0.08% or more.
  • Ni is particularly effective when Cu is contained because it has an effect of suppressing surface defects caused by Cu when it is contained together with Cu.
  • its content should be 0.50% or less. It is preferably 0.35% or less.
  • Sb 0.02% or less
  • Sb is an element that has the effect of suppressing the decarburization layer that occurs in the surface layer of the steel sheet and that contributes to the improvement of the crack resistance of the resistance weld by making the hardness distribution of the surface layer uniform.
  • its content is preferably 0.002% or more in order to obtain these effects. It is more preferably 0.005% or more.
  • the Sb content is 0.02% or less. It is preferably 0.015% or less.
  • Ca and REM are elements that reduce the adverse effect on the hole expansibility by spheroidizing the shape of the sulfide.
  • the content is preferably 0.0005% or more in order to obtain these effects. More preferably, it is 0.0008% or more.
  • the content is 0.0050% or less. Preferably, it is 0.0032% or less.
  • the balance other than the above is Fe and inevitable impurities.
  • the unavoidable impurities include, for example, Zn, Co, Sn, Zr, etc., and the permissible ranges of these contents are: Zn: 0.01% or less, Co: 0.10% or less, Sn: 0.10% or less, Zr:0.10. % Or less.
  • the microstructure is Volume fraction, 30% or more and 70% or less of ferrite, Martensite is 20% or more and 70% or less, Perlite is 10% or less (including 0%) and bainite is 20% or less (including 0%),
  • the ferrite has an average crystal grain size of 6 ⁇ m or less, and an average aspect ratio of 2.0 or less
  • the martensite has an average crystal grain size of 5 ⁇ m or less, and an average aspect ratio of 2.0 or less
  • the bainite has an average crystal grain size of 5 ⁇ m or less
  • the mean free path of the martensite is 8.0 ⁇ m or less, and, among the martensite, the martensite grains, martensite grains having a grain size of 0.1 ⁇ m or more and 10 or more carbides, the volume with respect to all martensite.
  • the ratio is 50% or more.
  • volume fraction such as volume fraction, average crystal grain size, aspect ratio, and mean free path:
  • the volume fractions of ferrite, martensite, bainite, and pearlite were 3000 times higher than that of the steel plate after the plate thickness cross section parallel to the rolling direction was polished and corroded with 3 vol.% Nital, using SEM (scanning electron microscope) At a magnification, 10 fields of view (1 field of view is 50 ⁇ m ⁇ 40 ⁇ m) are observed at a position 1/4 in the plate thickness direction from the surface, and the area fraction is determined by the point counting method (based on ASTM E562-83 (1988)). It is measured, and the area fraction is taken as the volume fraction.
  • the volume fraction thus obtained is the volume fraction of the entire hot press member.
  • the microstructure granular carbides are observed, but in the calculation of the area fraction of each phase, when carbides are present in each phase, the area including the areas of these carbides is also included. The percentage. The same applies to the following.
  • the average crystal grain size of ferrite, bainite, and martensite can be obtained by importing a photo in which the crystal grains of ferrite, bainite, and martensite have been identified in advance from the steel sheet micrograph using Image-Pro of Media Cybernetics. It is possible to calculate the area of each phase, calculate the circle equivalent diameter for the crystal grains of ferrite, bainite and martensite, and for each of the ferrite, bainite and martensite, it was determined by averaging the values of the circle equivalent diameter. It is a thing.
  • the volume fraction of martensite grains in which 10 or more carbides having a grain size of 0.1 ⁇ m or more are present in the martensite grains is obtained as follows. After polishing the plate thickness cross section parallel to the rolling direction of the steel plate, corroding it with 3 vol.% Nital and using a TEM (transmission electron microscope) at a magnification of 20000 times, about 1/4 position from the surface in the plate thickness direction , 10 fields of view (1 field of view: 0.5 ⁇ m ⁇ 0.5 ⁇ m) were observed, 25 martensite grains were selected in descending order of average crystal grain size, and the point count method (ASTM E562-83 ( 1988)), and calculate the area fraction to obtain the volume fraction.
  • TEM transmission electron microscope
  • the number of carbides having a grain size of 0.1 ⁇ m or more present in the grains is counted.
  • the particle size of the carbide is the equivalent circle diameter.
  • the area fraction of martensite grains in which 10 or more carbides having a grain size of 0.1 ⁇ m or more exist in the grains is determined as the volume fraction.
  • the mean free path of martensite is calculated by the following mathematical formula 1.
  • L M Martensite mean free path
  • d M Martensite mean crystal grain size
  • ⁇ Circularity
  • the volume fraction of ferrite If the volume fraction of ferrite exceeds 70%, it is difficult to achieve a tensile strength of 780 MPa. Therefore, the volume fraction should be 70% or less. It is preferably 65% or less. On the other hand, when the volume fraction is less than 30%, the second phase is excessively generated and voids are easily generated during punching in the hole expanding test, so that the hole expandability is deteriorated. Furthermore, if the volume fraction is less than 30%, the elongation will decrease. Therefore, the volume fraction should be 30% or more. It is preferably 35%, more preferably 40% or more.
  • the average crystal grain size of ferrite is 6 ⁇ m or less.
  • the lower limit is not particularly limited, it can be industrially set to 0.5 ⁇ m or more. It is preferably 5.5 ⁇ m or less, and more preferably 2 ⁇ m or more.
  • the average aspect ratio of ferrite exceeds 2.0, the voids generated during punching in the hole expansion test will be easy to connect during hole expansion, and the hole expandability will deteriorate. Therefore, the average aspect ratio is 2.0 or less.
  • the lower limit is 1.0, and may be 1.0. It is preferably 1.8 or less, and more preferably 1.3 or more. More preferably, it is 1.6 or less.
  • the volume fraction of martensite is 20% or more in order to secure the desired strength. It is preferably at least 23%.
  • the volume fraction should be 70% or less. It is preferably 65% or less, more preferably 60% or less.
  • the average crystal grain size of martensite is set to 5 ⁇ m or less.
  • the lower limit is not particularly limited, it can be industrially set to 0.2 ⁇ m or more. It is preferably 4.5 ⁇ m or less, and more preferably 2.5 ⁇ m or more.
  • the average aspect ratio of martensite exceeds 2.0, the voids generated during punching during the hole expansion test are likely to be connected during hole expansion, resulting in poor hole expandability. Therefore, the average aspect ratio is 2.0 or less.
  • the lower limit is 1.0, and may be 1.0. It is preferably 1.8 or less, and more preferably 1.3 or more.
  • the martensite grains having 10 or more carbides with a grain size of 0.1 ⁇ m or more in the grain should be 50% or more in the total martensite volume fraction. If the volume fraction is less than 50%, a large amount of high-hardness martensite will be generated, and void formation will excessively increase during punching, resulting in deterioration of hole expandability. In addition, hydrogen trap sites in the resistance weld and the heat affected zone (HAZ) are insufficient, so it is difficult to obtain sufficient delayed fracture resistance. Therefore, the volume fraction should be 50% or more. It is preferably 55% or more. On the other hand, the upper limit may be 95%, preferably 90% or less.
  • the mean free path of martensite exceeds 8.0 ⁇ m, void formation during punching in the hole expansion test increases excessively, resulting in deterioration of hole expandability. Furthermore, a concentration distribution of Mn, etc. occurs in the heat-affected zone (HAZ) after resistance welding, causing internal cracking during resistance welding and delayed fracture characteristics after resistance welding. Therefore, the mean free path of martensite is 8.0 ⁇ m or less.
  • the mean free path can be, for example, 3.0 ⁇ m or more.
  • the volume fraction should be 10% or less. It is preferably 5% or less, and may be 0%.
  • Bainite Since bainite contributes to strengthening, it may be generated in the microstructure, but since it contains a high dislocation density, if the volume fraction exceeds 20%, void formation after punching in the hole expanding test increases excessively. Therefore, the hole expandability deteriorates. Therefore, the volume fraction should be 20% or less. It is preferably 15% or less, and may be 0%.
  • the average grain size of bainite exceeds 5 ⁇ m, the toughness deteriorates due to the coarsening of grains during resistance welding, causing internal cracking and delayed fracture after resistance welding. Therefore, the average grain size of bainite is set to 5 ⁇ m or less.
  • the lower limit is not particularly limited, it can be industrially set to 0.5 ⁇ m or more. It is preferably 4 ⁇ m or less, and more preferably 2 ⁇ m or more.
  • the microstructure of the high-strength steel sheet of the present invention is basically composed of ferrite, martensite, pearlite and bainite (however, the volume fraction of pearlite and bainite may be 0%). Besides, residual austenite and unrecrystallized ferrite may be contained. Even in this case, if the ferrite, martensite, pearlite and bainite satisfy the above conditions, the object of the present invention can be achieved.
  • the residual structure other than ferrite, martensite, pearlite and bainite is preferably 5% or less, more preferably 3% or less, and may be 0%.
  • the high strength steel plate of the present invention may be provided with a plating layer.
  • the plating layer is not particularly limited and may be, for example, either a hot dip layer or an electroplating layer, or an alloyed plating layer.
  • the plating metal is not particularly limited and may be aluminum plating or the like in addition to zinc plating.
  • the high strength steel plate of the present invention includes a plated steel plate.
  • the plate thickness of the high-strength steel plate is not particularly limited, and can be 0.4 mm or more and 3.0 mm or less.
  • the plate thickness is preferably 0.5 mm or more, more preferably 0.55 mm or more, preferably 2.8 mm or less, and more preferably 2.6 mm or less.
  • the plate thickness refers to the plate thickness of the base steel plate excluding the plating layer.
  • the hot rolling start temperature of the steel slab is preferably 1100°C or higher and 1300°C or lower, and after the steel slab is cast, hot rolling is performed at a temperature of 1100°C or higher and 1300°C or lower without reheating. It is preferable to start hot rolling after starting or after reheating to 1100° C. or higher and 1300° C. or lower. That is, in the present invention, after the steel slab is manufactured, the steel slab is once cooled to room temperature and then reheated. An energy-saving process such as a method of immediately rolling after heating, or a method of direct-rolling/direct rolling in which casting is followed by rolling as it is can be applied without any problem.
  • the rolling reduction of the final pass of finish rolling is 12% or more, a large number of shear bands are introduced into the austenite grains, and The nucleation site of ferrite transformation increases, the structure of the hot-rolled steel sheet can be refined, the Mn band is eliminated, and the martensite in hot-rolled steel sheet, cold-rolled full-hard steel sheet, and high-strength steel sheet after annealing. It is necessary because the mean free path of can be reduced. It is also necessary for ferrite and martensite to obtain a predetermined aspect ratio.
  • the rolling reduction in the final pass is 13% or more.
  • the rolling reduction in the final pass is 16% or more.
  • the upper limit is not particularly limited, but by increasing the hot rolling load, the plate thickness variation in the width direction of the plate becomes large, and in order to avoid deterioration in resistance weld cracking resistance, 30% The following are preferred.
  • the rolling ratio of the pass immediately before the final pass 11% or more
  • the strain accumulation effect is further enhanced, and a shear band is formed in the austenite grains.
  • the nucleation site of ferrite transformation is further increased, and the structure of the hot-rolled steel sheet is further refined to eliminate the Mn band and reduce the mean free path of martensite. .. It is also necessary for ferrite and martensite to obtain a predetermined aspect ratio.
  • the rolling reduction immediately before the final pass is 12% or more.
  • the upper limit is not particularly limited, but by increasing the hot rolling load, the plate thickness variation in the width direction of the plate increases, and in order to avoid deterioration of the resistance weld cracking resistance, 16%, The following are preferred.
  • Edge ratio of the pass immediately before the final pass to the final pass reduction ratio is preferably 1.1 or less.
  • a predetermined aspect ratio may not be obtained for ferrite and martensite.
  • the number of passes other than the final pass, the pass immediately before the final pass, and the reduction rate are not particularly limited.
  • the number of passes may be two, the final pass and the pass immediately before the final pass.
  • the total number of passes including these two passes may be 4 or more, for example, 5 or more, and 20 or less. It is possible and 16 or less are preferable.
  • Hot rolling improves the resistance weld cracking resistance after annealing by uniformly refining the structure of the steel sheet and reducing the material anisotropy.
  • the finish rolling finish temperature is set to 840°C or higher. It is preferably 870°C or higher.
  • the finish rolling end temperature exceeds 950°C, the hot rolled structure becomes coarse and the crystal grains after annealing also become coarse. Therefore, the finish rolling finish temperature is set to 950°C or lower. It is preferably 930°C or lower.
  • the primary cooling is performed at a primary average cooling rate of 70° C./s or more to a first cooling stop temperature of 700° C. or less.
  • the primary average cooling rate is less than 70°C/s, ferrite and martensite are coarsened, so that the steel sheet structure of the hot-rolled steel sheet becomes inhomogeneous and the hole expandability, resistance weld cracking resistance and delayed fracture property are descend.
  • the primary average cooling temperature is 70°C/s or higher. It is preferably 75°C/s or more.
  • the upper limit of the first average cooling temperature is not particularly limited, but may be 150° C./s or less, for example. If the first cooling stop temperature in primary cooling exceeds 700°C, the structure of the hot-rolled steel sheet becomes coarse and the final microstructure becomes coarse, resulting in poor hole expansibility, resistance to weld cracking and delayed fracture properties. To do. Therefore, the first cooling stop temperature is set to 700°C or lower. It is preferably 680°C or lower. The first cooling stop temperature is not particularly limited as long as it is higher than the winding start temperature of 610° C. or lower, and may be 630° C. or higher, for example.
  • the secondary average cooling rate of 5° C./s or more and 50° C./s or less is used to cool from the cooling stop temperature to the winding start temperature of 610° C. or less.
  • the second average cooling rate is 5°C/s or more, preferably 8°C/s or more.
  • the second average cooling rate can be 45°C/s or less, preferably 40°C/s or less.
  • the temperature is cooled to a winding start temperature of 610° C. or lower, and then winding is started.
  • the lower limit of the winding start temperature is not particularly limited, but the temperature at the time of winding becomes too low, hard martensite is excessively generated, from the viewpoint of avoiding an increase in cold rolling load, 300 C. or higher is preferable.
  • -Winding start temperature 610°C or less
  • the winding start temperature is preferably 600° C. or lower.
  • the winding start temperature is preferably 300°C or higher.
  • the steel plate After the above-mentioned winding, the steel plate is cooled by air cooling etc. and used for the production of the cold rolled full hard steel plate described below.
  • the hot rolled steel sheet is an intermediate product to be traded, the hot rolled steel sheet is usually traded in a cooled state after winding.
  • the plate thickness of the hot-rolled steel sheet is not particularly limited, and is preferably 0.8 mm or more and 5.0 mm or less, more preferably 1.1 mm or more and 4.5 mm or less.
  • the cold rolled full hard steel sheet can be manufactured by cold rolling the hot rolled steel sheet obtained by the above manufacturing method.
  • the cold-rolled full-hard steel sheet refers to a cold-rolled steel sheet that is used for various purposes in a state where no annealing treatment is performed after cold rolling, that is, in a full-hard state.
  • Various applications include production of high-strength steel sheets, and high-strength steel sheets may be continuously produced after production of cold-rolled full-hard steel sheets.
  • the cold rolling conditions can be appropriately set, for example, from the viewpoint of desired thickness and the like.
  • the reduction ratio of cold rolling is usually 30% or less.
  • Pickling is performed before the cold rolling in order to remove the scale on the surface of the hot rolled steel sheet.
  • the pickling conditions can be set appropriately.
  • the plate thickness of the cold rolled full hard steel plate is not particularly limited, and is preferably 0.4 mm or more and 3.0 mm or less, more preferably 0.5 mm or more and 2.8 mm or less.
  • the high-strength steel plate can be manufactured by heating and cooling (annealing) a cold rolled full hard steel plate.
  • This annealing step is a step for advancing the recrystallization of ferrite and forming fine ferrite, martensite and bainite in the microstructure for increasing the strength, and is 0.5°C/min or more and 5.0°C/min or less. It is heated to a soaking temperature of 750° C. or higher and 900° C. or lower at an average heating rate, held at the temperature for 1 hour or longer, and then cooled to room temperature at an average cooling rate of 1.0° C./h or more and 100° C./h or less.
  • the average heating rate is 5.0°C/min or less. It is preferably 4.5°C/min or less.
  • the average heating rate is 0.5°C/min or more. It is preferably 1.0°C/min or more.
  • -Soaking temperature (holding temperature): 750°C or more and 900°C or less
  • the soaking temperature is the temperature range that is the two-phase region of ferrite and austenite or the austenite single phase region. If the temperature is less than 750°C, the ferrite fraction increases, and it becomes difficult to secure the strength. Further, martensite in which a predetermined amount of carbide is formed cannot be obtained, and good hole expandability and good delayed fracture properties cannot be obtained. Therefore, the soaking temperature is 750°C or higher. If the soaking temperature is too high, the growth of austenite crystal grains becomes remarkable, and the crystal grains become coarse, so that the crack resistance of the resistance weld zone deteriorates. Therefore, the soaking temperature is set to 900°C or lower. It is preferably 880°C or lower.
  • Soaking time 1 hour or more At the above soaking temperature, the soaking time is required to be 1 hour or more for the progress of recrystallization, spheroidization of crystal grains and austenite transformation of a part or all of the structure. .. Further, if the soaking time is less than 1 hour, the ferrite fraction increases, and it becomes difficult to secure the strength. Further, martensite in which a predetermined amount of carbide is formed cannot be obtained, and good hole expandability and good delayed fracture properties cannot be obtained. It is preferably 2 hours or more.
  • the upper limit is not particularly limited, but if the soaking time is excessively long, micro segregation of Mn is promoted, and hole expandability and resistance weld cracking resistance may be deteriorated, so 100 hours or less is preferable. It is more preferably 72 hours or less.
  • -Cooling conditions during annealing Cool to room temperature at an average cooling rate of 1.0°C/h or more and 100°C/h or less. After soaking, the soaking temperature to room temperature is 1.0°C/h or more and 100°C/h or less. It is necessary to cool at an average cooling rate. If the average cooling rate is less than 1.0°C/h, it is difficult to secure the strength because ferrite transformation proceeds during cooling and the volume fraction of the second phase decreases. Therefore, the average cooling rate is 1.0°C/h or more. It is preferably 1.3° C./h or more.
  • the average cooling rate exceeds 100°C/h, martensite is excessively formed, the TS is lowered and the El is lowered, and the desired aspect ratio of ferrite and martensite cannot be obtained, resulting in poor hole expansibility. .. Further, martensite in which a predetermined amount of carbide is formed cannot be obtained, and good delayed fracture rupture properties cannot be obtained. Therefore, the average cooling rate is 100°C/h or less. It is preferably 73° C./h or less.
  • the steel plate may be plated to form a plating layer.
  • the plated layer may be either a hot-dip plated layer or an electroplated layer, or may be an alloyed plated layer.
  • the plating metal is not particularly limited and may be aluminum plating or the like in addition to zinc plating.
  • the high-strength steel sheet of the present invention can secure hole expandability and resistance weld cracking resistance by controlling the above-described composition and microstructure without performing plating treatment.
  • Hot-dip galvanizing can be performed under normal conditions.
  • the temperature of the steel sheet immersed in the plating bath is preferably (hot dip galvanizing bath temperature ⁇ 40)° C. or higher and (hot dip galvanizing bath temperature +50)° C. or lower.
  • the temperature of the steel plate to be immersed in the plating bath is lower than the hot-dip galvanizing bath temperature -40°C, when the steel plate is immersed in the plating bath, part of the molten zinc solidifies and deteriorates the appearance of the plating. There is. Therefore, it is preferable to set the lower limit to (hot dip galvanizing bath temperature ⁇ 40)° C.
  • zinc plating can be alloyed in the temperature range of 450°C to 600°C.
  • the Fe concentration in the plating can be controlled to 7% to 15%, and the adhesion of the plating and the corrosion resistance after coating are improved.
  • the temperature is lower than 450°C, alloying does not proceed sufficiently, which leads to a reduction in sacrificial anticorrosion action and a reduction in slidability. ..
  • a galvanizing bath containing 0.10 mass% or more and 0.20 mass% or less of Al. After plating, wiping can be performed to adjust the basis weight of plating.
  • Temper rolling may be performed after annealing.
  • the preferable range of the elongation rate is 0.05% or more and 2.0% or less.
  • the hot rolling heating temperature is 1250°C and the finish rolling finish temperature (FDT) is finished under the conditions shown in Table 2.
  • Hot rolling was performed with a total of 7 passes of rolling to obtain a hot rolled steel sheet.
  • the first cooling stop temperature at the primary average cooling rate (cold speed 1) shown in Table 2
  • winding was performed to obtain a hot rolled steel sheet (HR).
  • the cold-rolled steel sheet thus obtained was annealed in a box-type annealing furnace (BAF) under the conditions shown in Table 2 to obtain a cold-rolled steel sheet (CR).
  • a portion of the alloy was hot dip galvanized and then alloyed to obtain a galvannealed steel sheet (GA).
  • the plating treatment is: zinc plating bath temperature: 460°C, zinc plating bath Al concentration: 0.14 mass% (when alloying treatment is applied), 0.18 mass% (when no alloying treatment is applied), plating adhesion per side The amount was 45 g/m 2 (double-sided plating).
  • Some of the steel sheets were non-alloyed galvanized steel sheets (GI) without galvanizing.
  • the microstructure (volume fraction, average crystal grain size, aspect ratio, volume fraction of specific martensite grains, average free path) of the obtained steel sheet was measured by the above-mentioned method. The results are shown in Table 3.
  • Tensile Strength (TS) and Elongation (El) From the obtained steel sheet, a JIS No. 5 tensile test piece was sampled so that the direction perpendicular to the rolling was the longitudinal direction (tensile direction) of the JIS No. 5 tensile test piece, and the tensile strength (JIS Z 2241:2011) was used to determine tensile strength ( TS) and elongation (El) were measured. A steel sheet having a good elongation was obtained when El (%) was 14% or more.
  • Hole expandability Based on JIS Z 2256:2010, punch a 10 mm ⁇ hole with a clearance of 12.5% from the obtained steel plate, set it on the testing machine so that the burr is on the die side, and then use a 60° conical punch.
  • the hole expansion ratio ( ⁇ ) was measured by molding.
  • a steel sheet having ⁇ (%) of 35% or more was defined as a steel sheet having good hole expandability.
  • Resistance weld cracking resistance As a longitudinal direction perpendicular to the rolling direction of the obtained steel sheet, using one test piece cut into 50 ⁇ 150 mm, the other using a 590 MPa class hot dip galvanized steel sheet with a plate thickness of 1.2 mm, Resistance welding (spot welding) was performed. Resistance spot welding was carried out using a servo motor pressure type single-phase direct current (50Hz) resistance welding machine with the plate set of two steel plates stacked at an angle of 6°. The welding conditions were a pressure of 3.5 kN and a hold time of 0.1 seconds. The welding current and welding time were adjusted so that the nugget diameter was 4.2 mm. After welding, the test piece was cut in half and the cross section was observed with an optical microscope. Those in which no crack of 0.1 mm or more was observed were evaluated as good resistance to cracking in the resistance weld ( ⁇ ).
  • resistance welding spot welding
  • spot welding was performed using two tensile shear test pieces obtained according to JIS Z 3136:1999.
  • Resistance spot welding was carried out on a plate stack of two steel plates using a servo motor pressure type single-phase DC (50 Hz) resistance welder.
  • the welding conditions were a pressure of 3.5 kN and a hold time of 0.1 seconds.
  • the welding current and welding time were adjusted so that the nugget diameter was 4.2 mm.
  • the resulting welded body was subjected to a tensile shear test in accordance with JIS Z 3136:1999 to measure the load when the steel sheet peeled off.
  • a tensile shear test piece was prepared in the same manner as above, and a load of 0.8 ⁇ FS was applied. Then, it was immersed in a 3.0% NaCl+0.3% NH 4 SCN solution at room temperature, and hydrogen was added thereto by cathodic electrolytic charging. The current density was 1.5 mA/cm 2 , and the counter electrode was platinum. Those that did not fracture even after 100 hours were rated as good ( ⁇ ) for delayed fracture resistance.

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Abstract

La présente invention concerne une tôle d'acier à haute résistance présentant une aptitude au façonnage exceptionnelle et une résistance exceptionnelle à la rupture d'une pièce soudée par résistance.　La présente tôle d'acier à haute résistance est caractérisée en ce qu'elle comprend : une composition de composant définie ; et une microstructure qui a, en termes de fraction volumique, de 30 à 70 % inclus de ferrite, de 20 à 70 % inclus de martensite, 10 % ou moins (y compris 0 %) de perlite, et 20 % ou moins (y compris 0 %) de baïnite, la ferrite ayant un diamètre de grain cristallin moyen de 6 µm ou moins et un rapport d'aspect moyen de 2,0 ou moins, la martensite ayant un diamètre de grain cristallin moyen de 5 µm ou moins et un rapport d'aspect moyen de 2,0 ou moins, la baïnite ayant un diamètre de grain cristallin moyen de 5 µm ou moins, le trajet libre moyen de la martensite étant de 8,0 µm ou moins, et la martensite étant telle que les grains de martensite, dans lesquels 10 carbures ou plus ayant une taille de grain de 0,1 µm ou plus sont présents dans les grains de martensite, ont un rapport de 50 % ou plus en termes de fraction volumique par rapport à la totalité de la martensite.
PCT/JP2020/002392 2019-02-21 2020-01-23 Tôle d'acier à haute résistance, procédé de fabrication d'une tôle d'acier laminée à chaud, procédé de fabrication d'une tôle d'acier crue laminée à froid, et procédé de fabrication de tôle d'acier à haute résistance WO2020170710A1 (fr)

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Cited By (4)

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WO2022210396A1 (fr) * 2021-03-31 2022-10-06 日本製鉄株式会社 Plaque d'acier, procédé de production de plaque d'acier, et procédé de production de plaque d'acier intermédiaire
WO2022269742A1 (fr) * 2021-06-22 2022-12-29 日本製鉄株式会社 Tôle d'acier laminée à chaud et son procédé de fabrication
WO2023008516A1 (fr) * 2021-07-28 2023-02-02 日本製鉄株式会社 Tôle d'acier, et procédé de fabrication de celle-ci
CN116209781A (zh) * 2020-09-30 2023-06-02 日本制铁株式会社 钢板

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WO2018030500A1 (fr) * 2016-08-10 2018-02-15 Ｊｆｅスチール株式会社 Tôle d'acier mince à haute résistance et son procédé de fabrication
WO2018043452A1 (fr) * 2016-08-30 2018-03-08 Ｊｆｅスチール株式会社 Tôle d'acier à haute résistance et son procédé de fabrication

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WO2017169940A1 (fr) * 2016-03-31 2017-10-05 Ｊｆｅスチール株式会社 Tôle d'acier mince et tôle d'acier plaquée, procédé de production de tôle d'acier laminée à chaud, procédé de production de tôle d'acier entièrement durcie laminée à froid, procédé de production de tôle d'acier traitée thermiquement, procédé de production de tôle d'acier mince et procédé de production de tôle d'acier plaquée
WO2018030500A1 (fr) * 2016-08-10 2018-02-15 Ｊｆｅスチール株式会社 Tôle d'acier mince à haute résistance et son procédé de fabrication
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Publication number Priority date Publication date Assignee Title
CN116209781A (zh) * 2020-09-30 2023-06-02 日本制铁株式会社 钢板
WO2022210396A1 (fr) * 2021-03-31 2022-10-06 日本製鉄株式会社 Plaque d'acier, procédé de production de plaque d'acier, et procédé de production de plaque d'acier intermédiaire
WO2022269742A1 (fr) * 2021-06-22 2022-12-29 日本製鉄株式会社 Tôle d'acier laminée à chaud et son procédé de fabrication
WO2023008516A1 (fr) * 2021-07-28 2023-02-02 日本製鉄株式会社 Tôle d'acier, et procédé de fabrication de celle-ci

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