WO2023037878A1 - Tôle d'acier laminée à froid et son procédé de fabrication - Google Patents

Tôle d'acier laminée à froid et son procédé de fabrication Download PDF

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WO2023037878A1
WO2023037878A1 PCT/JP2022/031939 JP2022031939W WO2023037878A1 WO 2023037878 A1 WO2023037878 A1 WO 2023037878A1 JP 2022031939 W JP2022031939 W JP 2022031939W WO 2023037878 A1 WO2023037878 A1 WO 2023037878A1
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steel sheet
cold
rolled steel
rolling
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PCT/JP2022/031939
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Japanese (ja)
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卓史 横山
智史 広瀬
裕之 鶴岡
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日本製鉄株式会社
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Priority to KR1020247004143A priority Critical patent/KR20240032929A/ko
Priority to JP2023546872A priority patent/JPWO2023037878A1/ja
Priority to CN202280047647.2A priority patent/CN117616144A/zh
Publication of WO2023037878A1 publication Critical patent/WO2023037878A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D3/00—Electroplating: Baths therefor
    • C25D3/02—Electroplating: Baths therefor from solutions
    • C25D3/22—Electroplating: Baths therefor from solutions of zinc
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present disclosure relates to cold-rolled steel sheets and manufacturing methods thereof.
  • Hydrogen embrittlement cracking is a phenomenon in which steel members, which are subjected to high stress during use, suddenly break due to hydrogen entering the steel from the environment. This phenomenon is also called delayed fracture from the mode of occurrence of fracture. Generally, it is known that hydrogen embrittlement cracking of steel sheets is more likely to occur as the tensile strength of steel sheets increases. It is believed that this is because the higher the tensile strength of the steel sheet, the greater the stress remaining in the steel sheet after part forming. This susceptibility to hydrogen embrittlement cracking (delayed fracture) is called hydrogen embrittlement resistance.
  • Patent Document 1 it has a predetermined chemical composition, and the value of the solid solution B amount solB [mass%] and the prior austenite grain size D ⁇ [ ⁇ m] in the steel is expressed by the formula (1): solB D ⁇ 0 .0010, and the area ratios are polygonal ferrite of 10% or less, bainite of 30% or less, retained austenite of 6% or less, and tempered martensite of 60% or more.
  • the Fe carbide number density of 1 ⁇ 10 6 /mm 2 or more, the average dislocation density of the entire steel is 1.0 ⁇ 10 15 /m 2 or more and 2.0 ⁇ 10 16 /m 2 or less, and the effective grain
  • An ultra-high-strength cold-rolled steel sheet having a tensile strength of 1300 MPa or more and excellent hydrogen embrittlement resistance is disclosed, which is characterized by having a steel structure with a diameter of 7.0 ⁇ m or less.
  • Patent Document 2 discloses that tempered martensite and bainite have a predetermined chemical composition, a total area ratio of 95% or more and 100% or less to the entire structure of tempered martensite and bainite, and are distributed in the rolling direction and/or in a dotted pattern.
  • One or more long axes composed of inclusion particles of 0.3 ⁇ m or more, and when the inclusion particles are composed of two or more, the distance between the inclusion particles is 30 ⁇ m or less, and the rolling direction Inclusion groups with total length exceeding 120 ⁇ m are 0.8/mm 2 or less, aspect ratio is 2.5 or less, and major axis is 0.20 ⁇ m or more and 2 ⁇ m or less, mainly composed of Fe
  • the number of carbides is 3500/mm 2 or less, the number of carbides with a diameter of 10 to 50 nm distributed in the tempered martensite and/or the bainite is 0.7 ⁇ 10 7 /mm 2 or more, and prior ⁇ grains
  • a cold-rolled steel sheet having an average grain size of 18 ⁇ m or less, a thickness of 0.5 to 2.6 mm, and a tensile strength of 1320 MPa or more is disclosed. Further, Patent Document 2 describes that with the above configuration, it is possible to obtain an ultra-high-strength cold-rolled steel sheet having a
  • Patent Document 3 it has a predetermined chemical composition, and has a structure consisting of martensite: 90% or more and retained austenite: 0.5% or more in terms of area ratio to the entire structure, and the local Mn concentration is An ultra-high-strength steel sheet having an area ratio of 1.1 times or more of the total Mn content of 2% or more, a tensile strength of 1470 MPa or more, and excellent delayed fracture resistance at the cut edge. disclosed.
  • Patent Document 4 describes an ultra-high-strength cold-rolled steel sheet having a predetermined chemical composition, a martensite single phase metal structure, a tensile strength of 980 MPa or more, and a flatness of 10 mm or less, and a method for producing the same. disclosed.
  • Patent Document 5 discloses a method for producing a high-strength cold-rolled steel sheet having a metal structure with a predetermined chemical composition and a tempered martensite content of 65 area% or more.
  • a secondary cooling step of cooling in seconds a tertiary cooling step of rapidly cooling from the secondary cooling stop temperature to room temperature at an average cooling rate of more than 100 ° C./s, and heating to a temperature range of 150 to 300 ° C. for 30 to 1500 seconds.
  • a method for producing a high-strength cold-rolled steel sheet having an excellent steel sheet shape is disclosed, which includes a holding overaging treatment step in this order.
  • JP 2016-50343 A WO2016/152163 JP 2016-153524 A JP 2011-202195 A JP 2013-227657 A
  • Patent Documents 4 and 5 do not improve the shape of the steel sheet with the intention of improving the hydrogen embrittlement resistance of the sheared portion. not enough to improve.
  • the "maximum warpage height" is used as an index for evaluating the quality of the steel plate shape. It has been found that the hydrogen embrittlement resistance of is not necessarily excellent.
  • an object of the present invention is to provide a cold-rolled steel sheet with improved resistance to hydrogen embrittlement while having high tensile strength and total elongation.
  • the present inventor believes that in order to improve the hydrogen embrittlement resistance of sheared parts, it is necessary to improve not the "maximum warpage height" of the steel sheet, but the "curvature”, which is the amount that indicates the degree of curvature of the curved surface. I found something. Then, as a result of examining a method of manufacturing a steel sheet necessary for improving the curvature of the steel sheet, the following findings were obtained. (1) In the hot rolling process, the edge portion is reheated after rough rolling. This suppresses fluctuations in the strength of the hot-rolled steel sheet in the width direction of the steel sheet. Furthermore, the steel sheet after finish rolling is wound up in an appropriate temperature range. As a result, the shape of the steel sheet after cold rolling is improved.
  • the forward tension and the backward tension in each rolling stand when passing through the rolling rolls are set to an appropriate range according to the yield strength of the hot-rolled steel sheet before cold rolling and the reduction ratio in each rolling stand. to control. Furthermore, the cumulative cold rolling reduction is controlled within an appropriate range. This improves the shape of the steel sheet after cold rolling.
  • the average cooling rate at 300 ° C. or less is limited to a predetermined range, gas is used as a coolant, and heat diffusion is prevented in the cooling process. Allow to cool to encourage. Furthermore, the average cooling rate between 300 and 700° C. and the cooling stop temperature must also be controlled within an appropriate range.
  • the steel plate tension during cooling is controlled within an appropriate range. This improves the shape of the steel sheet after heat treatment. When all of the above requirements (1) to (3) are satisfied, a steel sheet with an excellent level of shape that could not be achieved by existing techniques can be obtained.
  • the present invention has been realized based on the above findings, and is specifically as follows.
  • FIG. 2 is a schematic diagram of shearing related to hydrogen embrittlement testing.
  • C is an essential element for ensuring the strength of the steel sheet.
  • the C content is made 0.16% or more.
  • the C content may be 0.18% or more, 0.20% or more, or 0.22% or more.
  • the C content should be 0.40% or less.
  • the C content may be 0.37% or less, 0.33% or less, or 0.30% or less.
  • Si silicon
  • Si is an element that suppresses the formation of iron carbide and contributes to the improvement of strength and formability.
  • the Si content should be 0.05% or more.
  • the Si content may be 0.10% or more, 0.20% or more, or 0.40% or more.
  • excessive addition may lower toughness, weldability, and hydrogen embrittlement resistance. Therefore, the Si content should be 2.00% or less.
  • the Si content may be 1.60% or less, 1.30% or less, or 1.00% or less.
  • Mn 0.50 to 4.00%
  • Mn manganese
  • Mn is a strong austenite stabilizing element, and is an effective element for increasing the strength of steel sheets.
  • the Mn content is made 0.50% or more.
  • the Mn content may be 0.80% or more, 1.00% or more, or 1.30% or more.
  • excessive addition may deteriorate workability such as press formability, weldability, and hydrogen embrittlement resistance. Therefore, the Mn content should be 4.0% or less.
  • the Mn content may be 3.0% or less, 2.5% or less, or 2.0% or less.
  • Phosphorus (P) is a solid-solution-strengthening element that is effective in increasing the strength of steel sheets, but excessive addition degrades weldability and toughness. Therefore, the P content is limited to 0.050% or less.
  • the P content is preferably 0.045% or less, 0.035% or less or 0.020% or less.
  • the P content may be 0%, the lower limit is preferably set to 0.001% from the viewpoint of economy, because the cost of removing P increases in order to extremely reduce the P content.
  • S sulfur
  • S is an element contained as an impurity, and forms MnS in steel to deteriorate toughness and hole expansibility. Therefore, the S content is limited to 0.0100% or less as a range in which deterioration of toughness and hole expansibility is not remarkable.
  • the S content is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less.
  • the S content may be 0%, the lower limit is preferably set to 0.0001% from the viewpoint of economy, because desulfurization cost increases to extremely reduce the S content.
  • Al 0.001 to 1.00%
  • Al aluminum
  • the Al content may be 0.005% or more, 0.01% or more, or 0.02% or more.
  • the upper limit of the Al content is 1.00%.
  • the Al content may be 0.80% or less, 0.60% or less, or 0.30% or less.
  • N nitrogen
  • nitrogen is an element contained as an impurity, and when the content is large, coarse nitrides are formed in the steel, which may deteriorate bendability and hole expandability. Therefore, the N content is limited to 0.0100% or less.
  • the N content is preferably 0.0080% or less, 0.0060% or less or 0.0050% or less. Although the N content may be 0%, it is preferable to set the lower limit to 0.0001% from the viewpoint of economic efficiency, because the cost of removing N increases in order to extremely reduce the N content.
  • O oxygen
  • oxygen is an element contained as an impurity, and if the content is large, it may form coarse oxides in the steel, deteriorating bendability and hole expandability. Therefore, the O content is limited to 0.0100% or less.
  • the O content is preferably 0.0080% or less, 0.0060% or less or 0.0050% or less. Although the O content may be 0%, the lower limit is preferably 0.0001% from the viewpoint of manufacturing costs.
  • the basic chemical composition of the cold-rolled steel sheet according to the embodiment of the present invention and the slab used for its production are as described above. Furthermore, the cold-rolled steel sheet and slab may contain the following optional elements as necessary. In addition, the lower limit of the content when the arbitrary element is not included is 0%.
  • Cr 0-2.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, B: 0-0.0100%, Co: 0- 1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-1.00%, Nb: 0-0.100%, Ti: 0-0.200% and V: 0 to 0.50%]
  • Cr chromium
  • Mo molybdenum
  • Cu copper
  • Ni nickel
  • B boron
  • Co cobalt
  • W tungsten
  • Sn tin
  • Sb antimony
  • Nb (niobium), Ti (titanium), and V (vanadium) are alloy carbide forming elements, and contribute to increasing the strength of the steel sheet by precipitating as fine carbides in the steel sheet. Therefore, one or more of these elements may be added as required. However, excessive addition of these elements saturates the effect, unnecessarily leading to an increase in cost.
  • the contents are Cr: 0 to 2.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, B: 0 to 0.0100% , Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-1.00%, Nb: 0-0.100%, Ti: 0- 0.200% and V: 0-0.50%.
  • Each element may be 0.001% or more, 0.005% or more, or 0.010% or more.
  • the B content may be 0.0001% or more or 0.0005% or more.
  • Ca [Ca: 0-0.0100%, Mg: 0-0.0100%, Ce: 0-0.0100%, Zr: 0-0.0100%, La: 0-0.0100%, Hf: 0- 0.0100%, Bi: 0 to 0.0100% and REM other than Ce and La: 0 to 0.0100%]
  • Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf (hafnium), and REMs (rare earth elements) other than Ce and La are used to finely disperse inclusions in steel.
  • Bismuth (Bi) is an element that contributes to reducing the microsegregation of substitutional alloying elements such as Mn and Si in steel.
  • each element may be added, if necessary, because they each contribute to the improvement of the workability of the steel sheet. However, excessive addition causes deterioration of ductility. Therefore, the upper limit of its content is 0.0100%. Moreover, each element may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • the balance other than the above elements consists of Fe and impurities.
  • Impurities are components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when cold-rolled steel sheets are industrially manufactured.
  • the desired tensile strength can be obtained by using mainly martensite (as-quenched martensite + tempered martensite).
  • the area ratio of martensite is set to 90.0 to 99.5%, and the ratio of tempered martensite to the total martensite is set to 80 to 100%.
  • the lower limit of the area ratio of martensite is preferably 93.0% or more, more preferably 95.0% or more.
  • the upper limit of the area ratio of martensite may be 99.0% or less or 98.0% or less.
  • the lower limit of the ratio of tempered martensite to all martensite is preferably 85% or more, more preferably 90% or more.
  • the upper limit of the proportion of tempered martensite in the total martensite may be 98% or less or 95% or less.
  • the area ratio of ferrite is set to 0 to 5%.
  • the upper limit of the area ratio of ferrite is preferably 4% or less, preferably 2% or less, and ideally 0%.
  • the area ratio of retained austenite is set to 0.5 to 7.0%.
  • the lower limit of the area ratio of retained austenite is preferably 1.0% or more, and may be 2.0% or more.
  • the upper limit of the area ratio of retained austenite is preferably 6.0% or less, and may be 5.0% or less or 4.0% or less.
  • the steel structure may contain residual structures in addition to martensite, ferrite and retained austenite. Bainite, for example, can be exemplified as the residual structure. The area ratio of the remaining tissue is exemplified as 0 to 9.5%.
  • the area ratio of each structure other than retained austenite is evaluated by SEM-EBSD method (electron beam backscatter diffraction method) and SEM secondary electron image observation.
  • SEM-EBSD method electron beam backscatter diffraction method
  • SEM secondary electron image observation First, a sample is collected by using a plate thickness section parallel to the rolling direction of the steel sheet as an observation surface, and the observation surface is mechanically polished to a mirror finish, and then electrolytically polished.
  • SEM-EBSD method for a total area of 3000 ⁇ m 2 or more in one or more observation fields in the range of 1/8 thickness to 3/8 thickness centering on 1/4 thickness from the surface of the steel plate on the observation surface Crystal structure and orientation analysis are performed by "OIM Analysys 7.0" manufactured by TSL is used for analysis of data obtained by the EBSD method. Also, the distance between scores (step) is set to 0.03 to 0.20 ⁇ m. A grain boundary map is obtained with the boundary having a crystal orientation difference of 15 degrees or more as the grain boundary. Next, the same sample is subjected to nital etching. After that, a secondary electron image is taken using an FE-SEM for the same field of view as the field of view for crystal orientation analysis by EBSD.
  • crystal grains in which neither the substructure nor the iron-based carbide are recognized and the crystal structure is BCC are judged to be ferrite.
  • crystal grains in which a substructure is observed and iron-based carbides are precipitated in a single variant, or crystal grains in which iron-based carbides are not observed are judged to be bainite.
  • crystal grains in which cementite is precipitated in lamellar form are judged to be pearlite.
  • perlite is not included in the present invention.
  • the remainder is judged to be martensite and retained austenite. By subtracting the area ratio of retained austenite, which will be described later, from the area ratio of the remainder, the area ratio of martensite is obtained.
  • crystal grains in which substructures are recognized and two or more iron-based carbides precipitated in multiple variants are recognized in the secondary electron image are judged to be tempered martensite.
  • the area ratio of retained austenite is calculated by measurement using X-rays. That is, mechanical polishing and chemical polishing are performed to remove the steel plate from the plate surface to the depth of 1/4 position in the plate thickness direction. Diffraction peaks of (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase obtained using MoK ⁇ 1 rays as characteristic X-rays for the polished sample From the integrated intensity ratio of , the structure fraction of retained austenite is calculated, and this is defined as the area ratio of retained austenite.
  • the cold-rolled steel sheet according to the embodiment of the present invention has a high strength, for example, a high strength of 1470 MPa or more, but has a very high flatness. Also in , the end face properties of the sheared portion are very good, and as a result, excellent hydrogen embrittlement resistance can be achieved.
  • a steel sheet shape having such a high degree of flatness in the present invention is defined using the maximum value of the curvature 1/R corresponding to the reciprocal of the curvature radius R (mm). More specifically, the maximum value of the curvature 1/R in the present invention is defined by the following formula (1) using two principal curvatures ⁇ 1 and ⁇ 2 on the curved surface.
  • the maximum value of the curvature 1/R is controlled to 0.010 or less.
  • the curvature in the present invention is the larger absolute value of the principal curvatures ⁇ 1 and ⁇ 2 on the curved surface.
  • the principal curvatures ⁇ 1 , ⁇ 2 are measured using a common shape measuring machine and estimated from three-dimensional geometric data with reduced measurement noise.
  • ATOS 3D scanner manufactured by GOM can be used for measurement.
  • the curvature distribution in the cold-rolled steel sheet is obtained by measuring each point in an area of the entire width of the cold-rolled steel sheet and the length of 300 mm.
  • the term "full width” refers to the length of the steel sheet in the direction perpendicular to the longitudinal direction of the cold-rolled steel sheet (cold-rolled coil).
  • the maximum value of curvature distribution measured in this manner is 0.010 or less. For example, if the cold-rolled steel sheet is warped or wavy and the maximum value of the curvature distribution exceeds 0.010, an angle will be formed between the punch and the cold-rolled steel sheet during shearing, and the sheared part will be damaged. As a result, the hydrogen embrittlement resistance of the sheared portion deteriorates.
  • the maximum value of curvature 1/R may be, for example, 0.008 or less, 0.006 or less, 0.004 or less, or 0.002 or less.
  • the lower limit is not particularly limited, but the maximum value of the curvature 1/R is, for example, 0.0005 or more, 0.0006 or more, 0.0007 or more, 0.0008 or more, 0.0009 or more, or 0.001 or more. good too.
  • a very high flatness can be achieved in spite of the high strength of 1470 MPa or more, and the extremely high flatness exceeding 1800 MPa as specifically shown in the examples. Even with very high tensile strengths, it is possible to achieve flatness with a maximum value of curvature 1/R of 0.001. Therefore, for lower tensile strengths, e.g. closer to 1470 MPa, one skilled in the art will further reduce the maximum value of curvature 1/R, e.g. It will be readily appreciated that flatness can be achieved.
  • the measurement of the curvature distribution described above is not limited to any specific conditions regarding the timing of measurement and the like. Alternatively, it may be performed on as-manufactured cold-rolled steel sheets that have not undergone any specific mechanical flattening treatment. For example, in the case of a conventional cold-rolled steel sheet having a very high tensile strength of 1470 MPa or more, the maximum value of the curvature 1/R described above is controlled to 0.010 or less even if the flattening treatment is simply performed with a leveler or the like. is extremely difficult.
  • a slab having a predetermined chemical composition is used to produce a cold-rolled steel sheet by appropriately controlling the conditions of the hot rolling process, the cold rolling process, and the heat treatment process, as will be described later in detail.
  • the coating layer does not particularly affect the measurement of the curvature distribution. performed for
  • TS Tensile strength
  • TS tensile strength
  • TS tensile strength
  • TS tensile strength
  • the upper limit is not particularly limited, for example, the tensile strength may be 2000 MPa or less, 1900 MPa or less, or 1800 MPa or less.
  • total elongation (El) According to the cold-rolled steel sheet according to the embodiment of the present invention, high total elongation (El) can be achieved, and more specifically, total elongation of 6.0% or more can be achieved.
  • the total elongation is preferably 7.0% or more, more preferably 8.0% or more.
  • the upper limit is not particularly limited, for example, the total elongation may be 20.0% or less or 15.0% or less.
  • the tensile strength and total elongation of the cold-rolled steel sheet were obtained by collecting a JIS No. 5 tensile test piece from a direction perpendicular to the rolling direction of the steel sheet in the atmosphere at room temperature (25 ° C.), and specified in JIS Z 2241: 2011. It is measured by a tensile test.
  • [Hole expansion ratio ( ⁇ )] According to the cold-rolled steel sheet according to the embodiment of the present invention, high hole expansibility can be achieved, and more specifically, a hole expansibility ( ⁇ ) of 20% or more can be achieved.
  • the hole expansion rate is preferably 25% or more, more preferably 30% or more.
  • the upper limit is not particularly limited, for example, the hole expansion ratio may be 80.0% or less or 70.0% or less.
  • the hole expansion rate ( ⁇ ) is measured according to the Japan Iron and Steel Federation standard "JFS T 1001:1996 hole expansion test method".
  • a cold-rolled steel sheet according to an embodiment of the present invention is characterized in that cracks do not occur in a hydrogen embrittlement test by the following method. Shearing is performed by the method shown in FIG. A sample of T (thickness) x 50W (width) x 50L (length) (unit: mm) is taken from the steel plate so as to include the portion where the maximum value of curvature 1/R is obtained. The shear angle ⁇ is 1 degree, and the clearance CL is 0.15 ⁇ T. A plate pressing pressure of at least 1 ton or more is applied. After cutting the above sample by shearing, the steel plate on the product side (plate holding side) is heat-treated at 170° C. for 10 minutes.
  • the steel plate is immersed in an aqueous solution of ammonium thiocyanate at room temperature with a concentration of 0.3 g/L for 48 hours to introduce the generated hydrogen into the steel plate. After that, the sheared surface is observed with a microscope or the like to evaluate the presence or absence of cracks.
  • Heat treatment at 170° C. for 10 minutes simulates heat treatment such as paint baking treatment.
  • a cold-rolled steel sheet according to an embodiment of the present invention has a thickness of, for example, 0.5 to 3.0 mm.
  • the plate thickness may be 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more.
  • the plate thickness may be 2.8 mm or less, 2.6 mm or less, or 2.3 mm or less.
  • a cold-rolled steel sheet according to an embodiment of the present invention has a width of, for example, 500 mm or more.
  • the plate width may be 700 mm or more, 800 mm or more, or 900 mm or more.
  • the upper limit of the plate width is not particularly limited, the plate width may be 2000 mm or less, 1800 mm or less, 1600 mm or less, 1400 mm or less, 1300 mm or less, 1200 mm or less, or 1100 mm or less.
  • the cold-rolled steel sheet according to the embodiment of the present invention may have a coating layer on both sides or one side, preferably both sides.
  • the plating layer is typically exemplified by an electrogalvanizing layer, a hot-dip galvanizing layer, or an alloyed hot-dip galvanizing layer.
  • These galvanized layers may have any composition known to those skilled in the art, and may contain additive elements such as Al and Mg in addition to Zn.
  • the amount of the plating layer to be deposited is not particularly limited, and may be a general amount of deposition.
  • Rough rolling is performed on the heated slab before finish rolling.
  • Rough rolling conditions are not particularly limited, but it is preferable to carry out rough rolling so that the total rolling reduction is 60% or more at 1050°C. If the total rolling reduction is less than 60%, recrystallization during hot rolling becomes insufficient, which may lead to heterogeneity in the structure of the hot-rolled steel sheet.
  • the above total rolling reduction may be, for example, 90% or less.
  • the width edge portion of the steel sheet that has completed rough rolling is reheated so that the temperature (Te) of the width edge portion is higher than the temperature (Tc) of the width center portion by 10 to 150°C.
  • the width edge portion is hardened more than the width center portion because the subsequent cooling rate is higher in the width edge portion than in the width center portion.
  • a shape defect called “middle elongation” occurs in which the width center portion is elongated compared to the width edge portion.
  • the curvature in the final product is degraded.
  • the width edge portion is excessively heated, the width edge portion becomes excessively soft, resulting in a shape defect called "ear wave" in which the edge portion extends from the center portion in the subsequent cold rolling process.
  • the edge is heated so that the temperature of the width edge portion is 10 to 150° C.
  • Heating (reheating) of the width edge portion can be performed by any suitable means known to those skilled in the art and is not particularly limited, but can be performed using an edge heater, for example.
  • finish rolling After reheating the edge portion, finish rolling is performed.
  • the conditions are not particularly limited. desirable.
  • the finish rolling entry temperature is lower than 950°C
  • the finish rolling exit temperature is lower than 850°C, or the total rolling reduction is higher than 95%
  • the texture of the hot rolled steel sheet develops, so the final product sheet Anisotropy in may become apparent.
  • the finish rolling entry temperature exceeds 1050 ° C.
  • the finish rolling exit temperature exceeds 1000 ° C., or the total rolling reduction is less than 70%
  • the crystal grain size of the hot rolled steel sheet becomes coarse, and the final This may cause coarsening of the product plate structure.
  • the shape of the steel sheet after cold rolling can be improved by coiling the steel sheet after finish rolling at a coiling temperature of 450 to 650°C. If the coiling temperature is lower than 450°C, the strength of the hot-rolled steel sheet increases, and the shape of the steel sheet after cold rolling deteriorates. On the other hand, if the coiling temperature exceeds 650° C., cementite coarsens and undissolved cementite remains, which may impair workability.
  • pickling After hot rolling, if necessary, pickling is performed to remove scales.
  • the pickling method should just follow a conventional method.
  • pretreatment such as skin pass rolling or shot blasting may be performed before pickling.
  • the cold rolling process includes cold rolling the obtained hot rolled steel sheet using a tandem mill consisting of N (N ⁇ 3) rolling stands, and the cumulative cold rolling reduction is A cold rolling process is performed that is 30% or more and satisfies the following equations (2) and (3).
  • Equation (2) means that the value increases when a large reduction is applied in a state where the difference between the forward tension/flow stress and the rear tension/flow stress is large.
  • the difference between the forward tension/flow stress and the rear tension/flow stress should be reduced.
  • the left side of formula (2) is 3.0 or more, the shape of the steel sheet after cold rolling deteriorates significantly, and the curvature of the final product no longer satisfies formula (1).
  • the lower limit is not particularly limited, for example, the left side of Equation (2) may be 0.1 or more or 0.2 or more.
  • Formula (2) is one preferable index for realizing stable cold rolling without rolling defects such as slip by well balancing the tension and the yield strength of the hot-rolled steel sheet before and after each rolling stand. Therefore, in order to realize such stable cold rolling, it is possible to use other control methods instead of the control method according to equation (2).
  • the cumulative cold rolling reduction ratio 30% or more in order to obtain a good steel plate shape with high flatness. If the cumulative cold rolling reduction is less than 30%, the shape of the steel sheet after cold rolling is not sufficiently improved, and as a result the curvature of the final product does not satisfy the formula (1).
  • the cumulative cold rolling reduction may be 40% or more or 50% or more. Although the upper limit is not particularly limited, since excessive reduction causes an excessive rolling load and increases the burden on the cold rolling mill, the cumulative cold rolling reduction may be 75% or less or 70% or less.
  • the obtained cold-rolled steel sheet is subjected to a predetermined heat treatment in the heat treatment step.
  • a predetermined heat treatment in order to sufficiently promote austenitization, heating is performed at Ac 3° C. or higher for 10 seconds or longer. If the heating temperature is less than Ac3°C or the holding time is less than 10 seconds, the austenitization is not sufficient, so the desired steel structure mainly composed of martensite cannot be obtained, and sufficient strength cannot be obtained. . On the other hand, if the heating temperature exceeds 950° C. or the holding time exceeds 500 seconds, the crystal grain size will become coarse, and in addition, fuel costs will increase and equipment will be damaged.
  • Ac3 (°C) is calculated by the following formula.
  • T1 110 to 250° C.
  • the cooling stop temperature may be 120°C or higher and/or may be 220°C or lower.
  • Average cooling rate between 300-700°C: 20-150°C/s By controlling the average cooling rate between 300 to 700 ° C. (average cooling rate 1) in the range of 20 to 150 ° C./s, it is possible to suppress the increase in temperature deviation in the steel plate, so the curvature of the steel plate can be reduced. improvement is possible. If the average cooling rate in the above section is less than 20°C/s, the martensite fraction becomes low and the desired tensile strength cannot be obtained. On the other hand, if it exceeds 150° C./s, the curvature of the steel sheet is deteriorated due to an increase in the temperature deviation within the steel sheet. It should be noted that the average cooling rate in the present invention is a rate including the cooling time described later.
  • Average cooling rate between T1 and 300°C: 1.0 to 20°C/s and refrigerant: gas By setting the average cooling rate between T1 and 300°C (average cooling rate 2) to 1.0 to 20°C/s, and using a gas (for example, nitrogen gas) as a refrigerant for relatively gentle cooling, Since an increase in temperature deviation in the steel sheet can be suppressed, the curvature of the steel sheet can be improved. If the average cooling rate in the section is less than 1.0° C./s, the martensite fraction becomes low, making it impossible to obtain the desired tensile strength. On the other hand, if it exceeds 20° C./s, the curvature of the steel sheet deteriorates due to an increase in the temperature deviation within the steel sheet. Moreover, it is necessary to use a gas as the coolant from the viewpoint of reliably suppressing an increase in the temperature deviation within the steel sheet.
  • a gas for example, nitrogen gas
  • Ms (°C) is calculated by the following formula. The mass % of the element concerned is substituted for the symbol of the element in the following formula. 0% by mass is substituted for elements that are not contained.
  • Ms (°C) 561-474 x C-33 x Mn-17 x Cr-21 x Mo-7.5 x Si + 10 x Co
  • the cold-rolled steel sheet obtained by the cold-rolled steel sheet manufacturing method according to the embodiment of the present invention may be subjected to a post-process such as a plating process for forming a coating layer on one or both sides of the cold-rolled steel sheet.
  • a post-process such as a plating process can be performed by a conventional method.
  • steel having the chemical composition shown in Table 1 was cast to produce a slab.
  • the balance other than the components shown in Table 1 is Fe and impurities.
  • These slabs were subjected to hot rolling including rough rolling and finish rolling under the conditions shown in Table 2 to produce hot rolled steel sheets. Heating (reheating) of the width edge portion after rough rolling was performed using an edge heater.
  • the hot-rolled steel sheet was pickled to remove surface scales, and cold-rolled under the conditions shown in Table 2 using a tandem mill consisting of five rolling stands.
  • the sheet thickness after cold rolling was 1.6 mm, and the sheet width was 1000 mm.
  • the obtained cold-rolled steel sheets were heat-treated under the conditions shown in Table 2. Cooling between the cooling stop temperature T1 and 300° C. was carried out at a predetermined average cooling rate (average cooling rate 2 in Table 2) using nitrogen gas (water in Comparative Example 24) as a coolant.
  • JIS No. 5 tensile test piece was taken from the direction perpendicular to the rolling direction of the steel sheet in the air at room temperature (25°C), and a tensile test was performed in accordance with JIS Z 2241:2011. Tensile strength (TS) and total elongation (El) were measured. In addition, the "JFS T 1001: 1996 hole expansion test method" of the Japan Iron and Steel Federation standard was performed to measure the hole expansion rate ( ⁇ ).
  • the maximum value of curvature 1/R was determined as follows. First, cold-rolled steel sheets, which have not been subjected to a specific mechanical flattening process, are measured at each point in an area of full width x 300 mm length using an ATOS 3D scanner manufactured by GOM. Curvature distribution in the steel plate was obtained. Next, in the curvature distribution thus measured, the larger absolute value of the principal curvatures ⁇ 1 and ⁇ 2 was determined as the maximum value of the curvature 1/R.
  • the hydrogen embrittlement resistance was evaluated by the hydrogen embrittlement test using the shearing shown in Fig. 1. Specifically, first, a sample of T (thickness) ⁇ 50W (width) ⁇ 50L (length) (unit: mm) was taken from the steel plate so as to include a portion where the maximum value of curvature 1/R was obtained. . The shear angle ⁇ was 1 degree, the clearance CL was 0.15 ⁇ T, and the plate pressing pressure was 1 ton or more. After cutting the above sample by shearing, the steel plate on the product side (plate holding side) was heat-treated at 170° C. for 10 minutes.
  • the steel sheet was immersed in an aqueous ammonium thiocyanate solution having a concentration of 0.3 g/L and a concentration of 3 g/L at room temperature for 48 hours to introduce hydrogen into the steel sheet.
  • the sheared surface was observed with a microscope to evaluate the presence or absence of cracks. Those in which cracks were observed at 0.3 g / L were x (failed), cracks were not observed at 0.3 g / L, but cracks were observed at 3 g / L, ⁇ (accepted), 0 3 g/L and 3 g/L were evaluated as ⁇ (accepted) when cracks were not observed.
  • Comparative Example 2 the maximum value of the curvature 1/R increased and the hydrogen embrittlement resistance deteriorated because the expression (2) was not satisfied in the cold rolling process.
  • Comparative Examples 3 and 12 the difference between the temperature of the width edge portion and the temperature of the width center portion of the steel plate after rough rolling was not appropriate in the hot rolling process, so the maximum value of the curvature 1/R increased, and the hydrogen resistance increased. Embrittlement properties decreased.
  • Comparative Example 4 since the cumulative cold rolling reduction was low in the cold rolling process, the maximum value of the curvature 1/R was increased and the hydrogen embrittlement resistance was lowered.
  • Comparative Example 5 since the cooling stop temperature T1 was low in the heat treatment step, retained austenite was not sufficiently formed, and El decreased.
  • Examples 1, 20 to 22, and 25 to 44 of the present invention by having a predetermined chemical composition and steel structure and further controlling the maximum value of the curvature 1/R to 0.010 or less, , a cold-rolled steel sheet having high tensile strength and total elongation and improved hydrogen embrittlement resistance could be obtained.

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Abstract

La présente invention concerne une tôle d'acier laminée à froid caractérisée en ce qu'elle présente une composition chimique prescrite et en ce qu'elle est telle que : une structure en acier est composée, en termes de pourcentage surfacique, de 90,0 à 99,5 % de martensite, de 0 à 5 % de ferrite et de 0,5 à 7,0 % d'austénite résiduelle, le reste étant de la bainite ; le pourcentage de martensite revenue dans la martensite totale est de 80 à 100 % ; la valeur maximale d'une courbure 1/R représentée par la formule (1) ci-dessous et obtenue en mesurant la forme d'une zone égale à la largeur totale multipliée par une longueur de 300 mm est inférieure ou égale à 0,010 ; et la résistance à la traction est supérieure ou égale à 1470 MPa.
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JP2013227657A (ja) 2012-03-29 2013-11-07 Kobe Steel Ltd 鋼板形状に優れた高強度冷延鋼板の製造方法
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JP2016153524A (ja) 2015-02-13 2016-08-25 株式会社神戸製鋼所 切断端部での耐遅れ破壊特性に優れた超高強度鋼板
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