WO2024203411A1 - 鋼板、部材およびそれらの製造方法 - Google Patents

鋼板、部材およびそれらの製造方法 Download PDF

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Publication number
WO2024203411A1
WO2024203411A1 PCT/JP2024/010116 JP2024010116W WO2024203411A1 WO 2024203411 A1 WO2024203411 A1 WO 2024203411A1 JP 2024010116 W JP2024010116 W JP 2024010116W WO 2024203411 A1 WO2024203411 A1 WO 2024203411A1
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Prior art keywords
less
temperature
steel sheet
cooling
average
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PCT/JP2024/010116
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English (en)
French (fr)
Japanese (ja)
Inventor
琴未 野口
洋一郎 松井
三周 知場
英之 木村
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JFE Steel Corp
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JFE Steel Corp
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Priority to JP2024542204A priority Critical patent/JP7635896B1/ja
Priority to EP24779540.4A priority patent/EP4660347A1/en
Priority to CN202480020355.9A priority patent/CN120936735A/zh
Priority to KR1020257031251A priority patent/KR20250150126A/ko
Publication of WO2024203411A1 publication Critical patent/WO2024203411A1/ja
Priority to MX2025011271A priority patent/MX2025011271A/es
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • 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
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to high-strength steel plates and components with excellent formability that are used in a variety of applications, such as automobiles and home appliances, and are suitable for energy absorbing components, as well as methods for manufacturing the same.
  • Patent Document 1 specifies, in mass %, C: 0.15% or more and 0.30% or less, P: 0.040% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0060% or less, one or two of Si and Al: 0.70% or more and 2.50% or less in total, one or two of Mn and Cr: 1.50% or more and 3.5 0% or less, Mo: 0% or more, 1.00% or less, Ni: 0% or more, 1.00% or less, Cu: 0% or more, 1.00% or less, Nb: 0% or more, 0.30% or less, Ti: 0% or more, 0.30% or less, V: 0% or more, 0.30% or less, B: 0% or more, 0.0050% or less, Ca: 0% or more, 0.0400% or less, Mg: 0% or more, 0.0400% or less, and RE It is disclosed that the steel contains M: 0% or more and 0.0400% or less, with
  • Patent Document 2 discloses that a steel composition contains, in mass %, C: 0.07 to 0.20%, Si: 0.1 to 2.0%, Mn: 2.0 to 3.5%, P: 0.05% or less, S: 0.05% or less, and Sol.Al: 0.005 to 0.1%, with the balance being Fe and unavoidable impurities, and the steel structure contains, in area ratio, ferrite: 60% or less, tempered martensite: 40% or more, and fresh martensite: 10% or less, and the void number density of a bent portion in a VDA bending test is 1500 voids/mm2 or less , thereby obtaining a high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and excellent fracture resistance characteristics during a collision.
  • Patent Document 3 discloses that a high-strength cold-rolled steel sheet with excellent strength, ductility, and hole expandability can be obtained by containing, by mass%, C: 0.10-0.40%, Mn: 0.5-4.0%, Si: 0.005-2.5%, Al: 0.005-2.5%, Cr: 0-1.0%, with the remainder being iron and unavoidable impurities, with P limited to 0.05%, S limited to 0.02%, and N limited to 0.006%, with the steel structure containing, by area ratio, 2-30% retained austenite and limited to 20% martensite, with the average grain size of cementite being 0.01 ⁇ m or more and 1 ⁇ m or less, and with the cementite containing 30% or more and 100% or less of cementite with an aspect ratio of 1 or more and 3 or less.
  • Patent Document 1 granular bainite is used to minimize the difference in hardness between different phases in a dual-phase steel sheet, and the decrease in stretch flangeability that accompanies an increase in ferrite is suppressed. This makes it possible to manufacture steel sheets with excellent ductility and stretch flangeability, but does not take into account axial crush properties.
  • Patent Document 2 describes how reducing void density makes it possible to manufacture steel sheets with excellent axial crushing properties; however, because it does not contain the bainite phase, which has a hardness intermediate between ferrite and tempered martensite, there is a large difference in hardness between the different phases, resulting in poor stretch flangeability.
  • Patent Document 3 describes how controlling the shape of cementite makes it possible to obtain steel sheets with excellent ductility and stretch flangeability, but does not take into account axial crushing properties, and cracks occur starting from the cementite during bending deformation, resulting in poor axial crushing properties.
  • the present invention has been made to solve the above problems, and aims to provide steel plates and components having a tensile strength (TS) of 780 MPa or more, high uniform elongation and stretch flangeability, and excellent axial crushing properties, as well as methods for manufacturing the same.
  • TS tensile strength
  • the tensile strength is measured by a tensile test conforming to JIS Z 2241 (2011).
  • high ductility refers to a uniform elongation (U-El) measured in a tensile test according to JIS Z 2241 (2011): (A) When TS is in the range of 780 MPa or more and less than 980 MPa, U-El is 16.0% or more, (B) When TS is in the range of 980 MPa or more and less than 1180 MPa, U-El is 11.0% or more, (C) It means that U-El is 8.0% or more in the range where TS is 1180 MPa or more.
  • excellent axial crushing properties are defined as steel sheets having a VDA bending angle ⁇ of 70° or more in VDA bending.
  • the VDA bending test refers to a bending test (Verband der Automobilindustrie: VDA bending test) in accordance with the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry, and is a three-point plate bending test characterized by very narrowly spaced rolls and a sharp punch.
  • the VDA bending test uses a 60 mm x 60 mm square test piece, with the bending ridge direction parallel to the rolling direction, supported by rolls with a roll diameter D of 30 mm and a roll distance L of (plate thickness a 0 x 2) + 0.5 mm, and is performed by pressing from above with a punch with a tip r of 0.4 mm at a stroke speed of 20 mm/min.
  • the VDA bend angle ⁇ is the bend angle (° (more specifically, the unit is "°/mm", but hereinafter will be expressed as "°")) calculated using the stroke S (mm) at the maximum load in the bending test using formulas (1) to (5), and can be used as an index of axial crush properties.
  • the stroke S (mm) at the maximum load is the length (mm) traveled by the punch from the start of the test to the point at which the maximum load is obtained.
  • the present inventors have conducted extensive research into a steel plate having high strength, excellent ductility and stretch flangeability, and further excellent axial crush resistance, and have reached the following conclusions.
  • an annealing treatment is performed at an annealing temperature of 750°C or more and Ac3 temperature (°C) or less, and cooling is performed at a predetermined cooling rate from the annealing temperature, thereby controlling the total area ratio of ferrite and bainitic ferrite to 10% or more and 60% or less.
  • the cooling stop temperature is set to 200° C.
  • the martensite transformation start temperature Ms point (° C.) ⁇ 30° C.) or lower thereby partially replacing austenite with martensite and/or bainite.
  • the martensite and bainite thus produced enrich carbon in the surrounding untransformed austenite in the overaging treatment step after cooling is stopped, thereby ensuring a predetermined C concentration in the retained austenite and contributing to improved ductility.
  • a retention treatment can be performed during cooling, and by retaining the material in a temperature range from 500°C to the martensitic transformation start temperature Ms and 320°C or higher for 10 s to 60 s, more bainite can be secured and further ductility can be improved.
  • the steel sheet is heated to a heating temperature of 300°C or more and 450°C or less.
  • the steel sheet is cooled to a temperature 30°C or more and 150°C or less lower than the heating temperature at an average cooling rate of 0.5°C/s or more, and further cooled to a temperature of 150°C or more and 350°C or less at an average cooling rate of 0.1°C/s or more.
  • the steel sheet is held in a temperature range of 150°C or more and 350°C or less for 20 s or more and 1000 s or less.
  • the heat treatment process (a process of heating to 300 to 450°C after cooling is stopped, and then finally holding the temperature in the range of 150°C to 350°C for 20 s to 1000 s) promotes carbon distribution from the surrounding structure to the untransformed austenite region, thereby making it possible to further increase the carbon concentration of the retained austenite and improve ductility.
  • the present invention has been made based on the above findings, and specifically provides the following. [1] In mass%, C: 0.10% or more and 0.30% or less, Si: 0.5% or more and 2.0% or less, Mn: 1.5% or more and 3.0% or less, P: 0.10% or less, S: 0.020% or less, sol.
  • the composition further includes, in mass%, Ti: 0.100% or less, Nb: 0.100% or less, V: 0.100% or less, B: 0.0050% or less, Cr: 1.000% or less, Cu: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less,
  • a steel slab having the composition according to the above [1] or [2] is hot-rolled and cold-rolled, and then the obtained cold-rolled steel sheet is subjected to Annealing temperature: After annealing at 750 ° C. or higher and A c3 temperature (° C.) or lower, Cooling is performed at an average cooling rate CR1 of 3° C./s or more and 100° C./s or less in the range from the annealing temperature to a temperature T1 of 200° C. or more and (Ms point (° C.)-30° C.) or less, Heating the temperature range from the temperature T1 to a temperature T2 of 300° C. or more and 450° C.
  • a method for manufacturing a steel plate comprising cooling the steel plate to a temperature of 50°C or less at an average cooling rate of CR4: 1°C/s or more.
  • a steel slab having the composition according to the above [1] or [2] is hot-rolled and cold-rolled, and then the obtained cold-rolled steel sheet is subjected to Annealing temperature: After annealing at 750 ° C. or higher and A c3 temperature (° C.) or lower, Cooling the steel sheet in the temperature range from the annealing temperature to 500° C.
  • Cooling is performed at an average cooling rate CR7 of 3° C./s or more and 100° C./s or less in a temperature range from the retention end temperature T5 to a temperature T1 of 200° C.
  • a method for manufacturing a steel plate comprising cooling the steel plate to a temperature of 50°C or less at an average cooling rate of CR4: 1°C/s or more.
  • the present invention it is possible to obtain a high-strength steel sheet having a tensile strength (TS) of 780 MPa or more and excellent ductility, stretch flangeability and axial crush resistance.
  • the steel plate of the present invention has excellent axial crushing properties and is therefore suitable for energy absorbing members.
  • FIG. 1 is a photograph of the structure of an example of the steel sheet of the present invention.
  • FIG. 2 is a diagram for explaining the method for manufacturing a steel sheet according to the present invention.
  • FIG. 3 is a diagram for explaining a method for calculating the VDA bending angle.
  • the steel plate of the present invention has, in mass %, C: 0.10% to 0.30%, Si: 0.5% to 2.0%, Mn: 1.5% to 3.0%, P: 0.10% or less, S: 0.020% or less, sol.
  • the steel sheet has a component composition containing Al: 1.00% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities, and has a steel structure containing, by area ratio, a structure consisting of one or more of ferrite and bainitic ferrite: 10% or more and 60% or less, a structure consisting of one or more of tempered martensite and lower bainite: 20% or more and 80% or less, retained austenite: 5% or more, and fresh martensite: 10% or less, wherein the average C concentration in the retained austenite is 0.60 mass% or more, and the content of Fe element present as carbides in the tempered martensite in a region within 100 ⁇ m from the steel sheet surface in the sheet thickness direction is 0.20 mass% or less on average.
  • C 0.10% or more and 0.30% or less
  • C is essential for securing the amount of tempered martensite and the amount of retained austenite that is stable at room temperature.
  • C stabilizes the retained austenite by concentrating in the retained austenite, thereby improving ductility. If the C content is less than 0.10%, these effects cannot be fully exerted, and it becomes difficult to ensure the strength and workability of the steel sheet. It is necessary to make it 0.10% or more, preferably 0.12% or more, and more preferably 0.15% or more.
  • the C content must be 0.30% or less, preferably 0.28% or less, and more preferably 0.25% or less.
  • Si 0.5% or more and 2.0% or less
  • Silicon is an element that is useful for improving ductility and axial crushing properties by suppressing the formation of carbides in martensite and bainite, promoting the formation of retained austenite, and improving the stability of retained austenite.
  • the Si content must be 0.5% or more, preferably 0.6% or more, and more preferably 0.7% or more.
  • the Si content exceeds 2.0%, excessive strength is increased, which leads to a decrease in ductility and stretch flangeability, and also to an increase in the rolling load during hot rolling and a decrease in the strength of the welded material to the galvanized material.
  • the Si content is set to 2.0% or less, preferably 1.5% or less, and more preferably 1.0% or less.
  • Mn 1.5% or more and 3.0% or less
  • Mn is effective in securing a predetermined area ratio of tempered martensite and/or bainite to ensure strength.
  • Mn stabilizes the retained austenite by lowering the Ms point of the retained austenite, thereby reducing the residual austenite.
  • Mn is an important element for increasing the area ratio of austenite and improving ductility. Therefore, the Mn content must be 1.5% or more, preferably 1.8% or more, and more preferably 1.5% or more. It is 2.0% or more.
  • the Mn content exceeds 3.0% not only will excessive strength be obtained, but the bainite transformation will be significantly delayed, resulting in a decrease in ductility.
  • the Mn content is set to 3.0% or less, preferably 2.8% or less, and more preferably 2.5% or less.
  • P 0.10% or less
  • P is an element that strengthens steel, but if its content is high, it deteriorates spot weldability. Therefore, the P content is set to 0.10% or less, and preferably 0.02% or less. P may not be contained, but the P content is preferably 0.001% or more from the viewpoint of production costs.
  • S has the effect of improving the scale peeling property during hot rolling and the effect of suppressing nitriding during annealing, but it is an element that has a significant adverse effect on bendability, hole expandability, and spot weldability.
  • the S content is set to at least 0.020% or less.
  • the S content is set to 0.0020% or less from the viewpoint of suppressing the deterioration of bendability and hole expandability. It is preferable that the S content is less than 0.0010%, and more preferable that the S content is less than 0.0010%.
  • the S content is preferably 0.0001% or more from the viewpoint of production costs. The content is preferably 0.0005% or more.
  • sol. Al 1.00% or less
  • Al is an element that is effective in suppressing the formation of carbides and promoting the formation of retained austenite. It is also an element that is added as a deoxidizer in the steelmaking process. Therefore, although there is no particular lower limit for the sol. Al content, it is preferably 0.005% or more, and more preferably 0.01% or more in order to perform stable deoxidation. On the other hand, if the sol. Al content exceeds 1.00%, the amount of inclusions in the steel sheet increases, deteriorating the ductility. Therefore, the sol. Al content is set to 1.00% or less, preferably 0.50% or less. The sol. Al content is more preferably 0.25% or less, and further preferably 0.10% or less.
  • N is an element that forms nitrides such as BN, AlN, and TiN in steel, and reduces the hot ductility of steel and the surface quality. If the N content exceeds 0.015%, the surface quality is significantly deteriorated. Therefore, the N content is set to 0.015% or less, and preferably 0.010% or less. The N content is more preferably 0.005% or less, and further preferably 0.002% or less. Although N does not necessarily have to be contained, the N content is preferably 0.0001% or more from the viewpoint of production costs, and more preferably 0.001% or more.
  • the balance other than the above is Fe and unavoidable impurities. It is preferable that the steel sheet of the present invention contains the above basic components, and the balance has a composition consisting of iron (Fe) and unavoidable impurities.
  • composition of the steel sheet of the present invention may contain the following optional elements as appropriate in addition to the above components.
  • Ti, Nb and V form fine precipitates during hot rolling or annealing to increase strength.
  • Ti is 0.010% or more and Nb is 0.
  • the content of V is 0.020% or more, and the content of V is 0.020% or more.
  • the content of Ti, Nb and V exceeds 0.100%, the formability decreases. Therefore, when Ti, Nb and V are added, their contents should be 0.100% or less. It is preferable that each of them is 0.080% or less, more preferably 0.080% or less, and further preferably 0.050% or less.
  • B is an element that improves the hardenability of steel and has the advantage of easily forming a predetermined area ratio of tempered martensite and/or bainite. To obtain such an effect, B is 0.0005%. It is preferably 0.0010% or more, and more preferably 0.0010% or more. On the other hand, if the content of B exceeds 0.0050%, not only does the effect saturate, but it also causes a significant decrease in hot ductility and causes surface defects. The amount is preferably 0.0050% or less, more preferably 0.0040% or less, and further preferably 0.0025% or less.
  • Cr and Cu not only play a role as solid solution strengthening elements, but also stabilize austenite during the cooling process during annealing and facilitate the formation of a composite structure. Therefore, it is not necessary to include Cr and Cu. However, in order to obtain such effects, the Cr and Cu contents are each preferably 0.005% or more, more preferably 0.008% or more, and further preferably 0.010% or more. On the other hand, if the Cr content or Cu content exceeds 1.000%, the formability of the steel sheet decreases. Therefore, when Cr and Cu are added, their contents are preferably 1.000% or less, The content of each of Cr and Cu is preferably 0.800% or less, and more preferably 0.400% or less. The content of each of Cr and Cu is preferably 0.200% or less, and more preferably 0.100% or less.
  • Sb and Sn are elements effective in suppressing decarburization in a region of the steel sheet surface of about several tens of ⁇ m that occurs due to nitridation and oxidation of the steel sheet surface. Therefore, it is not always necessary to add Sb and Sn, but if added, the content of Sb and Sn is The amount of each of these elements is preferably 0.002% or more, more preferably 0.004% or more, and further preferably 0.006% or more. On the other hand, if the content of any of these elements exceeds 0.200%, it will cause a decrease in toughness. Therefore, when Sb and Sn are added, the content of Sb and Sn should be 0.200% or less. is preferable, more preferably 0.100% or less, and further more preferably 0.040% or less.
  • Ta 0.100% or less
  • Ta like Ti and Nb, produces alloy carbides and alloy carbonitrides, contributing to high strength.
  • Ta is partially dissolved in Nb carbides and Nb carbonitrides to produce composite precipitates such as (Nb,Ta)(C,N), which is thought to have the effect of significantly suppressing the coarsening of precipitates and stabilizing the contribution rate to the improvement of the strength of the steel sheet by precipitation strengthening.
  • Ta is preferably 0.005% or more.
  • the Ta content is preferably 0.100% or less, more preferably 0.080% or less, and further preferably 0.050% or less.
  • Ca 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less
  • Ca, Mg and REM are elements used for deoxidization, and are also effective in spheroidizing the shape of sulfides and improving the adverse effects of sulfides on local ductility and stretch flangeability.
  • the Ca content is 0.0001% or more
  • the Mg content is 0.0001% or more
  • the REM content is 0.0001% or more.
  • when Ca, Mg and REM are added in excess of 0.0050% they cause an increase in inclusions and lead to defects on the surface and inside.
  • the contents of Ca, Mg and REM are set at The content is preferably 0.0050% or less, more preferably 0.0025% or less, and further preferably 0.0010% or less.
  • the Ca content is more preferably 0.0008% or less.
  • the Mg content is more preferably 0.0008% or less.
  • the REM content is more preferably 0.0008% or less.
  • REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
  • the REM content in the present invention is the total content of one or more elements selected from the above-mentioned REM. There are no particular limitations on the REM, but it is preferable that they are La and/or Ce.
  • the optional elements contained in amounts less than the preferred lower limit do not impair the effects of the present invention. If the optional elements are contained in amounts less than the preferred lower limit, the optional elements are considered to be contained as unavoidable impurities.
  • ferrite and bainitic ferrite [Area ratio of structure consisting of one or more of ferrite and bainitic ferrite: 10% to 60%]
  • the ferrite and bainitic ferrite formed during annealing and cooling contribute to improving ductility, and also contribute to stabilizing the retained austenite by concentrating carbon in the surrounding untransformed austenite. This effect can be obtained by setting the total area ratio of ferrite and bainitic ferrite to 10% or more.
  • the total area ratio of ferrite and bainitic ferrite exceeds 60%, a difference in hardness with the surrounding hard phases such as martensite occurs, and cracks may develop from the interface with the hard phase during bending, resulting in a decrease in stretch flangeability and axial crushing properties.
  • the area ratio of a structure consisting of one or more of ferrite and bainitic ferrite is set to 10% or more and 60% or less. This area ratio is preferably 15% or more, and more preferably 20% or more. Moreover, this area ratio is 55% or less, and more preferably 50% or less. Furthermore, although not particularly limited, of ferrite and bainitic ferrite, the area ratio of ferrite in the entire structure is preferably 15% or more, more preferably more than 15%, and even more preferably 20% or more. Furthermore, of ferrite and bainitic ferrite, the area ratio of ferrite in the entire structure is preferably 40% or less, and more preferably 30% or less.
  • the total area ratio of tempered martensite and lower bainite is set to 20% or more, preferably 30% or more, and more preferably 40% or more.
  • the total area ratio of tempered martensite and lower bainite exceeds 80%, ductility decreases due to excessively high strength, so the total area ratio of tempered martensite and lower bainite is set to 80% or less, preferably 70% or less, and more preferably 60% or less.
  • the area ratio of the retained austenite to the entire steel structure is set to 5% or more, more preferably 7% or more, and further preferably 10% or more.
  • the amount of retained austenite includes the retained austenite present between bainitic ferrite.
  • the area ratio of the retained austenite present between bainitic ferrite can be measured in the same manner as the area ratios of other retained austenite by the measurement method described later.
  • the area ratio of retained austenite is preferably 20% or less, and more preferably 15% or less.
  • the area ratio of fresh martensite is set to 10% or less, preferably 8% or less, and more preferably 5% or less. Also, the area ratio of fresh martensite may be 0%.
  • the total proportion of the four types of tempered martensite, lower bainite, retained austenite, and fresh martensite in the remaining structure of the above ferrite and bainitic ferrite be 60% or more.
  • the average C concentration in the retained austenite is set to 0.60 mass% or more, and preferably 0.80 mass% or more.
  • the average C concentration in the retained austenite is preferably 2.0 mass% or less, and more preferably 1.5 mass% or less.
  • the content of Fe element present as carbide in tempered martensite is preferably 0.20 mass% or less on average, more preferably 0.15 mass% or less.
  • the content of the Fe element is substantially 0.001 mass% or more.
  • the steel structure of the present invention is a steel structure containing ferrite, bainitic ferrite, tempered martensite, lower bainite, retained austenite, and fresh martensite (including 0%).
  • the steel structure of the present invention may be a steel structure consisting of ferrite, bainitic ferrite, tempered martensite, lower bainite, retained austenite, and fresh martensite (including 0%).
  • the steel structure of the present invention may contain up to 5% of pearlite or cementite as phases other than those mentioned above, and these do not impair the effects of the present invention.
  • the area ratios of a structure consisting of one or more of ferrite and bainitic ferrite, a structure consisting of one or more of tempered martensite and lower bainite, and a massive structure MA (Martensite Austenite Constituent) consisting of fresh martensite and retained austenite described below are measured by cutting out a cross section of the sheet thickness perpendicular to the steel sheet surface and parallel to the rolling direction, mirror polishing it, and then etching it with 3 vol% nital, and observing it at a position 1/4 of the sheet thickness with a magnification of 5000 using an SEM with a field of view of 20 ⁇ m ⁇ 20 ⁇ m, in 10 fields of view.
  • FIG. 1 shows an example of a SEM photograph of the steel structure of a steel plate.
  • ferrite and bainitic ferrite are regions that are almost free of carbides and appear blackest in the relatively equiaxed SEM.
  • Tempered martensite (see symbol TM in FIG. 1) and lower bainite are regions that are accompanied by lath-shaped substructures and carbide precipitation in the SEM.
  • MA fresh martensite and/or retained austenite
  • FIG. 1 are massive regions that appear white in the SEM with no substructure visible inside.
  • the area fraction of fresh martensite (Fresh Martensite: FM) is calculated by the following formula (7) and is a value obtained by subtracting the area fraction of retained austenite (Retained Austenite: RA) from the area fraction of MA (fresh martensite and/or retained austenite).
  • the "area fraction" of RA (retained austenite) can be considered to be equal to the "volume fraction" of retained austenite determined by XRD measurement.
  • the average content of Fe element present as carbide in the tempered martensite in the region within 100 ⁇ m from the steel sheet surface in the sheet thickness direction is determined by extraction residue analysis. Electrolysis is performed using 10 mass% AA electrolytic solution (acetylacetone-1 mass% tetramethylammonium chloride-methanol) as the electrolytic solution, with a current density of 20 mA/cm 2 and an electrolysis time of 30 min, to dissolve the region from the steel sheet surface to 100 ⁇ m.
  • AA electrolytic solution acetylacetone-1 mass% tetramethylammonium chloride-methanol
  • the sample is immersed in methanol, and the residue (precipitates remaining after electrolytic treatment) peeled off by ultrasonic stirring is filtered using a filter with a pore size of 0.2 ⁇ m, and the amount of Fe element in the collected residue is determined by ICP emission spectrometry and converted into mass% in the steel sheet.
  • the collected residues are composed of precipitates present in the region within 100 ⁇ m from the steel sheet surface, and in this case, the amount of carbides outside the tempered martensite (bainite, ferrite, etc.) is extremely small compared to the amount of carbides in the tempered martensite.
  • Fe element does not exist as an iron-based compound other than carbon-based compounds. Therefore, the above measurement can provide an average content of Fe element that exists as carbides in tempered martensite in a region within 100 ⁇ m from the steel sheet surface in the sheet thickness direction.
  • the above-mentioned steel sheet of the present invention may be a steel sheet having a zinc plating layer on the surface (one side or both sides).
  • the plating layer may be either a hot-dip plating layer or an electroplating layer.
  • the steel plate of the present invention preferably has a thickness of 0.5 mm or more. Also, the thickness is preferably 2.0 mm or less.
  • temperatures specified in each process in this invention refer to the surface temperature of the slab (steel slab) or steel plate.
  • FIG. 2 is a diagram for explaining the steel plate manufacturing method of the present invention, and in particular shows the change in surface temperature of the slab (steel slab) or steel plate over time. Details of each process, including the change in temperature over time, are described below.
  • FIG. 2(a) shows the change in surface temperature of the slab (steel slab) or steel plate over time in the steel plate manufacturing method of the first embodiment (when retention treatment is not performed).
  • FIG. 2(b) shows the change in surface temperature of the slab (steel slab) or steel plate over time in the steel plate manufacturing method of the second embodiment (when retention treatment is performed).
  • a steel slab having the above-mentioned composition is hot-rolled and cold-rolled, and then the obtained cold-rolled steel sheet is annealed at an annealing temperature of 750° C. or more and A c3 temperature (° C.) or less, and then cooled at an average cooling rate CR1 of 3° C./s or more and 100° C./s or less in the range from the annealing temperature to a temperature T1 of 200° C.
  • the temperature range up to T3 is cooled at an average cooling rate CR2: 0.5°C/s or more, and the temperature range from temperature T3 to temperature T4 which is 150°C or more and 350°C or less and equal to or less than temperature T3 is cooled at an average cooling rate CR3: 0.1°C/s or more, the temperature range from temperature T3 to temperature T4 which is 150°C or more and 350°C or less and equal to or less than temperature T3 is held for 20 s to 1000 s, and the temperature range is cooled to a temperature of 50°C or less at an average cooling rate CR4: 1°C/s or more.
  • the hot rolling may be performed according to a conventional method.
  • a steel slab having the above-mentioned predetermined component composition may be heated to a slab heating temperature of 1100° C. or higher, and hot rolled at a finish rolling delivery temperature of the Ar3 transformation point or higher for a soaking time of 20 min or more, and then coiled at a coiling temperature of 400° C. or higher.
  • the slab heating temperature may be 1300° C. or less.
  • the soaking time may be 300 min or less.
  • the finish rolling delivery temperature may be Ar3 transformation point+200° C. or less.
  • the coiling temperature may be 720° C. or less.
  • the coiling temperature is preferably controlled from the viewpoint of suppressing thickness fluctuation and stably ensuring high strength.
  • the coiling temperature is preferably 430° C. or higher.
  • the coiling temperature is preferably 530° C. or lower.
  • the cold rolling may be performed, for example, at a rolling ratio (cumulative rolling ratio) of 30% or more.
  • the rolling ratio may be 85% or less. It is preferable to control the rolling ratio from the viewpoint of stably securing high strength and reducing anisotropy. Specifically, the rolling ratio is preferably 35% or more.
  • the rolling load is high, it is possible to perform softening annealing treatment at 450°C or more and 730°C or less in a CAL (continuous annealing line) or BAF (box annealing furnace).
  • An annealing process A steel slab having a predetermined composition is hot-rolled and cold-rolled, and then annealed under the following conditions.
  • the annealing equipment is not particularly limited, but it is preferable to use a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL) from the viewpoints of productivity and ensuring the desired heating and cooling rates.
  • CAL continuous annealing line
  • CGL continuous hot-dip galvanizing line
  • the annealing temperature is set to 750°C or higher and A c3 temperature (°C) or lower.
  • the annealing temperature is set to A c3 temperature or lower, that is, austenite + ferrite two-phase annealing.
  • the annealing temperature is preferably (A c3 temperature (°C) - 5°C) or lower, and more preferably (A c3 temperature (°C) - 10°C) or lower.
  • the annealing temperature is excessively decreased, in addition to excessive generation of ferrite and bainitic ferrite, the desired tempered martensite cannot be obtained, and this causes a decrease in strength and stretch flangeability.
  • the annealing temperature is less than 750°C, there is a risk that the hot-rolled structure will be inherited, such as insufficient recrystallization and residual carbides generated during hot rolling. Therefore, the annealing temperature is set to 750°C or higher and Ac3 temperature (°C) or lower.
  • the annealing temperature is preferably 755°C or higher, more preferably 760°C or higher.
  • the A c3 temperature can be determined by measuring the volume change when a cylindrical test piece (diameter 3 mm ⁇ height 10 mm) is heated from room temperature (25°C) to the austenite single phase region using a Formaster testing machine.
  • First cooling treatment step cooling from the annealing temperature to a temperature T1 of 200° C. or more and (martensitic transformation start temperature Ms point (° C.) ⁇ 30° C.) or less at an average cooling rate CR1: 3° C./s or more and 100° C./s or less] After the annealing, it is necessary to cool quickly in order to obtain a predetermined area ratio of tempered martensite.
  • the average cooling rate CR1 in the temperature range from the annealing temperature to a temperature T1 (cooling stop temperature T1) of 200°C or more and (martensitic transformation start temperature Ms (°C) -30°C) or less is less than 3°C/s, ferrite is excessively generated in the cooling process, which reduces strength, stretch flangeability, and axial crush properties. Therefore, from the viewpoint of controlling the amount of ferrite generated, the average cooling rate CR1 in the temperature range from the annealing temperature to a cooling stop temperature T1 of 200°C or more and (Ms point (°C) -30°C) or less is set to 3°C/s or more.
  • the average cooling rate CR1 is preferably 5°C/s or more, more preferably 8°C/s or more. On the other hand, if the average cooling rate CR1 is too high, the sheet shape deteriorates, so the average cooling rate CR1 is set to 100° C./s or less, and preferably 50° C./s or less.
  • the temperature T1 (cooling stop temperature T1) is set to 200°C or higher.
  • the temperature T1 (cooling stop temperature T1) is preferably 220°C or higher, and more preferably 240°C or higher. If the cooling stop temperature T1 exceeds (Ms point (°C) - 30°C), a large amount of clumpy untransformed austenite remains, the amount of fresh martensite at the time of final cooling increases, and stretch flangeability decreases. Therefore, the cooling stop temperature T1 is set to (Ms point (°C) - 30°C) or lower. The cooling stop temperature T1 is preferably (Ms point (°C) - 35°C) or lower.
  • the martensitic transformation start temperature Ms point (°C) can be determined by using a Formaster testing machine, using a cylindrical test piece similar to that used in measuring the Ac3 temperature, holding the test piece at a predetermined annealing temperature, and then quenching the test piece with helium gas to measure the volume change.
  • the average cooling rate CR1 is "(annealing temperature (cooling start temperature) (°C) - (Ms point (°C) - 30°C) (cooling stop temperature (°C))) / (cooling time (seconds) from annealing temperature to (Ms point (°C) - 30°C))".
  • Heat treatment process heating from temperature T1 to temperature T2 of 300° C. or more and 450° C. or less at an average heating rate of 2° C./s or more]
  • T1 cooling stop temperature T1: 200°C or higher and (martensitic transformation start temperature Ms point (°C) - 30°C or lower)
  • T2 300°C or higher and 450°C or lower
  • carbide precipitation can be suppressed and at the same time, carbon can be concentrated from the surrounding structure to the untransformed austenite, thereby ensuring high ductility.
  • the temperature T2 is set to 300°C or higher and 450°C or lower.
  • the temperature T2 is preferably 330°C or higher, and more preferably 350°C or higher.
  • the temperature T2 is preferably 420°C or lower, and more preferably 400°C or lower.
  • the average heating rate in the temperature range from temperature T1 (cooling stop temperature T1) to temperature T2 of 300°C or more and 450°C or less is set to 2°C/s or more.
  • the average heating rate is preferably 5°C/s or more, and more preferably 10°C/s or more. There is no particular upper limit to the above average heating rate, but it is preferably 50°C/s or less, and more preferably 30°C/s or less.
  • the average heating rate is "Temperature T2 (heating stop temperature) (°C) - Temperature T1 (heating start temperature) (°C)) / (Heating time from temperature T1 to temperature T2 (seconds))".
  • the rate of carbide precipitation tends to slow down as the heating temperature after cooling is stopped decreases, so by quickly cooling to a temperature range where carbide precipitation can be suppressed at a certain cooling rate or higher, it is possible to suppress carbide precipitation and improve axial crush properties.
  • carbon concentration in the untransformed austenite and tempering of fresh martensite is inhibited, which may result in an excessive increase in strength and deterioration of ductility and stretch flangeability.
  • a second cooling treatment step under specific conditions is performed between the heat treatment step and the third cooling treatment step.
  • the temperature T3 (cooling stop temperature T3) in the second cooling treatment step is set to be equal to or higher than (T2-150°C) and equal to or lower than (T2-30°C).
  • the temperature T3 is preferably equal to or higher than (T2-120°C), and more preferably equal to or higher than (T2-100°C).
  • the temperature T3 is preferably equal to or lower than (T2-50°C), and more preferably equal to or lower than (T2-70°C).
  • the average cooling rate CR2 is 0.5°C/s or more, preferably 0.6°C/s or more, and more preferably 0.7°C/s. Although there is no particular upper limit, a high average cooling rate tends to delay carbon distribution to the retained austenite and tempering of fresh martensite, decreasing ductility and stretch flangeability, so it is preferable that the average cooling rate CR2 is 5°C/s or less.
  • the average cooling rate CR2 is "(temperature T2 (°C) (cooling start temperature) - temperature T3 (°C) (cooling stop temperature)) / (cooling time (seconds) from temperature T2 to temperature T3)".
  • temperature T4 is set to 150° C. or higher and 350° C. or lower. Temperature T4 is preferably 180° C. or higher, and more preferably 200° C. or higher. Temperature T4 is preferably 300° C. or lower, and more preferably 250° C. or lower.
  • the average cooling rate CR3 is 0.1°C/s or more, preferably 0.2°C/s or more, and more preferably 0.3°C/s or more. There is no particular upper limit, but from the viewpoint of sufficiently promoting carbon distribution to untransformed austenite, it is preferable that the average cooling rate CR3 is equal to or less than the average cooling rate CR2 and equal to or less than 1°C/s.
  • the average cooling rate CR3 is "(temperature T3 (°C) (cooling start temperature) - temperature T4 (°C) (cooling stop temperature)) / (cooling time (seconds) from temperature T3 to temperature T4)".
  • the holding temperature is set to be equal to or lower than the above temperature T4. If the holding temperature is less than 150°C, the tempering of fresh martensite does not proceed sufficiently, resulting in an excessive increase in strength, and furthermore, the amount of carbon enriched in the untransformed austenite decreases, which is disadvantageous from the standpoint of improving ductility and stretch flangeability.
  • the holding temperature is set to 150° C. or higher and 350° C. or lower, and is set to be equal to or lower than temperature T4.
  • the holding temperature is preferably 180° C. or higher, and more preferably 200° C. or higher.
  • the holding temperature is preferably 300° C. or lower, and more preferably 250° C. or lower.
  • the holding time is set to 20 s or more and 1000 s or less.
  • the holding time is preferably 100 s or more, and more preferably 200 s or more.
  • the holding time is preferably 700 s or less, and more preferably 500 s or less.
  • the first cooling treatment step (after cooling at an average cooling rate CR1: 3°C/s or more and 100°C/s or less), i.e., before or after any of the steps from the heating treatment step from temperature T1 to temperature T2 described above to the holding treatment step of holding at 150°C or more and 350°C or less, or during any of the steps, it is possible to perform hot-dip galvanizing treatment or further alloying treatment to form a zinc-plated layer on the steel sheet surface. In this case, it is preferable to immerse the steel sheet in a zinc plating bath at 440°C to 500°C, perform hot-dip galvanizing treatment, and then adjust the coating weight by gas wiping or the like.
  • a zinc plating bath with an Al content of 0.10 mass% to 0.22 mass%. Furthermore, after the hot-dip galvanizing treatment, an alloying treatment of the zinc plating can be performed. When performing alloying treatment for zinc plating, it is preferable to carry out the treatment at a temperature range of 470°C to 590°C, and heating up to this temperature range is acceptable since it does not impair the effects of the present invention.
  • the steel sheet can be subjected to skin pass rolling in order to stabilize press formability, such as adjusting the surface roughness and flattening the sheet shape, and to increase the yield strength.
  • the skin pass elongation is preferably 0.1% or more and 0.5% or less.
  • the sheet shape can also be flattened with a leveler.
  • the average cooling rate CR4 from the above-mentioned holding end temperature to a temperature of 50° C. or less is 1° C./s or more, and preferably 5° C./s or more.
  • no upper limit is specified for the average cooling rate CR4, but from the viewpoint of production and from the viewpoint of suppressing transformation from retained austenite to fresh martensite, it is preferably 10° C./s or less.
  • the average cooling rate CR4 is "(temperature between 150°C and 350°C and below temperature T4 (°C) (holding end temperature (cooling start temperature)) -50°C or lower (cooling stop temperature))/(cooling time (seconds) from holding end temperature to cooling stop temperature))".
  • low-temperature heat treatment In order to improve stretch flange formability, it is also possible to perform a low-temperature heat treatment at 100 to 300°C for 30 seconds to 10 days after the above heat treatment or after skin-pass rolling. This treatment causes hydrogen that has entered the steel sheet during tempering or annealing of the martensite formed during final cooling or skin-pass rolling to be released from the steel sheet. Low-temperature heat treatment can reduce hydrogen to less than 0.1 ppm by mass.
  • the steel sheet may be subjected to electrogalvanization.
  • electroplating it is preferable to apply the above-mentioned low-temperature heat treatment from the viewpoint of reducing hydrogen in the steel.
  • a steel slab having the above-mentioned composition is hot-rolled and cold-rolled, and then the obtained cold-rolled steel sheet is annealed at an annealing temperature of 750° C. or more and Ac3 temperature (° C.) or less, and then cooled at an average cooling rate CR5 of 5° C./s or more and 100° C./s or less in a temperature range from the annealing temperature to 500° C. (first cooling treatment before retention treatment), and retained for 10 s to 60 s in a temperature range from 500° C.
  • the material is heated at an average heating rate of 2°C/s or more in a temperature range of 300°C or more to a temperature T2 of 300°C or more and 450°C or less, cooled at an average cooling rate CR2 of 0.5°C/s or more in a temperature range from temperature T2 to a temperature T3 of (T2-150°C) or more and (T2-30°C) or less, cooled at an average cooling rate CR3 of 0.1°C/s or more in a temperature range from temperature T3 to a temperature T4 of 150°C or more and 350°C or less and temperature T3 or less, held in the temperature range of 150°C or more and 350°C or less and temperature T4 or less for 20 s or more and 1000 s or less, and cooled to a temperature of 50°C or less at an average cooling rate CR4 of 1°
  • the hot rolling, cold rolling, and annealing treatment can be performed under the same conditions as in the first embodiment.
  • the treatment in the first cooling treatment step in the first embodiment is a first cooling treatment before the retention treatment, a retention treatment, and a first cooling treatment after the retention treatment.
  • the heating treatment, the second cooling treatment, the third cooling treatment, the holding treatment, and the fourth cooling treatment after the retention treatment and the first cooling treatment can be carried out under the same conditions as the heating treatment, the second cooling treatment, the third cooling treatment, the holding treatment, and the fourth cooling treatment in the first embodiment, respectively.
  • the hot-dip galvanizing treatment can be performed under the same conditions as in the first embodiment, except that, whereas in the first embodiment, the hot-dip galvanizing treatment is performed after the first cooling treatment step (after cooling at an average cooling rate CR7: 3°C/s or more and 100°C/s or less), in the second embodiment, the hot-dip galvanizing treatment is performed after the retention treatment and the first cooling treatment step (after cooling at an average cooling rate CR7: 3°C/s or more and 100°C/s or less).
  • other conditions including the low-temperature heat treatment after the total heat treatment and the conditions for the electroplating treatment can be the same as those in the first embodiment.
  • the first cooling process before the retention process, the retention process, and the first cooling process after the retention process will be mainly described below.
  • the steel is first cooled in the temperature range from the annealing temperature to 500°C at an average cooling rate CR5 of 5°C/s to 100°C/s. If the average cooling rate CR5 is lower than 5°C/s, a large amount of ferrite is generated, resulting in a decrease in strength and a decrease in ⁇ .
  • the average cooling rate CR5 is preferably 8°C/s or more.
  • the average cooling rate CR5 is 100° C./s or less, preferably 50° C./s or less, and more preferably less than 30° C./s.
  • the average cooling rate CR5 is "(annealing temperature (cooling start temperature) (°C) - 500°C (cooling stop temperature)) / cooling time from annealing temperature to 500°C (seconds)".
  • the temperature range is set to be equal to or higher than the Ms point (°C) and equal to or higher than 320°C and equal to or lower than 500°C.
  • the temperature range is preferably equal to or higher than the Ms point (°C) + 15°C, and more preferably equal to or higher than Ms (°C) + 30°C.
  • the temperature range is preferably equal to or higher than 350°C, and more preferably equal to or higher than 380°C.
  • the temperature range is preferably equal to or lower than 480°C, and more preferably equal to or lower than 460°C.
  • the average cooling rate CR6 exceeds 10°C/s, the amount of bainite transformation decreases. Therefore, the average cooling rate CR6 is set to 10°C/s or less. Furthermore, if the residence time is less than 10 s, the desired amount of bainitic ferrite cannot be obtained, and if it exceeds 60 s, C will concentrate from the bainitic ferrite to the lumpy untransformed austenite, leading to an increase in the amount of remaining lumpy structure. Therefore, the residence time is set to 10 s or more and 60 s or less. From the viewpoint of ensuring bainitic ferrite and retained austenite, a residence time of 20 s or more is preferable. Furthermore, from the viewpoint of improving stretch flangeability by reducing the lumpy structure, a residence time of 50 s or less is preferable.
  • the average cooling rate CR6 is "(500°C (retention start temperature) (°C) - (retention stop temperature (°C) above the Ms point (°C) and above 320°C)))/(retention time (seconds) from the retention start temperature to the retention stop temperature)".
  • the average cooling rate CR7 in the temperature range from the retention stop temperature T5 to the temperature T1 (cooling stop temperature T1) which is 200°C or higher and (martensitic transformation start temperature Ms point (°C) - 30°C) or lower is less than 3°C/s, ferrite is generated during the cooling process, which reduces the strength, stretch flangeability, and axial crushing properties. Therefore, from the viewpoint of suppressing the generation of ferrite, the average cooling rate CR7 in the temperature range from the retention stop temperature T5 to the cooling stop temperature T1 which is 200°C or higher and (Ms point (°C) - 30°C) or lower is set to 3°C/s or higher.
  • the average cooling rate CR7 is preferably 5°C/s or higher, more preferably 8°C/s or higher. On the other hand, if the average cooling rate CR7 is too high, the sheet shape deteriorates, so the average cooling rate CR7 is set to 100° C./s or less, and preferably 50° C./s or less.
  • the temperature T1 (cooling stop temperature T1) is set to 200°C or higher.
  • the temperature T1 (cooling stop temperature T1) is preferably 220°C or higher, and more preferably 240°C or higher. If the cooling stop temperature T1 exceeds (Ms point (°C) - 30°C), a large amount of clumpy untransformed austenite remains, the amount of fresh martensite at the time of final cooling increases, and stretch flangeability decreases. Therefore, the cooling stop temperature T1 is set to (Ms point (°C) - 30°C) or lower. The cooling stop temperature T1 is preferably (Ms point (°C) - 35°C) or lower.
  • the average cooling rate CR7 is "(retention stop temperature T5 (cooling start temperature) (°C) - (Ms point (°C) - 30°C) (cooling stop temperature (°C)))/(cooling time from retention stop temperature T5 to cooling stop temperature (seconds))".
  • the member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining processes.
  • the manufacturing method of the member of the present invention also includes a step of subjecting the steel plate of the present invention to at least one of forming and joining processes to form the member.
  • the steel plate of the present invention has a tensile strength of 780 MPa or more, high ductility, excellent stretch flange formability, and excellent axial crushing properties. Therefore, members obtained using the steel plate of the present invention are also high in strength, and have higher ductility, excellent stretch flange formability, and excellent axial crushing properties compared to conventional high-strength members. Furthermore, the use of the members of the present invention makes it possible to reduce weight. Therefore, the members of the present invention can be suitably used, for example, for vehicle body frame parts.
  • the members of the present invention also include welded joints.
  • general processing methods such as pressing can be used without restrictions.
  • general welding methods such as spot welding and arc welding, riveting, crimping, etc. can be used without restrictions.
  • Cold rolled steel sheets having the composition shown in Table 1 and a thickness of 1.4 mm were treated under the annealing conditions shown in Tables 2 and 3 to produce steel sheets according to the present invention and comparative steel sheets.
  • the cold-rolled steel sheet was obtained by subjecting a steel slab having the chemical composition shown in Table 1 to hot rolling (heating temperature: 1250°C, soaking time: 60 min, finish rolling delivery temperature: 1150°C, coiling temperature: 550°C) and cold rolling (rolling ratio (cumulative reduction ratio): 50%).
  • Table 2 shows the conditions under which no retention treatment was performed
  • Table 3 shows the conditions under which retention treatment was performed.
  • hot-dip galvanized steel sheets were subjected to hot-dip galvanizing treatment in any step from heating at temperature T1 until holding at 150° C. or lower and 350° C. or lower, to obtain hot-dip galvanized steel sheets (GI).
  • the steel sheet was immersed in a zinc plating bath at 440°C to 500°C to perform hot-dip galvanizing, and then the coating weight was adjusted by gas wiping or the like.
  • a zinc plating bath containing 0.10% to 0.22% Al was used for the hot-dip galvanizing.
  • some of the hot-dip galvanized steel sheets were subjected to an alloying treatment after the hot-dip galvanizing treatment to obtain alloyed hot-dip galvanized steel sheets (GA).
  • the alloying treatment was performed in a temperature range of 460°C to 590°C.
  • some of the steel sheets (cold-rolled steel sheets: CR) were electroplated to obtain electrogalvanized steel sheets (EG).
  • Table 4 shows the conditions without retention treatment
  • Table 5 shows the conditions with retention treatment
  • tensile strength, ductility From the obtained steel plate, JIS No. 5 tensile test pieces and hole expansion test pieces were taken, and tensile tests (in accordance with JIS Z2241 (2011)) were performed.
  • the tensile strength TS and uniform elongation U-El are shown in Tables 4 and 5. Those having a tensile strength of 780 MPa or more were judged to have excellent strength. Furthermore, the uniform elongation U-El was judged to be excellent when it was 16.0% or more when TS was less than 980 MPa, when it was 11.0% or more when TS was 980 MPa or more and less than 1180 MPa, and when it was 8.0% or more when TS was 1180 MPa or more.
  • the stretch flange formability was evaluated by a hole expansion test in accordance with the provisions of the Japan Iron and Steel Federation standard JFST1001. That is, after punching a sample of 100 mm x 100 mm square size using a punching tool with a punch diameter of 10 mm and a die diameter of 10.3 mm (clearance 13%), the hole was expanded using a conical punch with an apex angle of 60 degrees so that the burrs generated during the formation of the punched hole were on the outside until a crack penetrating the plate thickness occurred.
  • d 0 initial hole diameter (mm)
  • d hole diameter at the time of crack occurrence (mm)
  • the hole expansion ratio ⁇ (%) was calculated as ⁇ (d - d 0 ) / d 0 ⁇ ⁇ 100.
  • the hole expansion ratio ⁇ is shown in Tables 4 and 5. Steel sheets having a ⁇ of 30% or more were judged to have excellent stretch flangeability.
  • FIG. 3 is a diagram for explaining a method for calculating the VDA bending angle.
  • the VDA bending angle ⁇ was evaluated as an evaluation of the axial crushing properties by a bending test (Verband der Automobilindustrie: VDA bending test) in accordance with the VDA standard (VDA238-100) defined by the German Association of the Automotive Industry.
  • the VDA bending test is a three-point plate bending test characterized by a very narrowly spaced roll 10 and a sharp punch 11.
  • the VDA bend angle ⁇ is the bend angle (°) calculated using the formulas (1) to (5) from the stroke S (mm) at the maximum load in the bending test, and can be used as an index of axial crush characteristics. Steel plates with an ⁇ of 70° or more were judged to have excellent axial crush properties.
  • the examples of the present invention shown in Tables 4 and 5 have U-El of 16.0% or more when TS is 780 MPa or more and less than 980 MPa, U-El of 11.0% or more when TS is 980 MPa or more and less than 1180 MPa, U-El of 8.0% or more when TS is 1180 MPa or more, and ⁇ of 30% or more.
  • ⁇ is 70° or more, and the examples have excellent strength, ductility, stretch flangeability, and axial crush properties, whereas the comparative examples are inferior in any of these.
  • the components obtained by forming the steel plate of the present invention, by joining the steel plate, and by forming and joining the steel plate have high strength, high ductility, excellent stretch flange formability, and excellent axial crushing properties, just like the steel plate of the present invention, because the steel plate of the present invention has high strength, high ductility, excellent stretch flange formability, and excellent axial crushing properties.
  • the present invention makes it possible to manufacture steel sheets that are excellent in ductility, stretch flangeability, and axial crushing properties and are used in components for automobiles and home appliances, and are particularly suitable for energy absorbing components in automobiles, and can be preferably applied to the press forming of these components.

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